Cyclopropyl building blocks for organic synthesis, 37: Syntheses and Diels-Alder reactions of 2-substituted 2-cyclopropylidene-acetates in comparison with allenecarboxylate and ordinary acrylates

Article abstract of DOI:10.1002/jlac.199619961208

Several new 2-substituted 2-cyclopropylideneacetates 1a-X (X = F, N₃, 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, N₃), 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. VCH Verlagsgesellschaft mbH, 1996.