Asymmetric biomimetic oxidations of phenols using oxazolidines as chiral auxiliaries: The enantioselective synthesis of (+)- and (-)-dehydrodiconiferyl alcohol

Stereoselective bimolecular radical coupling reactions of phenylpropenoid phenols are described. Evans's 2-oxazolidinone 11a-d derivatives of ferulic acid were prepared and oxidized to give dimeric benzofuran neolignan structures 12-13a-d in 40-50% overall yields. The chiral phenols were dimerized either enzymatically with hydrogen peroxide and horseradish peroxidase (HRP) or with silver oxide. The enantioselectivity after reductive cleavage of the chiral auxiliaries to give dehydrodiconiferyl alcohol ranged from 18% to 62% enantiomeric excess. The conformational analysis and the activation energy using semiempirical PM3 calculations on the intermediate quinomethides is used to explain the observed stereoselectivity. Copyright (c) 2006 John Wiley & Sons, Ltd