Diastereoselective additions of titanium enolates from N-glycolyl thiazolidinethiones to acetals.

The stereochemical outcome of the Lewis acid-mediated glycolate addition of the titanium enolates from protected N-hydroxyacetyl-4-isopropyl-1,3-thiazolidine-2-thiones to dimethyl and dibenzyl acetals depends on the hydroxyl protecting group. Particularly, the pivaloyl protected glycolate derivative provides the reluctant anti adducts in high yields and diastereomeric ratios, which can be isolated and further converted in enantiomerically pure form to ß-methoxy or ß-benzyloxy α-pivaloyloxy carbonyl fragments in a straightforward manner.