An Approach to Modify 14-Membered Lactone Macrolide Antibiotic Scaffolds

[Image: see text] A ketolide derivative with (12R)-configuration was obtained via a novel ketene acetal in acidic conditions. The structure of this atypical β-keto ketene acetal intermediate within the macrocyclic system has been determined by NMR and X-ray methods. The use of basic conditions at an elevated temperature yielded new, doubly α,β-unsaturated ketone macrolide derivatives with (4E)-configuration as two conformational isomers of folded-in or folded-out conformations.