87(PA3-7) 連翹の新多価アルコールSUSPENOLの構造とそのエナンチオ選択的合成(ポスター発表の部)

抄録

Methanol extract of Forsythia suspensa fruits, an Oriental drug RENGYO, afforded a new tetraol, suspenol C_8H_<16>O_4, of the novel C_6-C_2 type non-aromatic carbon skeleton. The stereostructure of suspenol was determined as 4 based on the spectral data of its triacetate (6) and was confirmed by the synthesis, i.e. epoxidation and subsequent reductive cleavage by LAH of 2-dehydrorengyol (8), prepared by photosensitized dioxygenation and LAH reduction from ethyl 1,3-cyclohexadienylacetate (9). The absolute configuration was assigned as (2S, 4S) from the CD spectrum of its tribenzoate [α]_D+20.0°. A newly invented efficient asymmetric induction reaction has been applied to the enantioselective synthesis of the natural polyol 4. Photo-oxidation of L-tyrosine in 0.2% aq. Na_2CO_3 with Rose Bengal at 40° yielded a lactam 16, mp 168°, [α]_D-138°, [θ]_<228> -58800. Since the magnitude of the specific rotation of 16 remained unchanged after several recrystallization, and moreover, esterification of 16 with (-)-camphanyl chloride yielded only one product (500MHz), the photoproduct 16 was considered enantiomerically pure. This remarkable high efficiency of the asymmetric induction is ascribed to the cyclization of the photointermediate 13, via the electronically more stable transition states B-C before decarboxylation. The lactam 16 may be served for synthesis of either of the suspenol enantiomers or both, since it seems possible to interconvert between 16 and its enantiomeric structure, without affecting their optical purity, by conbinations of Michael addition and β-elimination reactions. Modification of 16 into 1-suspenol was examplified as follows. Catalytic reduction and successive NaBH_4 reduction gave diol (19) as the majour product, which was in turn hydrolysed with 10% aq. NaOH to a new unnatural amino acid (20). The amino acid 20 was transformed into 1-suspenol by oxydative deaminatin with 1% CrO_3 and reduction with LAH. The synthetic 4 [α]_D-1.8°, afforded the tribenzoate [α]_D +21.3°.