GB2110676A

GB2110676A - Preparation of 3-alkoxycycloalk- 2-en-1-ones

Info

Publication number: GB2110676A
Application number: GB08232596A
Authority: GB
Inventors: Robin Gerald Shepherd
Original assignee: John Wyeth and Brother Ltd
Current assignee: John Wyeth and Brother Ltd
Priority date: 1981-12-02
Filing date: 1982-11-15
Publication date: 1983-06-22
Also published as: GB2110676B

Abstract

Compounds of formula (I> <IMAGE> wherein n is 1 or 2, X is hydrogen, C1-6 alkyl, chlorine or bromine, R<2> is C1-6 alkyl and R and R<1> each represent H, C1-6 alkyl, aryl, arylalkyl or C1-6 alkoxy, are prepared by reacting a compound of formula (II> <IMAGE> with a C1-6 alkyl orthoformate in an alcohol solvent in presence of a catalytic amount of an acid ion exchange resin.

Description

SPECIFICATION 3-Alkoxycycloalk-2-en-1 -ones This invention relates to a process for the preparation of 3-alkoxycycloalk-2-en-1 -ones.

3-Alkoxycyclohex-2-en-l -ones and 3-alkoxycyclopent-2-en-ones are valuable intermediates, for example, for the preparation of 3-substituted cyclohexenones or 3-substituted cyclopentenones, 4substituted cyclohexenones or 3-unsubstituted cyclohexenones. In particular, 3-alkoxycyclohex-2-en 1 -ones and 3-alkoxy-2-halocyclohex-2-en-1 -ones are useful as precursors to pharmacologically active meta substituted phenols as described, for example, in UK Patent Specification Nos. 1,593,888 and 1,593,889 and in our UK Patent application filed concurrently herewith under the title "m Hydroxyphenyl substituted compounds" (our ref: H-312/327) which claims priority from UK Patent Applications 8136364 and 8220721.The concurrent application describes a process for preparing meta substituted phenols by dehydrohalogenating 3-substituted-2-halocyclohex-2-en-l -ones in the presence of strong nucleophilic acid catalysts. The 3-substituted-2-halocyclohex-2-en-l -ones may be prepared from 3-alkoxy-2-halocyclohex-2-en-l -ones.

3-Alkoxycycloalk-2-en- l-ones have, in the past, been prepared from the corresponding 1,3diones with a wide variety of reagents, e.g. alchohol/acid catalyst, alcohol/acid/azeotropic removal of water, alkyl halide/alkoxide, diazomethane, trialkyl orthoformate/sulphuric acid or via silver salts or enone mesylates. All of these known methods suffer from one or more disadvantages such as expensive or highly toxic reagents, indifferent yields, 2-step sequences, lack of applicability to acid sensitive substrates or to methyl ethers or the necessity of an aqueous workup (many of the 3 alkoxycycloalk-2-en-1 -ones are hydrolytically unstable or water soluble).A novel method of preparing 3-alkoxycycloalk-2-en-l -ones has now been found which enables the products to be obtained in good yields and by a very convenient process.

Accordingly, the present invention provides a process for preparing a 3-alkoxycycloalkyl-2-en-1 one of the general formula (I)

where n is 1 or 2, X is hydrogen, lower alkyl, chlorine or bromine, R and R1 are the same or different and each represent hydrogen, lower alkyl, aryl, aryl(lower)alkyl or lower alkoxy and R2 represents lower alkyl, which process comprises reacting a compound of general formula (II)

with a lower alkyl orthoformate of general formula (R2O)3CH in an alcohol solvent, preferably of formula R2OH, in presence of a catalytic amount of an acid ion exchange resin.

Although compound (II) has been illustrated in the enol form it is to be understood that the compound may also exist in the dione form and the formula and the names in this specification are used to include both the enol form and dione form and mixtures of the two.

The term "lower" as used herein means that the radical referred to contains 1 to 6 carbon atoms.

The radical preferably contains 1 to 4 carbon atoms. For example, a lower alkyl radical is preferably methyl, ethyl, propyl, or butyl. Examples of lower alkoxy groups are methoxy, ethoxy, propoxy and butoxy. Examples of aryi(lower)alkyl are phenethyl and benzyl in which the phenyl group may be substituted by one or more substituents such as halogen, alkyl, alkoxy, trifluoromethyl or other substituents common in medicinal chemistry. Examples of aryl include heterocyclic or carbocyclic aromatic radicals, particularly phenyl groups which may be substituted by one or more substituents, such as those mentioned above.

Preferably n is 2. R2 is preferably methyl or ethyl. R and R' are preferably both hydrogen. X is preferably hydrogen, chlorine or bromine.

The acid ion exchange resin may be a strongly acid ion exchange resin such as a sulphonic acid ion exchange resin in H+ form. Examples of ion exchange resins include Amberlite IR120H, Amberlite IR100, Amberlite-1 5, Dowex 50, Zeocarb 222 and Nafion H. The words 'Amberlite', 'Dowex' and 'Zeocarb' are Registered Trade Marks.

