[4 + 2] Cycloaddition of thiophene S-monoxides to activated methylenecyclopropanes

Thiophene S-oxides 4 have been shown to undergo [4 + 2] cycloadditions with methylenecyclopropanes 2 with one or two electron acceptor substituents, i.e. even the tetrasubstituted 2-chloro-2-cyclopropylideneacetate 2c reacted well. However, for the tetrasubstituted alkene bi(cyclopropylidene) 2f high pressure had to be applied to make it react with 4. Only one diastereoisomer was formed in all the reactions. X-Ray crystal structure analyses of two of the cycloadducts, 3a and 3f, have been performed, their relative configuration determined as being endo,syn. The Wittig olefination of cyclopropanone hemiacetal to generate the methylenecyclopropanes and the subsequent cycloaddition can be carried out in a one-pot operation. This procedure is one of many potential one-pot or multi-component reactions involving stabilized phosphoranes. A further example of this type of reaction is shown in the novel desilylation-Wittig olefination of 1-ethoxy-1-trimethylsilyloxycyclopropane 5 to yield in one step cyclopropylideneacetates, e.g. 2a.