DESC:FIELD OF THE INVENTION
[0001] The present disclosure generally relates to the technical field of organic synthesis. In particular, the present invention relates to the synthesis of esters of formula (I)
.

BACKGROUND OF THE INVENTION
[0002] Esters are derivatives of carboxylic acids having general formula R-COOR', where R and R' are alkyl or aryl groups. They are prepared by reaction of alcohols or phenols with carboxylic acids, acid chlorides or acid anhydrides. In the trans-esterification process alkyl group of an ester is replaced by another alkyl group under base catalyzed conditions. Acids like sulfuric acid, phosphoric acid, methanesulfonic acid, p-toluenesulfonic acid and cation exchange resins are used as catalyst in esterification reaction of carboxylic acid with alcohol. The reaction is reversible in nature. Water formed as a by-product is continuously removed to drive the reaction in the forward direction.
[0003] Aryl esters are prepared by reacting phenols with acid chlorides under basic conditions. The reaction of acid anhydrides with phenols under basic condition gives esters. The esters are industrially very useful compounds for a variety of purposes. Several esters are used as herbicides and pesticides. Many halogenated benzoic acid esters are phytotoxic and are used as herbicides. The phenoxy herbicides are primarily alkyl esters of dihalo-phenoxyacetic acid, alkyl-halo-phenoxyacetic acid, and hydroxy-phenoxy propionic acid. Because of their low toxicity, high selectivity, and relatively short life in the soil, phenoxy herbicides are widely used. They are used for controlling weeds in a large number of grass crops such as corn, small grains, sorghum, rice and sugarcane. Many esters of acetic acid, phenylacetic acid, substituted benzoic acid and fatty acids are used as flavours and fragrance compounds. Phenylethyl acetate and phenylethyl phenyl acetate having rose like odour are extensively used as perfumery agents. Some alkyl esters of methoxyphenyl acetic acid are used as flavouring agents in food. Allyl esters of some fatty acids are widely used as aroma ingredients.
[0004] Esters are used as protecting groups for hydroxyl and carboxylic acid groups since esters can be easily hydrolysed. Many parent drugs have been converted to esters to generate prodrugs which after metabolism in human body release the parent drug, in order to overcome some undesirable property, such as bitter taste, poor absorption, and poor solubility. For example, antibiotics such as chloramphenicol and clindamycin have been derivatized as their palmitate esters in order to minimize their bitter taste.
[0005] Patent documents such as GB1246326, CA733997, CN101941905, CN103508889, CN108947837, CN 102835394, and CN102835393 disclose traditional commercial processes for preparation of alkyl esters of acetic acid derivatives by esterification in presence of a catalyst like mineral acid or cation exchange resin.
[0006] US patent, US2596089 discloses a process for preparation of alkyl ester of acetic acid derivative by heating with high boiling alcohols like diethylene glycol monomethyl ether and diethylene glycol monobutyl ether
[0007] Research papers such as World J. of Pharmacy and Pharmaceutical Sc., 7(7), 992-1003 (2018); Bioorg. & Med. Chem. Lett., 27(4), 1017-1025 (2017); Ind. J. of Het. Chem., 14(1), 51-54 (2004); J. of Chemical Res., (6), 416-417 (2004) also disclose preparation of alkyl esters of acetic acid derivatives by esterification of the corresponding acetic acid compounds with suitable alcohol in presence of a catalyst.
[0008] Patent applications such as CN104447320 and IN2764/DEL/2007 disclose the esterification of fatty acid with allyl alcohol in the presence of catalyst consisting of p-toluenesulfonic acid and perchloric acid treated-SiO2 respectively.
[0009] Research papers such as Enzyme and Microbial Technology, 32(5), 589-595 (2003) and Journal of Organic Chemistry, 74(16), 5967-5974 (2009) also disclose the esterification of fatty acid with allyl alcohol in presence of lipase enzyme as catalyst.
[00010] US patent application US20020137660 discloses the synthesis of alkylcarboxylic allyl esters by the reaction of acid and allyl alcohol in the presence of an acid catalyst such as p-toluenesulfonic acid.
[00011] Patent publications like CN101606522, CN105837523, CN103265538, CN10467216, CN 109651350, CN103275066 and US20190300471 disclose the synthesis of substituted benzoic acid alkyl ester by the reaction of substituted benzoic acid and alkanol in the presence of an acid catalyst.
[00012] Further, research papers such as Bioorganic & Medicinal Chemistry, 13(16), 4842-4850 (2005); Letters in Drug Design & Discovery, 6(3), 186-192 (2009); Bioorganic & Medicinal Chemistry Letters, 24(1), 192-194 (2014); RSC Advances, 5(17), 12807-12820 (2015); Tetrahedron, 70(12), 2190-2194 (2014); Journal of Organic Chemistry, 60(21), 7072-4 (1995); and Journal of Medicinal Chemistry, 27(12), 1565-70 (1984) disclose the synthesis of substituted benzoic acid alkyl ester by the reaction of substituted benzoic acid and alkanol in the presence of an acid or other catalysts.
[00013] The prior art esterification processes suffer from one or other drawbacks like, large reaction volumes, lengthy work up procedures, large quantities of solvents and incomplete reactions. It is also very difficult to isolate and recycle unreacted acid as well as catalyst used in the process. Therefore, there is a need in the art to have an esterification process that does not suffer from the drawbacks of the prior art processes.

