Total Synthesis of Sphingofungin E and 4,5-Di-epi-sphingofungin E

Abstract

Total synthesis of sphingofungin E and 4,5-di-epi-sphingofungin E was achieved from an intermediate same as that of myriocin and mycestericin D via antipodal stereoselective dihydroxylations.

Myriocin (1),^1–4) mycestericin D (2),^5,6) sphingofungins E (3),⁷⁾ and F⁷⁾ are classified as members of a unique family in sphingosine-related natural products (Fig. 1). They have an additional carbon substituent at the C-2 position and contiguous stereogenic centers. They act as antifungal and immunosuppressive agents through the inhibitory activity of serine palmitoyl transferase.^8–12) Because of interesting structural features and potent biological activities, these natural products have attracted considerable attention of synthetic organic chemists and several total syntheses have already been reported.^13–44) Recently, we reported total synthesis of 1 and 2 from the same intermediate 5, which was derived from commercially available diethyl D-tartrate using rhodium(II)-catalyzed C–H amination, following stereoselective vinylation^45,46) (Chart 1). As part of our synthetic investigations of sphingosine natural products,^47–51) herein, we report the total synthesis of 3 and 4,5-di-epi-sphingofungin E (4) from the intermediate 5 via antipodal stereoselective dihydroxylations.

Fig. 1. Structures of Sphingosine and Its Related Natural Products Having an Additional Carbon Substituent at the C-2 Position

Chart 1. Synthesis of Myriocin (1) and Mycestericin D (2) via Aldehyde 5

Results and Discussion

Our synthetic plan for 3 and 4 is illustrated in Chart 2. By expanding the utility of our recently reported synthetic procedure^45,46) for the synthesis of sphingosine natural products having C-2 quaternary centers, we planned to investigate the synthesis of the more structurally complicated analog 3 and its 4,5-di-epimer 4 from the common intermediate 5. After conversion of aldehyde into alkene, the stereoselective introduction of a diol group via dihydroxylation^52–54) of 7 would result in both vinyl intermediates 8 and 9. Such antipodal diastereoselectivity has already been demonstrated in dihydroxylations of substrates having an allylic and/or homoallylic functionality.^55–62) Finally, cross metathesis reaction with known alkene 6⁶³⁾ would afford 3 and 4.

Chart 2. Synthetic Plan for 3 and 4

The stereoselective dihydroxylation of 7 was investigated, as illustrated in Chart 3. First, we examined a reagent-controlled dihydroxylation method. The Wittig reaction of 5⁴⁵⁾ afforded the α,β-unsaturated ester 7a⁴⁶⁾ (R=CO₂Me). The reactions of 7a with an AD-mix β under several conditions did not proceed at all, probably due to the steric hindrance of the allylic and homoallylic substituents of 7a. Subsequently, we investigated dihydroxylation of 7a under usual reaction conditions [using OsO₄ and N-methylmorpholine N-oxide (NMO)]. The OsO₄/NMO dihydroxylation usually results in diastereoselectivity under steric or stereoelectronic control.^52–54) The reaction of 7a with catalytic amounts of OsO₄ in the presence of NMO proceeded with the complete facial selectivity to produce diol 11 with 89% yield as a single diastereomer.⁶⁴⁾ Donohoe conditions [OsO₄ and tetramethylethylenediamine (TMEDA)]^55–62) were tested with the expectation of antipodal facial selectivity via a hydrogen-bonding mediated process. The dihydroxylation of 7a under Donohoe conditions formed a mixture of products; the major product was also diol 11. Fortunately, the mixture could be separated via chromatography to afford 10 in 16% yield and 11 in 77% yield. The two-step dedihydroxylation⁶⁵⁾ of 11 was achieved by thiocarbonation (93%) of diol and the following reduction of the resulting thiocarbonate 12 with P(OMe)₃ (92%) to afford 7a which can be used for reoxidation to 10 under Donohoe dihydroxylation conditions.

Chart 3. Dihydroxylation of 7a

Second, we investigated synthesis of 3 and 4 from 10 and 11, respectively (Chart 4). The diol moiety of 10 was protected as an acetonide group by treatment with 2,2-dimethoxypropane in the presence of pyridinium p-toluenesulfonate (PPTS) to produce 13. The ester group of 13 was reduced with diisobutylaluminum hydride (DIBAL-H), followed by the Wittig reaction to afford the terminal alkene 8a in 63% yield from 10. The resultant 8a required five equivalents of 6⁶³⁾ in the presence of 30 mol% of Grubbs second generation catalyst to react with reflux methylene chloride due to its low reactivity against the cross-metathesis reaction. (E)-Alkene 14 was selectively produced in 79% yield. The desilylation of 14 with tetrabutylammonium fluoride (TBAF), followed by the sequential oxidation of 15 (Swern oxidation, followed by Pinnick oxidation) to carboxylic acid through 16, and the final deprotection of the amino and hydroxy groups with trifluoroacetic acid (TFA) were achieved to synthesize sphingofungin E (3), which was purified by the acetylation–deacetylation process via 17 (3: [α]_D²⁰ +49.8 (c=0.60, CHCl₃); lit.^33,34) [α]_D^23.2 +48.6 (c=0.27, CHCl₃), 17: [α]_D¹⁹ −6.17 (c=0.60, CH₃OH); lit.³¹⁾ [α]_D²⁴ −5.43 (c=0.48, CH₃OH)). The diastereomeric diol 11 was also effectively converted into 4 in a similar manner.

Chart 4. Synthesis of 3 and 4

Reagents and conditions: (a) Me₂C(OMe)₂, PPTS, benzene; (b) DIBAL-H, CH₂Cl₂, −80°C; (c) Ph₃PCH₃Br, NaH, THF; (d) 6 (5 eq), Grubbs 2nd (30 mol%), CH₂Cl₂, reflux; (e) TBAF, THF, r.t.; (f) DMSO, (COCl)₂, Et₃N, CH₂Cl₂; (g) NaClO₂, NaH₂PO₄·H₂O, 2-methyl-2-butene, tert-BuOH–H₂O; (h) TFA, THF–H₂O; (i) Ac₂O, pyridine, r.t.; (j) 10% NaOH, MeOH; (k) Ph₃PCH₃Br, tert-BuOK, THF; (l) 6 (4 eq), Grubbs 2nd (5 mol%), CH₂Cl₂, reflux.

The OsO₄/TMEDA complex is known to function as a hydrogen-bond acceptor to the dihydroxylate substrate directly in the presence of potential hydrogen-bond donors, such as allylic hydroxy groups.^55–62) We investigated dihydroxylation reactions of alcoholic compounds 18–24 under Donohoe conditions in order to achieve higher stereoselectivity. The results are summarized in Table 1. The oxidation of esters 18 and 19 afforded lactones 25b and 26b with the same stereoselectivity as that in the reaction of 7a (Entries 1, 2). Additionally, the similar reactions of allyl ethers 20–24 showed higher antipodal stereoselectivity than those of 18 and 19, and formed 1 : 1.3–1 : 2.5 mixtures of diastereofacial isomers 27–31 in high yields (Entries 3–7). The resultant oxidized product was converted into the sphingofungin E intermediate (Chart 5). The acetonide protection of the 1 : 1.3 mixture of tetraols 27a and b afforded chromatographically separable di-acetonide 32a and b in 36 and 48% yields, respectively. The deacetylation of 32a with DIBAL-H reduction afforded alcohol 33, which was converted into the key intermediate of 3, terminal alkene 8a, via Swern oxidation, followed by the Wittig reaction.

