GB1578191A - 2,5 - dimenthyl - 3-carbalkoxy - tetrahydrofuren - 4 - ones and their manufacture - Google Patents

Description

(54) 2 ,5-DIMETHYL-3-CARBALKOXY-TETRAHYDROFURAN-4-ONES AND THEIR MANUFACTURE (71) We, BASF AKTIENGESELLSCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: The present invention relates to 2 ,5-dimethyl-3-carbalkoxy-tetrahydrofuran-4-ones. The present invention provides such compounds of the general Formula I

where R is alkyl of 1 to 6 carbon atoms, preferably alkyl of 1 to 4 carbon atoms, and especially methyl or ethyl.

The present 2 ,5-dimethyl-3-carbalkoxy-tetrahydrofuran-4-ones possess interesting o]factory properties. They have a spicy, tarry intense scent reminiscent of birch tar oil and can therefore be used as scents with leathery, woody and chypre-type notes.

The present compounds are also of great interest as intermediates or starting materials in further syntheses.

For example, 2,5-dimethyl-tetrahydrofuran-3-one can be obtained in 70% yield by boiling these compounds with dilute acids, so that they undergo hydrolysis and decarboxylation, whilst hitherto it has only been obtainable in poor yield from hex-3-yne-2,5-diol, with the aid of mercury catalysts.

The invention furthermore provides a process for the manufacture of the 2,5-dimethyl-3carbalkoxy-tetrahydrofuran-4-ones of the general formula I, wherein a lactic acid ester of the general formula II

where R has the above meaning, is reacted, in the presence of an approximately molar amount of a strong base, in an excess of the lactic acid ester or in a solvent which is inert under the reaction conditions, at a temperature of from -20 to 1200C, with a crotonic acid ester of the general formula III

where R has the above meaning and is preferably equal to the R in the lactic acid ester.

The lactic acid esters and crotonic acid esters required as starting materials for processes within the invention are conventional commercial compounds. If it is desired to produce a single tetrahydrofuranone ester, the lactic acid and crotonic acid esters of the same alcohol should be used (i.e., R in II and III should be equal), as otherwise a mixture of different tetrahydrofuranone esters of formula I is obtained due to partial reesterification.

Suitable strong bases for the present process are alkali metals, e.g. Na or K, and strongly basic alkali metal compounds, e.g. the hydrides, amides or alcoholates of sodium or potassium. Examples which may be mentioned are NaH, NaNH2, NaOCH3, NaOC2H5, KOCH3, KOC2Hs and KO-tert.-C4H9. When using an alcoholate as the strong base it is expedient to employ an alcoholate of the alcohol contained in the lactic and/or crotonic acid ester, as otherwise a mixture of different tetrahydrofuranone esters of formula I may result.

Any ester mixtures formed can however be reesterified in a simple and conventional manner by heating with the desired alcohol in the presence of traces of a base. By "an approximately molar amount" we mean a molar amount or an excess of up to 5 percent over the molar amount.

The strong base is in general used in the form of a solution or suspension in an excess of the lactic acid ester of the formula II or, preferably, in a solvent which is inert under the reaction conditions. Suitable inert solvents are, in general, aliphatic hydrocarbons, e.g.

pentane, hexane, cyclohexane and methylcyclohexane, aromatic hydrocarbons, e.g.

benzene, toluene and xylene, and open-chain and cyclic ethers, e.g. diethyl ether, diisopropyl ether, glycol dimethyl ether, glycol diethyl ether and tetrahydrofuran. More strongly polar solvents, e.g. dimethylformamide, are rather unsuitable. When using alcoholates it is advisable - in order to increase the yield - to remove the alcohol, as it is formed, from the reaction mixture; this may be achieved, for example, by azeotropically distilling off the alcohol with a suitable solvent. The amounts of solvent used are, in general, from 500 to 5,000 ml per mole of ester.

The reaction temperature is from -20 to +120"C, preferably from + 10 to 800C.

The reaction time is suitably from 3 to 24 hours, preferably from 6 to 12 hours.

To carry out a process within the invention, the procedure followed is in general that either the appropriate alkali metal alcoholate of the lactic acid ester is first prepared by reacting the said ester with the alkali metal or the alkali metal compound, the crotonic acid ester is then added and the reaction mixture is allowed to continue reacting for from 6 to 12 hours at the reaction temperature, or a mixture of the lactic acid ester and the crotonic acid ester is added to the solution or suspension of the strongly basic condensing agent.

