Stereoselective Synthesis of 1,3-Disaccharides through Diels-Alder Reactions: Part 2: Convenient Protecting Groups for Heterodienes and Conformational Evaluations

Improved synthesis of 1,3-disaccharides, obtained diastereomerically pure by [4+2] hetero cycloadditions between glycals and -thiono- -keto- -lactones, has been reported. The choice of appropriate protecting groups for -keto- -lactones, stable under cycloaddition conditions employed, sensibly shortened the synthesis of heterodienic precursors. The first example of a -oxy-imino- -lactone synthesized from -keto- -lactones was also reported. Molecular modeling and conformational evaluations about the cycloaddition features allowed tentatively rationalizing the experimental results.