[4+3] CYCLOADDITION REACTION OF SOME SILYL ENOL ETHERS HAVING A CONJUGATED CARBONYL FUNCTIONALITY

The Lewis acid catalyzed [4 + 3] cycloaddition reactions of 2-(trimethylsiloxy)propenal (1), with cyclopentadiene and furan were investigated. The vinylogous 4-(trimethylsiloxy)-2,4-pentadienal (5) reacted with cyclopentadiene in the presence of SnCl4at -78 °C to give 2-(formylmethyl)-substituted bicyclo[3.2.1]oct-6-en-3-one (11). This [4 + 3] cycloadduct arose as a result of the perispecific reaction at C3 and C5 rather than at C2 and C3 where the Diels-Alder reaction is normally expected to occur. With a homologous system, (1-(trimethylsiloxy)vinyl)oxirane (7, R = H) underwent a similar [4 + 3] cycloaddition reaction to give a 2-(hydroxymethyl)-substituted bicyclic product. In this case, trimethylsilyl triflate was a useful catalyst. The silyl enol ether 9, conjugated with a 1,1-cyclopropanedicarboxylate was also reactive; thus, a catalyzed ring opening followed by the cycloaddition reaction gave a bicyclic diester. These cycloaddition reactions may be explained by the formation of a key oxyallyl cation-like intermediate and provide a method for constructing a functionalized bicyclo[3.2.1]octane system. © 1990, American Chemical Society. All rights reserved.