Perhydropyrido oxazine, dipole

However, in many other series results have been obtained that are compatible with those from other methods and that gave the dipole-moment method an appearance of general reliability now known to be unjustified. Such compatible results include spiropiperidines (Section III,A,4), tropanes (Section III,B,4), 2-alkyltetrahydro-l,2-oxazines (Section III,C,2), perhydro-pyrido[l,2-c][l,3]oxazines (Section III,D,IX perhydropyrido[l,2-c][l,3]thi-azines (Section III,D,2), dialkylhexahydropyrimidines and perhydropyrido-[l,2-c]pyrimidines (Section III,D,3), 5-alkyldihydro-l,3,5-dithiazines (Section III,G,3), 3,5-dialkyltetrahydro-l,3,5-thiadiazines (Section III,G,4) and, in part, l,2,4,5-tetraalkylhexahydro-l,2,4,5-tetrazines (Section III,H,4) as well as piperidines, tetrahydro-l,3-oxazines, and tetrahydro-l,3-thiazines containing an N-H group. 

The H-NMR spectra of perhydropyrido[l,2-c][l,3]oxazines and Bohl-mann-band measurements (Section II,E,1) show the predominance of the trans-fused conformer 265,135 and comparison of -NMR chemical shifts in the locked perhydropyrido[3,2,l-i/][3,l]benzoxazines 269 and 270,45 taken as models for the trans- and cis-fused conformers, indicates 90% 265 in the equilibrium at 25°C (AG° 1.3 kcal mol-1). This is lower than that (AG° 2.6 kcal mol ) for the corresponding quinolizidine equilibrium. This estimate is consistent with the results of dipole moments, which indicate predominantly trans conformer,61 and of a low-temperature H-NMR study of ci s(4a-H,7-H)-7-ethylperhydropyrido[l,2-c][l,3]oxazine (271 272),61... 

Dipole moments of perhydropyrido[l,2-c][l,3]oxazine (20, X = O) and its four alkyl derivatives were calculated from measurements in benzene at 25°C and were used to estimate the conformational preferences of several perhydropyrido[l,2-c][l,3]oxazines [76JCS(P2)418]. The traaj-fused and O-outside c/s-fused conformers possess identical dipole moments (2.08 D), whereas that of the 0-inside cw-fused conformers is 1.30 D. 

---