Stereochemistry of erythro- and threo-syringylglycerol-8-O-4΄- (sinapyl alcohol) ethers and their enzymatic formation with optical activity in Eucommia ulmoides

In this study an enzymatic system, horseradish peroxidase (HRP)-hydrogen peroxide was used as catalysts for the enzymatic formation of enantiospecific syringulglycerol-8-O-4′ (sinapyl alcohol) ethers (SGSEs) from enzyme preparations of Eucommia ulmoides with sinapyl alcohol (SA) as a monolignol precursor. Reversed phase HPLC analysis of the erythro- and threo- SGSE showed that the ratio of erythro: threo was 47: 53. Both isomers were isolated by HPLC and their structural confirmation was done by 1H NMR spectra. Chiral column HPLC analysis of the erythro- and threo-SGSE showed that their enantiomeric compositions were as follows: (+)- erythro: (-)-erythro = 46.7:53.3 (6.6% e.e), and (+)-threo: (-)-threo = 45.2: 54.8 (9.6% e.e). To elucidate the stereochemistry of erythro and threo- SGSEs, we have determined absolute configurations of the four stereoisomers, (+)-erythro-, (-)-erythro-, (+)-threo-, and (-)-threo- SGSEs as (7R, 8S), (7S, 8R), (7S, 8S), and (7R, 8R), respectively, by Mosher’s method through the ^1H NMR spectroscopy of (+)-(R)-α-methoxy- α- trifluoromethylphenylacetate (MTPA) esters of α, γ and γ’ positions.