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 of six-membered) rings can be isolated. In all other cases the corresponding dx.irazu'-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 m,m-cycloalkadienes 5 as was proven by X-ray analysis in the case of 5c. Some compounds 5 rearrange further to the isomeric ris.tis-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-cyc\ononaáienes 5d, 5j, 5k, and 51 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 l-(l-pyrrolidinyl)bicyclo[n.2.0]alkenes 7-9 rearrange to the corresponding pyrrolizines [10][11][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 cw,/raní-cycloalkadienes 4c and 4d in methanol. As a result of our work a number of structures of reaction products of enamines and DMAD reported in the literature will have to be revised.
1 -(1 -Pyrrolidinyl)-1,3-butadienes (2, 3, 5, and 7), all having an electron-withdrawing substituent at C-3, undergo a thermal rearrangement to pyrrolizine derivatives (1, and 10-14). The rate of reaction generally increases with increasing polarity of the solvent; in aprotic solvents Lewis acids act as catalysts. The rate of reaction is also dependent on the configuration of the l-( 1-pyrrolidinyl)-1,3-butadiene (k^y¡/k(Z)-5 = 5.2). These thermal rearrangements take place in two steps. The first step comprises a concerted antarafacial [1,6] hydrogen shift that generates a 1,5-dipolar species, e.g., 15a, that may undergo stereomutation to 15b; in the second step a disrotatory electrocyclization takes place to give the corresponding pyrrolizines
The thermal valence isomerization of (hetero) cyclobutenes to (helero) 1,3-butadienes is a classic example of a stereospecific reaction, the conrotatory mode of which is in agreement with the principle of conservation of orbital symmetry in electrocyclic reactions.1,2 A large amount of work has been carried out on the thermal isomerization of compounds in which the cyclobutene moiety is cis annulated to another ring system. It is generally accepted that when the annulated ring possesses less than eight atoms, ring opening must occur by way of the symmetry-forbidden disrotatory mode3 or by homolytic4 or heterolytic5 pathways, all
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