Bicyclo proline synthesis

Hexenyl radicals cyclize to cyclopentylmethyl radicals (see Volume 4, Chapter 4.2). Thus radical decarboxylation of 6-heptenoic acids, by whatever means, usually results in die formation of five-mem-v beied rings. Although this fact had been appreciated previously it is only recendy, widi the advent of the 0-acyl thiohydroxamates, that it has been exploited from a syndietic point of view. An example is provided by the synthesis of bicyclo[4.3.0]proline derivatives from aspartic acid carried out by the Barton group (equation 51). It will be noted that activation of die C—C double bond acting as a radical trap is not necessary in these intramolecular reactions. 

Pro line tetrazole catalysts (category C in Figure 1.1) are readily accessible from L-proline (1) [158]. They are remarkably useful in asymmetric synthesis [159]. As shown in Figure 1.2, the pfCj of tetrazole is very similar to that of carboxyUc acid. Moreover, the advantage of tetrazole catalysts is their robust and lipophihc nature compared to L-proline (1) itself, which allows them to escape parasitic bicyclo-oxazolidinone formation [160]. 

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