Stereospecific synthesis and hydrolysis of optically active diaryl(acylamino)(acyloxy)spiro-λ 4-sulfanes and related cyclic diaryl(acylamino)sulfonium salts

Abstract

The stereospecific synthesis of diaryl(acylamino)(acyloxy)spiro-λ 4-sulfanes ( S)-(+)- 2, ( R)-(+)- 5, ( S)-(+)- 8, and their conversion into related diaryl(acylamino)sulfonium tetrafluoroborates ( R)-(+)- 3, ( S)-(+)- 6, ( R)-(+)- 9, respectively, is described. The enantiomers of spiro-λ 4-sulfanes ( S)-(+)- 2, ( R)-(+)- 5 and ( S)-(+)- 8 were prepared by dehydration of the corresponding optically active sulfoxide–carboxylic acids ( R)-(+)- 1, ( R)-(−)- 4 and ( S)-(+)- 7, respectively, which were obtained from the racemic forms by diastereoisomeric salt separation with homochiral organic bases. The stereomechanism of the hydrolysis reaction of spiro-λ 4-sulfanes and sulfonium tetrafluoroborates that depends on pH, the nature of the axial heteroatom, the size of the spiro rings and carboxyl neighbouring group participation is also discussed.