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Studies on the benzoxazine series. 2—Preparation and 1H and 13C NMR structural study of some substituted 1,2-dihydro-4H-3,1-benzoxazines

✍ Scribed by Kari Neuvonen; Riitta Pohtola; Kalevi Pihlaja

Publisher
John Wiley and Sons
Year
1989
Tongue
English
Weight
800 KB
Volume
27
Category
Article
ISSN
0749-1581

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✦ Synopsis

In addition to the parent compounds, nine methyl-substituted l,Mibydro-4H-3,l-benzoxazines with and without N-methyl substitution were prepared. The chain tautomer could only be detected in the case of 1,2-dihydro-2+ nitrophenyl)-4H-3,1-benzoxazine in solution. The configurational and conformational assignments were based on 'H and 13C NMR data. The calculations of the methyl substituent effects on the heterocyctic ring (half-chair structure) carbons gave a good fit between the observed and calculated shifts and made it possible to estimate that 1,2-dihydro-4-metbyl-4H-3,l-benzoxazine is a 75:25 and its N-methyl derivative a 69:31 mixture of the 4eq'-and 4ax'-forms. 1,2-Dihydro-trans-1,2,4-trimethyl-4H-3,1-benzoxazine is also not conformationally homogeneous, but is a 43:57 mixture of the 2eq,4axf-and 2ax,4eq'-forms. A comparison of the ' ' C chemical shift correlations with those of 3,4-dihydr0-2H-l,fbenzoxazines also improves the structural knowledge about the latter series. Whereas the N-methyl group in 3,Qdihydro-ZH-1,3-benzoxazin~ are axially orientated, model calculations reveal that the N-methyl substitutions in the title compounds can be orientationally heterogeneous.

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