Domino Heck-Diels-Alder reactions of monosubstituted bicyclopropylidenes

A three-component domino Heck-Diels-Alder reaction involving pinacol bicyclopropylideneboronate (8b), iodobenzene (9) and methyl acrylate (12) under Jeffery conditions [Pd(OAc)₂, PPh₃, K₂CO ₃, Et₄NCl, MeCN] produced a mixture of phenylspiro[2.5]octeneboronates syn/anti-(E)-14b and methyl phenylspirooctenecarboxylate 25 in 25 and 38% yield, respectively. The major product 25 was most probably formed via the homoallylpalladium complex 23b undergoing deboropalladation rather than dehydropalladation. Similarly, reactions of tributylstannyl- and hydroxydimethylsilyl-substituted bicyclopropylidenes 8c-d with 9 and tert-butyl acrylate gave the tert-butyl phenylspirooctenecarboxylate 26 via the diene 24 formed by demetallopalladation processes. The reaction of methyl 1,1′-bicyclopropylidene-2-carboxylate (8e) with iodobenzene (9) in the presence of tert-butyl acrylate (13) furnished a mixture of regioisomeric and diastereomeric spirooctenes syn/anti-(E)-15e and syn/anti-(Z)-15e in 69 and 6% yield, respectively. The structures of the major pair of diastereomers syn-(E)-15e and anti-(E)-15e were rigorously proved by X-ray crystal structure analyses.