Cyclopropyl Building Blocks for Organic Synthesis, 37. Synthesis and Diels—Alder Reactions of 2‐Substituted 2‐Cyclopropylideneacetates in Comparison with Allenecarboxylate and Ordinary Acrylates

Abstract

Abstract Several new 2‐substituted 2‐cyclopropylideneacetates 1a ‐X (X=F, N 3 , SPh, OTBDMS, OTBDPS) have been prepared. Their cycloadditions and those of some previously described compounds of type 1a ‐X (X=H, Br, Cl) with furan ( 5 ) and/or 6,6‐dimethylfulvene ( 7 ) are reported. Several of these peculiar acrylates 1a ‐X (X=H, Br, Cl, F, N 3 ), as well as allenecarboxylate 1b , regular acrylate 1c , crotonate 1d , and 3,3‐disubstituted acrylates 1e ‐X (X=H, Cl), were reacted with furan ( 5 ) and 6,6‐dimethylfulvene ( 7 ) in two series of competition experiments at 64°C. The [2 + 4] cycloaddition of 1a ‐Cl and 1a ‐Br to furan ( 5 ) are both about 16 times as fast and to dimethylfulvene ( 7 ) 230 and 210 times as fast as that of the parent acrylate 1c , while allenecarboxylate 1b reacts with 4 only 30 times as fast as 1c , and the 3‐substituted acrylates 1d and 1a ‐X (X=H, Cl) were all too sluggish to react under these conditions. The kinetic data obtained for the cycloadditions of 1a ‐X suggest a mechanism involving either diradicals or zwitterions as intermediates.