CA1059526A - Process for the oxidation of primary allylic alcohols - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Improved conversions of 3-substituted and 3,3-disubstituted allyl alcohols to the corresponding aldehydes are obtained in an Oppenauer oxidation process, under Oppenauer oxidation conditions, by carrying out the oxidation employing furfural as the hydrogen acceptor. The invention is particularly applicable to the oxidation of geraniol and nerol to citral, which can be converted directly to pseudoionone without purification.

Description

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The present invention relates to the Oppenauer oxidation of primary allylic alcohols to their corresponding aldehydes. The invention is particularly applicable to the Oppenauer oxidation of 3-substituted and 3,3-disubstituted allyl alcohols, and will be described primarily with respect to the Oppenauer oxidation of geraniol (trans) and nerol (cis) (3,7-dimethyl^2,6-octadien-1-ol) to citral (3,7-dimethyl-2,6-octadienal), although it will be apparent to those skilled in the art that the present invention has other applications.
Citral which is a mixture of citral-a (geranial) and citral-b (neral) is a highly useful intermediate in the synthe-ses of several desirable products. The syntheses of ionones and methyl ionones begin with citral. It may be converted via a condensation with acetone, directly to pseudoionone, which in turn can be converted into a-ionone and ~-ionone. The latter is a key intermediate in the production of Vitamin A.
Pseudoionone is useful in the production of Vitamin E. Citral has many other uses, for instance, in synthetic flavors.
The Oppenauer oxidation of secondary alcohols to ketones is a useful and well-known textbook reaction. The oxidation is carried out generally in the presence of an aluminum catalyst such as aluminum tert-butoxide or aluminum isopropo*ide employing a large excess of acetone as a hydrogen acceptor. The general application of this reaction is, however, for secondary alcohols. It is reported in Or~anic Reactions, Vol. VI, chapter 5, on "The Oppenauer Oxidation", (pages 222-223) by Carl Djerassi, John Wiley and Sons Inc., 1951: "Until very recently the Oppenauer reaction, except in isolated instances, has not been used as a preparative method for the oxidation of primary alcohols to aldehydes because the aldehydes condensed with the hydrogen acceptor." As indicated by Djerassi, experimental modifications in the usual Oppenauer -1- ~

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procedure are necessary. These include the use of expensive or difficult to come by hydrogen acceptors, use of stoichiometric amounts of catalyst and careful distillation of the product as it is formed. The methods are expensive, difficult to carry out on a large scale and are employed only when no other method is available.
Previous observations of the oxidation of geraniol and nerol with acetone as a hydrogen acceptor show that the aldehydes produced undergo a subsequent aldol condensation reaction with the acetone and little aldehyde (citral) is actually recoverable. Although the end product of the aldol condensation of citral is pseudoionone, two major problems have kept this reaction from being employed in the production of pseudoionone commercially. One problem is that the aldol con-densation reaction produces water as a by-product which hydrolyzes and consumes the aluminum catalyst. This requires nearly stoichiometric quantities (as compared to catalytic quantities~
of the aluminum catalyst (notice pase 224 of Djerassi, supra).
In addition, the hydrolyzed catalyst is in the form of a gel-like precipitate which is difficult to dispose of and which alsopresents mechanical problems in carrying out the oxidation reaction. Still further, large amounts of solvent are required to dissolve the correspondingly large amount of catalyst employed for the oxidation reaction. A second disadvantage is that if the reaction is carried to high conversion, the yield tends to fall of~.
Substituting hydrogen acceptors such as cyclohexan-one, which are less likely to undergo an aldol condensation, for the acetone may improve the aldehyde yield. Still relatively high reaction temperatures are required when using ketones as , :10~55~;

