A very efficient and fast energy transfer (k = 4.7 × 108 s−1) is observed from the Ru‐ to the Os‐based moiety in 1. In the presence of oxygen a photoreaction occurs that leads to a strong decrease (about 80 times) in the rate of the energy transfer process. R  CH2C6H5, bpy = 2,2′‐bipyridine.

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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.

Starting from the N-butadienyl-N-alkylpropionamides 1a-1c the corresponding N,O-trimethylsilylacetals could be obtained using the mixture of LDA and trimethylsilyl chloride in THF. The unexpected reaction sequence Diels-Alder reaction/acylation between the N-butadienyl-N-alkylketene N,O-trimethylsilylacetal of propionamide (2a-2b) and N-phenylmaleimide produced tricyclic products rac-5a-rac-5b and bicyclic products rac-6a–rac-6b with high diastereoselectivity. The reaction of the N,O-trimethylsilylacetals 2a and 2c with acryloyl chloride in a similar sequence gave the bicyclic products rac-8a and rac-8c. The stepwise synthesis of bicyclic systems of this general structure could only be successfully executed in 26% yield treating the Diels-Alder product rac-10 with LDA.