Triterpenoide. XIV. † [2,3-d]Isoxazoltriterpenderivate

The X-ray crystal structure analyses of two triterpene isoxazoles, namely olean-2,12-dieno[2,3-d]isoxazol-28-oic acid methyl ester {methyl isoxazolo[4,5-b]olean-2,12-dien-28-oate, C 32 H 47 NO 3 , (2a)} and 19β,28-epoxy-18α-olean-2-eno[2,3-d]isoxazole {19β,28-epoxy-isoxazolo[4,5-b]-18α-olean-2-ene, C 31 H 47 NO 2 , (4a)} are described. These compounds were obtained by treatment of 2-formyl- or 2-hydroxymethylene derivatives of 3-oxoolean-12-en-28-oic methyl ester, (la) and (1b), and 19β,28-epoxy-18Hα-oleanan-3-one, (3a) and (3b), with hydroxylamine hydrochloride according to the method described by Govardhan, Reddy, Ramaiah & Rao [J. Indian Chem. Soc. (1983), pp. 858-860] for the synthesis of ring-A-fused triterpene [3,2-c]isoxazoles. The structures of the obtained [2,3-d]isoxazoles, (2a) and (4a), were confirmed by the comparison of the bord lengths and bord angles found for the isoxazole rings of compounds (2a) and (4a) with the corresponding data given in the literature. The isoxazole rings in both compounds are planar. Ring A in (2a) and (4a) has a conformation between the half-chair and sofa form. Rings B. D and E in (2a), and B, C, D and E in (4a) have the chair conformation. Ring C in (2a) has a distorted sofa form. In (2a), the rings A/B, B/C and C/D are trans-fused and the rings DIE cis-fused. In (4a), the A/B, B/C, C/D and DIE ring junctions are trans. The axial 02 and C28 atoms in (4a) are β oriented and form, together with the C17. C18 and C19 atoms, the five-membered ring F, which has an envelope conformation.