hydrogen acceptors to carry out the oxidation to high conversion in a reasonable time. High temperatures would be a disadvantage with heat-sensitive aldehydes such as citral, as these are capable of self-condensation at high temperatures, In addition, other ketonic hydrogen acceptors present problems of availability or low equilibrium constants, the latter necessitating a large excess of hydrogen acceptor which causes difficulty in subsequent isolation of products, Djerassi on page 230 points out: "Until recently aldehydes have been used only infrequently as hydrogen acceptors." Aldehydes are traditionally difficult products to make, being unstable and subject to side reactions. Use of an aldehyde as a reactant or hydrogen acceptor is subject to the same problems, being equally unstable and subject to side reactions. In the Oppenauer oxidation process, it is likely to undergo both aldol and Tischenko condensation reactions, with both itself and with the Oppenauer oxidation product.
A number of studies have been conducted by Adkins and others (for instance, Adkins et al, J. Am. Chem. Soc., Vol. 71, pages 3622-3629) to determine the apparent oxidation potentials of various compounds (primarily ketones). Although it can be concluded that a high oxidation potential is desirable, a relatively low one (as pointed out by Djerassi on page 228) can be offset by using a large excess of hydrogen acceptor, and other factors such as rate of reaction and potential for side reactions may be more controlling. For instance, ?cetone has a relatively low oxidation potential but is inexpensive and can be used in large excess, Cyclohexanone on the other hand has a higher oxidation potential, but in comparative tests conducted with this compound, the oxidation of geraniol resulted in only 15% conversion to citral.

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The present invention resides in the discovery that furfural unexpectedly constitutes a superior oxidi~ing agent or hydrogen acceptor for the conversion of 3^substituted and 3,3-disubstituted allyl alcohols to their corresponding aldehydes, and particularly for the conversion of geraniol and nerol to citral.
Accordingly, the present invention provides in an Oppenauer oxidation for converting a 3-substituted or 3,3-disubstituted allyl alcohol of the general formula R'\
jC ~ CH20H
R
wherein R is an aliphatic or aromatic group and R' is hydrogen, an aliphatic group or an aromatic group into the corresponding olefin aldehyde, in the presence of an Oppenauer oxidation catalyst and hydrogen acceptor under mild temperature Oppenauer oxidation conditions, the improvement for obtaining increased conversion and yield of aldehyde .
which comprises utilizing furfural as said hydrogen acceptor in a molar ratio of furfural to allyl alcohol in the range of 10:1 to 1:10 and forming : a reaction product mixture containing the aldehyde corresponding to the ; allyl alcohol and as a by-product, furfuryl alcohol.
In particular, the reaction of the present invention is carried out under mild Oppenauer oxidation conditions in the presence of an aluminum catalyst, and can be represented by the following equation:
(I~ ~ OH ~ + Aluminum furfural geraniol nerol ~ furfuryl 2 cgeraniol) (neral) alcohol _ 4 _ lOS9SZfà

Under the mild reaction conditions, it was discovered that furfural does not undergo a Tischenko reaction as is common with many aldehydes. Moreover, furfural, having no alpha-protons, does not undergo an aldol condensation with the aldehyde product, for instance citral, as does acetone. Hence, the amount of aluminum catalyst required for the reaction is greatly reduced to catalytic quantities, eliminating the attendant mechanical and pollution problems, and reducing catalyst cost.
Surprisingly the reaction of furfural with substituted allyl alcohols such as nerol and geraniol has a high rate of reaction which permits it to be carried out under very mild conditions. This is important for such heat-sensitive compounds as citral. Specifically, at the mild - 4a -,~ ..

l~sss~æ6 conditions o~ the reaction of the present invention, citral, itself, under~oes no aldol self-condensation. Also, at the mild conditions of the present invention, no appreciable side reactions occur of the type resulting in such co~pounds as isocitrals and dimers.
The reaction of the present invention also has a fortuitous high equilibrium constant so that it results in high conversion and yields of citral without employing a large excess of furfural, The citral reaction product is readily recovered and is sufficiently pure that it can be employed for most uses without purification. In this regard, furfuryl alcohol, which is also a product of the oxidation reaction, has a relatively low boiling point allowing its convenient separation from citral and recovery by distillation. It is a very valuable product priced about 10% above furfural and is useful in the ~aking of certain resins, rendering the process of the present invention even more favorable economically.
A particular advantage of the present invention is that the citral reaction product is sufficiently pure that it can be used without purification for the production of pseudoionone. This is carried out in the presence of acetone in accordance with the following equation:

I

~ ~ 3 OCH

citral pseudoionone ' ~ , , This reaction can be carried out with citral following separation from the other reaction products of equation (1), or alternatively can be carried out in a single "~
;... .. . .

