A-Bisnorsteroids

Similar procedures are also successful in the construction of cyclobutane rings in other stereochemical environments, including the formation of bicyclo[3.1.1]heptane (67)and bridged tricyclic systems such as (68), (69) ° and (70). ° A variety of annelated cyclobutanes have been prepared by the photochemical procedure as above, yields are sometimes poor or only moderate. Examples include bicy-clo[2.1.0]pentanes (71), °° bicyclo[2.2.0]hexanes (72)"° and cyclobuteno aromatics (e.g, 73), " o-nor-steroids e.g. 74)," A-bisnorsteroids" and triterpenes" as well as the highly strained tricyclo[4.2.0.0 ]octane system (75)," and the [4.4.4.5]fenestrane (76)." ... 

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