Absolute configuration of arylglycerol-β-aryl ethers obtained by asymmetric reduction of the correspondingα-ketonic compound with intactFusarium solani cells

Abstract

When (±)-α-oxo-guaiacylglycerol-β-(vanillic acid) ether (1) is degraded byFusarium solani M-13-1, theα-ketone is initially reduced to giveerythro andthreo guaiacylglycerol-β-(vanillic acid) ethers (2), arylglycerol-β-aryl ethers, both of which are enantiomerically pure. The absolute configuration in each2 was determined by Mosher's method; the products were converted toα,γ-di-(R)-α-methoxy-α-trifluoromethylphenylacetates (MTPA esters) (3′) oferythro (-)- andthreo (+)-veratrylglycerol-β-(methyl vanillate) ethers (3), whose1H nuclear magnetic resonance (NMR) spectra were examined and compared with those of four di-(R)-MTPA ester (3′) diastereomers from chemically synthesizederythro (±)-3 andthreo (±)-3. To assign theα- andγ-MTPA-OCH3 peaks, the1H NMR scans of several compounds that have substructures of 3′ and their 3,4,5-trimethoxyphenyl analogues were examined. When a racemic alcohol reacts with (R)-MTPA to give a pair of (R)-MTPA ester diastereomers, the Δδ value was defined as the absolute value of the difference in the1H chemical shifts of the peak between the diastereomers. It was found that the Δδ values ofα-MTPA-OCH3 were larger than those ofγ-MTPA-OCH3 owing to a shielding effect of the veratryl ring located on theα-MTPA-OCH3, and that theα-MTPA-OCH3 peaks in the 3,4,5-trimethoxyphenyl compounds shifted downfield relative to those in the veratryl compounds. On the basis of the1h NMR data of (R)-MTPA esters, the absolute configuration of the four chemically prepared diastereomers (3′) were determined. The catabolicerythro 3′ [fromerythro (-)-3] andthreo 3′ [fromthreo (+)-3] were identical to (R, αS, βR)-erythro 3′ and (R, αS, βS)- threo 3′, respectively. An hydrogen species in the fungal reduction would attack theα-ketone fromre-face of both (βR)-1 and (βS)-1, givingerythro (αS, βR)-2 andthreo (αS, βS)-2, respectively.