~)55'~S~i pot pr~cess in which ace~one and a base are added directly to the reaction ~ixture of equation (1).
Preferably, the reaction of equation (1) is carried out itl the presence of furfural with a molar ratio of furfural to primary allylic alcohol of about 1:10 to about 10:1. A
preferred range for nerol/geraniol is about 1:2 to about 3:1 which results in high yields of the desired products without excessive amounts of unreacted starting materials. The specific ratio selected, however, depends on the end products desired. In the production of pseudoionone, furfural also undergoes aldol condensation, so that it is desirable to employ a molar ratio of furfural to nerol/geraniol in the order of about 3:4 to avoid the necessity of separating unreacted furfural prior to the conversion of citral to pseudoionone. On the other had, if it is desired to recover citral, a relatively high molar ratio of furfural to nerol/
geraniol in the order of 2:1, is employed to maximize the conversion.
Preferably, a catalytic amount of about 1-15 mol %
(based on the weight of primary allylic alcohol charged) of an aluminum catalyst such as aluminum isopropoxide is employed, although this depends in part on the amount of water present in the reaction mixture.
For nerol/geraniol, the use of less than 2 mol %
catalyst is possible if extreme care is taken with regard to the water content. Other factors dictating the amount of catalyst employed include rate of reaction desired, amount of coincidental acid present in the reaction mixture, and the amount of water or acid produced in the course of the reaction.
As a general rule~ about 5 mol % catalyst based on nerol/geraniol charged, gives optimum reacting conditions and efficiency;
e.g., equilibrium in two to three hours at about 40C, .

~3S95~6 Any aluminunl alkoxide or aluminum aryloxide catalyst useful in an Oppenauer reaction, such as aluminum tert-butoxide r Al(t-OC4Hg)3_ 7, may be used. Aluminum isopropoxide is preferred as it offers a cost advantage and an advantage in availability, although some furfural is consumed by oxidation of the isopropoxide to acetone. In this regard, it is reported in the aforementioned Organic Reactions, Vol VI, page 209 (Djerassi) and also in Physical Or~anic Chemistry, by Hine, that the true active catalyst in the oxidation is an aluminum alkoxide which is generated in situ, and thus is dependent upon the reactants and conditions. It is normally generated by the addition of aluminum isopropoxide, aluminum t-butoxide or aluminum phenoxide. It may also be generated in situ by addition of an alkyl aluminum compound such as triisobutyl-aluminum (Djerassi also lists a number of other suitablecompounds). These compounds, although they are normally referred to as the catalyst, are merely the source of the aluminum alkoxide. Hence the choice of aluminum source is largely one of convenience. For the purposes of this applica-tion, the term "Oppenauer Oxidation Catalyst" shall be deemedto embrace all of the above compounds.
In its broadest aspect, the present invention relates to the Oppenauer oxidation of 3-substituted and 3,3-disubstituted allyl alcohols, as represented by the following formula:

~ C = CH-CH20 wherein the substitution is by either an aliphatic or aromatic group, or groups. In a preferred aspect of the present invention, the allyl alcohol may be disubstituted at the number 3 carbon atom with aliphatic groups. Representative ~- ~ ', . ' , ~o~

compounds of this class are nerol/geraniol (supra); farnesol (3,7,11-trimethyl-2,6,10-dodecatrien-1-ol); prenol (3-methyl-

2-buten-1-ol)~ and Vitamin A alcohol. In another class of compounds to which the present invention is directed, the allyl alcohol is substituted at the number 3 carbon atom with an aryl group. Representative of this class of compounds are cinnamyl alcohol (C6H5CH-CHCH2OH); and polyhydroxyl aromatic carbinol compounds such as coniferyl alcohol r p-HOC6H4CH=
C~CH20H_7 .
Still a third class of compounds within the scope of the present invention are 3-substituted allyl alcohols wherein the substitution is by a single aliphatic group. A
representative compound within the group is 2-hexen-1-ol.
Structures of representative alcohols are as follows:
CH=cH-cH2OH
~ OH

Geraniol Cinnamyl alcohol :~.
~ : .
OH ~ OH

2-hexen-1-ol Farnesol H=CH-CH2OH

OH
Prenol Coniferyl alcohol .. ,. . ~ . ... .
- ~

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The following examples illustrate the present invention and its practice, but should not he considered as limit-ing it. In this specification, all percentages are by weight, all parts are parts by weight, and all temperatures are in degrees Centigrade unless othersie specified.

