For the mechanism, reaction of pyrrolidine with the carbonyl donor can give enamine a that could proceed reaction with the re -face of the aldehydes to give the iminium ion b (Scheme \PageIndex6...For the mechanism, reaction of pyrrolidine with the carbonyl donor can give enamine a that could proceed reaction with the re -face of the aldehydes to give the iminium ion b (Scheme \PageIndex6). The attack of the enamine on the si -face of the aldehyde leads to the unfavorable transition state c . Thus, the position of the carboxylic acid group on the pyrrolidine ring directs the stereoselection of the catalyzed reaction providing either syn - or anti -Mannich products.