Table 1. Dihydroxylation of Alkenes 18–24 under Donohoe Conditions

Entry	Alkene	R1	R2	Diol	Yield (%)	a : ba)
1	18	H	CO2Me	25b)	67	25b
2	19	MOM	CO2Me	26b)	74	26b
3	20	H	CH2OAc	27	93	1 : 1.3
4c)	21	H	CH2OAc	28	84	1 : 2.0
5	22	THP	CH2OPiv	29	92	1 : 2.1
6	23	H	CH2OBz	30	85	1 : 1.8
7d)	24	H	CH2OBn	31	86	1 : 2.5

a) Determined by the ¹H-NMR spectrum of a mixture. b) Corresponding γ-lactone was obtained. c) OsO₄ (2.2 eq) and TMEDA (2.6 eq) were used. d) OsO₄ (3.3 eq) and TMEDA (3.9 eq) were used.

Chart 5. Conversion of 27 into 8a

Reagents and conditions: (a) Me₂C(OMe)₂, PPTS, benzene; (b) DIBAL-H, CH₂Cl₂, −80°C; (c) DMSO, (COCl)₂, Et₃N, CH₂Cl₂; (d) Ph₃PCH₃Br, NaH, THF.

In summary, we achieved total synthesis of 3 and 4 via the same synthetic intermediate 5 by the antipodal stereoselective dihydroxylation of alkene. The common dihydroxylation conditions resulted in complete stereoselectivity to afford diol 11, which was converted into 4. Donohoe conditions exhibited the antipodal selectivity; the highest selectivity was obtained by the reaction of the dihydroxyallyl ether 20. Sphingofungin E (3) was synthesized from the antipodal diastereoisomers 10 and 27a.

Experimental

General

Melting points are uncorrected. IR spectra were recorded on a JASCO FT/IR-460 Plus spectrophotometer and absorbance bands are reported in wavenumber (cm⁻¹). All NMR spectra were recorded using a JEOL JNM-ECX400P spectrometer. ¹H-NMR spectra were recorded at 400 MHz. Chemical shifts are reported relative to internal standard (tetramethylsilane at δ_H 0.00, CDCl₃ at δ_H 7.26, or CD₃OD at δ_H 3.31). Data are presented as follows: chemical shift (δ, ppm), multiplicity (s=singlet, d=doublet, t=triplet, m=multiplet, br=broad), coupling constant and integration. ¹³C-NMR spectra were recorded on JEOL JNM-ECX400P (100 MHz) spectrometer. The following internal reference was used (CDCl₃ at δ 77.0 or CD₃OD at δ 49.0). All ¹³C-NMR spectra were determined with complete proton decoupling. High-resolution (HR)-MS were determined with JEOL JMS-GCmate II and JEOL JMS-AX505HAD instruments. Column chromatography was performed on Silica Gel 60 PF₂₅₄ (Nacalai Tesque, Kyoto, Japan) and Kanto silica gel 60 N (63–210 mesh) under pressure. Analytical TLC was carried out on Merck Kieselgel 60 F₂₅₄ plates. Visualization was accomplished with UV light and phosphomolybdic acid stain solution followed by heating.

All reagents are commercially available and were purchased from suppliers such as Sigma-Aldrich Co. (St. Louis, MO, U.S.A.); Wako Pure Chemical Industries, Ltd. (Osaka, Japan); Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan); Nacalai Tesque, Inc. Dehydrated CH₂Cl₂, THF, benzene, acetone, dimethyl sulfoxide (DMSO) and pyridine were purchased from Wako Pure Chemical Industries, Ltd. and Kanto Chemical Co., Inc. (Tokyo, Japan). Pentadec-1-en-9-one (6)⁶³⁾ was prepared according to literature procedures.

Methyl [2R,3S,3(4R,5R)]-3-[5-(tert-Butyldiphenylsilyloxy)methyl-2,2-dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-1,3-dioxane-4-yl]-2,3-dihydroxybutanoate (10) and Methyl [2S,3R,3(4R,5R)]-3-[5-(tert-Butyldiphenylsilyloxy)methyl-2,2-dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-1,3-dioxane-4-yl]-2,3-dihydroxybutanoate (11)

OsO₄ (112 mg, 0.44 mmol) in CH₂Cl₂ (0.6 mL) was added dropwise to a solution of α,β-unsaturated ester 7a⁴⁶⁾ (234 mg, 0.40 mmol) and TMEDA (60 mg, 0.52 mmol) in CH₂Cl₂ (1.5 mL) at −78°C. After stirring for 1 h, the solvent was removed in vacuo. NaHSO₃ (230 mg, 2.2 mmol) was added to a solution of resulting residue in pyridine–H₂O (1 : 1, 2.4 mL). After stirring at room temperature overnight, the reaction was quenched with water and the whole mixture was extracted with EtOAc (3×10 mL). Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 10% EtOAc in hexane) to provide 10 (40 mg, 16%) as a colorless oil and 11 (190 mg, 77%) as a colorless oil. 10: ¹H-NMR (400 MHz, CDCl₃) δ: 7.69–7.65 (4H, m), 7.45–7.36 (6H, m), 6.41 (1H, s), 4.38 (3H, br s), 4.22 (1H, d, J=4.4 Hz), 3.96 (1H, d, J=10.4 Hz), 3.83 (1H, m), 3.79 (3H, s), 3.28 (1H, d, J=10.8 Hz), 3.12 (1H, s), 1.42 (3H, s), 1.40 (3H, s), 1.37 (9H, s), 1.07 (9H, s). ¹³C-NMR (100 MHz, CDCl₃) δ: 173.4, 155.1, 135.7, 135.6, 132.7, 132.6, 129.8, 127.8, 127.7, 99.3, 77.2, 71.4, 70.2, 68.5, 63.4, 56.5, 52.6, 28.4, 27.9, 26.8, 19.6, 19.2. IR (film) cm⁻¹: 3506, 3388, 1740, 1713, 1511. FAB-MS m/z: 618.3123 (Calcd for C₃₂H₄₈NO₉Si: 618.3098). [α]_D²⁶ −4.23 (c=0.5, CHCl₃). 11: ¹H-NMR (400 MHz, CDCl₃) δ: 7.65–7.63 (4H, m), 7.45–7.35 (6H, m), 5.32 (1H, s), 4.90 (1H, br s), 4.47 (1H, d, J=10.0 Hz), 4.38 (1H, d, J=7.8 Hz), 4.05 (1H, d, J=8.8 Hz), 3.96–3.81 (4H, m), 3.78 (3H, s), 2.86 (1H, d, J=7.8 Hz), 1.43 (9H, s), 1.40 (6H, s), 1.05 (9H, s). ¹³C-NMR (100 MHz, CDCl₃) δ: 174.2, 156.7, 135.61, 135.59, 132.8, 132.6, 129.84, 129.79, 127.8, 127.7, 99.2, 80.1, 77.2, 71.9, 70.8, 66.7, 63.1, 56.6, 52.4, 28.2, 26.8, 19.3. IR (film) cm⁻¹: 3540, 3431, 3381, 1747, 1692, 1511. FAB-MS m/z: 618.3137 (Calcd for C₃₂H₄₈NO₉Si: 618.3098). [α]_D²⁰ −6.70 (c=0.5, CHCl₃).

