title: 1H, (13)C, and (15)N NMR stereochemical study of cis-fused 7a(8a)-methyl and 6-phenyl octa(hexa)hydrocyclopenta[d][1,3]oxazines and [3,1]benzoxazines.
abstract: Four 7a-methyl octa(or hexa)hydrocyclopenta[d][1,3]oxazines, five 8a-methyl octa(or hexa)hydro[3,1]benzoxazines, two 6-phenyl hexahydro[3,1]benzoxazinones, and 8a-methyl hexahydro[1,3]benzoxazinone, all cis-fused, were prepared and their stereostructures studied by various one- and two-dimensional (1)H, (13)C, and (15)N NMR spectroscopic methods. In solution, the cyclopentane-fused 2-oxo derivatives and the 1,3-benzoxazinone were found to attain exclusively the N-in/O-in conformation, whereas the 6-phenyl 2-oxo/thioxo derivatives were found to be present predominantly in the N-out conformation. The C-2 unsubstituted and the 2-oxo/thioxo 7a/8a-methyl derivatives were all present in solution as a rapidly interconverting equilibrium of the N-in and N-out conformations. The C-2 methyl derivatives were each found to be interconvertable mixtures of epimers (at C-2) with the N-in conformer predominating for one epimer and the N-out conformer predominating for the other, with both predominating conformers having the C-2 methyl group equatorially orientated. The substituent on the nitrogen (H or Me) was found to be always predominantly equatorial with respect to the heteroring, except for the epimeric 2-methyl derivatives with N-out conformations where steric constraints and the generalized anomeric effect resulted in the axial orientation of the C-2 methyl being favored.
authors.affiliation: Department of Chemistry, University of Turku, Turku, Finland.
authors.forename: Petri
authors.initials: P
authors.lastname: Tähtinen
authors.forename: Jari
authors.initials: J
authors.lastname: Sinkkonen
authors.forename: Karel D
authors.initials: KD
authors.lastname: Klika
authors.forename: Ville
authors.initials: V
authors.lastname: Nieminen
authors.forename: Géza
authors.initials: G
authors.lastname: STájer
authors.forename: Zsolt
authors.initials: Z
authors.lastname: Szakonyi
authors.forename: Ferenc
authors.initials: F
authors.lastname: FüLöp
authors.forename: Kalevi
authors.initials: K
authors.lastname: Pihlaja