1.3- Oxazines tetrahydro-, dipole moments

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. 

Dipole-moment measurements indicate 62% N-H-axial for tetrahydro-1,3-oxazine and 74% (V-H-axial for 5,5-dimethyltetrahydro-l,3-oxazine,269 but these estimates could well be low. The predominant /V-H-axial conformation is clearly demonstrated by the IR spectrum in the first overtone N-H... 

Dipole-moment measurements were interpreted as favoring the N-alkyl equatorial conformation for a range of N-alkyl tetrahydro-l,3-oxazines, giving 58% eq N-Me, 68% eq N-Et, 86% eq AMPr and 100% eq N-t-Bu,271 but these conclusions can no longer be regarded as reliable. 

Tetrahydro-l,3-oxazines are normally assumed to adopt a chair conformation in which the NH bond has an axial orientation. This view is substantiated by NMR spectroscopy and also by dipole moment analysis <73JCS(P2)325). The ring system is not stable and, for example, when the parent molecule is allowed to stand, it slowly ring opens and then forms a trimer (Scheme 33) (78AF937). In acidic media ring fission is accelerated and the open-chair imines may then hydrolyze. This property has been utilized in the synthesis of aldehydes as previously noted (see Section 2.27.2.2.4(0). 

Eliel et a/.268 seem to confirm the observation that the nitro group in this type of heterocyclic system is preferred in the axial position. Katritzky et a/.269-271 have found on the basis of dipole moment measurements that tetrahydro-l,3-oxazines without a 5-nitro group have V-alkyl in the preferred equatorial position, although the axial N-CH3 and -C2H5 are important minor contributors (42 and 32%, respectively). 

Infrared spectra and dipole moments led to the conclusion that the N-H in tetrahydro-l,3-oxazines is predominantly axial.270... 

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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