Olefinic compounds, exchange labeling

In principle any except tetrasubstituted olefins could be labeled by catalytic exchange, but in practice it is difficult to find experimental conditions under which significant tritium incorporation can be catalyzed without reducing the double bond. However, if the olefinic starting material is separable from the reduced product, it is sometimes possible to find reaction conditions that disfavour complete reduction, then isolate the unreduced olefin which has become labeled. Two examples with which this has been accomplished are pleuromutilin (71) and cyclosporin A (72) " °. Treatment with a deficiency of tritium gas for 60-90 min in the presence of 10% Pd/C in ethyl acetate (71) or DMF (72 followed by removal of reduced product by HPLC, provided the compounds, labeled as indicated, at specific activities of 10 and 19Ci/mmol, respectively. Unfortunately, it proved to be difficult to separate the cis/trans mixture formed with 72, which stimulated the development of a tritide labeling approach (see Chapter 4, Section 4.3.1). 

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