6,7,8,9,10,10-hexahalo-1,5,5a, 6,9,9a-hexahydro-6,9-methano- 2, 4, 3-benzodioxathiepin-3-oxide are of the formula I: Formula I wherein X may be a halogen such as fluorine, chlorine or bromine.

PRIOR ART t US Patent No 2799685 describes unsaturated polycyclic sulfites of the general formula II: Formula 11 wherein X may be hydrogen or halogen or aLkyl group and Y may be hydrogen or alkyl group, and their derivatives containing two hydrogen atoms in 5,6-positions. These compounds are reported to exhibit insecticidal activity. The process for the preparation of these compounds comprises heating a diol of the general formula III :

Formula III wherein X and Y are each as defined above, with thionyl chloride under heating optionally in an inert organic solvent. The product is reported to be a mixture of two isomers. The desired isomer may be resolved from the mixture, for instance by fractional crystallisation using petroleum ether.

US Patent No 3251856 describes a process for the resolution/separation of the two isomers viz a and p-isomers of 6,7,8,9,10,10-hexachloro-1, 5, 5a,6,9,9a-hexahydro-6,9-methano-2,4,3- benzodioxathiepin-3-oxide, commonly known as endosulfan of the formula IA: Formula IA The resolution process comprises contacting the endosulfan isomeric mixture with a halogenated hydrocarbon solvent at 40-50°C followed by cooling the mixture to 20-25°C. The resulting insoluble portion in the mixture is reported to predominantly contain the high melting isomer viz P-

isomer of endosulfan whereas the solution portion when subjected to evaporation results in a residue which is reported to predominantly contain the low melting isomer viz a-isomer of endosulfan. The a and (3-isomers so obtained may be purified by crystallisation from alcohol.

Both the isomers of endosulfan are reported to exhibit different insecticidal properties. For instance, in short exposure periods, a-endosulfan is more effective against flies than P-endosulfan. In the case of fruit flies Drosophila melanogaster, the killing times (LT50) with the use of a- endosulfan and p-endosmlfan are in the ratio 1 : 3 respectively. Insecticidal effectiveness or efficacy of a-endosulfan against wood boring insects is more as compared to P-endosulfan. Insecticidal effectiveness limit per m3 wood with the larvae of the house longhorn beetle Hylotupes bqulus L is more in the case of ß-endosulfan as compared to a-endosulfan (Mater. Org. 1983, 18 (2), 81-91, Kuehne Helmut et aL-CA 100: 134245a). Both the isomers differ in insecticidal persistence and biodegradability. α-Endosulfan decomposes rapidly by soil microorganisms when compared to p-endosulfan (Karachi Univ J. Sci; 1985, 13 (2), 191-7, Akhtar Shahida et al; CA 107 : 91836u). α-Endosulfan is degradable by both bacteria and fungi, whereas ß- endosulfan is degradable mainly by bacteria (Int. J Environ. Stud, 1981,16 (3-4), 171-80, El Beit, I 0 D et al CA 94: 151865c). ß-endosulfan has greater half life than oerendosulfan under varying environmental conditions (J.

Environ. Sci. Health, Part B, 1995, B30 (2), 221-32, Ceron JJ et al CA 122 : 180980p). Therefore, treatment with ß-endosulfan is particularly preferred to achieve desired efficacy over a long term.

The a and P-isomers of endosulfan are reported to be obtained in the average isomeric ratio of about 2: I (US Patent No 3251856). Due to the differences in the physical, chemical and biological properties of a and ß- endosulfan and a and p-benzodioxathiepin compounds in general, it is advantageous to have increased quantity of the desired stereoisomer in the isomeric mixture depending upon the intended specific application of the benzodioxathiepin compound. In order to obtain required quantity of a desired isomer of the benzodioxath. epiri compound, correspondingly large quantities of the substrate viz the diol compound is required. Therefore, the above process is uneconomical. Besides, it also generates the undesired isomer in the ratio 2: 1 and its efficiency is low.

OBJECTS OF THE INVENTION : An object of the invention is to provide a process for the stereoselective preparation of 6,7,8,9,10,10-hexahalo-1,5,5a, 6,9,9a- hexahydro-6, 9-methano-2, 4,3-benzodioxathiepin-3-oxide having insecticidal activity, which results in increased quantity of the desired stereoisomer in high purity without using additional quantity of starting material.

