Reaction of isocembrol and alcohols on clay

Abstract

Isocembrol (1) is a diterpene component of Pinus sibirica gum and a representative of a broad group of cembranoids. It incorporates a 14-membered ring, a tertiary alcohol, and three trans-double bonds in the ring. The principal sources of cembranoids are coniferous plants, tobacco, and marine invertebrates, especially corals. Cyclic alcohol 1 was first isolated from the neutral part of P. sibirica Mayr gum [1] and was described under the name thunbergol as the principal component of the neutral part of Pseudotsuga menziessi Mirb. Franco gum [2]. Compound 1 was also detected in marine corals [3], tobacco leaves [4], and propolis [5]. We improved the method for isolating 1 from P. sibirica gum. The previously reported method [1] was used as a prototype. According to that method, gum is saponified by NaOH solution (1%). Neutral unsaponified substances are extracted by Et2O and fractionated by vacuum distillation. The residual after distillation is chromatographed twice successively over basic Al2O3 (1:15 and 1:50 ratios). Compound 1 is isolated in 1.9% yield of the neutral fraction and 0.5% of the total gum mass. We proposed isolating 1 from the neutral part of the gum by extraction with nonpolar solvents (hexane, petroleum ether) and successive purification of the target product by column chromatography. This method has several advantages over the known methods. The solvents and gum extraction method are selected such that the resulting mixture containing 1 is fluid and easily subjected to either chromatography or vacuum distillation. The yield of target product using the proposed method was 2.9–3.8% of the total gum mass and 18–22% of the hexane-extract mass. This was significantly greater than the yield of known methods. The spectral characteristics of isolated 1 agreed with those published [6]. The properties of 1 are insufficiently studied despite its relative availability. Isomerization of 1 in formic acid that leads to the formation of biand tricyclic compounds [7], epoxidation by per-acids [8], and hydroxylation [9] were studied. Epoxidation of 1 by t-BuOOH in the presence of VO(acac)2 was performed efficiently [10]. Further transformations of the resulting epoxy derivative were studied. Oxidation of 1 by pyridinium chlorochromate produced conjugated enones [11]. Herein results of a study of the reactions of 1 with allyl and methyl alcohols on K-10 clay are reported. Adding 1 to an allyl alcohol–clay system gave an intermolecular product, the allyl ether of isocembrol 2, as the principal product. The reaction was conducted without solvent. Adding CH2Cl2 to the system increased significantly the amount of isomerization products. Thus, adding 1 to a CH2Cl2–allyl alcohol–clay system gave 3 as the principal product (Scheme 1). Compound 3 was formed as a result of intramolecular cyclization and subsequent reaction with allyl alcohol. Furthermore, a mixture of cembrenes was formed.