Some N.M.R. techniques used in conformational analysis, the principal factors peculiar to heterocyclic conformational analysis, and the background to conformational effects in alkyl substituted hydroxylamines are reviewed.
The synthesis of N-carbethoxy- and N-methyl-tetrahydro-1,2- oxazines is discussed. The mechanism of formation of N-carbethoxytetrahydro-1,2-oxazines from the condensation of 1,4 dibromo-alkanes with N-hydroxyurethan under basic conditions is found to proceed by initial attack of the oxy-anion tautomer, formed from N-hydroxyurethane, on the more reactive bromo-substituted carbon.
The preferred conformations of several N-methyl-tetrahydro1,2-oxazines are deduced from their N.M.R. spectra, and a measure of the free energy difference between the equatorial and axial positions for a methyl group substituted on any one of the four ring carbon atoms is estimated. A qualitative rationalisation of the variation in free energy difference with the point of substitution is given. The N-methyl group is found to strongly prefer the equatorial position.
The stereochemistry of the B/C ring junction of the alkaloid geneserine is deduced from its N.M.R. spectra, and is found to be cis.
The conformational equilibrium at nitrogen in some N,N-dimethyl-hexahydropyrimidines is studied by N.M.R. For N,N-dimethyl hexahydropyrimidine and l,3,5-trimethyl-hexahydropyrimidine a free energy difference between an equatorial and axial N-methyl group of ca. 0.5 kcal./mol. is found.
A new method of measuring rate constants by N.M.R. using a small digital computer is tested by studying the barriers to internal rotation in the N,N-dimethyl amides of the three pyridine carboxylic acids. The method is found to be satisfactory, and the free energies of activation for the rotational process at 25°C are 17.9 kcal./mol. for N ,N-dimethyl picolinamide, 15,9 kcal./mol. for N,N-dimethyl nicotinamide, and 16.6 kcal./mol. for N,N-dimethyl isonicotinamide. The error on these values is considered to be less than 0.1 kcal./mol.