Methyl [2S,3SR,3(4R,5R)]-3-[5-(tert-Butyldiphenylsilyloxy)methyl-2,2-dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-1,3-dioxane-4-yl]-2,3-thiocarbonylbutanoate (12)

1,1′-Thiocarbonyldiimidazole (374 mg, 2.1 mmol) and N,N-dimethyl-4-aminopyridine (DMAP) (64 mg, 0.525 mmol) were added to a solution of diol 11 (650 mg, 1.05 mmol) in CH₂Cl₂ (16 mL). After stirring at reflux for 5 h, the reaction mixture was quenched with saturated NH₄Cl and the whole mixture was extracted with EtOAc (3×10 mL). The combined organic layers were washed with water (10 mL) and brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 10% EtOAc in hexane) to provide 12 (646 mg, 93%) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 7.66–7.62 (4H, m), 7.46–7.40 (6H, m), 5.45 (1H, dd, J=4.4, 2.8 Hz), 5.38 (1H, d, J=4.4 Hz), 5.05 (1H, s), 4.27 (1H, d, J=2.8 Hz), 4.00 (1H, d, J=10.8 Hz), 3.94 (1H, d, J=10.8 Hz), 3.90 (1H, d, J=12.0 Hz), 3.82 (3H, s), 3.80 (1H, d, J=12.0 Hz), 1.43 (3H, s), 1.39 (3H, s), 1.38 (9H, s), 1.09 (9H, s). ¹³C-NMR (100 MHz, CDCl₃) δ: 189.8, 166.5, 155.2, 135.64, 135.57, 132.3, 132.1, 130.11, 130.10, 128.0, 100.7, 83.0, 80.4, 77.9, 72.4, 64.8, 63.6, 56.3, 53.3, 28.2, 27.1, 26.9, 20.2, 19.2. IR (film) cm⁻¹: 3430, 1759, 1712, 1504, 1163. FAB-MS m/z: 660.2645 (Calcd for C₃₃H₄₆NO₈SiS: 660.2662). [α]_D²⁰ +12.3 (c=0.5, CHCl₃).

Conversion of 12 into 7a

P(OMe)₃ (6.0 mL) was added to 12 (646 mg, 0.98 mmol). After stirring at reflux for 5 h, the solvent was removed in vacuo. The resulting residue was purified by column chromatography (silica gel, 10% EtOAc in hexane) to provide 7a (526 mg, 92%) as a colorless oil.

[1R,1(4R,5R),2S,3E]-1-[5-(tert-Butyldiphenylsilyloxy)methyl-2,2-dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-1,3-dioxane-4-yl]-1,2-isopropylidenedioxy-3-butene (8a)

2,2-Dimethoxypropane (64 mg, 0.615 mmol) and PPTS (5 mg, 0.0185 mol) were added to a solution of 10 (38 mg, 0.0615 mmol) in benzene (1.4 mL). After stirring at reflux for 2.5 h, the reaction was quenched with 5% NaHCO₃ and the whole mixture was extracted with EtOAc (3×5 mL). The combined organic layers were washed with water (5 mL) and brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo to provide crude product 13 (32 mg), which was used in the next step without further purification.

DIBAL-H (0.097 mL, 0.097 mmol) was added dropwise to a solution of crude 13 (32 mg) in CH₂Cl₂ (1.0 mL) at −78°C under nitrogen. After stirring for 1 h, the reaction was quenched with saturated potassium sodium tartrate and the whole mixture was extracted with EtOAc (3×5 mL). The combined organic layers were washed with water (5 mL) and brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was filtered through a pad of Celite to provide crude aldehyde (29 mg), which was used in the next step without further purification.

Ph₃PCH₃Br (191 mg, 0.535 mmol) was added slowly to a suspension of NaH (60% in mineral oil, 19 mg, 0.486 mmol) in THF (2.0 mL) at 0°C. After stirring for 3 h under nitrogen at room temperature, the reaction mixture was cooled to −80°C and crude aldehyde (29 mg) in THF (1.0 mL) was added dropwise to the reaction mixture, and the resulting mixture was warmed to room temperature and stirred for 2 h. The reaction mixture was quenched with saturated NH₄Cl and the whole mixture was extracted with EtOAc (3×5 mL). The combined organic layers were washed with water (5 mL) and brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 10% EtOAc in hexane) to provide 8a (24 mg, 63% for 3 steps) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 7.63–7.59 (4H, m), 7.45–7.35 (6H, m), 6.30 (1H, s), 5.71 (1H, ddd, J=16.8, 10.6, 8.0 Hz), 5.35 (1H, d, J=16.8 Hz), 5.25 (1H, d, J=10.6 Hz), 4.47 (1H, dd, J=8.4, 8.0 Hz), 4.38 (1H, d, J=11.8 Hz), 4.10–4.06 (2H, m), 3.99 (1H, d, J=10.6 Hz), 3.95 (1H, d, J=11.8 Hz), 3.87 (1H, d, J=10.6 Hz), 1.45 (6H, s), 1.42 (6H, s), 1.39 (9H, s), 1.05 (9H, s). ¹³C-NMR (100 MHz, CDCl₃) δ: 155.1, 135.62, 135.55, 135.1, 132.84, 132.79, 129.84, 129.80, 127.73, 127.70, 119.9, 110.1, 99.5, 78.9, 78.7, 78.4, 67.3, 63.8, 63.2, 57.3, 28.0, 27.3, 27.2, 26.9, 26.2, 20.3. IR (film) cm⁻¹: 3402, 3073, 1713, 1516, 990, 903. FAB-MS m/z: 626.3543 (Calcd for C₃₅H₅₂NO₇Si: 626.3513). [α]_D²¹ +8.30 (c=0.5, CHCl₃).

