α‐Amino Acids and N‐Protected α‐Amino Aldehydes by Stereoselective Additions of a Chiral Vinyllithium Reagent to Sulfonylimines

Abstract

AbstractThe chiral vinyllithium reagent (S)‐1b, readily generated from dibromoalkene (S)‐1a by bromine/lithium exchange, adds stereoselectively to mesitylsulfonylimines 2a–f so that the diastereomers 3 are obtained in ⩾98% d.e. after column chromatography. The bromoalkenes 3a–d are submitted to ozonolysis in methanol to give α‐mesitylsulfonylamino esters (S)‐8a–d which can be hydrolyzed to deliver N‐protected α‐amino acids in >95% e.e. On the other hand, α‐mesitylsulfonylamino aldehydes 12a–d are available when bromoalkenes 3a, d–f are first debrominated (→ 11a–d) and subsequently ozonized. In order to avoid reacemization, the aldehydes 12a–d are not purified but submitted to a Mukaiyamatype aldol addition whereby hydroxyesters 15a–d are formed as single diastereomers in a chelate‐controlled reaction. The relative configuration of the esters 15a–d is proven by conversion into the oxazolidinones 16a–d whose optical purity is determined to exceed 92% e.e. by 1H‐NMR measurements in the presence of chiral shift reagents. The sulfonylimine 21a and a series of para‐substituted derivatives 21b–h are also allowed to react with the vinyllithium reagent 1b to give mixtures of diastereomers 22/23. The logarithms of the diastereomeric ratios 22:23 correlate with Hammett's σ‐values.