Stereochemical studies with derivatives of octahydro-2H-pyrido[1,2-a]-pyrazine

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Abstract
Some substituted octahydro-2-phenyl-2H-pyrido[1,2-a]pyrazin-4- and -3-ones have been synthesised and their configurations assigned on the basis of their i.r. and n.m.r. spectra. These lactams have been converted to the corresponding octahydro-2H-pyrido[1,2-a]pyrazines all of which preferentially adopt the trans-fused ring conformation. An examination of the i.r. spectra of the 3,3- and 4,4-dideuteriated derivatives of this latter system permits an evaluation of the relative contributions of the various C–H bonds α to nitrogen to Bohlmann band formation. The stereochemistry of the related octahydropyrido[2,1-c][1,4]oxazine system is discussed with reference to the 220 MHz n.m.r. spectra of some methyl substituted derivatives.

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