Steric effects on regioselectivity in 1.3-Dipolar cycloaddition of C,N-dialyl nitrones with acceptor-substituted alkynes

Article abstract of DOI:10.1016/S0040-4020(01)88127-8

The 1.3 dipolar cycloaddition of alkynes 4-4 with acyclic nitrones, 1-3, 23 and 24 as well as with cyclic nitrones 27 is studied. As was found for the reaction of the aldonitrones 1-3 an increased portion of 5-regioisomers is formed with increasing steric demand of either of the two substituents, the C-alkyl substituent R⁹ and the N-alkyl substituent R¹. Thus the conclusion if drawn that cycloaddition of 1-3 proceeds not only via transition states arising from Z-nitrones but also via transition states developing from E-nitrones, although in the ground-state the Z-isomer is favored to a considerable extent. Int his context steric destabilization of the transition states is discussed qualitatively.