PROCESS FOR PREPARING 2- AND 4-ALKOXYCARBONYL THIOLAN-3-ONES

Abstract: The invention concerns a process for preparing 2- and 4-alkoxycarbonylthiolan-3-ones useful as intermediates, which process involves cyclising a 3-akloxycarbonylmethylthiopropanoic acid ester using lithium alkoxide. Novel lithium salts are also included.

Description: This invention relates to a novel process, more particularly to a novel process for the preparation of 2- and 4-alkoxycarbonylthiolan-3-ones which are useful chemical intermediates. Certain 2 and 4-alkoxycarbonylthiolan-3-ones of the type: wherein COOR represent an ester group such as methoxycarbonyl or ethoxycarbonyl are known or are analogous to known compounds described in the chemical literature. These compounds in general have utility as chemical intermediates. For example, 4-ethoxy- and 4-methoxy-carbonylthiolan-3-ones have been used as intermediates in drug syntheses and in the preparation of sweeteners - see for example R K Razdan et al, J. Med. Chem., 19 , 549 (1976);P A Rossy et al, J. Org. Chem., 45 , 617 (1980) and United States Patent No. 4143050 published 6 March 1979. Similarly 2-methoxycarbonylthiolan-3-one and other esters are described in US Patent No 4143050 as useful in the preparation of starting materials for the manufacture of drugs and sweeteners. The preparation of 4-methoxycarbonylthiolan-3-one via a Dieckmann cyclisation of 3-methoxycarbonylmethylthiopropanoic acid methyl ester (A) is described by P A Rossy et al, in J. Org. Chem., 45 , 617 (1980) at page 619 and involves the use of sodium methoxide in methanol to give the product in a yield from 50-55%. A similar reaction giving 4-methoxycarbonylthiolan-3-one in 67.5% yield is described by Hromatka et al in Monatsh, Chem., 104 , 1520-1525 1973. R B Woodward and R H Eastmann in J. Amer. Chem. Soc, 2229 (1946) also describe the Dieckmann cyclisation of the same starting material but using sodium methoxide in dry ether at room temperature to give 2-methoxycarbonylthiolan-3-one. However, at 80°-120°C using NaOMe in dry toluene Woodward and Eastmann found the main product was 4-methoxycarbonylthiolan-3-one prepared in about 49% yield (ca.30% isolated). F Duus, in Tetrahedron Vol 37, No 15 pp 2633 to 2640 (1981) describes the preparation of 4-ethoxycarbonylthiolan-3-one in 35% yield via a Dieckmann condensation of 3-ethoxycarbonylmethylthiopropanoic acid ethyl ester using a suspension of sodium ethanolate in boiling toluene. When ethanol was used as solvent and the reaction temperature was lowered to 0°C the product obtained was 2-ethoxycarbonylthiolan-3-one in 23-43% yield. It has now been found that by using lithium alkoxides in the Dieckmann cyclisation of 3-alkoxycarbonylmethylthiopropanoic acid esters then excellent yields of either the 2- or 4-alkoxycarbonylthiolan-3-ones or mixtures thereof can be obtained by choice of appropriate reaction times and temperatures. The use of lithium alkoxide gives advantages over sodium alkoxide which include higher yields and purer products. In addition the reaction may be carried out substantially free from unpleasant odours which occur when sodium alkoxide is used. Further if desired the 4-alkoxycarbonylthiolan-3-ones can be conveniently prepared substantially free from contamination by the corresponding 2-alkoxycarbonyl isomer and vice versa. Accordingly in one aspect this invention provides a process for preparing a compound of formula Ia or Ib or a lithium salt or mixtures thereof, which comprises cyclising a compound of formula wherein COOR represents the same or different ester functions, in the presence of a lithium alkoxide, eg.alkoxides of 1 to 6 carbon atoms such as lithium methoxide and if desired acidifying to give the compound of formula Ia or Ib or mixtures thereof. In the aforementioned