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

Abstract

The discrimination of enantiomeric cyclic hemiacetals and methyl acetals derived from hydroxamic acids and lactams with the 2H-1,4-benzoxazin-3(4H)-one and 2H-1,4-benzothiazin-3(4H)sne skeleton was investigated using Conditions for the enantiomeric discrimination of six configurationally stable methyl acetals are reported. 2,4-Dihydroxy-2H-1,4-benzoxazin-3(4~-one and its 7-methoxy derivative, bioactive aglucones from Gramineae species, are the first cyclic hemiacetals that could be differentiated into enantiomem by means of 'H NMR, despite their oxo-cyclo tautomerization that prevented enantioseparation by chromatography or capillary electrophoresis. However, 2-hydroxy-2H-1,4-benzothiazin-3(4~-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.

Zwitterionic Phosphorylated Quinines as Chiral Solvating Agents for NMR Spectroscopy

“…= 0.804 (5), 0.197 (5) for Bu in A and s.o.f. The diffraction patterns obtained from the same and/or different crystals of 2b × 2H 2 O at different temperatures were always similar.…”

“…DNB derivatives were introduced because the presence of an electron-poor aromatic ring enhances the interaction with the electron-rich quinoline fragment of the receptor. In general, the application of zwitterionic phosphorylated quinines (2a-c; ligand : CSA concentration 10 mM : 10 mM) resulted in discrimination of the enantiomers of eleven (3)(4)(5)(6)(7)(8)(9)(10)(11)15, and 16) of the 14 studied compounds (Table 2; Table S1 in the Supporting Information). In general, the application of zwitterionic phosphorylated quinines (2a-c; ligand : CSA concentration 10 mM : 10 mM) resulted in discrimination of the enantiomers of eleven (3)(4)(5)(6)(7)(8)(9)(10)(11)15, and 16) of the 14 studied compounds (Table 2; Table S1 in the Supporting Information).…”

“…Among them, enantiomers of N-DNB derivatives (3-8 and 11) were differentiated the most effectively (Table 2). The CSA 2c exhibited the highest affinity to the N-DNB derivatives: amino acids (3)(4)(5) and their methyl esters (6)(7)(8). When comparing the potency, the phenyl analog 2c appeared to be privileged to ethyl (2a) and butyl (2b) derivatives.…”

“…In particular, host 1c preferentially interacted with carboxylic acids (3)(4)(5), and this interaction was mediated by the formation of salt bridges. In particular, host 1c preferentially interacted with carboxylic acids (3)(4)(5), and this interaction was mediated by the formation of salt bridges.…”

Phosphorylation as a method of tuning the enantiodiscrimination potency of quinine—An NMR study

“…Accordingly, selected proton and fluorine resonances were indicative of enantiomers of substituted binaphthyl compounds, alkylarylcarbinols and others in the original works of Salvadori et al14, 15 Being inexpensive and soluble in standard NMR solvents, nonderivatized quinine has been applied for the characterization of composition of compounds and the enantioselectivity of selected reactions, mainly those involving hydroxyl derivatives. Thus, 1 H NMR has served to determine the optical purity of secondary alcohols obtained through the enantioselective addition of diethylzinc to furaldehydes,16 as well as for the analysis of β‐hydroxy esters17 and cyclic hemiacetals and methyl acetals 18. Secondary and tertiary α‐aryl‐α‐trifluoromethyl alcohols and hydroxyesters were studied by the use 19 F NMR,19 whereas 31 P NMR allowed enantiodiscrimination of 1‐hydroxy,20 1,2‐dihydroxyalkanephosphonates21 and N ‐benzyloxycarbonylamino‐phosphonates22 in our group.…”