The present invention relates to a novel process for producing vitamin A ester and especially to a process for producing all trans-vitamin A ester.
Vitamin A is a known compound. Its chemical formula is represented as following: 
Vitamin A ester of formula (I) such as, acetate or palmitate are used in great quantities as medicines, food additives and feed additives, etc. A variety of methods for the synthesis of vitamin A and its derivatives have been proposed.
Among those methods, there are three routines relating to Wittig or Wittig-Homer reaction: 
The disadvantage of routine 1 lies in the fact that the triphenylphosphine reactant required for the synthesis is relatively expensive and that the byproduct of the reaction, i.e., Ph3PO, is water insoluble, thus it is difficult to isolate the desired product. In routine 2, Wittig-Horner reaction is employed to overcome the shortages existing in routine 1, the byproduct phosphonate is soluble in water, and thus can easily be isolated from desired product. However, the expected product is retinoic acid ester rather than vitamin A ester. In routine 3, intermediate aldehyde (II) in routine 1 and intermediate phosphonate compound (III) in routine 2 were employed to synthesize vitamin A ester directly.
The phosphonate compound (III) can be obtained by the following reactions: (see U.S. Pat. No. 4,916,4250) 
Starting from routine 3, it is an object of this invention to provide an improved process for more conveniently producing vitamin A ester, especially all trans-vitamin A ester in good yields. The advantages of this invention will become apparent from the following descriptions.
This object can be reached by a process for preparation of vitamin A ester, especially all trans-vitamin A ester of formula (I) which comprises treating compound represented by the formula (IV) with aldehyde represented by the formula (II) in an organic solvent in the presence of a base. 
wherein R1=3-methyl-1,3-pentadienyl or 3-methyl-1, 4-pentadienyl, R2 and R3 are identical or different and are C1-C4-alkyl
R4 is C1-C15-hydrocarbyl.
Compound (IV) includes:
A; 3-methyl-5-(2, 6, 6-trimethyl-1-cyclohexen-1-yl)-1, 3-pentadienyl-phosphonic acid dialkyl ester 
B: 3-methyl-5(2, 6, 6-trimethyl-1-cyclohexen-1-yl)-1, 4-pentadienyl-phosphonic acid dialkyl ester or 
C: the mixture of A and B in any ratios
In CN 1097414A, compound (III), which was employed as starting material to synthesize vitamin A ester, was obtained by isomerization of compound (IV). However, according to the present invention, compound (IV) is directly employed as starting material to synthesize vitamin A ester. Due to the omission of isomerization step, vitamin A ester can be prepared in a more simply manner and higher yield according to the process of tie present invention,
In present invention, R2 and R3 are identical or different and represent C1-C4 alkyl, for example methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, sec-butyl and tert-butyl, preferably methyl, ethyl, isopropyl. R4 represents saturated or unsaturated C1-C15 hydrocarbyl, for example, alkyl, alkenyl etc., preferably C1-C15 alkyl, for example, C1-C4-alkyl as mentioned above, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl and the isomer thereof, especially preferably methyl, ethyl, pentadecyl.
It is believed that the mechanism of this reaction is as follows: 
According to the process of this invention, vitamin A ester can be obtained by the reaction of compound (II) with intermediate carbanion (V), which is generated when compound (IV) is treated with a proper base. Suitable base is inorganic base or organic base. Preferred inorganic base includes, for example, alkali metal hydrides such as sodium hydride and potassium hydride. Preferred organic base includes organic base of alkali metal, for example, alkali metal alkoxides such as sodium tert-butoxide alkali potassium tert-butoxide, alkali metal salt of sulfoxides such as dimsyl sodium and dimsyl potassium, organolithium compounds such as methyllithium and n-butyllithium; organomagnesium halides such as ethylmagnesium chloride.
The amount of the base is not critical, the molar ratio of base to the compound of formula (IV) is generally from about 1 to about 3, preferably from about 1 to about 2. The molar ratio of the compound of formula (II) to the compound of formula (IV) is generally from about 1 to about 2.
In general, the reaction is carried out in an organic solvent. Any organic solvents which do not adversely affect the reaction can be employed. Preferred organic solvents include two types of organic solvents: (1) nonpolar aprotic solvents, for example, hydrocarbons such as benzene, hexane, cyclohexane, toluene; ethers such as tetrahydrofuran, diisopropyl ether; (2) polar aprotic solvents, for example, sulfoxides such as DMSO, ketones such as acetone, nitrites such as acetonitrile, amides such as DMF, HMPT. These solvents may be used either singly or in combination. Preferably, two types of solvents are used in combination. The amount of the solvent is not critical. Generally, it is advantageous that from about 0.05 to about 1 mole, preferably from about 0.1 to about 0.5 mole of compound (IV) are employed per liter of solvent. The temperature at which the reaction is carried out may be varied in wide rang upon the type of the used base. Generally, the suitable temperature is from about xe2x88x9270xc2x0 C. to about 70xc2x0 C., preferably from about xe2x88x9270xc2x0 C. to about 0xc2x0 C. The reaction is preferably carried out in an atmosphere of an inert gas such as helium, nitrogen or argon The time of reaction is not critical, generally from 30 minutes to 5 hours, preferably from 2 to 4 hours. Preferably, compound (IV) and base are added in the solvent prior to compound (II).
By the aforesaid reaction, vitamin A ester predominantly containing all trans-vitamin A is formed in good yields from the compound of formula (IV).
The resulting vitamin A ester of formula (I) can be isolated and separated from the reaction mixture in a known manner. For example, water or an aqueous solution of ammonium chloride etc., is added to the reaction mixture, and the organic phase is separated from the mixture. If necessary, the organic phase is washed with water and dried over anhydrous sodium sulfate, and the solvent is evaporated under reduced pressure to separate vitamin A ester. If necessary, the product may be purified, for example, by recrystallization to give vitamin A ester of high purity.
According to the present invention, all trans-vitamin A derivatives can be obtained in good yields and high purity, the content of other isomers such as 9-cis, 11-cis, and 13-cis isomers is very low. It is easy to obtain all trans-vitamin A derivatives from the formed product by conventional purifying means such as recrystallization,
Compound of formula (IV) can easily be obtained according to known methods or in analogy to the known methods, for example, it can be obtained from starting material xcex2-ionone according to the method described in U.S. Pat. No. 4,916,250, 
As to the synthesis of compound of formula (II), reference is made to, for example,
Reif W. et al., Chemie. Ing. Techn, 1973; 45 (10a); 648;
Eletti-Bianchi G. et al., J. Org. Chem. 1976; Vol. 41: 1648;
J. Org. Chem, Vol. 42, 1977, 2939;
U.S. Pat. No. 5,527,952;
J. Org. Chem. Vol, 44, 1979, 1716