34 CHEMISTRY ON THE FUNGAL METABOLISM OF (±)-EPDXYFARNESOL : OPTICALLY ACTIVE ISOPRENOIDS
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The metabolic transformation of (±)-epoxyfarnesol by Helminthosporium sativum yielded three optically minus products, i.e., (-)-10, 11-dihydroxyfarnesol (2), (-)-10, 11-dihydroxyfarnesic acid (3) and (-)-9, 10-dihydroxygeranylacetone (4). Owing to the more facile consumption of (+)-epoxyfarnesol than its (-)-enantiomer, the latter could be recovered from the culture filtrate after interrupted the fermentation, although in optically incompletely state. That (-)-epoxyfarnesol has the same configuration around C-10 as (-)-dihydroxyfarnesol (2) was shown by the hydrolytic conversion of the former into the latter under the controlled condition, and, also, the S-configuration was evidenced for both compounds by means of cyclization of (-)-epoxyfarnesol into a drimane-type compound ((7) and (21)) by BF_3-etherate. S-(-)-10, 11-Dihydroxyfarnesol (2) thus obtained by the aid of fungal metabolism was chemically and stereospecifically transformed into R-(+)-epoxyfarnesol (12), [α]^<25>_D+1.8^o_1 (c, 1.91), or into S-(-)-epoxyfarnesol (16), [α]^25_D-1.8^o_3 (c, 1.70), either of which is difficult to obtain from the racemic compound by common methods of resolution. S-(-)-10, 11-Dihydroxyfarnesic acid (3) was similarly transformed into R-(+)-, [α]^<25>_D+3.7°, or into S-(-)-epoxyfarnesic acid (18), [α]^<25>_D-3.2°Both enantiomers of 2, 3-epoxysgualene, i.e. R-(+)-, (30), [α]^<26>_D+1.2°, and S-(-)-, (29), [α]^<26>_D- 1.2°, were first synthesized starting from 3. Interestingly, optically minus three products were shown to have been produced from both enantiomers of the substrate, presumably through the different hydrolytic cleavages for R- and S-epoxide ring as shown in Scheme VII.
