Stereochemistry and the mass spectra of some 1,3‐ and 3,1‐perhydrobenzoxazines

Abstract

Abstractcis‐Anellated 3‐methyl‐ (1) and cis (2)‐ and trans (3)‐anellated 2,3‐dimethyl‐1,3‐perhydrobenzoxazines and the corresponding 3,1‐perhydrobenzoxazines (4, 5a, 5b, and 6, respectively) have been prepared and their configurations assigned with 1H NMR spectroscopy. The electron‐impact induced fragmentations differentiate clearly the two sets of isomers since only the [MH]+ and [MMe]+ ions from 1–3 exhibit an appreciable retro Diels‐Alder reaction giving rise to the ions m/z 72 and/or m/z 86. Besides the main reaction, elimination of methyl radical, isomers 5a, 5b and 6 also eliminate C3H and C2H3O− forming the ions [C7H12NO]+ and [C8H16N]+, at m/z 126. The formation of the former is most favourable for 5a, 5b both for reasons of chemical composition and stereochemistry. The ion C6H10NO+ at m/z 112 represents the base peak of compound 4, i.e., a methyl group at the 2‐position is a better leaving group than a hydrogen atom and the ring nitrogen assists the α ‐cleavage of the cyclohexane ring in this case. Even the mass spectra of the isomeric cis‐ and trans‐annellated forms (2 vs 3 and 5a, 5b vs 6) can be distinguished from each other on the basis of some intensity differences caused by stereochemical effects. However, the two epimeric cis forms 5a and 5b gave indistinguishable spectra. The characteristic decompositions have been verified by low resolution, high resolution, metastable and collision‐induced dissociation techniques. The compounds studied could not be differentiated based on their ammonia or isobutane chemical‐ionization spectra.