OBJECTS OF THE INVENTION
[00014] The main objective of the present invention is to provide a new and improved process for preparation of esters of Formula (I), which overcomes one or more disadvantages of the prior art specified above.
[00015] Another objective of the present invention is to provide an improved process for the preparation of esters of Formula (I) by reacting corresponding acids and alcohols at an elevated temperature.
[00016] Another objective of the present invention is to provide a process for preparing esters of Formula (I) optionally in presence of catalyst.
[00017] Another objective of the present invention is to provide a commercially viable process for preparing esters of Formula (I).
[00018] Yet another objective of the present invention is to provide a process for preparing esters of Formula (I) in a batch, semi-batch or continuous mode.
[00019] Other objectives and advantages of the present invention will be more apparent from the following description, which is not intended to limit the scope of the present specification.

SUMMARY OF THE INVENTION
[00020] The present invention provides an improved process for preparation of esters of Formula (I).

R of formula (I) can be substituted phenyl or substituted phenoxy alkyl, wherein the substitution can be at least one moiety selected from the group consisting of chloro, bromo, methyl, ethyl, hydroxy and lower alkoxy, positioned at any position on the phenyl ring. The alkyl of the substituted phenoxy alkyl can be a lower alkyl group having C1 to C4 carbon atoms. In some embodiments, R can be linear or branched alkyl group having C2 to C8 carbon atoms.
R1 of formula (I) can be linear or branched alkyl group having C1 to C8 carbon atoms, or linear or branched alkenyl group having C2 to C5 carbon atoms.
[00021] In one aspect, the present invention provides a process for preparation of esters of Formula (I), which comprises reacting an acid of Formula (A) with an alcohol of Formula (B) at a temperature in the range of 50°C to 250°C.

wherein R and R1 are as defined above.
[00022] In another aspect of the present invention, the esterification reaction is carried out optionally in presence of a catalyst.
[00023] In another aspect of the present invention, the esterification reaction is preferably performed with simultaneous removal of water formed in the reaction e.g., by distillation, azeotropic distillation, etc.
[00024] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.

DETAILED DESCRIPTION OF THE INVENTION
[00025] The following is a detailed description of embodiments of the present invention. The embodiments are in such detail as to clearly communicate the invention. However, the amount of details offered are not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
[00026] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[00027] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[00028] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[00029] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, process conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.
[00030] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein.
[00031] All methods described herein can be performed in suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[00032] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
[00033] Various terms used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[00034] The terminology or phrases described herein are well known and disclosed in the prior arts.
[00035] The term, “linear or branched alkyl”, as used herein, refers to the radical of saturated aliphatic groups, including straight or branched-chain alkyl groups having eight or fewer carbon atoms in its backbone, for instance, C1-C8 for straight chain and C3-C8 for branched chain. Representative examples of alkyl include, but are not limited to, methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, isopropyl, sec-butyl, isobutyl, tert-butyl, isopentyl, 2-ethyl hexyl, 2-methylbutyl and 3-methylbutyl etc.
[00036] The term, “linear or branched alkenyl”, as used herein, refers to the radical of unsaturated aliphatic groups, including straight or branched-chain alkenyl groups having eight or fewer carbon atoms in its backbone, for instance, C2-C8 for straight chain and C3-C8 for branched chain. Representative examples of alkenyl include, but are not limited to, ethenyl, n-propenyl, n-butenyl, n-pentenyl, n-hexenyl, isopropenyl, sec-butenyl, isobutenyl, isopentenyl, 2-methylbutenyl and 3-methylbutenyl etc.
[00037] The term "substituted phenyl or aryl" as used herein refers to a phenyl ring bearing one or more functional groups selected from halogen, alkyl, hydroxy or lower alkoxy group or combination thereof at any of the place of a phenyl ring.
[00038] The term, "halogen" as used herein refers to chlorine, fluorine, bromine or iodine atom.
[00039] The term, “lower alkoxy" refers to a (C1-4)alkyl having an oxygen radical attached thereto. Representative examples of alkoxy groups include, but are not limited to, methoxy, ethoxy, propoxy, isopropoxy, n-butoxy, isobutoxy and tert-butoxy.
[00040] The present invention generally pertains to the technical field of preparation of esters, in particular to the synthesis of the esters of substituted or unsubstituted aryl or alkyl acid derivatives.
[00041] In one aspect, the present invention provides an improved process for preparation of esters of Formula (I).

