First discrimination of enantiomeric cyclic hemiacetals and methyl acetals derived from hydroxamic acids and lactams of Gramineae by means of 1H NMR using various chiral solvating agents
✍ Scribed by Jens Klein; Holger Hartenstein; Dieter Sicker
- Publisher
- John Wiley and Sons
- Year
- 1994
- Tongue
- English
- Weight
- 441 KB
- Volume
- 32
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
Abstract
The discrimination of enantiomeric cyclic hemiacetals and methyl acetals derived from hydroxamic acids and lactams with the 2__H__‐1,4‐benzoxazin‐3(4__H__)‐one and 2__H__‐1,4‐benzothiazin‐3(4__H__)‐one skeleton was investigated using (S)‐(−)‐phenylethylamine, (−)‐quinine, β‐cyclodextrin and, for the first time, (5__R__,11__R__)‐(+)‐2,8‐dimethyl‐6__H__,12__H__‐5,11‐ methanodibezo [b,f] [1,5]diazocine, a Troeger's base enantiomer, as chiral solvating agents (CSA). Conditions for the enantiomeric discrimination of six configurationally stable methyl acetals are reported. 2,4‐Dihydroxy‐2__H__‐1,4‐benzoxazin‐3(4__H__)‐one and its 7‐methoxy derivative, bioactive aglucones from Gramineae species, are the first cyclic hemiacetals that could be differentiated into enantiomers by means of ^1^H NMR, despite their oxo–cyclo tautomerization that prevented enantioseparation by chromatography or capillary electrophoresis. However, 2‐hydroxy‐2__H__‐1,4‐benzothiazin‐3(4__H__)‐ones (thiohemiacetals) could not be differentiated by the 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.