FIELD OF THE INVENTION AND RELATED ART STATEMENT 
     This invention relates to a method for the preparation of geometrical isomers which are important as constituents of synthetic pheromones, synthetic perfumes, terpenes and the like. More particularly, it relates to a method for the isomerization of cis-alkenyl compounds to their trans-isomers. 
     In recent years, the technique of utilizing synthetic sex pheromones to predict the breeding of harmful insects or disturb communications between harmful insects has made great progress for the main purpose of controlling harmful insects of the order Lepidoptera. Thus, the utilization of synthetic sex pheromones is now in the stage of practical application. 
     Sex pheromones produced by lepidoptera are composed chiefly of unsaturated aliphatic compounds, and most of them are alkenes and alkadienyl compounds. Moreover, the position of an unsaturated bond or bonds in these components, their geometrical structure (i.e., cis- or trans-form), and the mixing ratio of these components have already been determined. The results thus obtained constitute an important factor to be considered in the synthesis of sex pheromones. 
     Conventionally known techniques for the isomerization of cis-isomers to trans-isomers are as follows: 
     An isomerization reaction using a mercapto-containing compound or a disulfide as a catalyst is described in Japanese Patent Publication Nos. 57608/&#39;87 and 58337/&#39;87. 
     Moreover, an isomerization reaction using elemental selenium (Se) as a catalyst is described in Journal of Am. Oil Chem. Soc., 26, 83, 1949. 
     Furthermore, an isomerization reaction using a mineral acid (e.g., nitrous acid, sulfurous acid or the like) as a catalyst and conducted in an organic solvent is described in Journal of Am. Oil Chem. Soc., 26, 83, 1949. 
     In addition, an isomerization method using nitrous acid derived from sodium nitrite and a mineral acid as a catalyst is described in the Journal of the Japanese Society of Chemical Synthesis, 38(7), 643-646, 1987. 
     However, these conventional techniques have the following respective disadvantages. 
     In the isomerization reaction using a mercapto-containing compound or a disulfide as a catalyst, the mercapto-containing compound used as a catalyst imparts an unpleasant mercapto odor to the products. This is especially undesirable in the case of perfume products having fragrance for their commercial value. 
     In the isomerization reaction using elemental selenium (Se) as a catalyst, selenium (Se) falls under the class of poisons as provided for according to the Poisonous and Deleterious Substances Control Law in Japan and is undesirable from the viewpoint of safety, hygiene and environmental protection. 
     The isomerization reaction using a mineral acid (e.g., nitrous acid, sulfurous acid or the like) as a catalyst and conducted in an organic solvent is undesirable in that it involves a high cost and requires a complicated procedure owing to the necessity of recovering the organic solvent. Moreover, mineral acids exhibit a powerful isomerizing activity, but have the disadvantage that they tend to cause polymerization and/or a shift of the double bond in olefinic compounds, resulting in a reduction in yield and purity. In particular, a shift of the double bond may cause a subtle change of odor in perfumes, and may produce impurities exerting a serious influence on biological activities in sex pheromones. Accordingly, it is very important to minimize the shifts of the double bonds. 
     The isomerization reaction using nitrous acid derived from sodium nitrite and a mineral acid as a catalyst is also undesirable in that it involves a high cost and the use of a mineral acid produces the same disadvantage as described above. 
     Thus, conventional isomerization techniques have their respective disadvantages and there has been a demand for the development of a new technique. 
     In view of these circumstances, it is an object of the present invention to provide a method for the isomerization of cis-alkenyl compounds to their trans-isomers with a high degree of isomerization and a high selectivity. 
     SUMMARY OF THE INVENTION 
     The present inventors made an intensive investigation for the purpose of solving the above-described problem. As a result, it has unexpectedly been found that, by adding 0.5 to 3% by weight of nitric acid (as expressed in terms of pure HNO 3 ) to cis-alkenyl compounds and reacting them at temperature of 60° to 150° C. for a period of 0.5 to 5 hours, their trans-isomers can be obtained at a degree of isomerization of 50 to 81% and a selectivity of 99 to 100%. Although 61% nitric acid was used in the examples given below, 65%, 70% and 98% products are also commercially available. Any of these products may be used in the present invention without difficulty. 
     As used herein, the term &#34;degree of isomerization&#34; means the percentage of the trans-isomer in the reaction product (i.e., the resulting mixture of cis- and trans-isomers). The term &#34;selectivity&#34; means the ratio of the purity of the reaction product (i.e., the resulting mixture of cis- and trans-isomers) to that of the starting material, and indicates the degree of reduction in purity due to side reactions such as a shift of the double bond during isomerization reaction. 
     As described above, the use of nitric acid as the sole catalyst makes it possible to isomerize cis-alkenyl compounds to their trans-isomers with a high degree of isomerization and a high selectivity. Moreover, this can overcome the disadvantages possessed by conventional techniques, such as safety problems and high cost. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Several isomerization methods were compared with respect to cis-3-heptenol. The results thus obtained are summarized in Table 1 below. 
     