Another object of the invention is to provide a process for the stereoselective preparation of 6,7,8,9,10,10-hexahalo-1,5,5a, 6, 9, 9a-hexahydro- 6,9-methano-2,4,3-benzodioxathiepin-3-oxide having insecticidal activity, which is economical.

Another object of the invention is to provide a process for the stereoselective preparation of 6,7,8,9,10,10-hexahalo-l, 5, Sa, 6,9,9a-

hexahydro-6, 9-methano-2, 4, 3-benzodioxathiepin-3-oxide having insecticidal activity, which is efficient.

Another object of the invention is to provide a process for the stereoselective preparation of 6,7,8,9,10,10-hexahalo-1, 5,5a, 6,9,9a-hexahydro- 6,9-methano-2,4,3-benzodioxathiepin-3-oxide having insecticidal activity, without using any foreign reagent which may introduce impurity to the product.

DESCRIPTION OF INVENTION : According to the invention there is provided a process for the stereoselective preparation of insecticide 6,7,8,9,10,10-hexahalo- 1, 5, 5a, 6, 9,9a- hexahydro-6, 9-methano-2,4,3-benzodioxathiepin-3-oxide of the general formula 1 : Formula 1 wherein X is a halogen such as fluorine, chlorine or bromine, comprising reacting l, 4,5, 6,7,7-hexahalo-5-norbomene-2, 3-dimethanol of the general formula IV: Formula IV

wherein X is as defined above, with a cyclic sulfite ring forming reagent optionally in an inert organic solvent at ambient to 139°C, wherein the reaction is carried out in the presence of a stereoisomer directing agent comprising an isomer of the compound of the formula I different from the desired isomer, the molar ratio of the stereoisomer directing agent to the dimethanol compound of the formula IV being at least 0.07.

Preferably the process comprises preparation of a-isomer of the benzodioxat. iepin-3-oxide compound in which its R-isomer is used as the stereoisomer directing agent, in the molar ratio of 0. 2 to 0.8 to the dimethanol compound of the formula IV. Preferably the process comprises preparation of 0-isomer of the benzodioxathiepin-3-oxide compound in which its a- isomer is used as the stereoisomer directing agent, in the molar ratio of 1.7 to 4.3 to the dimethanol compound.

V' Preferably the process comprises preparation of a-isomer of 6, 7, 8,9,10, 10-hexachloro-1, 5,5a, 6,9,9a-hexahydro-6, 9-methano-2, 4,3- benzodioxathiepin-3-oJnde ie endosulfan (compound of the formula I wherein X is chlorine) in which its 0-isomer is used as the stereoisomer directing agent, the molar ratio of the p-isomer of endosulfan to 1, 4,5,6,7,7- hexachloro-5-norbomene-2, 3-dimethanol ie endosulfandiol (compound of the formula IV wherein X is chlorine) being 0.2 to 0.8. Preferably the process comprises preparation of ß-isomer of 6,7,8,9,10,10-hexachloro-1,5,5a, 6,9,9a- hexahydro-6, 9-methano-2,4,3-benzodioxathiepin-3-oxide ie endosulfan (compound of the formula I wherein X is chlorine) in which its a-isomer is used as the stereoisomer directing agent, molar ratio of the a-isomer of

endosulfan to 1, 4, 5, 6,7,7-hexachloro-5-norbornene-2,3-dimethanol ie endosulfandiol (compound of the formula IV wherein X is chlorine) being 1.7 to 4. 3.

The cyclic sulfite ring forming reagent is preferably thionyl chloride.

The inert organic solvent may be aromatic hydrocarbon such as toluene or isomer of xylene such as o-xylene, m-xylene p-xylene or mixture thereof, aliphatic hydrocarbon such as n-hexane, octane, chlorinated aliphatic or aromatic hydrocarbon such as 1,2-dichloroethane, 1,1,2-trichloroethane, carbontetrachloride or trichloromethane or cyclo aliphatic hydrocarbon such as cyclohexane or any such solvent of petroleum origin. Preferably, toluene, o-xylene or carbontetrachloride is used as the solvent. When an inert organic solvent is not used, the reaction may be carried out in the presence of excess of cyclic sulfite ring forming reagent.