process the reaction may be effected quickly in a time which is sufficiently short to produce the lithium salt of the compound of formula Ib substantially in excess of or preferably substantially free from, the corresponding lithium salt of the compound of formula Ia. Low reaction temperatures eg. less than about 50°C, such as room temperature or below, also assist in obtaining the lithium salt of the compound of formula Ib by slowing its conversion to formula Ia. However where it is desired to obtain the lithium salt of the compound of formula Ia substantially in excess of formula Ib then higher reaction temperatures, eg. above about 50°C or more, and/or longer reaction times eg. about 3 hours or more, depending on the reaction temperature, may be used. Mixtures may be obtained at intermediate times and temperatures from which individual 2- or 4- esters of formula Ia and Ib may be isolated by standard techniques. For the preparation of mixtures, the process of this invention employing lithium alkoxide is superior to use of sodium alkoxide in view of the high yields of purer combined products and the absence of obnoxious odours. In a preferred aspect this invention provides a process for preparing a compound of formula Ia or a lithium salt thereof, which comprises cyclising a compound of formula II as defined above in the presence of a lithium alkoxide, eg. alkoxides of 1 to 6 carbon atoms such as lithium methoxide and for a time sufficient to produce the product substantially free from the 2-isomer and if desired acidifying to give the compound of formula Ia. In the formulae Ia, Ib and II COOR may represent any ester function that is stable under the reaction conditions. Preferred values for R are alkyl eg. alkyls of 1 to 6 carbons (such as methyl, ethyl, propyl and butyl) and aralkyl such as aralkyl of 7 to 12 carbon atoms, eg. benzyl and phenethyl. Preferably both COOR groups are the same eg. methyl or ethyl. Most preferably the lithium alkoxide has the formula LiOR where R is the same group as R in the compound of formula II. The use of lithium alkoxides is also particularly advantageous since extended reaction times may be employed without deterioration of reaction product of formula Ia. This is especially important if it is desired to carry out the reaction on a large scale or in the plant where transfer and handling of bulk materials makes it difficult to achieve short reaction times. The reaction is conveniently carried out in a solvent comprising an alcohol, eg. ROH such as ethanol. A hydrocarbon and/or linear or cyclic ether may also be present, eg. toluene, tetrahydrofuran or dioxane. Sodium alkoxides under hot conditions degrade 2- and 4-akloxycarbonylthiolan-3-ones with the result that short reaction times are necessary if an optimum yield is desired. This invention also provides the lithium salts of compounds of formulae Ia and Ib as defined hereinabove. A preferred compound of the invention is the lithium salt of 4-methoxycarbonylthiolan-3-one. Acidification of the lithium salts provide compounds of formulae Ia and Ib. Accordingly this invention provides a process for preparing a compound of formula Ia or Ib which comprises acidifying the corresponding lithium salt. The following route is described in J. Org. Chem. 45 , 617 (1980) for preparing a sweetening agent, thiophene saccharin(B), sodium salt, using 4-methoxycarbonylthiolan-3-one: Similarly other esters of formula Ia may be used. Accordingly in a further aspect this invention provides thiophenesaccharin when prepared from a compound of formula Ia made according to a process of this invention. The following non-limiting Examples illustrate the invention: EXAMPLE 1 4-Methoxycarbonylthiolan-3-one a) Lithium (8.46g, 1.245M) in toluene (11) was treated with methanol (315ml). After all