alpha-amino acids and N-protected alpha-amino aldehydes by stereoselective additions of a chiral vinyllithium reagent to sulfonylimines

The 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 greater than or equal to 98% d.e. after column chromatography. The bromoalkenes 3a-d are submitted to ozonolysis in methanol to give alpha-mesitylsulfonylamino esters (S)-8a-d which can be hydrolyzed to deliver N-protected alpha-amino acids in >95% e.e. On the other hand, alpha-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 Mukaiyama-type 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 H-1-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 sigma-values.