BACKGROUND OF THE INVENTION 0001 The present invention relates to an improved and at the same time more economical process for the synthesis of 4,5-dihydro-1,3-thiazoles. 0002 4,5-Dihydro-1,3-thiazoles are materials that have been known for a long time and are used, inter alia, as key intermediates for the synthesis of dihydrothiazole- and thiazole-based active compounds in the agrochemical and pharmaceutical industries. 0003 An efficient synthetic route giving very good selectivities and yields is required for the preparation of 4,5-dihydro-1,3-thiazoles. The starting materials needed for this purpose must be available on an industrial scale. 0004 The class of 4,5-dihydro-1,3-thiazoles (I) is known and their synthesis is described, for example, in DE-A 1,964,276 and U.S. Pat. No. 3,678,064. In the synthetic route described, 1-amino-2-alkanethiols (II) are reacted with 2,2-dialkoxyalkanenitriles (III) to give the ketals (IV), which are converted by hydrolysis into the desired 4,5-dihydro-1,3-thiazoles (I). The preparation of 2,2-dialkoxyalkanenitriles (III) by the method of DE-A 1,964,276 requires an unacceptable reaction time of 40 days. An improved process described in Synthesis, 1983, 498-500, gives a yield of 83% by weight. The reaction time here is from 3 to 12 hours. 1

0005 TMSCNtrimethylsilyl cyanide 2

0006 In the formulas of Equations 1 and 2, R1, R2, R3, and R4 are each, independently of one another, hydrogen or an organic radical having from 1 to 10 carbon atoms. 0007 The first step in Equation 2 requires 1.57 equivalents of 1-amino-2-alkanethiol (II), which is very expensive. The disadvantage of the synthetic route described is the complicated work-up steps with isolation of the intermediates. With a view to industrial implementation, the hydrolysis using an excess of concentrated sulfuric acid (i.e., 15.5 equivalents) is particularly critical, since these large amounts of acid subsequently must be neutralized, which is highly exothermic. In addition, the neutralization forms a large quantity of salts, which is undesirable from an ecological point of view. Each step of the process described is followed by an aqueous work-up with subsequent purification. The aqueous work-up is always associated with formation of a considerable quantity of salts, which is likewise disadvantageous in an industrial process. 0008 It was therefore an object of the invention to improve the process so that it can be implemented industrially while taking into account ecological aspects and so that the disadvantages of the earlier process are overcome. This object has been able to be achieved according to the invention. 0009 It has surprisingly been found that the entire synthesis sequence can be carried out as a single-vessel synthesis without complicated work-up steps.

SUMMARY OF THE INVENTION 0010 The invention accordingly provides a process for preparing 4,5-dihydro-1,3-thiazoles of formula (I) 3

0011 where R1, R2, and R3 are each, independently of one another, 0012 hydrogen or an organic radical having from 1 to 10 carbon atoms, comprising 0013 (1) reacting a trialkoxyalkane of the formula 4

0014 where R3 and R4 are each, independently of one another, hydrogen or an organic radical having from 1 to 10 carbon atoms, 0015 with CN to form a 2,2-dialkoxyalkanenitrile of the formula 5

0016 where R3 and R4 are defined as above, 0017 (2) reacting the 2,2-dialkoxyalkanenitrile with an aminoalkanethiol of the formula 6

0018 where R1 and R2 are defined as above, to form a ketal of the formula 7

0019 where R1, R2, R3, and R4 are defined as above, and 0020 (3) hydrolyzing the ketal with an acid to form the 4,5-dihydro-1,3-thiazole of formula (I), 0021 wherein the entire reaction sequence is carried out in a single vessel without isolation of intermediates. 0022 The process of the invention can be summarized by the following reaction sequence: 8

DETAILED DESCRIPTION OF THE INVENTION 0023 Examples of functional groups by which the organic radicals R1, R2, and R3 can be substituted are alcohols and halogens. R1 and R2 are preferably hydrogen or alkyl groups having from 1 to 10 carbon atoms and are particularly preferably each hydrogen. R3 is preferably an alkyl group having from 1 to 10 carbon atoms and is particularly preferably ethyl. R4 is preferably an alkyl group having from 1 to 10 carbon atoms and is particularly preferably methyl, ethyl, or propyl. 0024 The overall yield in, for example, the synthesis of 2-propionyl-4,5-dihydro-1,3-thiazole (formula (I) in which R3 is C2H5) is 40%. In addition, the amount of 1-amino-2-alkanethiol (II) was able to be reduced from 1.57 equivalent to 1.1 equivalent. 1TABLEComparison of the yieldsAmountAmountOverallof (II)of (III)yieldProcessmolmolR1R2R3R4%DE-A1.571.0HHC2H5C2H5161,964,276Example 31.101.0HHC2H5CH340(according tothe invention)