The stereoisomer directing agent is separated from the reaction mass in known manner, for instance by the method described in US Patent No 3251856.

According to the invention practically only one of the desired isomers viz a or P isomer is selectively prepared in high purity by using the stereoisomer directing agent, depending upon the specific intended use thereof. For instance, to give treatments lasting a longer duration, compound of the formula I wherein X is chlorine ie endosulfan, enriched with the 0-

isomer is prepared ; whereas for treating wood a-endosulfan enriched product is prepared. Thus the process of the invention results in a product enriched with the desired isomer. Besides, for a given amount of the starting material the process produces the compound of the formula 1 enriched with the desired isomer practically devoid of the undesired isomer without consuming additional quantities of the starting material. Therefore, the process is economical and efficient. Since no foreign reagent is used in the reaction and the process uses the α or ß isomer of the compound of the formula I itself as the stereoisomer directing agent for the preparation of ß or α isomer respectively, formation of impurities is avoided. The stereoisomer directing agent in the product mixture is recovered and reused.

The following experimental examples are illustrative of the invention but not limitative of the scope thereof.

Selectivity of the processes for a-endosulfanJß-endosulfan was calculated by the equation : Selectivity = a-endosulfan/, ß-endosulfan obtained (g) X 100 endosulfan product (g) The weight of endosulfan product was computed after subtracting the weight of SIDC-A/SIDC-B added in the reaction mixture from the weight of the product mixture.

Example 1 Endosulfandiol (250 g, 0.693 mole), carbontetrachloride (750 ml) and ß-endosulfan [stereoisomer directing agent (SIDC-A), 20 g, 0. 049 mole, mole ratio of SIDC-A : endosulfandiol :: 0. 071] were heated to 65°C with stirring. Thionyl chloride (55 ml, 0. 7556 mole) was added to the reaction mixture and the reaction mixture was heated to 75°C for 2 hours. HCl generated during the reaction was scrubbed with alkali solution. The reaction mass was cooled to 30'C. Excess thionyl chloride was neutralised with dilute aqueous 6.5% weight/weight soda ash solution to pH 6.5. The aqueous layer was discarded. From the organic layer, carbontetrachloride was distilled off under reduced pressure. The product mixture weighing 290 g was analysed for a-endosulan content by Gas Chromatogrphy (GC) and was found to contain 202.7 g of a-endosulfan. SIDC-A in the product mixture was separated from the a-endosulfan as per the procedure of US Patent No 3251856 and reused. The selectivity of the process for a-endosulfan was found to be 75.07 %.

Examples 2 to 12 The procedure of Example I was followed using different quantities of SIDC-A and the results were as in the following Table 1 :

Table 1 <BR> <BR> <BR> <BR> <BR> <BR> <BR> Eg Endosul SIDC-A SIDC-A Endosulfan a-Fndosulfan Selectivity<BR> <BR> <BR> No fandiol (mol) Endosulf procuct (g) for (x-<BR> <BR> <BR> <BR> (mo1) andiol mole (g) endosulfan ratio 2 0. 693 0 0 273 190.6 69.81 3 0.693 0.098 0.141 275 220. 8 80.3 4 0.693 0.147 0. 212 271 235. 7 86.97 5 0. 693 0.197 0.284 271 245. 7 90.66 6 0.693 0. 246 0.355 271 264.2 97. 49 7 0.693 0.295 0. 426 277 277.1 100.0 8 0.693 0.344 0. 496 269 266.7 99.14 9 0.693 0.399 0.576 272 267. 8 98. 46 10 0.693 0.442 0.638 272 266.7 98. 05 11 0.693 0.491 0.709 273 264.4 96.85 12 0.693 0. 540 0.779 274 271. 2 98.98 Examples 13 to 24 The procedure of Examples 1 to 12 were followed using o- xylene as the solvent and the results were as in the following Table 2:

