A-Campholenic aldehyde

The photolysis of camphor (XXII) in aqueous alcoholic solution (11) has been observed to lead to a-campholenic aldehyde (XXXII) and a second isomer with a ketonic function. The structure of the latter has been found (30) to be 1,2,2-trimethyl cyclopent-3-enyl methyl ketone (XXXIII). The quantum yields at 3130 A. for the formation of the two isomers and of carbon monoxide in five different solvents have been determined (Table IV). It is interesting that the sum of the quantum... 

Rearrangement of a-pinene oxide, catalysed by zinc chloride or bromide, gives a product known as a-campholenic aldehyde. Aldol condensation of this with a second aldehyde or a ketone gives an a, (3-unsaturated carbonyl compound that can then be reduced to an... 

Aldol reaction of the campholenic aldehyde with 2-butanone gives the intermediate ketones from condensation at both the methyl group and methylene group of 2-butanone (Fig. 6). Hydrogenation results in only one of the two products formed as having a typical sandalwood odor (160). 

Aldol reaction of campholenic aldehyde with propionic aldehyde yields the intermediate conjugated aldehyde, which can be selectively reduced to the saturated alcohol with a sandalwood odor. If the double bond in the cyclopentene ring is also reduced, the resulting product does not have a sandalwood odor (161). Reaction of campholenic aldehyde with -butyraldehyde followed by reduction of the aldehyde group gives the aHyUc alcohol known commercially by one manufacturer as Bacdanol [28219-61 -6] (82). 

When the a,P-unsaturated ketone is hydrogenated to the alcohol, a product with an intense sandalwood odor is produced (162). Many other examples of useful products have been made by condensation of campholenic aldehyde with ketones such as cyclopentanone and cyclohexanone. 

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. 

Scheme 5.6 Conversion of a-pinene into campholenic aldehyde.

More recently spinning disc reactors have been used by Wilson et alP0) to carry out catalytic reactions using supported zinc triflate catalyst for the rearrangement of a-pinene oxide to yield campholenic aldehyde. The results of this study, presented in Table 20.1, suggest that by using a supported catalyst on a spinning disc reactor it is possible to... 

Rearrangement of a-pinene oxide to campholenic aldehyde Batch Reactor Catalysed SDR... 

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... 

Table 1 Comparison of the Best SDR Runs with Batch Results for Conversion of a-Pinene Oxide to Campholenic Aldehyde...

By using 200mg of 0.01mmolg-l Zn(OTf)2/Si02 the reaction rate was slowed so that 100% conversion was reached after 20 minutes, and a selectivity to campholenic aldehyde of 60% was obtained. On reuse only a slight decrease in catalyst activity was... 

Alpha-Pinene oxide 9 (Eq. 15.2.5) is known as a reactive molecule which rearranges easily under the influence of an acid catalyst (6, 7). Thereby many products can be formed. For example compounds such as the isomeric campholenic aldehyde 11, trans-carveol 12, trans-sobrerol 13, p-cymene 14 or isopinocamphone 15 are observed as main by-products. At temperatures higher than 200°C more than 200 products can be formed. The industrially most desired compound among these is campholenic aldehyde 10. It is the key molecule for the synthesis of various highly intense sandalwood-like fragrance chemicals (7, 8). 

A heterogeneous catalyzed process for the production of campholenic aldehyde from alpha-pinene oxide has been found, which is competitive with the homogeneous ZnBr2 system due to yields up to 85%. 

H. van Bekkum et al. (17) reported that the alpha-pinene oxide 9 can be succesfully converted to campholenic aldehyde 10 (Eq. 15.2.5) in the presence of a BEA-zeolite. Ti-BEA proves to be an excellent catalyst for the rearrangement of a-pinene oxide to campholenic aldehyde in both the liquid and vapor phase. This is mainly attributed to the presence of isolated, well-dispersed titanium sites in a Bronsted-acid-free silica matrix. Furthermore, the unique molecularsized pore structure of the zeolite may enhance selectivity by shape-selectivity. 

The absence of a solvent when working under vapor phase conditions strongly increases the intraporous alpha-pinene oxide concentration. This leads to a decreased campholenic aldehyde selectivity. A competitive inert co-adsorbate may be added to the reactor feed to control the alpha-pinene oxide concentration. When dichloroethane is chosen as a co-adsorbate, a very high selectivity to campholenic... 

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