Synthesis of 1,3-thiazolidines from aromatic thioketones and N-benzylidene α-amino acid esters via 1,3-dipolar cycloaddition of azomethine ylides

The reaction of N-benzylidenephenylglycine methyl ester (2a) and N-benzylidenalanine methyl ester (2b) with thiobenzophenone (5a) and fluorene-9-thione (5b) in acetonitrile in the presence of lithium bromide and DBU gave a mixture of two corresponding diastereoisomeric [2+3] cycloadducts of types (6) and (7). The products were formed regioselectively and in good yields from the in situ generated metallo-azomethine ylides (8). The relative configurations of the two products (6c) and (7c) formed from fluorene-9-thione (5b) and 2a were established by X-Ray crystallography