[1R,1(4R,5R),2S,3E]-1-[5-(tert-Butyldiphenylsilyloxy)methyl-2,2-dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-1,3-dioxane-4-yl]-1,2-isopropylidenedioxy-heptadec-3-en-11-one (14)

Pentadec-1-en-9-one (6)⁶³⁾ (85 mg, 0.405 mmol) and second generation Grubbs catalyst (21 mg, 0.0243 mmol) were added to a solution of alkene 8a (51 mg, 0.081 mmol) in CH₂Cl₂ (2.0 mL). After stirring at reflux overnight, the solvent was removed in vacuo. The resulting residue was purified by column chromatography (silica gel, 5% EtOAc in hexane) to provide 14 (52 mg, 79%) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 7.63–7.59 (4H, m), 7.45–7.36 (6H, m), 6.34 (1H, s), 5.79 (1H, dt, J=14.7, 7.6 Hz), 5.34 (1H, dd, J=14.7, 8.4 Hz), 4.45 (1H, dd, J=8.4, 8.0 Hz), 4.37 (1H, d, J=11.6 Hz), 4.08–3.99 (3H, m), 3.95 (1H, d, J=11.6 Hz), 3.86 (1H, d, J=10.4 Hz), 2.39–2.36 (4H, m), 1.98–1.94 (2H, m), 1.57–1.52 (4H, m), 1.44 (3H, s), 1.43 (3H, s), 1.42 (6H, s), 1.32–1.23 (12H, m), 1.05 (9H, s), 0.88 (3H, t, J=6.8 Hz). ¹³C-NMR (100 MHz, CDCl₃) δ: 211.4, 155.1, 137.9, 135.6, 135.5, 132.8, 129.9, 127.73, 127.68, 126.2, 109.6, 99.5, 78.9, 78.5, 77.2, 67.2, 63.8, 63.3, 57.4, 42.8, 42.7, 32.3, 31.6, 29.0, 28.9, 28.7, 28.4, 28.3, 27.5, 27.4, 27.1, 26.9, 26.2, 23.8, 23.7, 22.4, 20.5, 19.2, 14.0. IR (film) cm⁻¹: 3402, 1722, 1713, 1516, 971, 903. FAB-MS m/z: 822.5527 (Calcd for C₄₈H₇₆NO₈Si: 822.5340). [α]_D¹⁸ +0.97 (c=0.5, CHCl₃).

[1R,1(4R,5S),2S,3E]-1-[2,2-Dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-5-hydroxylmethyl-1,3-dioxane-4-yl]-1,2-isopropylidendioxy-heptadec-3-en-11-one (15)

Tetrabutylammonium fluoride (TBAF) (0.32 mL, 1 M in THF, 0.32 mmol) was added to a solution of 14 (87 mg, 0.106 mmol) in THF (1.5 mL) at 0°C. After stirring at room temperature for 24 h, the reaction was quenched with water and the whole mixture was extracted with EtOAc (3×10 mL). The combined organic layers were washed with water (10 mL) and brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 10% EtOAc in hexane) to provide 15 (60 mg, 97%) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 6.07 (1H, s), 5.76 (1H, dt, J=15.5, 6.4 Hz), 5.41 (1H, dd, J=15.5, 8.5 Hz), 4.45 (1H, br d, J=5.6 Hz), 4.37 (1H, dd, J=8.8, 8.5 Hz), 4.22 (1H, d, J=12.2 Hz), 3.92 (1H, d, J=11.6 Hz), 3.78 (1H, d, J=12.2 Hz), 3.71 (1H, d, J=8.8 Hz), 3.58 (2H, br s), 2.38 (4H, t, J=7.2 Hz), 2.09–2.04 (2H, m), 1.61–1.50 (4H, m), 1.49–1.34 (2H, m), 1.46 (9H, s), 1.44 (6H, s), 1.40 (6H, s), 1.35–1.27 (10H, m), 0.88 (3H, t, J=6.8 Hz,). ¹³C-NMR (100 MHz, CDCl₃) δ: 211.6, 157.1, 137.7, 126.4, 109.8, 99.2, 79.8, 78.8, 78.4, 69.3, 66.0, 63.5, 56.3, 42.8, 42.7, 32.1, 31.6, 29.0, 28.9, 28.8, 28.7, 28.4, 28.3, 27.4, 26.0, 23.8, 23.7, 23.5, 22.5, 19.1. IR (film) cm⁻¹: 3410, 1712, 970, 902. FAB-MS m/z: 583.4079 (Calcd for C₃₂H₅₇NO₈: 583.4084). [α]_D²⁰ +2.60 (c=0.5, CHCl₃).

[1R,1(4R,5S),2S,3E]-1-[2,2-Dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-5-formyl-1,3-dioxane-4-yl]-1,2-isopropylidenedioxy-heptadec-3-en-11-one (16)

A solution of DMSO (48 mg, 0.618 mmol) in CH₂Cl₂ (0.3 mL) was added dropwise to a solution of oxalyl chloride (52 mg, 0.412 mmol) in CH₂Cl₂ (0.6 mL) at −78°C under nitrogen and stirred for 5 min. A solution of alcohol 15 (60 mg, 0.103 mmol) in CH₂Cl₂ (0.7 mL) was added dropwise to the reaction mixture and stirred for 20 min. A solution of Et₃N (0.14 mL, 1.03 mmol) in CH₂Cl₂ (0.3 mL) was added dropwise to the reaction mixture at −78°C. The reaction mixture was warmed to −40°C and stirred for 1 h. The reaction was quenched with saturated NH₄Cl and the whole mixture was extracted with EtOAc (3×5 mL). The combined organic layers were washed with water (5 mL) and brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 10% EtOAc in hexane) to provide 16 (44 mg, 73%) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 9.69 (1H, s), 6.38 (1H, s), 5.74 (1H, m), 5.33 (1H, dd, J=15.0, 8.4 Hz), 4.36 (1H, t, J=8.4 Hz), 4.18 (1H, s), 4.17 (1H, m), 3.81 (1H, s), 3.57 (1H, d, J=8.4 Hz), 2.39 (4H, t, J=7.2 Hz), 2.06–2.01 (2H, m), 1.59–1.52 (4H, m), 1.48 (9H, s), 1.47 (3H, s), 1.46 (3H, s), 1.45 (3H, s), 1.44 (3H, s), 1.28–1.27 (12H, m), 0.88 (3H, t, J=6.4 Hz). ¹³C-NMR (100 MHz, CDCl₃) δ: 211.6, 201.8, 155.7, 138.1, 125.8, 110.2, 99.5, 80.4, 79.6, 78.3, 68.4, 63.7, 63.1, 42.8, 42.7, 32.1, 31.6, 29.0, 28.9, 28.8, 28.6, 28.4, 28.2, 28.1, 27.4, 25.9, 23.8, 23.5, 22.5, 19.0, 14.0. IR (film) cm⁻¹: 3419, 1726, 1713, 1704, 1495, 971, 903. FAB-MS m/z: 582.4003 (Calcd for C₃₂H₅₆NO₈: 582.4006). [α]_D¹⁸ +10.0 (c=0.25, CHCl₃).

(2S,3R,4R)-2-Acetamido-3-acetoxy-2-acetoxymethyl-4-[(E,S)-1-acetoxy-10-oxohexadec-2-en-1-yl)-4-butanolide (17)

A solution of NaClO₂ (55 mg, 0.605 mmol) and NaH₂PO₄·H₂O (42 mg, 0.302 mmol) in water (0.42 mL) was added to a solution of 16 (44 mg, 0.0756 mmol) and 2-methyl-2-butene (0.21 mL) in t-BuOH (1.3 mL). After stirring at room temperature for 1.5 h, the reaction was quenched with H₂O and the whole mixture was extracted with EtOAc (3×5 mL). The combined organic layers were washed with brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo to provide the crude carboxylic acid (48 mg), which was used in the next step without further purification.