The process of the invention is carried out in the alcohol solvent. Preferably the orthoformate is used in a stoichiometric amount/or in slight excess (for example 1.1 equivalents). The reaction in many instances takes place at ambient temperature although, if necessary or desired, the reaction may be carried out at higher temperatures, for example at reflux temperature. Isolation of the product from the reaction mixture is particularly convenient since the catalyst may be removed by filtration and the alcohol solvent may be evaporated. If required the recovered catalyst and solvent may be re-used.

The following Examples illustrate the invention: Example 1 3-Methoxycyclohex-2-en-l -one A mixture of cyclohexa-1,3-dione (224 g), trimethyl orthoformate (240 ml) and methanol (1 I.) was stirred at ambient temperature for 6 hours in the presence of Amberlite IR 1 20H ion exchange resin (10 g, prewashed with methanol). The ion exchange resin was removed by filtration and the solvent evaporated. The residue was distilled in vacuo to yield the title compound (223 g,92%) Bp.

9092 /3 mm.

Example 2 (a) 2-Chloro-1,3-cyclohexanedione Cyclohexa-1,3-dione (112 g, 1 M) in water (1 litre) was cooled in an ice bath and treated dropwise with a solution of chloramine T trihydrate (281 g 1 M) in water (2 litres). After 0.5 h. the resulting precipitate was filtered and washed with water. The combined filtrate and washings were acidified [conc. HCI (c. 80 ml)] and the precipitated product filtered. The filtrate was saturated with sodium chloride (about 1 kg) and refiltered. The second crop of product was washed with a small volume of water and both crops dried under vacuum. Total yield of product was 117 g, identical with authentic material (J. Prakt. Chem., 1963,20, 285-290).

(b) 2-Chloro-3-methoxycyclohexenone A mixture of 2-chlorocyclohexane-1 3-dione (50 g), trimethyl orthoformate (45 ml), methanol (250 ml) and Amberlite IR 1 20H (c. 1 g) was stirred for 3 hours at ambient temperature. The mixture was filtered and the volatile components removed in vacuum to give 55 g of product, which was recrystallised from ethyl acetate, m.p. 90-920C identical with authentic material (Bull. Soc. Chem.

Belges,1962 76, 330-334).

Example 3 (a) 2-Bromo-1 3-cyclohexanedione Cyclohexane-1 ,3-dione (11.2 g) in water (50 ml) and 48% HBr (15 ml) was treated dropwise with a solution of potassium bromate (5.7 g) in water (50 ml), the reaction being cooled in a water bath.

After 0.5 h. the mixture was filtered and the product washed well with water. Drying in vacuo gave the title compound (17 g), identical with authentic material (Izvest. Akad. Nauk SSR, OKN, 1959, 668- 72).

(b) ?rBromo-3-methoxycyclohexenone A mixture of 2-bromocyclohexane-1 ,3-dione (16.5 g), trimethyl orthoformate (10.5 ml), methanol (100 ml) and Amberlite IR 1 20H (0.5 g) was stirred for 4 h. at ambient temperature. The mixture was filtered and the volatile components were removed in vacuo to give the title compound (17 g) which was recrystallised from ethyl acetate, m.p. 85-860C, identical with authentic material (Izvest. Akad, Nauk SSR, OKN, 1959, 668-72).

Examples 4 to 10 Following the procedure of Examples 1,2b and 3b the following products were prepared:

Time of Temp. of Yield of B.p. (or m.p.) Example n R R R2 X reaction reaction product of product 4 2 H H Et H 12 ambient 95 120 /15mm 5 2 H H Et Cl 24 ambient1 94 (8284 ) 6 2 H H Et Br 24 ambient' 90 (98--1 000) 7 2 5-Me 5-Me Me H 24 reflux2 91 (579a) 8 2 5-Ph H Me H 6 ambient2 93 170 /4 mm 9 2 H H Me Me 24 reflux2 89 (44--6 0) 10 1 H H Me H 8 reflux2 75 (523 ) (1solvent=ethanol 2solvent=methanol)

Claims

1. A process for preparing a 3-alkoxycycloalk-2-en-1 -one of the general formula (I)

with a lower alkyl orthoformate of general formula (R20)3CH in an alcohol solvent in presence of a catalytic amount of an acid ion exchange resin.

2. A process as claimed in ciaim 1 in which n is 2.

3. A process as claimed in claim 1 or 2 in which R2 is methyl or ethyl.

4. A process as claimed in any one of the preceding claims in which R and R' are both hydrogen.

5. A process as claimed in any one of the preceding claims in which X is hydrogen, chlorine or bromine.

6. A process as claimed in any one of the preceding claims in which the alcohol solvent is of the formula R20H.

7. A process as claimed in any one of the preceding claims in which the acid ion exchange resin is a sulphonic acid ion exchange resin in H+ form.

8. A process for preparing 3-alkoxycycloalk-2-en-1 -ones substantially as hereinbefore described with reference to any one of Examples 1, 2(b), 3b and 4 to 10.

9. A 3-alkoxycycloalkyl-2-en-1 -one whenever prepared by the process claimed in any one of the preceding claims.