Structure of the ABC ring synthon of dendrobine

(1R,4S,8S,11R)-11-Methyl-3-[(R)-alpha-phenyl-ethyl]-3- azatricyclo[6.2.1.0(4.11)]undec-5-en-2-one, C19H23NO, Mr = 281.40, orthorhombic, P2(1)2(1)2(1), a = 9.534 (2), b = 11.055 (4), C = 14.372 (4) A, V = 1514.8 (8) A, Z = 4, Dx = 1.23 (163 K), Dm = 1.15 g cm-3 (295 K), lambda(Mo K alpha) = 0.71069 A, mu = 0.702 cm-1, F(000) = 608, T = 163 K, R = 0.0387 for 2310 reflections. The absolute configuration is assigned on the basis of internal comparison with the alpha-methylbenzylamine moiety. The tricyclic ring system is concave. The N appears to be sp2 hybridized [N is 0.0661 (1) A from the plane through C atoms bound to it] and conjugated through the carbonyl group [short C-N bond [1.351 (2) A]]. Ring strain appears to cause a slight increase in the C-C bond lengths of the center atom of the tricyclic ring system. The average bond length from this atom to the three other ring C atoms is 1.557 (1) A while the average bond length for all other sp3-C to sp3-C bonds is 1.529 (1) A.