                                           TABLE 1__________________________________________________________________________Isomeriza- 2-Mercap-       Selenium             HCl + NaNO.sub.2 +                         Nitriction method toethanol       (Se)  isopro-                   HCl   acid (as(catalyst         panol       pureand other                     HNO.sub.3)factor)Reaction Starting       Same as             Same as                   Same as                         Same asconditions material,       left  left  left  left 300 g (2.6 moles) 2-Mercap-       Se, 3 g             HCl, 15 g                   NaNO.sub.2, 9 g                         HNO.sub.3, 2 g toethanol,  Isopro-                   Aqueous 9 g         panol,                   HCl, 24 g             600 g Stirred at       Stirred at             Stirred at                   Stirred at                         Stirred at 90-100° C.       170-180° C.             80-90° C.                   80-90° C.                         80-90° C. for 4 for 3 for 3 for 3 for 3 hours hours hours hours hoursDegree of 58    62    52    66    77isomeriza-tion (%)Selectivi- 93    94    90    94    99ty (%)__________________________________________________________________________ 
    
     It can be seen from Table 1 that the isomerization method using nitric acid as a catalyst in accordance with the present invention is superior to the other methods in degree of isomerization and selectivity. 
     When nitric acid is used as the sole catalyst, the amount of catalyst added is generally in the range of 0.5 to 3% by weight (as expressed in terms of pure HNO 3 ), though it depends on the isomerizing tendency of the cis-alkenyl compound. For cis-alkenyl compounds having a relatively low molecular weight, it will suffice to add nitric acid in an amount of 0.5 to 1.5% by weight (as expressed in terms of pure HNO 3 ) based on the weight the cis-alkenyl compound. In some cases, the reaction may be conducted in the presence of toluene, xylene, n-hexane or the like. 
     The isomerization reaction is generally conducted at a temperature in the range of 80° to 100° C., though it depends on the structure of the cis-alkenyl compound. If the temperature is lower than 60° C., the degree of isomerization will be reduced to 5-10%, while if the temperature is unduly high, the selectivity will tend to lower. 
    
