Terpenes epoxidation

Polymerization of terpene epoxides over solid catalysts 97UK376. 

Table 15) highlights the stability of this system compared to the PS/MTO system (entry 6, Table 15), which shows a decrease in activity during recycling. This difference in behaviour may be due to the weaker interaction between MTO and the PS polymer, which is only accomplished by the physical envelopment of the benzene ring. The PVP/MTO combination was successfully used for other compounds of biological interest, such as ter-penes. Even highly sensitive terpenic epoxides, hke a-pinene oxide, can be obtained in excellent yields using polymer-supported MTO catalysts [73] (Scheme 20, Table 16). 

Terpene epoxides are very reactive compounds. Some products formed by isomerization of such epoxides are valuable raw materials for perfumes, synthetic flavourings and pharmaceuticals, and also provide useful intermediates in organic syntheses. The isomerization of isophorone oxide 23 (Eq. 15.2.9) was originally investigated by H.O. House and R.L. Wasson using boron trifluoride etherate as a homogeneous catalyst (24). 

The selectivity of the epoxidation, in conjunction with the availability of optically active terpenes from natural sources, has resulted in the application of terpene epoxides as starting materials for the synthesis of several natural products. Both enantiomers of carvone, (10) and (ent-l0), have been used for the synthesis of methyl trans- and c/.v-chrysanthemates 15 and ent-15. i+Hsy Carvone (10) was converted into hydrochlorinated compound 13 and the methylated derivative 11, which were selectively epoxidized with alkaline hydrogen peroxide, and further converted into methyl trum-chrysanthcmate 15. The same route led from (— )-(/ )-carvone ent-10) to m-chrysanthemate ent-1543 ent-13 was converted to ( + )-a-3,4-epoxycaran-2-one 1644. 

More typically, mixtures of products are formed. Terpene epoxides that have been subjected to ZnBr2-catalyzed rearrangement include the oxiranes derived from limonene, carvomenthene, other pinenes, 2,3-carvene, pulegone and piperitone. ZnCh is rarely employed, but has been compared with AICI3 in a study involving sever epoxides. In most instances there appears to be no evidence for special advantages of zinc halides compared with other more commonly used Lewis acids. 

P. N. Rao and J. C. Uroda, Tetrahedron Lett., 1964, 1117 and refs, therein J. Klinot, M. Krumpolc and A. VystrCil, Coll. Czech. Chem. Soc., 1966, 31, 3174 for a related terpene epoxide rearrangement. A steroidal p-5,6-epoxide gave the 6-ketone by hydride migration when treated with MgBr , J. J. Frankel, S. Julia and C. Richard-Neuville, Bull. Soc. Chim. Fr., 1968, 4870. 

A. L. Villa, B. E. Sels, D. E. De Vos, P. A. Jacobs, A heterogeneous tungsten catalyst for epoxidation of terpenes and tungsten-catalyzed synthesis of acid-sensitive terpene epoxides, /. Org. Chem. 64 (1999) 7267. 

Terpenes epoxides, prepared by the straightforward oxidation of their unsaturation [9], have also been submitted to cationic polymerization [5, 10] and to insertion copolymerization with C02 [11], but these studies were not systematic in their approach. 

Figure 16.1-23. Some examples of olefinic terpene epoxidation.

Terpene epoxides are very reactive compounds. Some are prepared conventionally by isomerization using homogeneous catalysts such as Bp3.Et20 [35,36]. The rearrangement of isophorone oxide (Figure 6) yields the keto aldehyde (13), which is an intermediate for fragrance chemicals. If the reaction is performed in the presence of zeolites the keto aldehyde (13) and the keto-enol form of the a-diketone... 

Isomerization of Isophorone Oxide - Terpene epoxides are very reactive compounds. Some products formed by isomerization of such epoxides are... 

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