Table 2 <BR> <BR> <BR> <BR> <BR> <BR> Eg Endosul SIDC-A SmC-A : Endosulfan a-Endosulfan Selectivity<BR> <BR> <BR> No fandiol (mol) Endosulf product (g) for a-endosulfan (mol) andiol mole (9) 0/0) ratio 13 0. 693 0 0 273 179.00 65. 57 14 0.693 O. 049 0.071 273 191.33 70.08 15 0.693 0.098 0.141 273 204. 86 75. 04 16 0.693 0.147 0. 212 273 217.45 79.65 17 0.693 0.197 0.284 273 230.69 84.50 18 0.693 0.246 0.355 273 243.94 89.36 19 0.693 0. 295 0.426 273 256. 0 93.77 20 0.693 0.344 0.496 273 270.02 98.91 21 0.693 0.399 0.576 273 269.76 98.81 22 0.693 0.442 0.638 272 269.17 98.96 23 0.693 0.491 0.709 273 270.79 99.19 24 0.693 0. 540 0.779 276 271.41 98.34 Examples 25 to 36 The procedure of Examples 1 to 12 were followed using toluene as the solvent and the results were as in the following Table 3: Table 3

. Eg Endosulf SIDC-A C-A : Endosulfan a-Endosulfan 9electivity<BR> <BR> No diol (mol) Fndosulf product (@ for a-endo<BR> <BR> (mol) andiol mole (g) sulfan ratio 25 0.693 0 0 273 175. 8 64. 4 26 0.693 0.049 0.071 273 188. 4 69.0 27 0.693 0.098 0. 141 273 201.9 74.0 28 0.693 0.147 0.212 273 213.8 78. 3 29 0.693 0.197 0.284 273 227.3 83.3 30 0.693 0.246 0.355 273 241.0 88. 3 31 0.693 0.295 0.426 273 252.3 92.4 32 0.693 0.344 0.496 273 265.6 97. 3 33 0.693 0.399 0. 576 273 269.8 98. 8 34 0. 693 0.442 0.638 273 265.2 97. 2 35 0.693 0.491 0.709 273 270.8 99.2 36 0.693 0. 540 0.779 273 269.8 98. 8 Examples 37 to 41 The procedure of example 8 was followed varying the temperatures and the results were as shown in the following Table 4.

Table 4 Example No 37 38 39 40 41 Endosulfandiol 0.693 0.693 0.693 0.693 0. 693 (mole) SIDC-A 0.344 0. 344 0. 344 0.344 0.344 (mole) SIDC-A to Endosutfandiol 0.497 0.497 0.497 0.497 0.497 moleratio Reaction temperature (°C) 15 25 35 SO 75 Reaction time (hrs) 8 6 4 2 1 EndosuHan product (gms) 269 268 270 269 266 a-Endosulfan content 266.6 265. 2 266.9 266.6 265. 5 in the product mixture (gms) Selectivity for α- 99.10 98.95 98.85 99.10 99.81 Endosulfan (%) Example 42 EndoSfandiol (25Q gQ69 mole), carbontetrachlonde (750 ml) and a-endosulfan [stereoisomer directing agent (SIDC-B), 100 g, 0.25 mole, SIDC-B: endosulfandiol : : 0. 355] were heated to 65°C with stimng. Thionyl chloride (55 ml, 0.7556 mole) was added to the reaction mixture and the reaction mixture was heated to 75°C for 2 hours. HCl generated during the reaction was scrubbed with alkali solution. The reaction mass was cooled to 30°C. Excess thionyl chloride was neutralised with dilute aqueous 6.5% weight/weigt soda ash solution to pH 6.5. The aqueous layer was discarded.

From the organic layer, cgbontetrachloride was distilled off under reduced pressure. The product mixture weighing 374 g was analysed for endosulfan content by Gas Chromatogrphy (GC) and was found to contain 106. 9 g of ß-endosulfan. SIDC-B in the product mixture was separated from the p-endosulfan as per the propcedure of US Patent No 3251856 and reused. The selectivity of the process for 0-endosulfan was found to be 39.01%.

