Structures of 3-cyanohexahydronaphth-[2,3-e]-1,2-oxazines part 3: Comparison of the conformations of some 3-cyanotetrahydro-1, 2-oxazine ring systems

Structural information, obtained from crystal structures and NMR investigations, on the range of 3-cyanohexahydronaphth-[2,3-e]-1,2-oxazines obtained from the cycloaddition reactions of N-cyclohexyl-N-propenylnitrosonium ions with 5-substituted 1,4-dihydronaphthalenes, are compared. The crystal structure of 6-chloro-3 α-cyano-2-cyclohexyl-4 α-methyl-1a α,3,4,4a α,5,10-hexahydronaphth-[2,3-e]-1,2-oxazine, an example of one of three types of stereoisomeric cyanide adducts, is reported. This compound, C₂₀H₂₅N₂OCl, M_r=344.88, crystallizes in the monoclinic space group P2₁/n unit-cell dimensions a=27.823(8), b=2.338(9), c=5.238(7) Å, β=90.33(5)°;V_c=1798.1 ų, Z=4, D_x=1.248 g cm^-3, μ(Cu Kα)=18.35 cm^-1. The final R was 0.052 for 1837 unique reflections with I>2σ(I). The analysis confirms that the oxazine rings of all three types of stereoisomers assume distorted chair conformations in the solid state, with the N-cyclohexyl groups equatorial and the cyano groups axial. The endocyclic torsion angles about the C(1A)-O(1) and O(1)-N(2) bonds, -64.6(3) and 71.9(3)° respectively, are still relatively large, but smaller than in the case of the other stereoisomers. An analysis of the¹H NMR spectra of a number of these naphthoxazines, representative of all three types of stereoisomers, indicates that their conformations in solution are similar to those which prevail in the solid state.