Photoinduced electron transfer
Recyclable chiral auxiliary
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
---PET activation of 1-(N-benzyl-N-methylglycyl)-(S)-prolinol (1) in dry acetonitrile, utilizing 1,4-dicyanonaphthalene (DCN) as a light-harvesting electron-acceptor and methyl viologen (MV++) as an electron-transfer mediator, leads to the formation of 3-[benzyl(methyl)amino]perhydropyrrolo[2,1-c][1,4]oxazin-4-one (3). When this photolysis is carried out in aqueous acetonitrile, exclusively 3-hydroxyperhydropyrrolo[2,1-c][1,4]oxazin-4-one (4) is produced. The formation of 3 can be rationalized in terms of intramolecular cyclization of the in situ generated iminium cation intermediate (2) by the OH moiety of (S)-prolinol, while 4 is generated by hydrolysis of 2 followed by acetalization. Nucleophilic alkylation of 3 and 4, using Grignard reagents and allyltrimethylsilane/TiCl4, provides 12a-d & 15 and 17a-c & 21, respectively, in a highly stereoselective manner. Hydrolysis of the resultant amides (12, 15, 17, and 21) provides α-amino acid derivatives (14) and α-hydroxy acids, respectively, in optically active form, along with the recovered (S)-prolinol chiral auxiliary in its recyclable form.
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