Examples 43 to 53 The procedure of Example 42 was followed with varying quantities of SIDC-B and the results were as in the following Table 5: Table 5 Eg Endosulf 3IDC-B SIDC-B: Endosulfan ß -Endosulfan Selectivity No andiol (mol) endosulfan product (@ for ß-endo (mol) diol mole oulfan ratio 43 0.693 0 0 273 77.8 28.5 44 0.693 0.49 0.709 274 135. 3 49.37 45 0. 693 0. 74 1.064 274 163.9 59. 82 46 0.693 0.98 1.418 275 193.0 70.18 47 0. 693 1.23 1.773 273 220.8 80.90 48 0.693 1. 47 2.127 273 249.3 91. 32 49 0. 693 1.72 2.482 275 272.0 98.91 50 0.693 1.97 2.836 274 273.7 99.89 51 0.693 2.21 3.191 274 272.7 99.53 52 0.693 2.46 3.545 275 274.0 99. 64 53 0. 693 2.95 4.255 274 273. 7 99.89

Examples 54 to 65 The procedure of Examples 42 to 53 were followed using o- xylene as the solvent and the results were as in the following Table 6: Table 6 <BR> <BR> <BR> <BR> <BR> <BR> Eg Endosu SIDC-B Smc-B : Endotdfm P-Endosulfan Selectivity<BR> <BR> <BR> <BR> No fandiol (mol) EndosuJfan product for p-endo<BR> <BR> <BR> (mol) diol mole (gaz ratio (%) 54 0.693 0 0 274 88.2 32.19 55 0.693 0.25 0.355 273 121.3 44.43 56 0.693 0.49 0.709 274 155. 0 56. 57 57 0.693 0.74 1. 064 274 188.4 68. 75 58 0.693 0.98 1.418 274 221.8 80.93 59 0.693 1.23 1.773 275 255. 5 92.90 60 0.693 1.47 2.217 273 271.7 99.52 61 0.693 1.72 2.482 273 272.4 99. 78 62 0. 693 1.97 2.836 275 274.0 99. 64 63 0.693 2.21 3.191 276 273.2 99.00 64 0.693 2.46 3.545 275 274.0 99.64 65 0.693 2.95 4.255 274 272.4 99.42 Examples 66 to 77 The procedure of Examples 42 to 53 were followed using toluene as the solvent and the results were as in the following Table 7 :

Table 7 Eg Endosul SIDC-B SIDC-B : Endosulfan ß -Endosulfan Selectivity No fandioi (mol) Endosulfan product (g) for ß-endo (mol) diol mole (g) sulfan <BR> <BR> ratio<BR> _, 66 0. 693 0 0 273 84.6 31.00 67 0.693 0.25 0.355 274 117.3 42. 81 68 0.693 0.49 0. 709 274 149.6 54. 59 69 0. 693 0.74 1.064 274 181.9 66.38 70 0.693 0.98 1.418 275 214.9 78.15 71 0.693 1.23 1.773 273 246.2 90.18 72 0. 693 1.47 2.217 273 271.7 99.52 73 0.693 1.72 2.482 275 273. 0 99.27 74 0.693 1.97 2.836 274 273. 7 99.91 75 0.693 2.21 3.191 274 272.8 99.55 76 0.693 2.46 3.545 275 274.0 99.64 77 0.693 2.95 4.255 274 273. 8 99.93 Examples 78-82 The procedure of example 61 was followed varying the temperature and the results were as shown in the following Table 8.

Table 8 Example No 78 79 80 81 82 Endosulfandiol 0.693 0.693 0.693 0.693 0. 693 (mole) SIDC-B 1.72 1. 72 1.72 1. 72 1.72 (mole) SIDC-B : Endosulfandiol 2. 482 2.482 2. 482 2.482 2.482 mole ratio Reaction temperature (°C) 15 25 35 50 75 Reaction time (hrs) 8 6 4 2 1 Endosulfan product (gms) 274 272.9 273.2 274 273 P-Endosulfan 270.9 270. 0 271.0 270.8 267.5 content in the product mixture (gms) Selectivity for ß-Endosulfan (%) 98.89 98.94 99.19 98.83 97.98 The results of Examples 1 to 36 as shown in Tables 1 to 3 clearly show that selectivity of the process for a-endosulfan depends on the molar ratio of SIDC-A to endosulfandiol. Similarly the results of Examples 42 to 77 as shown in Tables 5 to 7 clearly show that selectivity of the process for P- endosulfan depends on the molar ratio of SIDC-B to endosulfandiol. The results of Examples 37 to 41 and 78 to 82 as shown in Tables 4 and 8 clearly indicate that temperature has practically no effect on the selectivity of the process for a-endosulfan or p-endosulfan.