Reaction of allylsilane with 2, 3; 4, 5-di- O-isopropylidene-aldehydo-D-arabinose in the presence of boron trifluoride etherate afforded an unexpected product, which was identified as a tetrahydrofuran derivative, as a major component along with a small amount of homoallyl alcohol. This tetrahydrofuran formation probably proceeds via cyclization of the β-silyl cation intermediate accompanied by migration of the isopropylidene group. The scope of this reaction was investigated : When 2, 3-O-isopropylidene derivatives of D-erythrose, D-ribose, L-rhamnose, and D-mannose were used as the aldehyde in the reaction of allylsilane, the tetrahydrofuran derivatives were obtained in moderate yields, whereas aldehydes derived from D-xylose, 6-deoxy-D-glucose, and D-glucose gave the homoallyl alcohols as major products. Similar cyclization proceeded employing disubstituted terminal olefins, vinyl ethers, and vinyl sulfides instead of allylsilane to afford the corresonding tetrahydrofurans in good to high yields. Especially, the reaction using β-substituted vinyl ethers and sulfides provided a stereoselective route to 2-C-substituted 2-deoxyfuranosides, which were utilized for synthesizing 2-C-substi-tuted 2-deoxyhexoses and 2'-substituted 2'-deoxynucleosides.
Stereoselective synthesis of tri- and tetra-substituted olefins is reviewed based on original papers published mostly in the last two decades, during which a great progress has been made in methodology and selectivity. The methodology for the synthesis can be classified into the following five categories : 1) conversion of acetylenes into polysubstituted olefins, 2) reactions of alkenes with organo-transition metal species, 3) reactions involving carbonyl compounds, 4) elimination reactions, and 5) rearrangement reactions. Each category is devided, when needed, into several subclasses. In reviewing stereoselective methods, basic methodology and concept for achieving high selectivity are emphasized, and the application of these methods is only briefly illustrated. The most remarkable feature of the recent progress is establishment of highly stereocontrolled syntheses of isomeric pairs of olefins using common starting materials or intermediates.
Recent advances in the oxidation reaction of alcohols by use of 2, 2, 6, 6-tetramethylpiperidine-1-oxyl (TEMPO), and scope and limitations as well as characteristic feature of the method are surveyed. Emphasis is placed on the catalytic process by the aid of co-oxidants. Useful applications of this oxidation method to syntheses of various bioactive compounds and functionalized molecules are shown. Primary alcohols are oxidized to aldehdyes and to carboxylic acids, selectively. Secondary alcohols, more slowly than primary, can be oxidized to ketones in a slightly basic biphase solution.
Binuclear or mononuclear divalent manganese complexes, LMn (OH)2MnL (3), LMn (O)2MnL (4), LMn(OH)(OAc)MnL (7), LMn(OBz) (10), LMn(OBz) (3, 5-i-Pr2pzH) (11) were synthesized with a hindered N3 ligand (L = hydrotris (3, 5-diisopropyl-1-pyrazolyl) borate) as structual analogues for the active sites of Mn containing proteins including PS II OEC, Mn Catalase and Mn SOD. The structure and properties were characterized and their biological relevance is discussed. The reactivitiy of 3 and 7 with O2 or H2O2 were explored and the structures of oxidized products were determined by X-ray crystallography. The superoxide dismutation activities of 10 and 11 were examined by NTB method.
Organometallics of rare earths such as (C5Me5)2LnR (Ln=Y, Sm, Yb, Lu; R =H, Me) initiated the living polymerizations of alkyl methacrylates and alkyl acrylates for the first time to afford high molecular weight polymers (Mn>500, 000) with extremely narrow molecular weight distribution (Mw/Mn<1.05). The 1 : 2 reaction of SmH (C5Me5)2 with MMA provides a single crystal and its X-ray analysis revealed the formation of an 8-membered ring complex where one end is bound with enolate form and the other end by the coordination of ester group of MMA. Based on these findings we propose coordination anionic polymerization mechanism for this reaction. SmMe(C5Me5)2(THF) and YOMe(C5Me5)2(THF) were also able to conduct the living polymerizations of β-propiolactone, δ-valerolactone and ε-caprolactone. Organo rare earth metal (II) complex with bulky auxiliary ligand, Me2Si(2-Me3Si-4-tBu-C5H2)2Sm, as well as rare earth metal (III) complexes, Me2Si[2, 4-(Me3Si)2C5H2]2LnCH(SiMe3)2(Ln_=La, Sm), were found to conduct the polymerization of ethylene to give high moleculr weight polymers with narrow molecular weigt distribution.
It is shown that the ethylene bond in (E) -stilbenes is often observed unusually short by X-ray diffraction. Detailed analyses of the temperature dependent X-ray structures reveal that the short ethylene bond is an artifact caused by a dynamical disorder originating from the large amplitude torsional vibration of the C-Ph bonds, during which the movement of the benzene rings is restrained to be a minimum. Origin of the unusually short ethane bond in the X-ray structures of polyarylethanes is also discussed by a similar model.