Photoinduced electron transfer (PET) promoted oxidative activation of 1-(N-benzyl-N-methylglycyl)-(S)-prolinol: Development of novel strategies towards enantioselective syntheses of alpha-amino acids, their N '-methyl derivatives and alpha-hydroxy acids employing (S)-prolinol as a recyclable chiralauxiliary

PET activation of 1-(N-benzyl-N-methylglycyl)-(S)-prolinol (1) in dry aceto nitrile, utilizing 1,4-dicyanonaphthalene (DCN) as a light-harvesting elect ron-acceptor and methyl viologen (MV++) as an electron-transfer mediator, l eads 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 acetonitri le, exclusively 3-hydroxyperhydropyrrolo[2,1-c][1,4]oxazin-4-one (4) is pro duced. The formation of 3 can be rationalized in terms of intramolecular cy clization of the in situ generated iminium cation intermediate (2) by the O H moiety of (S)-prolinol, while 4 is generated by hydrolysis of 2 followed by acetalization. Nucleophilic alkylalion of 3 and 4, using Grignard reagen ts and allyltrimethylsilane/TiCl4, provides 12a-d & 15 and 17a-c & 21, resp ectively, in a highly stereoselective manner. Hydrolysis of the resultant a mides (12, 15, 17, and 21) provides alpha-amino acid derivatives (14) and a lpha-hydroxy acids, respectively, in optically active form, along with the recovered (S)-prolinol chiral auxiliary in its recyclable form.