Reactivity of Cis-Fused 3-(Dialkylamino)cyclobutenes in Polar and Apolar Solvents. Synthesis, X-ray Structures, and Reactions of cis,cis- and cis,trans-1,3-Cycloalkadienes

Article abstract of DOI:10.1021/ja00317a029

Contrary to previous reports in the literature, the reactivity of (2+2) cycloadducts of enamines of cyclic ketones and DMAD in apolar solvents depends strongly on the size of the ring fused to the cyclobutene ring and on the dialkylamino group at the bridgehead position.Only the cycloadducts (2) of five-membered (and in some cases six-membered) rings can be isolated.In all other cases the corresponding cis,trans-cycloalkadienes 4 are obtained at room temperature.The structure of 4c was proven by a single-crystal X-ray analysis.In two cases a rapid reversible interconversion of compounds 2 and 4 in solution was observed (2b ->/<- 4b and 2k ->/<- 4k).At higher temperatures the cis,trans-cycloalkadienes 4 rearrange by a thermal <1,5> hydrogen shift to give the cis,cis-cycloalkadienes 5 as was proven by X-ray analysis in the case of 5c.Some compounds 5 rearrange further to the isomeric cis,cis-cycloalkadienes 3.The structure of 3c was proven by X-ray analysis.Compounds 3 have been previously reported in the literature as the reaction product in apolar solvents of the enamines and DMAD at higher temperatures.The cis,cis-cyclononadienes 5d, 5j, 5k, and 5l undergo further electrocyclization under these conditions in a "symmetry nonallowed" fashion to the corresponding bicyclo<5.2.0>non-8-enes 6.The cis stereochemistry in the compounds was proven by X-ray analysis of 6b.Under the influence of light compounds 4 rearrange to 3 by trans to cis isomerization and/or to 5 by an antarafacial <1,5> sigmatropic reaction.In polar solvent the 1-(1-pyrrolidinyl)bicycloalkenes 7-9 rearrange to the corresponding pyrrolizines 10-12 with a deviating pathway of the mixture of 2b and 4b that gives the dimer 14, the structure of which was proven by single-crystal X-ray analysis.The conversion into pyrrolizines 15 and 16 was also observed upon reaction of the cis,trans-cycloalkadienes 4c and 4d in methanol.As aresult of our work a number of structures of reaction products of enamines and DMAD reported in the literature will have to be revised.