Bohlmann band

The presence of the Bohlmann bands (2812, 2778 and 2756/cm) in the infrared spectrum of 3-methylperhydropyrido[l,2-c][l,3]oxazine 81 identifies the fused conformation of the bicycle (97TA109). 

The presence or the lack of the Bohlmann bands in the infrared (IR) spectra of perhydropyrido[l,2-c][l,3]oxazines was used to identify /ram-fused <1997TA109, 2003TL8609> orm-fused <2005EJ01378> conformations of the bicycle. 

Deplancheine (7) was isolated first from the stem and bark of Alstonia deplanchei van Heurck and Muell. Arg. by Husson et al. (14). The structure of this novel type of alkaloid has been established from spectral evidence the trans annellation of rings C and D is supported by the presence of characteristic Bohlmann bands in the IR spectrum and also by the chemical shift of the C-3 proton below 3.7 ppm. The ethylidene side chain was also indicated by the H-NMR spectrum [8 1.52 (3H, d, J =6 Hz), 5.30 (1H, q, J = 6 Hz)] and its E configuration was confirmed by regio- and stereospecific total synthesis (14). 10-Methoxydeplancheine (8) has been found in the leaves of Alstonia lanceolifera... 

A review has appeared regarding the infrared and nuclear magnetic resonance spectroscopic investigations of quinolizidine alkaloids (303). The connection between the C/D ring junction and the existence of Bohlmann bands in the IR spectra of indolo[2,3-a]quinolizidines has been reinvestigated and interpreted (304). 

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. 

Measurements of the Bohlmann bands in piperidine derivatives139,140 have supported a preference for /V-Hcq in the piperidine equilibrium. The IR spectra of 2,6-dideuterated piperidine, A-methylpiperidine, and A-isopro-pylpiperidine showed two C—D absorptions at 2075-2030 cm-1 (anti-C—D... 

Investigations of the conformational equilibria by measurements of Bohlmann bands (Section II,E,1) and the chemical shift of the angular proton... 

The two types of cis conformations 201 and 203 may be distinguished by the 13C-NMR chemical shifts of the C-6 and C-7 nuclei (yax-effect on C-7 in 201 and yax-effect on C-6 in 203), and consideration of such shifts for the l-methylbenzo[a]quinolizidines along with Bohlmann-band measurements indicates220 76% cis-fused conformation 203 for the trans( 1 -//, 11 b-H)-1 -methyl isomer. 

The Bohlmann bands at 2805 and 2761 cm-1 in the infrared spectra indicated trans-fusion of the heterorings in 3,3-diethyl-l,3,4,6,7,116-hexahydro-2//-pyrimido[2,l-a]isoquinoline (69YZ649). The presence of Bohlmann bands in the infrared spectra of l,3,4,6,ll,llu-hexahydro-2//-pyrimido[l,2-6]-isoquinoline and its 2-oxo derivative justified the trans-type ring junction of the heterorings [68JCS(CC)1423]. 

The 5,7-dimethylindolizidines (9) and (10) show strong bands in the Bohlmann region which are much reduced in (11). The trans fused conformation (11) has only two correctly oriented C—H bonds necessary for strong Bohlmann bands, compared with three in (9) and (10). Isomer (12), on the other hand, shows only little absorption in the 2700-2800 cm-1 region, indicative of the cis fused conformation shown. 

The presence of Bohlmann bands in the infrared 2700 cm-1 region has been used as evidence of a trans-fused conformation in quinolizi-dines.278 These bands appear to be present when two or more C-H... 

In the spectra of dilute solutions, only 240 shows intramolecular hydrogen bonding, and all four isomers show strong Bohlmann bands. Hence all were assigned trans-ring conformations. These conformations were further supported by (i) NMR data—primarily the chemical shift and bandwidth of the carbinol proton (ii) pKa values—240 was found to be considerably more basic than 241, whereas there was less difference between the 7-hydroxy compounds (iii) chemical data—the product isomer ratios obtained when the conditions for the reduction of the ketones were altered were in accordance with the above assignments. The... 

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 IEs on amine basicity are also due to changes in vibrational frequencies, not only computationally but also experimentally. Gas-phase IEs of O.lOkcalmol-1 per CD3 group on basicities of methylamine, dimethy-lamine, and trimethylamine can be reproduced by ab initio force constants for C-H stretching, which increase on TV-protonation.100 Infrared spectra of amines show characteristic bands (called Bohlmann bands ) in the 2700-2800 cm-1 region, lower than the 2900 cm-1 of a typical C-H stretch.169,170 Upon /V-protonation these bands revert to a typical, higher frequency. Therefore the zero-point energy of the C-H increases on... 

The absence of Bohlmann bands in the infrared (IR) spectrum indicates that the nitrogen lone pairs of the di-iV-nitroso-l,4-diazepines 23 are delocalized <1995JOC7461>. While the absence of an amide moiety in the ring confers torsional freedom, the m-2,7-di-Ph groups reduce ring flexibility and the 111 NMR data are consistent with major and minor families of four twist chair conformers that project the C-7 Ph and C-6 R -substituents equatorially These interconvert by a pseudorotation that allows the C-2 phenyl and C-3 R substituents to adopt a pseudoaxial orientation in the preferred conformer family. 

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