TFA (0.63 mL) was added to a solution of the crude alcohol in THF (1.0 mL) at 0°C. After stirring at room temperature for 1 h, H₂O (1.0 mL) was added to the solution. After stirring at 50°C for 5.5 h, the reaction mixture was alkalified with K₂CO₃. Insoluble materials were filtered off, and the filtrate was concentrated in vacuo to provide the crude product (82 mg), which was used in the next step without further purification.

Ac₂O (1.0 mL) was added to a solution of the crude product in pyridine (1.0 mL) at 0°C. After stirring at room temperature overnight, the solvent was removed in vacuo. The resulting residue was solved with EtOAc (10 mL), and washed with water (5 mL) and brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 50% EtOAc in hexane) to provide 17 (24 mg, 56% for 3 steps) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 6.04 (1H, s), 5.87 (1H, dt, J=15.4, 6.4 Hz), 5.80 (1H, d, J=5.0 Hz), 5.54 (1H, dd, J=8.4, 7.6 Hz), 5.34 (1H, dd, J=15.4, 7.6 Hz), 4.76 (1H, dd, J=8.4, 5.0 Hz), 4.56 (1H, d, J=11.6 Hz), 4.49 (1H, d, J=11.6 Hz), 2.38 (4H, t, J=8.0 Hz), 2.12 (3H, s), 2.09 (3H, s), 2.02 (6H, s), 1.96–2.05 (2H, m), 1.55 (4H, br s), 1.34–1.27 (12H, m), 0.88 (3H, t, J=6.4 Hz). ¹³C-NMR (100 MHz, CDCl₃) δ: 211.6, 171.7, 170.2, 169.2, 168.2, 139.5, 122.0, 80.5, 77.2, 71.6, 70.4, 62.8, 62.5, 42.8, 42.6, 32.2, 31.6, 28.93, 28.90, 28.2, 23.8, 23.7, 22.6, 22.5, 21.1, 20.6, 20.5, 17.4, 14.0. [α]_D²⁰ +49.8 (c=0.6, CHCl₃) {lit^33,34) [α]_D^23.2 +48.6 (c=0.27, CHCl₃)}.

Sphingofungin E (3)

Aqueous NaOH solution (10%, 1.0 mL) was added to 17 (24 mg, 0.0423 mmol) in MeOH (1.0 mL) and stirring at 70°C for 2.5 h. The reaction mixture was cooled to 0°C, and then neutralized with Amberlite^® IRC-86 (H⁺ type). The insoluble material was removed by filtration and the filtrate was concentrated in vacuo. The resulting residue was purified by column chromatography (1 : 1 : 10 to 1 : 3 : 10 (gradient) H₂O–MeOH–CHCl₃ (lower phase)) to afford sphingofungin E (3) (12 mg, 67%) as white crystals. ¹H-NMR (400 MHz, CD₃OD) δ: 5.77 (1H, dt, J=15.3, 6.8 Hz), 5.45 (1H, dd, J=15.3, 7.2 Hz), 4.10 (1H, dd, J=7.2, 7.0 Hz), 3.97 (1H, d, J=10.8 Hz), 3.94 (1H, s), 3.85 (1H, d, J=10.8 Hz), 3.64 (1H, d, J=4.8 Hz), 2.43 (4H, t, J=7.2 Hz), 2.08–2.03 (2H, m), 1.55–1.52 (4H, m), 1.42–1.28 (12H, m), 0.89 (3H, t, J=6.4 Hz). ¹³C-NMR (100 MHz, CD₃OD) δ: 214.3, 173.3, 135.7, 130.3, 76.3, 75.5, 71.0, 64.9, 43.50, 43.47, 33.4, 32.8, 30.19, 30.16, 30.04, 30.01, 24.9, 24.7, 23.6, 14.4. IR (KBr) cm⁻¹: 3348, 3189, 2928, 2855, 1714, 1636. mp 144–147°C (lit.³¹⁾ mp 145–147°C). [α]_D²⁵ −6.17 (c=0.60, CH₃OH) {lit.³¹⁾ [α]_D²⁴ −5.43 (c=0.48, CH₃OH)}.

[1S,1(4R,5R),2R,3E]-1-[5-(tert-Butyldiphenylsilyloxy)methyl-2,2-dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-1,3-dioxane-4-yl]-1,2-isopropylidenedioxy-3-butene (4,5-Di-epi-8a)

2,2-Dimethoxypropane (0.46 mL, 3.77 mmol) and PPTS (7 mg, 0.038 mmol) were added to a solution of 11 (233 mg, 0.38 mmol) in benzene (11 mL). After stirring at 50°C for 9 h, the reaction was quenched with 5% NaHCO₃ and the whole mixture was extracted with EtOAc (3×10 mL). The combined organic layers were washed with water (10 mL) and brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo to provide crude product (224 mg), which was used in the next step without further purification.

DIBAL-H (0.68 mL, 1.0 M in hexane, 0.68 mmol) was added dropwise to a solution of crude product (224 mg) in CH₂Cl₂ (4.8 mL) at −78°C under nitrogen. After stirring for 1 h, the reaction was quenched with saturated potassium sodium tartrate and the whole mixture was extracted with EtOAc (3×10 mL). The combined organic layers were washed with water (10 mL) and brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was filtered through a pad of Celite to provide crude aldehyde (216 mg), which was used in the next step without further purification.

t-BuOK (381 mg, 3.4 mmol) in THF (7.0 mL) was added to a solution of Ph₃PCH₃Br (1.34 g, 3.74 mmol) in THF (5.0 mL) at −78°C. The reaction mixture was warmed to room temperature and stirred for 1 h. The reaction mixture was cooled to −78°C, a solution of crude product (216 mg) in THF (6.0 mL) was added dropwise to the reaction mixture, and the resulting mixture was warmed to 0°C and stirred for 1 h. The reaction mixture was quenched with saturated NH₄Cl and the whole mixture was extracted with EtOAc (3×10 mL). The combined organic layers were washed with water (10 mL) and brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 10% EtOAc in hexane) to provide 4,5-di-epi-8a (167 mg, 71% for 3 steps) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 7.71–7.65 (4H, m), 7.44–7.36 (6H, m), 5.88 (1H, ddt, J=16.8, 10.4, 4.8 Hz), 5.37 (1H, d, J=16.8 Hz), 5.18 (1H, d, J=10.4 Hz), 4.93 (1H, s), 4.32 (1H, t, J=7.4 Hz), 4.20 (1H, d, J=12.0 Hz), 4.15–4.09 (2H, m), 3.88 (1H, t, J=7.4 Hz), 1.4 (3H, s), 1.39 (3H, s), 1.38 (3H, s), 1.32 (3H, s), 1.35 (9H, s), 1.08 (9H, s). ¹³C-NMR (100 MHz, CDCl₃) δ: 155.1, 136.5, 135.7, 135.6, 133.2, 133.1, 129.7, 129.6, 127.7, 127.6, 116.5, 109.4, 99.1, 80.9, 78.3, 77.2, 73.8, 63.3, 56.4, 28.3, 27.8, 26.9, 26.8, 19.9, 19.3. IR (film) cm⁻¹: 3427, 2073, 1726, 1499, 998, 925. FAB-MS m/z: 626.3466 (Calcd for C₃₅H₅₂NO₇Si: 626.3513). [α]_D¹⁹ +17.6 (c=1.0, CHCl₃).