0025 A first advantage of the invention is the significantly better technical manageability, since a number of work-up and purification steps can be saved due to the single-vessel synthesis. Secondly, the amount of acid required was able to be reduced from 15 equivalents to 5 equivalents, which is a great advantage, particularly with a view to an industrial synthesis. The amount of salts formed in the neutralization is greatly reduced as a result. 0026 In the process of the invention for preparing 4,5-dihydro-1,3-thiazoles of the formula (I), preference is given to heating equimolar amounts of trialkoxyalkane and cyanide (preferably from trimethylsilyl cyanide) with addition of catalytic amounts of a Lewis acid (preferably ZnCl2) in a temperature range from 40 to 100 C. (preferably in a temperature range from 55 to 70 C.) for from 3 to 20 hours (preferably for a time of from 12 to 18 hours). After cooling, from 1.0 to 1.5 equivalents (preferably from 1.0 to 1.2 equivalents) of 1-amino-2-alkanethiol (II) in an organic solvent are added. As organic solvent, preference is given to using polar solvents, e.g., alcohols. The reaction mixture is then heated to from 40 to 100 C.; the reaction temperature is preferably from 60 to 80 C. The reaction time is from 3 to 20 hours, preferably from 12 to 18 hours. The solvent is preferably distilled off under reduced pressure. From 5 to 30 equivalents (preferably 5 to 15 equivalents, particularly preferably 5 to 7 equivalents) of an acid (preferably concentrated sulfuric acid) are added dropwise to the remaining reaction mixture at a temperature of from 10 C. to 10 C. (preferably from 0 C. to 5 C.). After stirring at the above-mentioned temperature for from 1 to 5 hours (preferably from 1 to 3 hours), the reaction mixture is neutralized by means of an aqueous base (preferably NaHCO3). After extraction of the 4,5-dihydro-1,3-thiazole (I) into an organic phase, preferably using dichloromethane or an organic ether (e.g., diethyl ether), as solvent, the desired compounds are isolated in a yield of about 40%. 0027 The following examples further illustrate details for the process of this invention. The invention, which is set forth in the foregoing disclosure, is not to be limited either in spirit or scope by these examples. Those skilled in the art will readily understand that known variations of the conditions of the following procedures can be used. Unless otherwise noted, all temperatures are degrees Celsius and all percentages are percentages by weight.

EXAMPLES

Example 1

2-(1,1-Dimethoxypropyl)-4,5-dihydro-1,3-thiazole (IV)Individual Synthesis Steps 0028 Under argon, 20.04 g of anhydrous ammonium acetate (260 mmol), 6.79 g (88 mmol) of cysteamine, and 10.33 g (80 mmol) of 2,2-dimethoxy-butyronitrile were dissolved in 80 ml of absolute methanol and refluxed for 16 h. After distilling off the solvent under reduced pressure, the reaction solution was added a little at a time to a mixture of 18.4 g of KOH, 164 ml of ice water, and 40 ml of diethyl ether. The phases were separated and the aqueous phase was extracted with diethyl ether (510 ml). After drying the combined organic phases over NaSO4 and KOH pellets, the solution was evaporated and could be converted directly into 2-propionyl-4,5-dihydro-1,3-thiazole. 0029 Crude yield: 13.89 g (73.4 mmol, 91.7%). 0030 1H-NMR (400 MHz; CDCl3): 0.85 (t, 3H, CH3); 1.94 (q, 2H, CH2); 2.27 (t, 8H, CH2S and OCH3); 4.38 (t, 2H, CH2N)

Example 2

2-Propionyl-4,5-dihydro-1,3-thiazole (I)Individual Synthesis Steps 0031 10.13 g (53.5 mmol) of 2-(1,1-dimethoxypropyl)thiazoline were added to 43 ml of sulfuric acid (96%) at 0 to 5 C. After stirring at this temperature for 20 min, the solution was added a little at a time to a mixture of 187 mg of NaHCO3, 965 mg of ice, and 64 ml of diethyl ether. After phase separation, extraction of the aqueous phase with CH2Cl2, and drying of the combined organic phases over Na2SO4, the solvent was removed under reduced pressure. 0032 The residue was distilled using a Vigreux column to give 3.099 g (21.6 mmol, 40% yield) of product having a purity of 98% according to gas chromatography (GC). 0033 1H-NMR (400 MHz; CDCl3): 1.14 (t, 3H, CH3); 2.95 (q, 2H, CH2); 3.33 (t, 2H, CH2S); 4.52 (t, 2H, CH2N).

Example 3

Single-vessel Synthesis of 2-propionyl-4,5-dihydro-1,3-thiazole (I)According to the Invention 0034 536 mg of 1,1,1-trimethoxypropane (4 mmol), 0.53 ml of trimethylsilyl cyanide (4 mmol), and 1 mg of ZnCl2 were heated at 60 C. under argon for 16 h. 339 mg of cysteamine (4.4 mmol), 154.2 mg of ammonium acetate (2.0 mmol), and 4 ml of methanol were added and the mixture was refluxed for a further 17 h. After removing the solvent under reduced pressure, 2.043 g of sulfuric acid (96%) were added dropwise at 0 to 5 C. After stirring at this temperature for 2 h, the reaction solution was added a little at a time to a mixture of 4.7 g of NaHCO3 (56 mmol), 75 ml of ice water, and 5 ml of diethyl ether. The aqueous phase was extracted with CH2Cl2, after which the combined organic phases were dried over NaSO4 and evaporated under reduced pressure. 0035 Yield (crude product): 229 mg (40%); purity according to GC: 84%.