Synthesis of an Enantiomerically Pure 1,3-Thiazole-5(4H)-thione and Its Stereoselective 1,3-Dipolar Cycloaddition with an Azomethine Ylide

Abstract

Starting from the enantiomerically pure 2H-azirin-3-amines (R,S)-4 and (S,S)-4, the enantiomeric, optically active 4-benzyl-4-methyl-2-phenyl-1,3-thiazole-5(4H)-thiones (R)-1 and (S)-1, respectively, have been prepared (Schemes 2 and 3). In each case, the reaction of 1 with N-(benzylidene)[(trimethylsilyl)methyl]amine (2) in HMPA in the presence of CsF and trimethylsilyl triflate gave a mixture of four optically active spirocyclic cycloadducts (Scheme 4). Separation by preparative HPLC yielded two pure diastereoisomers, e.g., (4R,5R,9S)-10 and (4R,5R,9R)-10. The regioisomeric compounds 11 were obtained as a mixture of diastereoisomers. The products were formed by a 1,3-dipolar cycloaddition of 1 with in situ generated azomethine ylide 3, which attacks 1 stereoselectively from the sterically less-hindered side, i.e., with (R)-1 the attack occurs from the re-side and in the case of (S)-1 from the si-side.