[1S,1(4R,5R),2R,3E]-1-[5-(tert-Butyldiphenylsilyloxy)methyl-2,2-dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-1,3-dioxane-4-yl]-1,2-isopropylidenedioxyheptadec-3- en-11-one (4,5-Di-epi-14)

Pentadec-1-en-9-one (6)⁶³⁾ (224 mg, 1.07 mmol) and second generation Grubbs catalyst (11 mg, 0.0134 mmol) were added to a solution of alkene 4,5-di-epi-8a (167 mg, 0.267 mmol) in CH₂Cl₂ (2.0 mL). After stirring at reflux for 5 h, the solvent was removed in vacuo. The resulting residue was purified by column chromatography (silica gel, 5% EtOAc in hexane) to provide 4,5-di-epi-14 (212 mg, 97%) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 7.71–7.65 (4H, m), 7.43–7.36 (6H, m), 5.75 (1H, dt, J=15.3, 6.8 Hz), 5.44 (1H, dd, J=15.3, 7.2 Hz), 4.92 (1H, s), 4.29 (1H, t, J=7.2 Hz), 4.20–4.17 (2H, m), 4.05 (3H, br s), 3.85 (1H, t, J=7.6 Hz), 2.38 (4H, t, J=7.2 Hz), 2.06–2.01 (2H, m), 1.59–1.52 (4H, m), 1.43 (3H, s), 1.37 (9H, s), 1.35–1.27 (21H, m), 1.08 (9H, s), 0.88 (3H, t, J=7.2 Hz). ¹³C-NMR (100 MHz, CDCl₃) δ: 211.6, 155.1, 135.7, 135.6, 134.8, 133.23, 133.15, 129.7, 129.6, 127.77, 127.69, 127.57, 108.9, 99.0, 80.9, 78.2, 77.2, 76.7, 73.6, 63.5, 56.6, 42.8, 42.7, 32.3, 31.59, 31.58, 29.1, 29.0, 28.91, 28.87, 28.3, 27.7, 26.9, 26.8, 23.81, 23.77, 23.76, 22.5, 20.0, 19.3, 14.0. IR (film) cm⁻¹: 3414, 1721, 1717, 1496, 972, 943. FAB-MS m/z: 822.5343 (Calcd for C₄₈H₇₆NO₈Si: 822.5340). [α]_D¹⁸ +14.3 (c=0.5, CHCl₃).

[1S,1(4R,5S),2R,3E]-1-[2,2-Dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-5-formyl-1,3-dioxane-4-yl]-1,2-isopropylidenedioxyheptadec-3-en-11-one (4,5-Di-epi-16)

Tetrabutylammonium fluoride (TBAF) (0.26 mL, 1.0 M in THF, 0.258 mmol) was added to a solution of 4,5-di-epi-14 (71 mg, 0.086 mmol) in THF (1.4 mL) at 0°C. After stirring at room temperature for 4 h, the reaction was quenched with water and the whole mixture was extracted with EtOAc (3×10 mL). The combined organic layers were washed with water (10 mL) and brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo to provide crude product 4,5-di-epi-15 (66 mg), which was used in the next step without further purification.

A solution of DMSO (40 mg, 0.516 mmol) in CH₂Cl₂ (0.3 mL) was added dropwise to a solution of oxalyl chloride (44 mg, 0.344 mmol) in CH₂Cl₂ (0.5 mL) at −78°C under nitrogen and stirred for 5 min. A solution of crude 4,5-di-epi-15 (66 mg) in CH₂Cl₂ (0.7 mL) was added dropwise to the reaction mixture and stirred for 20 min. A solution of Et₃N (88 mg, 0.86 mmol) in CH₂Cl₂ (0.3 mL) was added dropwise to the reaction mixture at −78°C. The reaction mixture was warmed to −40°C and stirred for 1 h. The reaction was quenched with saturated NH₄Cl and the whole mixture was extracted with EtOAc (3×5 mL). The combined organic layers were washed with water (5 mL) and brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 10% EtOAc in hexane) to provide 4,5-di-epi-16 (41 mg, 82% for 2 steps) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 9.58 (1H, s), 5.71 (1H, dt, J=15.2, 6.8 Hz), 5.45–5.40 (2H, m), 4.27 (1H, d, J=12.4 Hz), 4.23 (1H, t, J=7.7 Hz), 4.06 (1H, d, J=12.4 Hz), 3.96 (1H, d, J=7.7 Hz), 3.78 (1H, t, J=7.7 Hz), 2.38 (4H, t, J=7.2 Hz), 2.06–2.01 (2H, m), 1.58–1.52 (4H, m), 1.49 (9H, s), 1.41 (3H, s), 1.38 (6H, s), 1.34–1.26 (16H, m), 0.88 (3H, t, J=6.8 Hz). ¹³C-NMR (100 MHz, CDCl₃) δ: 211.5, 199.8, 156.1, 134.8, 127.4, 110.2, 99.0, 82.0, 81.0, 77.2, 76.7, 63.0, 61.9, 42.8, 42.7, 32.2. 31.6, 29.7, 29.0, 28.93, 28.90, 28.80, 28.79, 28.2, 27.2, 26.4, 23.8, 23.7, 22.5, 18.6, 14.0. IR (film) cm⁻¹: 3362, 1723, 1714, 965, 945. FAB-MS m/z: 582.3986 (Calcd for C₃₂H₅₆NO₈: 582.4006). [α]_D¹⁹ +20.2 (c=0.5, CHCl₃).

(2S,3R,4S)-2-Acetamido-3-acetoxy-2-acetoxymethyl-4-[(E,R)-1-acetoxy-10-oxohexadec-2-en-1-yl)-4-butanolide (4,5-Di-epi-17)

A solution of NaClO₂ (167 mg, 1.85 mmol) and NaH₂PO₄⋅H₂O (170 mg, 1.24 mmol) in water (1.6 mL) was added to a solution of 4,5-di-epi-16 (60 mg, 0.103 mmol) and 2-methyl-2-butene (0.28 mL) in t-BuOH (1.6 mL). After stirring at room temperature for 1.5 h, the reaction was quenched with H₂O and the whole mixture was extracted with EtOAc (3×5 mL). The combined organic layers were washed with brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo to provide the crude carboxylic acid (70 mg), which was used in the next step without further purification.

