Bohlmann Region 2800-2600 cm-1 of IR Spectra

In summary, it was stated that for trans bands to occur in a given conformational of a heterocyclic system it is necessary to have one a. axial C—H bond trans to the nitrogen lone pair. The intensity and complexity 

Similar correlations exist between the stereochemistry of yohimbines and related indole alkaloids and the appearance of bands in the 2800-2600 cm 1 region of their IR spectra.130 All compounds possessing an a-hydrogen at C-3, i.e., normal (82) and alio (83) compounds, show two or more distinct and characteristic peaks of medium intensity on the low wave-number side of 2800 cm 1 in addition to the normal C—H stretching bands. Those compounds having a jff-orientation of the C-3 hydrogen, i.e., pseudo (84) and epiallo (85) compounds such as / -yohimbine, ( + )-epialloyohimbane, and 3-isoajmalicine, exhibit only shoulders on the low wave-number side of the main C—H band. 

It was finally established131,132 that only those alkaloids possessing, in their preferred conformations), the C-3—H and at least one more adjacent C—H bond trans diaxial to the nitrogen lone pair will exhibit bands in their IR spectra between 2900 and 2700 cm-1, one or more absorbing below 2800 cm-1, whereas those alkaloids possessing in their preferred conformation(s) the C-3—H cis to the nitrogen lone pair will not. 

A study of isomeric monomethylquinolizidines revealed an apparent exception to the Bohlmann-bands criterion.133 1348 All the compounds adopted trans-fused conformations and all except the trans- 4-H,lO-H)-4-methyl derivative 86 showed pronounced Bohlmann bands in their IR spectra. The 4-methyl derivative 86 gave rise to only one Bohlmann band at 2805 cm 1. Thus the Bohlmann criterion should be applied to 4-substituted quinolizidines with care. 

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. 

---