Conformations of 4‐tert.‐butyloxy‐, 4‐trimethyl silyloxy‐ and 4‐trimethyl stannyloxy‐6‐methyl‐2‐pyrones in the crystalline state and in solution

The conformational studies in solution and the solid state of substituted 6-methyl-4-hydroxy-2-pyrones indicated that substitents on 4-hydroxy group clearly favored s- cis arrangement towards formal double bond. steady NOE measurements kinetic tert -butyl, trimethylsilyl trimethylstannyl derivatives showed a marked preference for this conformation solution. structures determined by X-ray diffraction demonstrated exclusive presence same state. In these three derivatives, pertinent dihedral angle deviates only slightly from value coplanarity with predictions τ-bond model are accordance observations. structure derivative tin is pentacoordinated short contacts to oxygens at 2 4 position neighboring molecules. plays both role protecting moiety Lewis acid 2-carbonyl oxygen. x-ray stannyl shows unique features which can be compared extended transition aldol reactions.