TFA (1.3 mL) was added to a solution of the crude alcohol in THF (1.3 mL) at 0°C. After stirring at room temperature for 1 h, H₂O (1.3 mL) was added to the solution. After stirring at 50°C for 6 h, the reaction mixture was alkalified with K₂CO₃. Insoluble materials were filtered off, and the filtrate was concentrated in vacuo to provide the crude product (152 mg), which was used in the next step without further purification.

Ac₂O (1.9 mL) was added to a solution of the crude product in pyridine (1.9 mL) at 0°C. After stirring at room temperature overnight, the solvent was removed in vacuo. The resulting residue was solved with EtOAc (10 mL), and washed with water (10 mL) and brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 50% EtOAc in hexane) to provide 4,5-di-epi-17 (28 mg, 48% for 3 steps) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 6.42 (1H, br s), 5.92 (1H, dt, J=12.0, 5.2 Hz), 5.47–5.35 (2H, m), 4.98 (1H, s), 4.72 (1H, s), 4.55 (1H, d, J=6.8 Hz), 4.30 (1H, s, J=6.8 Hz), 2.39 (4H, t, J=5.6 Hz), 2.17 (3H, s), 2.14–1.99 (2H, m), 2.12 (3H, s), 2.07 (3H, s), 2.05 (3H, s), 1.55 (4H, br s), 1.35–1.27 (12H, m), 0.88 (3H, t, J=5.6 Hz). ¹³C-NMR (100 MHz, CDCl₃) δ: 211.7, 171.0, 170.8, 170.2, 169.9, 169.8, 139.1, 122.5, 82.2, 73.7, 70.7, 64.6, 61.0, 42.8, 42.7, 32.2, 31.6, 29.0, 28.90, 28.87, 28.4, 23.8, 23.7, 22.5, 21.9, 21.1, 20.7, 20.4, 14.0. IR (film) cm⁻¹: 3321, 3209, 1795, 1760, 1747, 1713, 1688, 1668. FAB-MS m/z: 568.3123 (Calcd for C₂₉H₄₆NO₁₀: 568.3121). [α]_D²⁰ −99.4 (c=0.5, CHCl₃).

(E,2S,3R,4S,5R)-2-Amino-3,4,5-trihydroxy-2-hydroxymethyl-14-oxoicos-6-enoic Acid [4,5-Di-epi-sphingofungin E (4)]

Aqueous NaOH solution (10%, 0.48 mL) was added to 4,5-di-epi-17 (24 mg, 0.042 mmol) in MeOH (0.48 mL) and stirring at 70°C for 2 h. The reaction mixture was cooled to 0°C, and then neutralized with Amberlite^® IRC-86 (H⁺ type). The insoluble material was removed by filtration and the filtrate was concentrated in vacuo. The resulting residue was purified by column chromatography (1 : 1 : 10 to 1 : 3 : 10 (gradient) H₂O–MeOH–CHCl₃ (lower phase)) to afford 4 (12 mg, 70%) as white crystals. ¹H-NMR (400 MHz, CD₃OD) δ: 5.66 (1H, dt, J=15.6, 6.4 Hz), 5.52 (1H, dd, J=15.6, 6.4 Hz), 4.19 (1H, d, J=3.6 Hz), 3.94–3.83 (2H, m), 3.58–3.55 (2H, m), 2.34 (1H, t, J=6.8 Hz), 1.99–1.98 (2H, m), 1.50–1.43 (4H, m), 1,31–1.19 (12H, m), 0.80 (3H, t, J=6.4 Hz). ¹³C-NMR (100 MHz, CD₃OD) δ: 214.3, 137.8, 133.8, 131.3, 80.1, 76.9, 72.9, 71.1, 64.2, 43.50, 43.47, 32.8, 30.8, 30.20, 30.18, 30.17, 30.1, 30.0, 24.8, 23.6, 14.4. IR (KBr) cm⁻¹: 3472, 3266, 3205, 3123, 2928, 2851, 1705, 1639. FAB-MS m/z: 418.2802 (Calcd for C₂₁H₄₀NO₇: 418.2805). mp 141–143°C. [α]_D²⁰ −1.77 (c=0.25, DMSO).

(4S,5S,2E)-5-(tert-Butyldiphenylsilyloxy)methyl-4,6-dihydroxy-2,2-dimethyl-5-(1,1-dimethylethoxycarbonyl)amino-2-hexenyl Acetate (20)

DIBAL-H (0.62 mL, 0.62 mmol) was added dropwise to a solution of ester 7a⁴⁶⁾ (120 mg, 0.206 mmol) in CH₂Cl₂ (2.0 mL) at −78°C under nitrogen. After stirring for 1 h, the reaction was quenched with saturated potassium sodium tartrate and the whole mixture was extracted with EtOAc (3×10 mL). The combined organic layers were washed with water (10 mL) and brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was filtered through a pad of Celite to provide crude alcohol (101 mg), which was used in the next step without further purification.

Ac₂O (2.0 mL) was added to a solution of the crude product in pyridine (2.0 mL). After stirring at room temperature for 1.5 h, the solvent was removed in vacuo. The resulting residue was solved with EtOAc (10 mL), and washed with water (5 mL) and brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo to provide crude acetate (121 mg), which was used in the next step without further purification.

PPTS (14 mg, 0.0546 mmol) was added to a solution of crude product in MeOH (2.0 mL). After stirring at reflux for 2.5 h, the reaction was quenched with 5% NaHCO₃ and the whole mixture was extracted with EtOAc (3×10 mL). The combined organic layers were washed with water (10 mL) and brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 20% EtOAc in hexane) to provide 20 (71 mg, 61% for 3 steps) as a colorless oil. ¹H-NMR (400 MHz, CDCl₃) δ: 7.65–7.62 (4H, m), 7.48–7.39 (6H, m), 5.48–5.72 (2H, m), 5.38 (1H, s), 4.93 (1H, d, J=8.0 Hz), 4.50 (2H, d, J=4.8 Hz), 4.13 (1H, dd, J=8.6, 5.2 Hz), 3.94 (1H, dd, J=12.0, 5.2 Hz), 3.84 (1H, d, J=10.2 Hz), 3.70 (1H, dd, J=12.0, 8.8 Hz), 3.61 (1H, d, J=10.2 Hz), 3.18 (1H, br s), 2.03 (3H, s), 1.44 (9H, s), 1.09 (9H, s). ¹³C-NMR (100 MHz, CDCl₃) δ: 170.6, 156.8, 135.5, 132.3, 132.1, 131.9, 130.13, 130.11, 127.9, 127.1, 80.4, 73.3, 64.2, 63.9, 62.5, 61.9, 28.2, 26.9, 20.9, 19.2. IR (film) cm⁻¹: 3416, 1740, 1689, 1499, 970, 910. FAB-MS m/z: 598.3202 (Calcd for C₃₃H₄₈NO₇Si: 598.3200). [α]_D¹⁹ −17.2 (c=0.5, CHCl₃).

