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organic compounds
The X-ray crystal structure analyses of three new triterpene lactones (C29H42O6), namely 9β-hydroxy-28-methoxy-11,28-dioxo-1,2-dinor-10α-18β-olean-12-en-3,10β- olide, (3), its 18α- and 9α,18α-isomers, (4) and (5) [9β-hydroxy-28-methoxy-11,28-dioxo-1,2-dinor-10α-18α-olean-12-en-3,10β- olide and 9α-hydroxy-28-methoxy-11,28-dioxo-1,2-dinor-10α-18α-olean-12-en-3,10β- olide, respectively], are described. Compounds (3)–(5) are obtained by treatment of 3β-hydroxy-11-oxo-18α-olean-12-en-28-oic acid methyl ester, (1), with sodium dichromate in acetic acid. In compounds (3)–(5) the six-membered ring A of (1) is replaced by a γ-lactone ring. In contrast to compound (1), the C25 methyl group at C10 has α configuration. The newly introduced hydroxyl group at C9 is β-oriented in compounds (3) and (4), and α oriented in (5). Ring B in (3) has a boat form, but in (4) and (5) it has different equivalent chair conformations. Ring C in (3) has a sofa form, in (4), it has a slightly distorted half-chair form and in (5), it has a conformation intermediate between the half-chair and sofa forms. Rings D and E have chair conformations in all three compounds and are trans-fused in compounds (4), (5) and (1) and cis-fused in (3). In contrast to compound (1), rings A/B in compounds (3)–(5) and rings B/C in compounds (3) and (4) are cis-fused. In compounds (3)–(5), formation of hydrogen bonds has been observed. Inversion of the α configuration to a β configuration at C18 in oleanolic acid derivatives [transformation of (1) to (3)] has been observed for the first time. The triterpene lactones (3)–(5) are the first known oleanan derivatives with cis-fused A/B [(3)–(5)] and B/C rings [(3) and (4)].
