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.