2,4-Methanoproline synthesis

The presence of a pyrrolidine unit in complex systems such as the azabicy-clo[2.1.1]hexane unit of 2,4-methanoproline (56) lends itself to synthesis pertaining to the 1,3-disubstituted cyclobutane part of the molecule. Thus two syntheses of 56 have been published, relying on the [2 + 2] photochemical synthesis of cyclobutanes. A nontrivial problem of the synthesis of these compounds is liberation of the target molecule from its protective groups (Scheme... 

The intramolecular [2 + 2] photocycloaddition of an a-carboxy enamide with an olefin has been elegantly exploited independently by Pirrung and by Clardy and coworkers in the synthesis of the toxic amino acid 2,4-methanoproline via irradiation of 32 (equation 9)15. 

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