Perhydropyrido pyrimidine, Bohlmann

Bohlmann absorption is also characteristic of the trans-fused but not of the cis-fused conformations of the 1,3-heterocyclic systems 87,135 88, ss and 89.136 Bohlmann bands are intense in the trans-syn-lrans-perhydropyrido-[l,2-c 2, l -/]pyrimidine 90,137 and these are markedly reduced in the 6,6-dideutero derivative,138 showing the contribution of the interheteroatomic C-6—Hax to the formation of Bohlmann bands in 1,3-heterocyclic systems. 

The Bohlmann band appearing between 2650 and 2800 cm-1 in the IR spectra indicated that the unsubstituted perhydropyrido[l,2-n]pyrimidines contain the two rings in trans anellation.238,276,277... 

The appearance of Bohlmann bands in the IR spectrum of perhydropyr-ido[l,2-c]pyrimidine indicated that this compound exists predominantly in the trans-fused conformation (69JHC181), but the application of Bohl-mann s IR criterion to perhydropyrido[l,2-c]pyrimidines is greatly complicated by the presence of the two N atoms and in some cases the a-H atoms of the 2-alkyl group (70T701). 

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