This example illustrates the oxidation of geraniol/
nerol to citral using furfural as a hydrogen acceptor in accord-ance with the concepts of the present invention. A flask was charged with lO00 grams of redistilled geraniol/nerol (in a 60:40 ratio~ and 475 grams of distilled furfural providing a molar ratio of furfura] to nerol/geraniol of about 3:4. The mixture was heated to 35C. and a solution of 50 grams of aluminum isopropoxide (5% by weight of nerol/geraniol charged) in lO0 grams of toluene was added. The toluene may be omitted by predissolving the catalyst in the geraniol/nerol. Heating was discontinued, and the mixture was stirred at ambient temper-ature for about three (31 hours. The reaction was mildly exothermic. The mixture was analyzed by gas-liquid phase chrom-atography on a four-foot Carbowax* (a trade mark owned by Union Carbide Corporation to designate polyethylene glycol compounds) column using tetradecane as an internal standard.
Table I
Analysis of Reaction Product Weight Weight ~, Compound Percent (grams~

Furfural l.06 17.3 Furfuryl alcohol 30.l9 489.0 Neral 15.31 248.0 Geranial 24.70 400.l Nerol 7.68 124.4 Geraniol ll.70 189.6 -- -- _ _ Conversion of furfural 95.7%
Weight yield to furfuryl alcohol 105.2%
Conversion of geraniol/nerol 65.2%
Weight yield to citral 94.5%

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This exa~ple illustrates the course of the oxidation reaction as a function of time and shows that the o~idation of nerol/geraniol to citral may be carried out essentially to com-pletion at equilibrium under very mild conditions employingfurfural. The oxidation was carried out at room temperature and equilibrium was reached in about five hours. The initial molar ratio of furfural to nerol/geraniol was approxi~ately 5:3. Aluminum isopropoxide catalyst was employed, at ten percent (lOV/o) weight level based on the weight of nerol/geraniol charged. The product analyses were made by gas-liquid phase chromatograph,y using an internal standard.

Table II

Conversion Yield to Conversion of Yield to of Furfural Furfuryl Alc. Nerol, etc. Citral Time Hrs. (/O) (%) (~O) (%) 0:10 9.56 99.97 17.59 100.02 1:11 44.22 99.78 65.72 2:26 60.60 100.73 85.19 98.67 ~:38 63.03 99.09 90.46 97.08 5:40 63.20 101.54 91.44 96.47 It is apparent from the above data that even at room temperature conversions of about 85% of the nerol/geraniol to citral can be achieved in a relatively short time (2:26 hours) and that yields or recovery of citral of about 98% can be attained at that conversion. Even as the conversion is forced to about 90%, the drop in yield (percent recovery,of citral produced) is slight. Tests indicate that yield losses over a 24-hour period are less than 5% at room temperature, with recoveries consistently in the 95-98~/o range. Ma~jor impurities are believed to be isocitrals. The purity of the resultant * Analysis not available product, even following reaction to equilibriu~, renders it suitable for use in the production of pseudoionone.

This example illustrates that the equilibrium of the oxidation of nerol using furfural is decidedly in favor of citral and furfuryl alcohol. This permits the reaction to be carried out conveniently to a high degree of conversion (e.g., 95~/O of either nerol or furfural without using a large excess of one reactant.
A number of tests were conducted at different molar ratios of furfural and nerol. The same oxidation conditions ~;~
were employed in each test, namely, room temperature, 10%
aluminum isopropoxide, and atmospheric pressure. The results are given in the following Table III:
. ",', Table III

Molar Ratio Conversion Conversion Furfural:Nerol of Furfural _ of Nerol 2:4 97.9 44.6

3:4 95.2 68.0

4:4 87.2 79.4

5:4 81.2 89 6

6:4 71.0 93 0 7s4 60.4 94.2 j 8:4 53.8 97.6 10~4 44.2 98.2 The conversion of furfural is slightly higher than would be expected for the amount of nerol converted. The discrepancy is due to some oxidation of isopropyl alcohol (from aluminum isopropoxlde) to acetone. This reaction is much slower than the oxidation of nerol or geraniol and becomes apparent only when the nerol or ~eraniol oxidation approaches equilibrium.

; A conversion of furfural to furfuryl alcohol of more ~.. , . .. ", . . ... . . .

