Highly Enantioselective Synthesis of 11‐Substituted 10,11‐Dihydrodibenzo[b,f][1,4]thiazepines through a Proline‐Catalyzed Mannich Reaction of Seven‐Membered Cyclic Imines with Ketones

Abstract Highly enantioselective Mannich addition reactions of N,S‐heterocyclic dibenzo[ b,f ][1,4]thiazepines as seven‐membered cyclic imine substrates with ketones were developed by using proline as an organocatalyst. Substituted dibenzo[ b,f ][1,4]thiazepines and various unsymmetrical alkyl methyl ketones were used as substrates and optimized reaction conditions offered an efficient method to synthesize optically active 11‐substituted 10,11‐dihydrodibenzo[ b,f ][1,4]thiazepine derivatives that contained a carbonyl functional group in 91–99 % ee . Furthermore, we found that the reactions could also be performed on a gram scale, whilst maintaining high yields and enantioselectivities. The carbonyl group of the Mannich products also underwent diastereoselective reduction, thus yielding a new hydroxy‐substituted chiral stereogenic center with moderate‐to‐excellent diastereoselectivities, good yields, and without the loss of enantiomeric excess.