Bicyclic compounds, free radical synthesis

The wide variety of methods available for the synthesis of orga-noselenides,36 and the observation that the carbon-selenium bond can be easily cleaved homolytically to give a carbon-centered radical creates interesting possibilities in organic synthesis. For example, Burke and coworkers have shown that phenylselenolactone 86 (see Scheme 16), produced by phenylselenolactonization of y,S-unsaturated acid 85, can be converted to free radical intermediate 87 with triphenyltin hydride. In the presence of excess methyl acrylate, 87 is trapped stereoselectively, affording compound 88 in 70% yield 37 it is noteworthy that the intramolecular carbon-carbon bond forming event takes place on the less hindered convex face of bicyclic radical 87. 

In conclusion, free radical intramolecular addition in the CyS/Cy6 case appears to be a method of preparative value for the synthesis of fused bicyclic compounds. The observed regioselectivity is often very high but it is clear that other factors than those considered for monocyclization processes are operative, for in many instances only the (Cy6) compounds are observed. A new and interesting feature appears, namely, the high stereoselectivity often observed in these cyclizations. The origin on this stereoselectivity has been discussed in the reference eked, and is often considered to result from the last step, i.e., the transfer step being favored from the less hindered side of the molecule. In other cases, such as the one discussed in Scheme 113 where four asymmetric centers are formed stereoselectively, a synchronous mechanism has been proposed. Because of the very different nature of the substrates considered in this section it does not seem worthwhile to discuss further the origin of this stereoselectivity. 

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