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than 90% is desirable if the crude citral product is to be converted directly to pseudoionone without purification of the citral. Furfuryl alcohol present and unreacted nerol and geraniol do not interfere with the reaction. A small amount of furfurylidene acetone is formed from the unreacted furfural during the pseudoionone reaction, but it does not interfere with the production and recovery of pseudoionone.
In this example, 390 gra~s of crude citral from Example 1 (i.e., the reaction product of Example 1 without purification) was caused to react with 1650 grams of 90% acetone using 100 ml. of 10% aqueous sodium hydroxide as the cata]yst.
Contained in the reaction mixture was only 1.06 weight percent furfural resulting from a 95. 70/D conversion of furfural to furfuryl alcohol from the reaction of equation (1). The pseudoionone reaction, illustrated by equation (2) above, was carried out by stirring at 40 to 43C. for 2.5 hours. The product was neutralized with 20 ml. of acetic acid and acetone was stripped off at atmospheric pressure and a pot temperature of 100C. The aluminum compounds were extracted with dilute sulfuric acid followed by saturated sodium carbonate. The product was then distilled at about 1 mm mercury pressure.
Analyses showed a 105% weight yield of pseudoionone based on nerol/geraniol consumed, prior to the final distillation, and a 101% weight yield subsequent to the distillation.
Fractionation of furfuryl alcohol from pseudoionone should produce a theoretical yield of 113 grams of furfuryl alcohol (based on a 96% conversion of furfural). Analyses by gas-liquid phase chromatography showed 106.3 grams of furfuryl alcohol prior to final distillation and 95~1 grams after final distillation, for an 82% recovered weight yield from furfural.
Recovery in excess of 96% is feasible enhancing the economics of the process of this invention.

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This example illustrates the principles of the invention in the Oppenauer oxidation of a herniterpene alcohol, and in particular the oxidation of prenol (3-methyl-2-buten-1-ol) to prenal. Prenol is a commercially available productmade by catalytic hydration of isoprene in glacial acetic acid ; to prenyl acetate which is then readily hydrolyzed to the alcohol. It can also be made directly from dimethylvinyl-carbinol with dilute sulfuric acid. Prenal is a product useful as a synthetic flavor, having a desirable raspberry flavor.
The reaction of prenol to prenal may be represented by the following equation:

(3) ~ `

CH2OH + ~ ~
O CHO aluminum CHO
Prenol Furfural Prenal + ~CH20H '~
,: O .' Furfuryl alcohol A flask was charged with 2.6 grams of aluminum isopropoxide and 21.5 grams of prenol and was stirred for 15 minutes~ 4.8 grams (2 equivalents) of furfural was then added giving mol ratios of 1:20 catalyst to prenol, and ~ 2:1 furfural to prenol. The mixture was then allowed to react with , stirring at room temperature. Analyses by glpc showed about 20 70% conversion after 3 hours. Continuing the reaction overnight gave 98% conversion to prenal.

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This example illustrates application of the concepts of the present invention in the oxidation of 2-hexen-1-ol.

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A reactor was charged with five (5.0) gra~s of trans-2-hexen-1-ol, 5 grams of furfural and 2 ml. o~ a 20% solution of alumin~m isopropoxide in toluene. The mixture was allowed to react for 16 hours at room temperature. Analyses by gas-liquid phase chro~at~graphy showed 73% conversion of furfuralto furfuryl alcohol and 71% conversion of hexenol to trans-2-hexenal. Only trace amounts of by-products totalling less than 2% of the crude product were observed.

In this example, farnesol is the substrate. A reactor was charged with 4.4 grams of farnesol (a mixture of 4 isomers of 3,7,11-trimethyl-2,6,]0-dodecatrien-1-ol), 1.9 grams of furfural and 1 ml, of a 20% solution of aluminum isopropoxide in toluene. The 0ixture was allowed to react for 14 hours at room temperature. Analyses by gas-liquid phase chromatography showed a 78% conversion of furfural to furfuryl alcohol and a comparable conversion of farnesol to farnesal. (Partial peak overlap of some isomers of farnesol and farnesal prevented making a precise analysis of the conversion of farnesol to farnesal.) The only by-products noted were about 5% of what were believed to be isofarnesals.

A reactor was charged with 2.6 grams of cinnamyl alcohol, 2.0 grams of furfural and 1.0 ml. of a 20% solution of aluminum isopropoxide in toluene. The solution was allowed to react for 16 hours at room temperature. Analysis by gas-liquid chromatography showed 82% conversion of furfural to furfuryl alcohol and 85% conversion of cinnamyl alcohol to cinnamaldehyde.
Less than about 2% by-products were detected.

