Cage compounds, photocyclization

A dehcate stereochemical balance is clear in the high yield photocyclization of one cyclophane configuration (47) to a cage compound (48), whereas its spatial relative (49) leads only to higher molecular weight product (38). 

The most widely exploited photochemical cycloadditions involve irradiation of dienes in which the two double bonds are fairly close and result in formation of polycyclic cage compounds. Some examples of alkene photocyclizations are given in Scheme 6.9. Entry 1 is a transannular cyclization. The preference for the observed product over tricyclo[4.2.0.02,5]octane does not seem to have been analyzed in detail. Entries 2, 3, and 4 involve photolysis in the presence of Cu03SCF3. Entries 5 and 6 are cases in which the double bonds are in close proximity and can cyclize to caged structures. 

Irradiation of the bis-alkene 210 brings about the formation of the bishomocubane 211 in good yield98. The triene 212 is of interest and has been shown to be photochemically reactive, yielding the adduct 213 on irradiation. Several approaches to 212 have been reported over the years. One such approach follows the path of photocyclization of 214 to yield 215 that can be converted to the desired product 21299. A variety of sensitizers can be used for the excitation of alkenes to bring about the (2 + 2)-cycloaddition. Commonly, acetone has been used but, in at least one case, the formation of the cage compound 216 from the diene 217, tetraphenylporphine has been found to be of use100. 

Iwakuma, X, Hirao, K., and Yonemitsu, O., Photocyclization of N-chloroacetyltyramines. II. Flash photolysis and substituent effect studies on the formation of dimeric cage compounds and novel acid-catalyzed reversion,/. Am. Chem. Soc., 96, 2570,1974. 

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