Campholenic preparation

The preparation of campholenic aldehyde (12) from a-pinene oxide (13) is currently a commercial process in which zinc bromide is used as the catalyst. Ravasio et al. [20] have investigated replacing the zinc bromide with a commercial mixed co-gel solid-acid catalyst. They found that by use of Si02-Al203 (1.2 %) or Si02-Zr02 (4.7 %) they could readily achieve a 72 % yield of 12 under relatively mild conditions (25-60 °C, toluene) although small quantities of several side-products were also still formed (Scheme 2). 

Another group has also reported a potential zeolite (H-US-Y)-catalyzed production process for preparation of campholenic aldehyde from a-pinene oxide which is described as competitive with the traditional zinc bromide process [21]. Other groups have also looked at this transformation [22]. 

The members of the second family of synthetic sandalwood materials are derived from campholenic aldehyde (6.72). This aldehyde is prepared by treatment of a-pinene oxide (6.71) with a Lewis acid, usually zinc chloride or bromide. a-Pinene oxide is, in turn, prepared from a-pinene... 

Figure 6.18 shows the general scheme for the preparation of sandalwood materials from campholenic aldehyde (6.72) and also three typical members of the family. 

Production Even today many aroma chemicals are still isolated from essential oils, others are prepared semisynthetically from the components of the oils or from other suitable oiganic compounds. The proportion of natural and nature identical aroma chemicals used in the perfume industry is ca. 70%. The aroma chemicals not occurring in nature are often structural analogues of natural products that are difficult to synthesize, e.g., the sandalwood aroma chemicals prepared from campholene aldehyde or camphene and guaiacol as substitutes for the expensive sandalwood oil or santalols. 

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. ... 

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