FIRST DISCRIMINATION OF ENANTIOMERIC CYCLIC HEMIACETALS AND METHYL ACETALS DERIVED FROM HYDROXAMIC ACIDS AND LACTAMS OF GRAMINEAE BY MEANS OF H-1-NMR USING VARIOUS CHIRAL SOLVATING AGENTS

The discrimination of enantiomeric cyclic hemiacetals and methyl aceta ls derived from hydroxamic acids and lactams with the 2H-1,4-benzoxazi n-3(4H)-one and 2H-1,4-benzothiazin-3(4H)-one skeleton was investigate d using (S)-(-)-phenylethylamine, (-)-quinine, beta-cyclodextrin and, for the first time, R,11R)-(+)-2,8-dimethyl-6H,12H-5,11-methanodibenzo [b.f][1,5]diazocine, a Troeger's base enantiomer, as chiral solvating agents (CSA). Conditions for the enantiomeric discrimination of six c onfigurationally stable methyl acetals are reported. 2,4-Dihydroxy-2H- 1,4-benzoxazin-3(4H)-one and its 7-methoxy derivative, bioactive agluc ones from Gramineae species, are the first cyclic hemiacetals that cou ld be differentiated into enantiomers by means of H-1 NMR, despite the ir oxo-cyclo tautomerization that prevented enantioseparation by chrom atography or capillary electrophoresis. However, 2-hydroxy-2H-1,4-benz othiazin-3(4H)-ones (thiohemiacetals) could not be differentiated by t he use of these CSA. The influence of the structure of the enantiomers , CSA, temperature and concentration on the size of the chemical shift anisochrony is discussed.