Swern Oxidation of Alkenyl‐Substituted 2‐(tert‐Butyldimethylsiloxy)‐1‐(hydroxymethyl)cyclopropanes: A Novel and Flexibel Route to Functionalized 2,5‐Dihydrooxepines

Swern oxidation of 2‐alkenyl‐substituted 2‐siloxy‐1‐(hydroxymethyl)cyclopropanes 7 and 11 ‐ 16 provided 2,5‐dihydrooxepines 8 and 17‐21 , respectively. The corresponding cyclopropanecarboxaldehydes are highly plausible intermediates in this transformation, which give the seven‐membered heterocycles by a concerted [3,3]‐sigmatropic rearrangement. Some of the 2,5‐dihydrooxepines easily undergo ring contraction to 2‐vinyl‐2,3‐dihydrofurans. Direct formation of these furan derivatives was observed when (hydroxymethyl)‐ cyclopropanes 25‐28 were oxidized. In these cases a step‐wise rearrangement of the 2‐siloxycyclopropanecarboxalde‐hydes involving 1,3‐zwitterions as intermediates is proposed. Our results are compared with the few literature examples and reveal interesting substituent effects. Thus, the donor‐acceptor substitution shifts the equilibrium between the 2‐vinylcyclopropanecarboxaldehydes and the 2,5‐dihydrooxepines to the side of the seven‐membered heterocycles. Further mechanistic details are discussed.