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.
Solvent polarity and reaction temperature strongly influence the reactions of dimethyl acetylenedicarboxylate (DMAD) with I-pyrrolidinyl enamines of acyclic and cyclic ketones. Whereas DMAD and I-[l-phenylt-@henylthio)ethenyl]pyrrolidiae (3) give only a mixture of the isomeric I$-butadiencs (5) in apolar solvents, in methanol the main product is the pyrrolizine 7, together with 5. Again in methanol, DMAD reacts at 0-Y with 8.9 and 10 to give exclusively I : I adducts, tbe pyrrolizines 11, 12 and 13, respectively, whereas at-50" 8 and 9 give I : 2 (enamine : DMAD) adducts. the pyrrolixines 14 and 15, respectively; a siqle crystal X-ray analysis of 14 gave the structure of the I : 2 adducta. In the same solvent methyl propiolate and 8 give only the linear Michael adduct 17. The enamine-ketone 18 reacts with DMAD in propylene carbonate at W' to give, via (2 t Zj-cycloaddition and ring expansion, 19, and the linear Michael adduct 20. The mechanism of (2 t Zj-cycloaddition and pyrrolizine formation is discussed in terms of a common tied-ion pair intermediate formed in the first. ratedetermining step,
S u m m a r y 2,3-Dihydroazete 1-oxides (3b-e), the first examples of crystalline, 'stable,' four-membered cyclic nitrones, are formed by reaction of 1-nitroalkenes with ynamines, and other products are 3-nitrocyclobutenes ;the structure of the cyclic nitrone (3b) has been determined by X-ray crystallography.CYCLOBUTENES are formed thermally under mild conditions either by reaction of electron-rich alkenes (e.g. enamines) with electron-deficient acetylenes [acetylene(di)carboxylic acid esters' 9 , or 1-t-butyl-2-nitr~acetylene~] or by reaction of electron-deficient alkenes (e.g. 1-cyanoalkenes) with electron-rich acetylenes (ynamines4 9 5 ) . We describe here the reactions of 1-nitroalkenes and ynamines.1-Nitrocyclopentene (la)6 was treated with an equimolar amount of l-phenyl-2-(pyrrolidin-l-yl)acetylene (2a) in light petroleum for 16 h at room temperature. A tan precipitate was filtered off,? and orange crystals (m.p. 80-81 "C; 40% yield) were obtained from the filtrate. This product was identified as the bicycloheptene (4a); lH n.ni.
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