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Conformational analysis of substituted perhydro‐1,2‐oxazolo[3,2‐ c ] [1,4]oxazines by NMR spectroscopy
Author(s) -
Wazeer Mohamed I. M.,
AlMuallem Hasan A.,
Ali Sk. Asrof
Publication year - 1993
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610060603
Subject(s) - chemistry , nitrogen inversion , conformational isomerism , intramolecular force , oxazines , hydroxymethyl , cis–trans isomerism , nuclear magnetic resonance spectroscopy , carbon 13 nmr , stereochemistry , population , proton nmr , hydrogen bond , spectral line , spectroscopy , nitrogen , crystallography , molecule , organic chemistry , physics , demography , quantum mechanics , sociology , astronomy
The 13 C NMR spectra of most of the substituted perhydro‐1,2‐oxazolo[3,2‐ c ] [1,4]oxazines (3) at low temperature showed the presence of two isomers of unequal populations. The major isomer is shown to be the cis isomer {except in 2‐hydroxymethyl‐2‐methylperhydro‐1,2‐oxazolo[3,2‐ c ] [1,4] oxazine (3e)}, which is in equilibrium with the minor isomer ( trans conformer) by a relatively slow nitrogen inversion. Intramolecular hydrogen bonding in oxazines, having 2‐hydroxymethyl substituents, is shown to be an important factor in determining the population ratio of the two isomers. The barrier to nitrogen inversion was determined by detailed band‐shape analysis of proton and carbon NMR spectra and were in the range 66·3–72·9 kJ mol −1 . The chair inversion had been slowed down, in one case, trans ‐dimethylperhydro‐1,2‐oxazol[3,2‐ c ] [1,4]oxazine‐2,3‐dicarboxylate (3j), to show the presence of the two forms of the cis isomers. The barrier to chair inversion is 41·5 kJ mol −1 as determined by proton NMR band‐shape analysis of 3j.