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Wood extractives : the structure and chemistry of some triterpenes isolated from the bark of Sitka spruce (Picea stichensis) Roger, Ian Henry

Abstract

The neutral fraction of the petroleum ether extract of Sitka spruce (Picea sitchensis) bark was shown to be rich in compounds of the triterpene class. In particular three monomethyl ether derivatives of triterpene diols were isolated (compounds A, B and C). The structure of the main component, compound A, was proved by methylation studies to be 3β-methoxy-21β-hydroxy-∆¹⁴-serratene (LXXI) and the parent diol, 21-episerratenediol (LIV), and related ketone, 3β-methoxy-21 keto-∆¹⁴-serratene (LVIIa) were also isolated and identified. The other major component, compound B, was shown to have the structure 3α-methoxy-21β-hydroxy-∆¹⁴-serratene (LXXX) by appropriate correlation with known compounds of the serratene series. Compound C is isomeric with A and B but the double bond exists in the fully substituted ∆¹³ position. The most likely structure for this compound is 3α-methoxy-21β-hydroxy-∆¹³-serratene (LXXXI) as shown,by the correlation of its acetate with iso compound B acetate. Two other triterpenes, a ketol, C₃₀H₄₈O₂, and diaxial diol, C₃₀H₅₀O₂, were also isolated but their structures were not conclusively established. A diterpene alcohol, C₂₀H₃₄O, was separated and had spectral properties suggesting its identity with the known compound manool. The extract also contained fatty acid esters of β-sitosterol and presumably of glycerol. The identity of the acids was not investigated. A low melting wax ester was recovered but was not examined in detail. It is interesting to note that the fundamental triterpene skeleton of the above compounds is one in which ring C is seven-membered and only recently have such compounds been obtained in Nature. They undoubtedly present an interesting variation in triterpene biosynthesis.

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