Campholenic epoxide

Acid-catalysed rearrangement of epoxides is another widely used reaction in the fine chemicals industry. Here again the use of solid acid catalysts such as zeolites is proving advantageous. Two examples are shown in Fig. 2.25 the isomerization of rsophorone oxide (Elings et al., 1997) and the conversion of a-pinene oxide to campholenic aldehyde (Holderich et al., 1997 Kunkeler etal., 1998). Both products are fragrance intermediates. 

Isomerization of a-pinene epoxide to campholenic aldehyde, an intermediate for perfumery chemicals, has been carried out elegantly with ultra stable Y-zeolite. 

Another opportunity to combine two reaction steps towards a one pot synthesis is the epoxidation of a-pinene and the isomerization of the epoxide to campho-lenic aldehyde (Scheme 5.6). Zeolite Ti-Beta seems adequate to deal with both steps as a catalyst [24]. Campholenic aldehyde is the starting material for several sandalwood fragrances. 

Campholenic Aldehyde Manufacture. Campholenic aldehyde is readily obtained by the Lewis-acid-catalyzed rearrangement of a-pinene oxide. It has become an important intermediate for the synthesis of a wide range of sandalwood fragrance compounds. Epoxidation of (+)- Ct-pinene (8) also gives the (+)-o -a-pinene epoxide [1686-14-2] (80) and rearrangement with zinc bromide is highly stereospecific and gives (-)-campholenic aldehyde... 

Some of the chemistry developed by the industry more recently, to produce new monohydric alcohols, is just as interesting as the linalool chemistry. Sandalore, a recent new Givaudan chemical with a persistent, sandalwood odor is made according to the scheme in Figure 15 (9). Alpha-pinene, the starting material, is converted to the epoxide which is catalytically rearranged to campholen-ic aldehyde. Aldol condensation with methyl ethyl ketone followed by hydrogenation yields Sandalore . 

Epoxides can be isomerized to carbonyl compounds. Industrial examples include the isomerization of styrene oxide to phenylacctaldehyde and that of a-pinene oxide to campholenic aldehyde. As Ti-containing zeolites as TS-1 [77J and the Ti-Beta [78] are good catalysts for these isomerizations it is tempting to combine the epoxidation and the isomerization step. Several substituted styrenes have been subjected [79[ to such a two-step one-pot procedure. 

A number of different catalysts have been used in the epoxidation of monoterpenes with t-butyl hydroperoxide and the conditions optimized. While oxidation of a-pinene in the presence of V(acac)3 gave ds-epoxide (4.4%), campholenic aldehyde was also obtained in the presence of Mo(CO)e. 

Carvone-derived 2,3-epoxy alcohol derivatives (317) have been found to rearrange with stereoselective formation of ring-contracted ketones (318) in a process where the stereochemical result seems to be independent of epoxide configuration. On the basis of this study, the authors concluded that the rearrangement of tetrasubstituted cyclic epoxy alcohol derivatives remains a transformation for which both regiose-lectivity and stereochemical outcome are difficult to predict. New solid acid catalysts based on silica-supported zinc triflate have been prepared for use in the rearrangement of a-pinene oxide to campholenic aldehyde. ... 

Epoxidation of a-pinene gives a-pinene oxide, which can be rearranged by Lewis acid catalysts to give campholenic aldehyde (30). This is a key feedstock for the production of sandalwood ingredients, and further details will be found below under the heading campholenic aldehyde. 

Campholenic aldehyde (30) or campholene aldehyde occurs in nature, but in a limited range of species. It is produced commercially from a-pinene (20) by epoxi-dation and subsequent Lewis acid catalyzed rearrangement [298, 299]. Zinc bromide is the most effective catalyst for the rearrangement, but the chloride can also be used. As shown in Fig. 8.49, the epoxidation and rearrangement both proceed... 

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