The product formed is initially present in the reaction mixture as a salt of the ss-dicarbonyl compound and may be obtained in the free form by acidifying the reaction mixture to a pH of about 4 with any suitable acid, e.g. HCI, H2SO4, CH3COOH or CO2.

The acidified mixture may be worked up by conventional methods, e.g. by extraction with conventional water-insoluble solvents and fractionation of the resulting organic phase.

The present 2 ,5-dimethyl-3-carbalkoxy-tetrahydrofuran-4-ones may be useful as starting materials for further syntheses. For example, they permit the preparation of 2,5-dimethyl tetrahydrofuran-3-one in very good yields, whilst this compound was previously only obtainable by an expensive method and in poor yields.

To prepare the 2,5-dimethyl-tetrahydrofuran-3-one, the present 2,5-dimethyl-3 carbalkoxy-tetrahydrofuran-4-ones may be subjected to the conventional conditions for the acid hydrolysis of esters, such as are described, for example, in Houben-Weyl, "Methoden der Organischen Chemie", Volume 8, pages 418 et seq. For example, the 2,5-dimethyl-3 carbalkoxy-tetrahydrofuran-4-ones are heated in the presence of a mineral acid such as HC1 or H2SO4, with or without a solvent, the acid being used in amounts ranging from catalytic quantities to several times the molar amount of the ester. The reaction is continued until no further CO2 is evolved or no further ss-keto-ester is detectable (FeCI3 test). The mixture is worked up in the conventional manner by extracting or separating off the organic phase and washing, drying and distilling the extract. The yields are from 65 to 80% of theory.

Particular embodiments of the present invention will now be described by reference to specific Examples.

Example 1 48 g of ethyl lactate are slowly added dropwise, whilst cooling to 12 g of 80% strength NaH in 140 ml of tetrahydrofuran (THF) at from 20 to 300C. After the evolution of gas has ceased, 46 g of ethyl crotonate are added dropwise and the reaction mixture is stirred overnight at room temperature.

The reaction mixture is then acidified to pH 4 with dilute HCI, ether is added and the organic phase is separated off, washed with water and dried. After distilling off the solvent, the residue is distilled under reduced pressure.

54 g (71% of theory) of 2,5-dimethyl-3-carbpethoxytetrahydrofuran-4-one of boiling point 55"C/0.3 mm Hg and refractive index n25 = f.4315 are obtained.

Example 2 24 g of ethyl lactate are slowly added dropwise to 4.6 g of Na in 150 ml of THF, whilst cooling. When the evolution of gas has subsided, 23 g of ethyl crotonate are added dropwise. After about 60 minutes, all the sodium has dissolved. The mixture is kept for 12 hours at room temperature and is then worked up by the method described in Example 1.

The yield is 25 g, corresponding to 67% of theory.

Example 3 21 g of methyl lactate are added to 12 g of solid NacOCH3 in 150 ml of cyclohexane and methanol is distilled off azeotropically. When the boiling point of cyclohexane is reached, the reaction mixture is cooled and a solution of 20 g of methyl crotonate in 100 ml of THF is added dropwise, at room temperature. The mixture is then stirred overnight.

The batch is worked up by the method described in Example 1.

21 g (corresponding to 61% of theory) of 2,5-dimethyl-3-carbomethoxy-tetrahydrofuran4-one of boiling point 53"C/0.3 mm Hg and refractive index no9)1 = 1.4395 are obtained.

Example 4 45 g of the 2,5-dimethyl-3-carbomethoxy-furan-4-one obtained in Example 3 are boiled for 3 hours with 200 ml of 10% strength H2SO4 until no further CO2 is evolved. The mixture is then extracted with ether and washed with water and the organic phase is dried and concentrated. On distillation, 20 g of 2 ,5-dimethyl-tetrahydrofuran-3-one (corresponding to 67% of theory) of boiling point 83-85"C/100 mm Hg are obtained. The substance corresponds to a sample prepared from hexynediol.

