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18.2.4.2.2.1 Variation 1: [2+2]-Cycloaddition Reactions

DOI: 10.1055/sos-SD-018-00070

Braverman, S.; Cherkinsky, M.; Birsa, M. L.Science of Synthesis, (200518154.

Isocyanates can undergo [2+2]-cycloaddition reactions across the C=N bond with activated alkenes and afford β-lactam derivatives. Usually, an electron-donating substituent in the alkene and an electron-withdrawing substituent in the isocyanate are necessary for the cycloaddition to occur. Thus, highly electrophilic sulfonyl and acyl isocyanates give cycloadducts in good yields with vinyl esters, vinyl ethers, acetates, or enamines.[‌561‌] For example, the reaction of tosyl isocyanate with 3,4-dihydro-2H-pyran at 0°C leads to the formation of the bicyclic β-lactam 268; heating of the latter in benzene solution for 20 minutes induces heterolytic cleavage of the NC bond and formation of the α,β-unsaturated amide 269 (Scheme 143).[‌562‌] Various sugar vinyl ethers react with tosyl isocyanate in the same manner.[‌563‌] Similarly, 2,2,2-trichloroethylsulfonyl and 2,2,2-trichloroethoxysulfonyl isocyanates react cleanly and in good yield with methylenecyclohexane, 1-methylene-4-phenylcyclohexane, and styrene to give the corresponding β-lactams.[‌564‌]

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