[2S,3R,3(4R,5S)]-3-[5-(tert-Butyldiphenylsilyloxy)methyl-4,6-dihydroxy-2,2-dimethyl-5-(1,1-dimethylethoxycarbonyl)amino]-2,3-isopropylidenedioxy Propyl Acetate (32a) and [2R,3S,3(4R,5S)]-3-[5-(tert-Butyldiphenylsilyloxy)methyl-4,6-dihydroxy-2,2-dimethyl-5-(1,1-dimethylethoxycarbonyl)amino]-2,3-isopropylidenedioxy Propyl Acetate (32b)

OsO₄ (46 mg, 0.184 mmol) in CH₂Cl₂ (1.2 mL) was added dropwise to a solution of 27a and 27b (93 mg, 0.167 mmol) and TMEDA (25 mg, 0.217 mmol) in CH₂Cl₂ (2.5 mL) at −78°C. After stirring for 1 h, the solvent was removed in vacuo. NaHSO₃ (98 mg, 0.94 mmol) was added to a solution of resulting residue in pyridine–H₂O (1 : 1, 1 mL). After stirring at room temperature overnigt, the reaction was quenched with water and the whole mixture was extracted with EtOAc (3×10 mL). The combined organic layers were washed with brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo to provide crude tetraol (102 mg), which was used in the next step without further purification.

2,2-Dimethoxypropane (0.61 mL, 5.01 mmol) and PPTS (13 mg, 0.0501 mmol) were added to a solution of crude product in benzene (5 mL). After stirring at reflux for 2.5 h, the reaction was quenched with 5% NaHCO₃ and the whole mixture was extracted with EtOAc (3×10 mL). The combined organic layers were washed with water (5 mL) and brine (10 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 10% EtOAc in hexane) to provide 32a (40 mg, 36% for 2 steps) as a colorless oil and 32b (54 mg, 48% for 2 steps) as a colorless oil. 32a: ¹H-NMR (400 MHz, CDCl₃) δ: 7.72–7.71 (4H, m), 7.46–7.36 (6H, m), 5.07 (1H, s), 4.67 (1H, d, J=6.4 Hz), 4.40 (1H, d, J=9.4 Hz), 4.18 (1H, dd, J=9.6, 3.2 Hz), 4.13 (1H, d, J=8.2 Hz), 4.04 (1H, dd, J=9.6, 6.4 Hz), 3.95 (1H, m), 3.78 (1H, d, J=9.6 Hz), 3.74 (1H, m), 3.69 (1H, d, J=8.2 Hz), 2.03 (3H, s), 1.44 (9H, s), 1.36 (6H, s), 1.20 (6H, s), 1.08 (9H, s). ¹³C-NMR (100 MHz, CDCl₃) δ: 170.9, 154.6, 135.9, 135.7, 133.1, 132.7, 129.8, 129.8, 127.8, 127.6, 102.1, 101.6, 77.2, 68.5, 68.0, 67.2, 63.1, 61.4, 60.2, 58.0, 28.42, 28.37, 26.8, 25.6, 24.9, 24.3, 24.2, 23.4, 21.0, 19.3. IR (film) cm⁻¹: 3447, 1734, 1719, 1491. FAB-MS m/z: 672.3541 (Calcd for C₃₆H₅₄NO₉Si: 672.3568). [α]_D²⁰ +0.55 (c=0.47, CHCl₃). 32b: ¹H-NMR (400 MHz, CDCl₃) δ: 7.71–7.60 (4H, m), 7.44–7.36 (6H, m), 4.96 (1H, s), 4.51 (1H, dd, J=11.6, 1.6 Hz), 4.19 (1H, dd, J=11.6, 9.4 Hz), 4.12 (1H, d, J=8.0 Hz), 4.05 (1H, m), 3.98–3.90 (3H, m), 3.95 (1H, d, J=8.0 Hz), 2.10 (3H, s), 1.44 (3H, s), 1.41 (3H, s), 1.38 (12H, s), 1.30 (3H, s), 1.08 (9H, s). ¹³C-NMR (100 MHz, CDCl₃) δ: 170.7, 155.0, 135.6, 133.2, 133.1, 129.7, 129.6, 127.6, 127.5, 110.1, 99.1, 78.7, 77.2, 74.3, 73.9, 65.3, 63.1, 56.3, 28.4, 28.2, 27.9, 26.9, 26.7, 20.9, 19.6, 19.2. IR (film) cm⁻¹: 3414, 1747, 1716, 1505. FAB-MS m/z: 672.3588 (Calcd for C₃₆H₅₄NO₉Si: 672.3568). [α]_D²⁰ +18.4 (c=0.60, CHCl₃).

Conversion of 32a into 8a

DIBAL-H (0.076 mL, 0.076 mmol) was added dropwise to a solution of ester 32a (17 mg, 0.0253 mmol) in CH₂Cl₂ (0.8 mL) at −78°C under nitrogen. After stirring for 1 h, the reaction was quenched with saturated potassium sodium tartrate and the whole mixture was extracted with EtOAc (3×5 mL). The combined organic layers were washed with water (5 mL) and brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was filtered through a pad of Celite to provide crude alcohol (14 mg), which was used in the next step without further purification.

A solution of DMSO (16 mg, 0.20 mmol) in CH₂Cl₂ (0.3 mL) was added dropwise to a solution of oxalyl chloride (17 mg, 0.133 mmol) in CH₂Cl₂ (0.5 mL) at −78°C and stirred for 5 min. A solution of crude product in CH₂Cl₂ (0.6 mL) was added dropwise to the reaction mixture and stirred for 20 min. Et₃N (34 mg, 0.333 mmol) in CH₂Cl₂ (0.3 mL) was added dropwise to the reaction mixture and stirred at −78°C for 2.5 h, and the reaction mixture was allowed to warm to room temperature and stirred for 30 min. The reaction was quenched with saturated NH₄Cl and the whole mixture was extracted with CH₂Cl₂ (3×5 mL). The combined organic layers were washed with water (5 mL) and brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo to provide crude aldehyde (12 mg), which was used in the next step without further purification.

Ph₃PCH₃Br (87 mg, 0.244 mmol) was added slowly to a suspension of NaH (60% in mineral oil, 9 mg, 0.222 mmol) in THF (0.8 mL) at 0°C. After stirring for 3 h under nitrogen, crude product in THF (0.7 mL) was added dropwise to the reaction mixture, and the resulting mixture was warmed to room temperature and stirred for 2 h. The reaction mixture was quenched with saturated NH₄Cl and the whole mixture was extracted with EtOAc (3×5 mL). The combined organic layers were washed with water (5 mL) and brine (5 mL), and dried over anhydrous MgSO₄. Filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel, 7% EtOAc in hexane) to provide 8a (5 mg, 31% for 3 steps) as a colorless oil.

Acknowledgments

This research was supported, in part, by a Grant-in-Aid for Scientific Research (C) (Grant No. JP16K08158) from the Japan Society for the Promotion of Science (JSPS) and JSPS Core-to-Core Program, B. Asia-Africa Science Platforms.

Conflict of Interest

The authors declare no conflict of interest.

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