Example 5 35 g of methyl lactate are added to 18 g of NaOCH3 powder in 500 ml of xylene and the mixture is distilled at 11 mm Hg. A mixture of xylene and methanol passes over; in total, 450 ml are distilled off. 200 ml of tetrahydrofuran are added to the distillation residue, whereupon a clear solution is formed. 30 g of methyl crotonate are added dropwise, with slight cooling, and the reaction mixture is stirred overnight. It is then worked up by the method described in Example 1.

36 g (corresponding to 62% of theory) of 2,5-dimethyl-3-carbomethoxy-tetrahydrofuran4-one are obtained.

Example 6 25 g of K tert.-butylate are introduced into 100 ml of tetrahydrofuran. At from 25 to 30"C, 24 g of methyl lactate are added dropwise in the course of 10 minutes, followed by 20 g of methyl crotonate. The reaction mixture is then stirred overnight and worked up by the method described in Example 1. On distillation, 14 g (corresponding to 35% of theory) of 2,5-dimethyl-3-carbomethoxy-tetrahydrofuran-4-one are obtained.

Example 7 7.7 g of Na, in small pieces, are introduced into 700 ml of tetrahydrofuran.A mixture of 35 g of methyl lactate and 30 g of methyl crotonate is then added dropwise in the course of 3 hours, with slight cooling. After a further 2 hours, all the Na has dissolved. The reaction mixture is stirred overnight and worked up by the method described in Example 1. 40 g (corresponding to 70% of theory) of 2,5-dimethyl-3-carbomethoxy-tetrahydrofuran-4-one are obtained.

Example 8 36.5 g of n-hexyl lactate are slowly added dropwise, whilst stirring, to 6.3 g of 80% strength NaH in 140 ml of THF, at from 20 to 30"C. After the evolution of gas has ceased, 35.6 g of n-hexyl crotonate are added dropwise and the reaction mixture is stirred overnight at room temperature. It is then worked up by the method described in Example 1. 36.6 g (corresponding to 72% of theory) of 2,5-dimethyl-3-n-hexyloxycarbonyl-tetrahydrofuran-4- one of boiling point 100 - 103"C/0.3 mm Hg and refractive index n25 = 1.4408 are obtained.

Claims (11)

WHAT WE CLAIM IS:

1. A 2,5-dimethyl-3-carbalkoxy-tetrahydrofuran-4-one of the general formula I

where R is alkyl of 1 to 6 carbon atoms.

2. 2 ,5-Dimethyl-3-carbomethoxy-tetrahydrofuran-4-one.

3. 2, 5-Dimethyl-3-carboethoxy-tetrahydrofuran-4-one.

4. A process for the manufacture of a 2,5-dimethyl-3-carbalkoxy-tetrahydrofuran-4-one as claimed in claim 1, wherein a lactic acid ester of the general formula II

where R has the meaning given in claim 1, is reacted with a crotonic acid ester of the general formula III

where R has the meaning given in claim 1, in the presence of an approximately molar amount of a strong base, in an excess of the lactic acid ester or in a solvent which is inert under the reaction conditions, at a temperature of from -20 to 1200C.

5. A process as claimed in claim 4 wherein R in the starting material of general formula II has the same meaning as R in the starting material of general formula III.

6. A process as claimed in claim 4 wherein Na, K or a hydride, amide or alcoholate of Na or K is used as the strong base.

7. A process for the manufacture of a compound as claimed in claim 1 carried out substantially as described in any of the foregoing Examples, 1 to 3 and 5 to 8.

8. A 2,5-dimethyl-3-carbalkoxy-tetrahydrofuran-4-one when manufactured by a pro cess as claimed in any of claims 4 to 7.

9. An odorous composition containing a 2,5-dimethyl-3-carbalkoxy-tetrahydrofuran-4- one claimed in claim 1, 2, 3 or 8 as odoriferous constituent.

10. A process for the manufacture of 2,5-dimethyl-tetrahydrofuran-3-one, wherein a 2,5-dimethyl-3-carbalkoxy-tetrahydrofuran-4-one claimed in claim 1 is subjected to hydrolysis and decarboxylation.

11. A process as claimed in claim 10 wherein the 2,5-dimethyl-3-carbalkoxy- tetrahydrofuran-4-one is heated in the presence of a mineral acid to effect hydrolysis and decarboxylation.