Stereoselective One-Pot Synthesis of Constrained N,O-Orthogonally Protected C-Glycosyl Norstatines [C(1')-Aminoglycosyl-1,3-dioxolan-4-ones].

A procedure for the synthesis of conformationally constrained C-glycosyl norstatines has been developed. The key step of the reaction is the addition of (S)-N-sulfinyl azomethines to chiral (2S)-enolates of dioxolanones which exploits Seebach's "SRS" principle. The trisubstituted C-glycosyl-alpha-hydroxy-beta-aminoacids are formed as N,O-orthogonally protected 1'-glycosyl-sulfinylamino-dioxolan-4-ones, usually with high diastereomeric excesses. Both the sulfinyl group at the nitrogen atom and the acetal moiety of the dioxolanone ring were selectively removed, thus demonstrating the orthogonality of the two protecting groups. In fact, the MeO- induced methanolysis of the acetal group gave the corresponding methyl C-glycosyl-sulfinylamino-isoserinates, while the acid induced removal of the sulfinyl group gave the N-deprotected 1'-glycosylamino-dioxolan-4-ones, which were in several cases subjected to a one-pot base-induced cyclization yielding the corresponding glycosyl-beta-lactams. This allowed the stereochemical configuration assessment of the parent 1'-glycosyl-sulfinylamino-dioxolan-4- ones by chemical correlation methods or by NOE experiments performed on the beta-lactams.