Campholenic aldehyde

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). 

Fig. 6. Campholenic aldehyde (81) reacts with 2-butanone to produce ketones that are hydrogenated to alcohols having the odors indicated.

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). 

Condensation of campholenic aldehyde with ethyl acetoacetate with subsequent saponification and decarboxylation gives the intermediate unsaturated ketones. 

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. 

Camouflaged boranes, 4 190-191 Campanulales, alkaloids in, 2 75 Camphene, 3 231 24 477, 478, 497-498 Campholenic aldehyde, 24 496, 534—536 sandalwood materials made from, 24 535-536... 

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... 

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... 

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

Figure 25 Batch reaction conversion and selectivity towards campholenic aldehyde. 

Unfortunately ZnCl2/Si02 cannot be recycled after reaction, due to irreversible catalyst deactivation which is believed to occur via hydrolysis of the Zn-Cl bond. Indeed chlorinated campholenic aldehyde was observed to form during the reaction, which would occur through reaction with HC1 from hydrolysed ZnCl2. It was anticipated that the enhanced water stability of the metal triflate would thus be advantageous for this reaction. 

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... 

The selectivity towards campholenic aldehyde can be boosted further to 65% if the O.Olmmolg 1 Zn(0Tf)2/Si02 catalyst is pretreated under N2 at 200°C prior to use. This increase in selectivity is attributed to loss of Bronsted acid sites by dehydration of the catalyst surface, which in turn reduces the amount of side reactions. 

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