Shahrokh Saba

Conformational analysis of saturated heterocycles. Part 85. Perhydrodipyrido[1,2-b;1′,2′-e]-1,4,2,5-dioxadiazine

The title compound is shown to be the trans-isomer which exists both in the crystal (X-ray) and in solution (n.m.r) in the tetraequatorial conformation.

The conformational analysis of saturated heterocycles. Part LXV. Low temperature nuclear magnetic resonance studies on 2-methyl-,2-ethyl-, and 2,3,3-trimethyl-2,3,5,6-tetrahydro-1,4,2-dioxazines

The N-alkyl equatorial conformers of 2-methyl-, 2-ethyl-, and 2,3,3-trimethyl-2,3,5,6-tetrahydro-1,4,2-dioxazines predominate, with ΔG°191 values of 1·03, 0·72, and 0·61 kcal mol–1, respectively.

The conformational analysis of saturated heterocycles. Part LXIII. Tetrahydro-1,2-oxazines 2and related acyclic hydroxylamines

Infrared intensities show that the proton attached to nitrogen in tetrahydro-1,2-oxazine exists predominantly in the equatorial position. For an N-methyl group, the equatorial position is favoured by ca. 1·9 kcal mol–1, as deduced from electric dipole moments. The conformations of di- and tri-methylhydroxylamines are discussed.

Preparation and conformational analysis of 2,3,3-trimethyl- and 2-methyl-tetrahydro-1,4,2-dioxazine

The title compounds exist preferentially with N-methyl equatorial to the extent of ΔG°191 of 0·61 and 1·05 kcal mol–1, respectively.

The conformational analysis of saturated heterocycles. Part LXVII. 2-Alkyl-3,6-dihydro-2H-1,2-oxazines and 3-alkyl-3,4-dihydro-1H-2,3-benzoxazines

Low temperature n.m.r. studies of the title compounds indicate that nitrogen inversion is slowed. Peak area measurements show that 2-methyl substituents prefer the equatorial position by a factor of ca. 10 : 1 and 2-t-butyl substituents by a considerably larger factor. This considerable equatorial preference is due in part to lone pair–lone pair repulsion in the 2-axial conformers. The conclusions from n.m.r. are compared with measured and calculated dipole moments.

The conformation of the NH-groups in piperazines, hexahydropyrimidines, tetrahydro-1,2- and 1,3-oxazine, and tetrahydro-1,3-thiazine

I.r. band shapes and electric dipole moments show that the predominant conformers for tetrahydro-1,3-oxazine, tetrahydro-1,3-thiazine, and 1-t-butylhexahydropyrimidine are those with the NH axial; the predominant conformer for tetrahydro-1,2-oxazine is that with NH equatorial as determined by the same methods.