Conformations of 4-tert-Butyloxy-, 4-(Trimethylsilyl)oxy- and 4-(Trimethylstannyl)oxy-6-methyl-2H-pyran-2-ones in the Crystalline State and in Solution

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