Cycloaddition of isocyanates with alkenes

The reaction of isocyanates with alkenes toward azetidin-2-ones requires activation of the former by electro-with-drawing groups or activation of the alkenes by electron-donating groups 2004COR463 . 

The classical reaction with chlorosulfonyl isocyanate has been extended to it-vinyl sulfide 449 to give a 2.5 1 diastereomeric mixture of 4-(phenylthio)azetidin-2-ones 343 and 450 (Equation 180) 2000MI935 . The facial selectivity in the cycloaddition has been explained by the conformational preference of the allylic groups in the transition structure. A similar reaction with styrene resulted into synthesis of the racemic 4-aryl-azetidin-2-one (Equation 181) 2000TA2351 . The divinyl ether 451 reacted with acid-free chlorosulfonylisocyanate to form 4-vinyloxyazetidin-2-one 452 (Equation 182) 1996SL895, 1997TA2553, 1998TL8349 . Most of the results in the reactions of isocyanate with vinyl ethers could be rationalized by a -conformational preference of the ether in 

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