the lithium had dissolved, 3-methoxycarbonylmethylthiopropionic acid methyl ester (216g 1.12M) was added over 0.5 hour with internal temperature about 70°C. After the addition the temperature of the mixture was raised and methanol was distilled off to a final temperature of 110°C (18 hours). The mixture was cooled to room temperature and the resulting lithium salt of 4-methoxycarbonylthiolan-3-one filtered and washed with toluene. b) The product was suspended in water/dichloromethane and acidified with acetic acid (90g) to convert from lithium salt to free keto ester. The organic phase was dried and evaporated to yield the title compound (139g, 83%) as a pale yellow solid, m.p 40-41°C. EXAMPLE 2 4-Methoxycarbonylthiolan-3-one Lithium metal (6.94g, 1M) was dissolved in methanol (400ml) then treated dropwise with 3-methoxycarbonylmethylthiopropanoic acid methyl ester (192g, 1M) over ½ hour. The mixture was then refluxed for 6 hours. The methanol was removed under reduced pressure and cold water (500 ml) added. The resulting lithium salt was removed by filtration. The crude lithium salt was slurried in water (1l) and treated with acetic acid (60 ml, 1m) and the keto ester extracted with di-isopropyl ether (2 x 500 ml). The organic phase was dried, evaporated and the residue recrystallised from hexane to give the title compound. (126g 79%) mp 40-1°C. EXAMPLE 3 2-Methoxycarbonylthiolan-3-one 3-Methoxycarbonylthiopropanoic acid methyl ester (19.2g, 0.1M) was added dropwise to a solution of lithium methoxide (5.7g, 0.15M) in methanol (150ml) at 25°C. After 2 hours tlc examination indicated that reaction was complete, giving a product more polar than that of Examples 1 and 2. (None of the product of Examples 1 and 2 was present). Acetic acid (9ml, 0.15M) in methanol (20ml) was added and the mixture poured onto water (500 ml). The product was extracted into di-isopropyl ether (2x250ml), and the organic phase washed with water and evaporated. Distillation gave the title compound (13.8g 86%) Bp. 90-5°C/lmbar. EXAMPLE 4 4-Methoxycarbonylthiolan-3-one 3-Methoxycarbonylthiopropanoic acid methyl ester (192g) was added to a solution of lithium methoxide (38g) in methanol (800 ml). The mixture was refluxed for 6 hours, cooled to 20°, treated with acetic acid (60 ml) and then poured on to water (21). The product was extracted with di-isopropyl ether and the organic extracts dried and evaporated. Recrystallisation of the rendue from hexane gave the title compound (130g, 81%) mp 40-1°C. EXAMPLE 5 4-Ethoxycarbonylthiolan-3-one Following the procedure of Example 4 but using as reactants 220g 4-ethoxycarbonylthiopropanoic acid ethyl ester, 52g lithium ethoxide in 1l ethanol, the title compound was obtained, (yield 139g) Bp 120-2°/10mbar. EXAMPLE 6 2-Ethoxycarbonylthiolan-3-one Following the procedure of Example 3 but using as reactants 22 g of 3-ethoxycarbonylmethylthiopropanoic acid ethyl ester and 7.8g lithium ethoxide in ethanol (250 ml) the title compound was obtained, 14.3g, Bp 96-8°/2mbar. EXAMPLE 7 4-Methoxycarbonylthiolan-3-one A solution of 2-methoxycarbonylthiolan-3-one (16g, 0.1m) in methanol (50 ml) was added to lithium methoxide (3.8g, 0.1M) in methanol (50 ml) then the mixture was refluxed for 5 hours. The mixture was cooled and treated successively with acetic acid (6 ml) and water (250ml). The product was extracted with di-isopropyl ether (2x250ml) and the organic phase dried and evaporated. Recrystallisation of the residue from hexane gave the isomeric title compound (14.5g) Mp 40-1°C. EXAMPLE 8 4-n-Butoxycarbonylthiolan-3-one Following the procedure of Example 3 but using as reactants 200g of 3-n-butoxycarbonylthiopropanoic acid n-butyl ester and 50g of lithium n-butoxide in 1 litre of n-butanol the title compound was obtained after 5 hours heating at 80°C.

