3-Cyano-11-oxo-3,4-seco-12a-aza-C-homoolean-4(23)-en-28-oic acid methyl ester

The title compound, C31H48N2O3, is a Beckmann rearrangement product. The isopropenyl and methoxycarbonyl groups have β-orientations, whereas the 2-cyanoethyl group has an α-orientation. In the triterpenoid skeleton, the seven-membered lactam ring, as well as the three six-membered carbocyclic rings, have chair conformations. In the crystal, molecules are linked via nonclassical C—H⋯O hydrogen bonds into layers parallel to the ab plane.

The title compound, C 31 H 48 N 2 O 3 , is a Beckmann rearrangement product. The isopropenyl and methoxycarbonyl groups have -orientations, whereas the 2-cyanoethyl group has anorientation. In the triterpenoid skeleton, the seven-membered lactam ring, as well as the three six-membered carbocyclic rings, have chair conformations. In the crystal, molecules are linked via nonclassical C-HÁ Á ÁO hydrogen bonds into layers parallel to the ab plane.

Comment
The title compound was obtained from 3,12-dioxo-18β-olean-28-oic acid methyl ester as a product of the two-step synthesis.
In the first step the diketone derivative mentioned above undergone the condensation with hydroxylamine hydrochloride to give the oxime derivative as a product. The latter reacted with POCl 3 (Beckmann rearrangement reaction) (Bednarczyk-Cwynar, 2006). The results of the X-ray analysis showed that the final product is 3-cyano-11-oxo-3,4-seco-12a-aza-C-homoolean-4(23)-en-28-oic acid methyl ester, (I), (Fig. 1). Molecular structure obtained in the course of the X-ray investigation showed that oxime derivative formation and Beckmann rearrangement reaction took place within two triterpenoid rings, i.e.
A and C. In ring C Beckmann rearrangement reaction took place while in ring A Beckmann fragmentation was observed.
As a result of Beckmann fragmentation C3-C4 bond cleavage and ring A opening were observed. In this process two new functions were formed. In The axial methyl group C25 adopts β-orientation while hydrogen atom in C5 position reveals α-orientation. Thus, both of these substituents retain the orientation observed in oleanolic acid molecules (Froelich & Gzella, 2010).
In the molecule of (I) the original six-membered carbocyclic ring C has been transformed into the seven-membered lactam ring in which nitrogen atom connects carbonyl group (C12═O1) and tertiary carbon atom C13. The planar ester group in C17 is attached axially to ring D and equatorially to ring E. Its carbonyl (C28═O2) group is synperiplanar (-sp) with respect to C17-C18 bond belonging to both D and E rings [torsion angle C18-C17-C28-O2: In the crystal lattice of (I) molecules are linked by nonclasical hydrogen bonds C15-H15B···O2 i and C31-H31B···O1 ii (Tab. 1, Fig. 2) into layers parallel to the ab plane.
In the molecule of (I) fourteen short H···H contacts are observed. The distances between related hydrogen atoms lie within the range of 1.92 -2.20 Å. The short contacts are mainly the consequence of the presence of axial methyl groups C25, C26 and C27.

Experimental
The title compound was synthesized according to the procedure described by Bednarczyk-Cwynar (2006) and dissolved in hot ethanol. The solution was set aside to crystallize at room temperature. After a week block-shaped colourless single crystals suitable for X-ray experiments were obtained.