Photolysis of N-bromo-N-tert-butyl-Nα-phthaloylphenylalaninamide gave a 1∶1 mixture of the diastereomers of 3-bromo-N-tert-butyl-Nα-phthaloylphenylalaninamide. Reactions carried out with various concentrations of the N-bromoamide, and in the presence of 4-tert-butyltoluene, showed that the ratio of the product bromophenylalanine derivatives to 4-tert-butylbenzyl bromide varied as a function of the concentration of the N-bromoamide, indicating that the bromophenylalanine derivatives are formed through an intermolecular process and not by intramolecular 1,4-hydrogen atom transfer of the corresponding amidyl radical. Results of reactions of N-bromo-N-tert-butyl-Nα-phthaloyl-p-methylphenylalaninamide are also consistent with this interpretation. Reactions of stereoselectively β-deuteriated derivatives of N-bromo-N-tert-butyl-Nα-phthaloylphenylalaninamide established that the pro-S benzylic hydrogen of the phenylalaninamide is selectively abstracted, by a factor of ca. 3.8, in this intermolecular process.
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