Photodimerization and Photocycloaddition Reactions Yielding Cyclobutanes

This chapter contains a review of some photodimerizations and photocondensations leading to the formation of substituted cyclobutanes. Since the literature in this field is indeed vast, it would be impossible to present here a comprehensive review of all the reactions reported to lead to such products. Instead, it is our goal to present a general survey of the various types of compounds known to produce cyclobutane compounds upon irradiation and to discuss mechanisms which have been proposed to account for some of these products. 

It could be recognized at this point that dimerization of an unsymmetri-cally substituted olefinic bond can potentially lead to a total of 12 different isomeric dimers, depending upon whether the dimerization proceeds in a 

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