1,3-Dipolar cycloaddition reactions of cis-3,4-dichlorocyclobutene with several nitrile oxides yield mixtures of syn- and anti- 4-substituted 6,7-dichloro-2-oxa-3-azabicyclo[3.2.0]hept-3-enes. The structures of the cycloadducts are deduced from n.m.r. and dipole moment data and a possible explanation for the abnormally high proportion of the sterically disfavoured syn-isomer is presented.
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Bianchi et al. (1974) studied this question.