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This example illustrates the advantages of the inven-tion using furfural as co~pared to the use of other hydrogen acceptors, namely cyclohexanone and benzaldehyde. The compara-tive reactions were carried out under the same conditions withequal equivalents of hydrogen acceptor, substrate and catalyst.
With benzaldehyde: A flask was charged with 3.1 grams of geraniol, 0.25 grams of aluminum isopropoxide and 2.1 grams (1 equivalent) of benzaldenyde and stirred for 16 hours at ambient temperature. Analysis by glpc showed only a 5% conver-sion of geraniol to citral.
With cyclohexanone: A flask was charged with 3.1 grams of geraniol, 0.25 grams of aluminum isopropoxide, and 1.9 grams (1 equivalent) of cyclohexanone and stirred for 16 hours at ambient temperature. Analysis by glpc showed only a 15%
conversion to citral.
With furfural: A flask was charged with 3.1 grams of geraniol, 0.25 grams of aluminum isopropoxide, and 1.9 grams (1 equivalent) of furfural and stirred for 16 hours at ambient temperature. Analysis by glpc showed a 79% conversion of geraniol to citral.

In this example, comparative tests were conducted with furfural and isobutyraldehyde which has a higher oxidation potential under the same conditions. Two flasks were each charged with a solution of 0.5 grams of aluminum isopropoxide in 10 grams of geraniol. To one flask was added 1.5 equivalents of furfural and to the other 1.5 equivalents of isobutyraldehyde.
Both flasks were allowed to react for 20 hours at ambient temperature. Analysis by glpc gave the following results:

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Hydrogen/O of Geraniol ~/O Yield Acceptor Consumed to Citral Furfural 94~0~/O 98+%
Isobutyraldehyde 78.2% 88.1%

.
This exa~ple illustrates the use of triisobutyl-aluminum to generate the aluminum alkoxide catalyst. A flask was charged with 80 grams of nerol and 10 ml. of a 25% solution of triisobutylaluminum in toluene. To the solution was added 39 grams (0.75 equivalents) of furfural and the solution was allowed to react at ambient temperature for 18 hours. Rnalysis by glpc showed that 6~% of the nerol had been consumed to give citral in 98~% yield.

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Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In an Oppenauer oxidation for converting a 3-substituted or 3,3-disubstituted allyl alcohol of the general formula wherein R is an aliphatic or aromatic group and R' is hydrogen, an aliphatic group or an aromatic group into the corresponding olefin aldehyde, in the presence of an Oppenauer oxidation catalyst and hydrogen acceptor under mild temperature Oppenauer oxidation conditions, the improvement for obtaining increased conversion and yield of aldehyde which comprises utilizing furfural as said hydrogen acceptor in a molar ratio of furfural to allyl alcohol in the range of 10:1 to 1:10 and forming a reaction product mixture containing the aldehyde corresponding to the allyl alcohol and as a by-product, furfuryl alcohol.

2. The oxidation process of claim l wherein said Oppenauer reaction is carried out with a molar ratio of furfural to allyl alcohol in the range of 2:1 to 1:2.

3. The oxidation process of claim 1 wherein said allyl alcohol is substituted with aliphatic or aromatic groups.

4. The oxidation process of claim 1 wherein said allyl alcohol is disubstituted with aliphatic groups.

5. The oxidation process of claim 4 wherein said allyl alcohol is nerol/geraniol (3,7-dimethyl-2,6-octadien-1-ol), said reaction product mixture containing citral.

6. The oxidation process of claim 5 wherein the citral reaction product is caused to undergo an aldol condensation by reaction with acetone and base without intermediate purification and the aldol condensation product formed is pseudoionone.

7. The oxidation process of claim 6 wherein said ketone is acetone and the aldol condensation product is pseudoionone, the molar ratio of furfural to nerol/geraniol being more than about 1:1.

8. The process of claim 4 wherein said allyl alcohol is prenol (3-methyl-2-buten-1-ol), oxidized to prenal.

9. The process of claim 4 wherein said allyl alcohol is farnesol (3,7,11-trimethyl-2,6,10-dodecatrien-1-ol), oxidized to farnesal.

10. The process of claim 1 wherein said allyl alcohol is mono-substituted with an aromatic group.

11. The process of claim 10 wherein said allyl alcohol is cinnamyl alcohol (C6H5CH=CHCH2OH).

12. The process of claim 1 wherein said allyl alcohol is mono-substituted with an aliphatic group.

13. The process of claim 12 wherein said allyl alcohol is 2-hexen-1-ol.