Claims: Claims for the following Contracting States : AT, BE, CH, DE, FR, GR, IT, LI, LU, NL, SE A process for the preparation of a compound of formula Ia or Ib or a mixture thereof or lithium salts thereof which comprises cyclising a compound of formula wherein COOR are the same or different ester functions, in the presence of a lithium alkoxide and if desired acidifying the lithium salt or salts obtained to give the compound of formula Ia or Ib or mixtures thereof. A process according to claim 1 in which the cyclisation reaction is carried out for a time sufficient to produce the lithium salt of the compound of formula Ia substantially in excess of the lithium salt of the compound of formula Ib, and if desired acidifying to give the corresponding compound of formula Ia substantially in excess of the compound of formula Ib. A process according to Claim 2 in which the cyclisation reaction is carried for a time sufficient to produce the lithium salt of the compound of formula Ia substantially free from the lithium salt of the compound of formula Ib. A process according to Claim 2 or Claim 3 in which the cyclisation reaction is carried out at a temperature greater than about 50°C. A process according to any one of Claims 1 to 4 in which isolation of the compound of formula Ia or the lithium salt thereof is effected about 3 hours or more after initiation of the process. A process according to Claim 1 in which the cyclisation reaction is carried out for a time sufficiently short to produce the lithium salt of the compound of formula Ib substantially in excess of the lithium salt of the compound of formula Ia, and if desired acidifying to give the corresponding compound of formula Ib substantially in excess of the compound of formula Ia. A process according to Claim 6 in which the cyclisation reaction is carried out for a time sufficiently short to produce the lithium salt of the compound of formula Ib substantially free from the lithium salt of the compound of formula Ia. A process according to any one of Claims 1 to 7 in which the COOR groups represent the same or different alkyl or aralkyl ester functions. A process according to Claim 8 in which the lithium alkoxide has the formula LiOR where R is an alkyl group the same as in the starting material of formula II. A process according to any one of Claims 1 to 9 which is carried out in a solvent comprising an alcohol and optionally a hydrocarbon or a linear or cyclic ether. The lithium salt of a compound of formula Ia or Ib wherein COOR is as defined in Claim 1. A process for preparing a compound of formula Ia or Ib as defined in Claim 1 which comprises acidifying a compound as claimed in Claim 11. A compound as claimed in Claim 11 which is the lithium salt of 4-methoxycarbonyl- thiolan-3-one, or the lithium salt of 2-methoxycarbonyl- thiolan-2-one. Claims for the following Contracting State : ES A process for the preparation of a compound of formula Ia or Ib or a mixture thereof or lithium salts thereof which comprises cyclising a compound of formula wherein COOR are the same or different ester functions, in the presence of a lithium alkoxide and if desired acidifying the lithium salt or salts obtained to give the compound of formula Ia or Ib or mixtures thereof. A process according to claim 1 in which the cyclisation reaction is carried out for a time sufficient to produce the lithium salt of the compound of formula Ia substantially in excess of the lithium salt of the compound of formula Ib, and if desired acidifying to give the corresponding compound of formula Ia substantially in excess of the compound of formula Ib. A process according to Claim 2 in which the cyclisation reaction is carried for a time sufficient to produce the lithium salt of the compound of formula Ia substantially free from the lithium salt of the compound of formula Ib. A process according to Claim 2 or Claim 3 in which the cyclisation reaction is carried out at a temperature greater than about 50°C. A process according to any one of Claims 1 to 4 in which isolation of the compound of formula Ia or the lithium salt thereof is effected about 3 hours or more after initiation of the process. A process according to Claim 1 in which the cyclisation reaction is carried out for a time sufficiently short to produce the lithium salt of the compound of formula Ib substantially in excess of the lithium salt of the compound of formula Ia, and if desired acidifying to give the corresponding compound of formula Ib substantially in excess of the compound of formula Ia. A process according to Claim 6 in which the cyclisation reaction is carried out for a time sufficiently short to produce the lithium salt of the compound of formula Ib substantially free from the lithium salt of the compound of formula Ia. A process according to any one of Claims 1 to 7 in which the COOR groups represent the same or different alkyl or aralkyl ester functions. A process according to Claim 8 in which the lithium alkoxide has the formula LiOR where R is an alkyl group the same as in the starting material of formula II. A process according to any one of Claims 1 to 9 which is carried out in a solvent comprising an alcohol and optionally a hydrocarbon or a linear or cyclic ether.

Inventors: SHEPHERD, ROBIN GERALD
Applicants: JOHN WYETH & BROTHER LIMITED
Application year: 1989
Main classification: C07D333
IPC codes: C07D333, C07C323