NMR and X-ray structural study of saturated ( p-chlorophenyl)pyrrolo[1,2-a][3,1]benzoxazin-1-ones prepared from aroylisobutyric acid and cyclic amino alcohols. High energy barriers for hindered rotation of bridgehead phenyl groups

From 2-methyl-3-(p-chlorobenzoyl)propionic acid with stereoisomeric cyclic saturated or partly saturated cis and trans 1,3-amino alcohols and bicyclic amino alcohols, tri- and tetracyclic methyl substituted (p-chlorophenyl)pyrrolo[1,2-a][3,1]benzoxazin-1-ones and methylene bridged derivatives were prepared. For comparison, the bicyclic oxazolone and oxazinone analogs were also prepared. In each case isomeric pairs, which differ in the mutual positions of the aryl and methyl groups, were formed. For the methylene bridged derivatives, the isomers were separated. For evaluation of the structure in solution H-1, C-13{H-1}, NOE difference, COSY and HMQC NMR methods were used, and for the crystal structure determinations X-ray diffraction measurements were used. An unusually high free energy barrier of the restricted rotation of the bridgehead p-chlorophenyl group was measured for a cis-, a diendo- and a diexo-fused compound.