1.3- Thiazines, tetrahydro-, conformation

Following the trend observed for the oxazine analog 296, the fusion of a benzene ring onto the [/] position of perhydropyrido[l,2-c][l,3]thiazine, as in 1,6,7,116-tetrahydro-2//,4//-[l,3]thiazino[4,3- ]isoquinoline 316, pushes the equilibrium almost exclusively to the S-inside cis conformation 317.284... 

I.r. bandshapes and dipole-moment measurements were used by Katritzky s group to study the conformations of tetrahydro-1,3- (35) and tetrahydro-1,2-oxazines (36) as well as of tetrahydro-1,3-thiazine (37) and 1-t-butylhexa-hydropyrimidine (38). Compounds (35), (37), and (38) have an axial NH... 

Simple 1,3-Thiazines.—In connection with a study on the NH conformation in the analogues of piperidine in which a ring CH2 group is replaced by a hetero-atom, the equilibrium position of tetrahydro-l,3-thiazine [(19a) dj(l9b)] has been examined." I.r. measurements of the KNH) first overtone and electric dipole moments indicate that the predominant conformer is that with the NH axial. This preference, which applies also for tetrahydro-l,3-oxazine, may be interpreted in terms of attractive forces between the lone pair of the sulphur atom and axial NH in the conformer (19a), and repulsive rabbit ear forces between the two axial lone pairs in the conformer (19b). 

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