Facial selectivity due to a chiral auxiliary

Carboxylates with different non-racemic chiral auxiliaries have been anodically de-carboxylated to explore the face-selective hetero- and homocoupling of the intermediate radicals. 2-Substituted malonamides were subjected to heterocoupling with different coacids. In the acids 30a-c, (2/ ,5.R)-2,5-dimethylpyrrolidine served as chiral amido group. 

Heterocoupling with the coacids 31-35 led to the amides 36a-g with different diastereomeric excess (Eq. 6), Table 7] [97.  

Side-products besides 36 were compounds arising from hydrogen abstraction and further oxidation of the radical to a carbocation. This was especially pronounced in the case of 361, where these compounds became the major products. 

Replacement of the methyl group by the methoxymethyl group in the auxiliary of 30b in the coelectrolysis with 33 leads to the coupling product 38 in a similar yield and diastereoselectivity to that with 30b [100]. 

The energy profile for the heterocoupling of the radicals from 30b and 33 has been modeled with the semiempirical method PM3 (BIRADICAL LET GNORM = 0.001 EF). The reaction pathways to the diastereomers 2R- and 2S -36d show a flat maximum at about 270 pm. The energy maximum leading to the major (2R)-diastereomer is calculated to be 0.34 kcal/mol lower than that leading to the minor (25)-diastereomer [100]. 

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