    
     Specific examples of cis-alkenyl compounds to which the method of the present invention can be applied include alkenes such as cis-3-hexene, cis-3-heptene and cis-3-decene; alkenols such as cis-3-hexenol, cis-3-heptenol, cis-3-octenol and cis-6-nonenol; alkenyl esters such as methyl oleate, cis-8-dodecenyl acetate and cis-3-hexenyl acetate; alkenyl halides such as cis-3-hexenyl chloride, cis-3-heptenyl chloride, cis-3-octenyl chloride and cis-3-decenyl chloride; and alkenoic acids such as oleic acid and cis-5-tetradecenoic acid. However, it is to be understood that the present invention is not limited thereto. 
     EXAMPLE 1 
     (Isomerization of cis-4-tridecenyl chloride with the aid of nitric acid) 
     500 g (2.3 moles) of cis-4-tridecenyl chloride and 7.5 g of nitric acid (as expressed in terms of pure HNO 3 ) were placed in a 2-liter reactor and stirred at 80°-85° C. for 3 hours. Thereafter, the mixture was washed with 500 g of a 5% aqueous solution of sodium hydroxide and then twice with 500-g portions of purified water. 
     The resulting substance was distilled to obtain 495 g of an oily liquid (with a vapor pressure of 2-3 mmHg at 130° C.). Then, this product was examined for degree of isomerization and selectivity by gas chromatography. 
     Thus, a mixture of trans-4-tridecenyl chloride and cis-4-tridecenyl chloride was obtained at a degree of isomerization of 78% and a selectivity of 99%. 
     EXAMPLES 2-6 
     Attempts were made to isomerize other cis-alkenyl compounds in the same manner as in Example 1. The starting materials and reaction conditions employed are shown in Tables 2 and 3 below. Comparative Example 1 
     (Isomerization of cis-4-tridecenyl chloride with the aid of 2-mercaptoethanol) 
     500 g (2.3 moles) of cis-4-tridecenyl chloride, which was used for reaction in Example 1, was placed in a 2-liter reactor, and 25 g (0.32 mole) of 2-mercaptoethanol was added thereto. This mixture was stirred at 90°-95° C. for 3 hours. Then, the isomerization reaction was stopped by cooling the mixture to room temperature. Thereafter, the mixture was subjected to after-treatments as described in Example 1. 
     Thus, a mixture of trans-4-tridecenyl chloride and cis-4-tridecenyl chloride was obtained at a degree of isomerization of 62% and a selectivity of 93%. 
     Comparative Example 2 
     (Isomerization of cis-8-dodecenyl acetate with the aid of NaNO 2  and HCl) 
     500 g (2.2 moles) of cis-8-dodecenyl acetate, 15 g of NANO 2 , and 40 g of a 20% aqueous solution of HCl were placed in a 2-liter reactor and stirred at 80°-90° C. for 3 hours. Then, the isomerization reaction was stopped by cooling the mixture to room temperature. Thereafter, the mixture was subjected to after-treatments as described in Example 1. 
     Thus, a mixture of trans-8-dodecenyl acetate and cis-8-dodecenyl acetate was obtained at a degree of isomerization of 65% and a selectivity of 93%. 
     The above-described results are summarized in Tables 2 and 3 below. 
     
                       TABLE 2______________________________________     Example            Example  Example  Example     1      2        3        4______________________________________cis-Alkenyl com-       cis-4-Tri                cis-3-   cis-3- cis-3-pound       decenyl  Hexene   Heptenol                                Octenyl       chloride                 chlorideWeight percentage       1.5      1.0      1.0    1.0of nitric acid(as pure HNO.sub.3)based on startingmaterialReaction tempera-       80-85    60-65    80-85  85-90ture (°C.)Reaction time       3.0      3.0      3.0    3.0(hrs)Degree of iso-       78       70       74     81merization (%)Selectivity (%)       99       100      99     99______________________________________ 
    
     
                       TABLE 3______________________________________                     Compar-                     ative    Compara-     Example            Example  Example  tive     5      6        1        Example 2______________________________________cis-Alkenyl com-       Oleic    cis-8-   cis-4-Tri-                                cis-8-pound       acid     Dode-    decenyl                                Dodecenyl                cenyl    chloride                                acetate                acetateWeight percentage       1.5      1.5      --     --of nitric acid(as pure HNO.sub.3)based on startingmaterialReaction tempera-       85-90    90-95    90-95  80-90ture (°C.)Reaction time       3.0      3.0      3.0    3.0(hrs)Degree of iso-       78       80       62     65merization (%)Selectivity (%)       99       99       93     93______________________________________ 
    
     Application Example 1 
     (Synthesis of trans-4-tridecenyl acetate) 
     495 g (2.2 moles) of the mixture of trans-4-tridecenyl chloride and cis-4-tridecenyl chloride synthesized in Example 1, 432 g (4.4 moles) of potassium acetate, and 264 g (4.4 moles) of acetic acid were mixed under an atmosphere of nitrogen (N 2 ) and stirred at 180°-185° C. for 10 hours. Thereafter, this mixture was washed with 600 g of purified water and then with 600 g of a 5% aqueous solution of sodium bicarbonate. The resulting organic layer was purified by distillation. Thus, there was obtained 548 g (2.2 mole) of 4-tridecenyl acetate having a trans to cis ratio of 78:22 and a purity of 98%. This product is a sex pheromone component of the tomato pinworm (Keiferia lycopersicella) known as an insect pest of tomato fields principally in the United States of America.
An object of the present invention is to provide a method for the preparation of geometrical isomers which are important as constituents of synthetic pheromones, synthetic perfumes, terpenes and the like (i.e., a method for the isomerization of cis-alkenyl compounds to their trans-isomers). In this method, the above-described object is accomplished by reacting a cis-alkenyl compound in the presence of nitric acid used as the sole catalyst.