R of formula (I) can be substituted phenyl or substituted phenoxy alkyl, wherein the substitution can be at least one moiety selected from the group consisting of halogen, alkyl, hydroxy and lower alkoxy, positioned at any position on the phenyl ring. The halogen moiety can be any halogen including, but not limited to, chloro and bromo. The alkyl moiety can include, but not limited to, methyl and ethyl. The alkyl of the substituted phenoxy alkyl can be a lower alkyl group having C1 to C4 carbon atoms. In some embodiments, R can be linear or branched alkyl group having C2 to C8 carbon atoms.
In various embodiments, R1 of formula (I) can be linear or branched alkyl group having C1 to C8 carbon atoms, or linear or branched alkenyl group having C2 to C5 carbon atoms.
[00042] In an embodiment of the present invention, the process for the preparation of esters of formula (I), as described in the scheme-1, comprises the step of heating the compound of formula (A) and the compound of formula (B) at a temperature in the range of 50?C to 250°C to obtain the compound of formula (I),
Scheme-1

In various embodiments, R of formula (A) is substituted phenyl or substituted phenoxy alkyl, wherein the substitution is at least one moiety selected from the group consisting of halogen, alkyl, hydroxy and lower alkoxy, positioned at any position on the phenyl ring. The halogen moiety can be any halogen including, but not limited to, chloro and bromo. The alkyl moiety can include, but not limited to, methyl and ethyl. The alkyl of the substituted phenoxy alkyl can be a lower alkyl group having C1 to C4 carbon atoms. In some embodiments, R is linear or branched alkyl group having C2 to C8 carbon atoms.
In various embodiments, R1 of formula (B) is linear or branched alkyl group having C1 to C8 carbon atoms, or linear or branched alkenyl group having C2 to C5 carbon atoms. In formula (I), R and R1 are the same as defined above for formulas (A) and (B).
[00043] In some embodiments, R and R1 are independently as described in the below Table-1.
Table-1
R R1 X Y
(X)(C6H4)- -CH3 p-OCH3 ----
-C2H5 p-OCH3 ----
-n-C3H7 p-OCH3 ----
-CH(CH3)2 p-OCH3 ----
(X)(Y)(C6H3)OCH2- -CH3 o-CH3 p-Cl
-C2H5 o-CH3 p-Cl
-CH(CH3)2 o-CH3 p-Cl
-CH2(CH2)2CH3 o-CH3 p-Cl
-CH2CH(C2H5)-(CH2)3-CH3 o-CH3 p-Cl
-CH3 o-Cl p-Cl
-C2H5 o-Cl p-Cl
-CH(CH3)2 o-Cl p-Cl
-CH2CH(C2H5)-(CH2)3-CH3 o-Cl p-Cl
-CH2CH(CH3)2 o-Cl p-Cl
-CH2(CH2)2CH3 o-Cl p-Cl
CH3(CH2)4- -CH3 ---- ----
-C2H5 ---- ----
-CH2CH=CH2 ---- ----
CH3(CH2)5- -CH3 ---- ----
-C2H5 ---- ----
-CH2CH=CH2 ---- ----

[00044] In certain embodiments, the acid, represented by Formula (A) can be selected from the compounds listed in Table-2.
Table-2
Compound No Formula (A)
A01 X(C6H5)-COOH; where X is p-OCH3
A02 (X)(Y)(C6H3)OCH2-COOH;
where X is o-CH3 and Y is p-Cl
A03 (X)(Y)(C6H3)OCH2-COOH;
where X is o-Cl and Y is p-Cl
A04 CH3(CH2)4-COOH
A05 CH3(CH2)5-COOH

[00045] In another embodiment of the present invention, the alcohol, represented by Formula (B) is preferably a linear or branched, saturated or unsaturated, aliphatic alcohol having 1 to 8 carbon atoms. In an exemplary embodiment, the alcohol of formula (B) can be selected from the group consisting of methanol, ethanol, isobutanol, 2-ethylhexanol and allyl alcohol.
[00046] In an embodiment, the acid of formula (A) is reacted with the alcohol of formula (B) at a temperature in the range of 100?C to 250°C to obtain the compound of formula (I).
[00047] In certain embodiment of the present invention, the esterification reaction is performed optionally in presence of a catalyst.
[00048] In another embodiment of the present invention, the catalyst, which can be used optionally in the reaction is selected from mineral acid, phosphoric acid, methanesulfonic acid, p-toluenesulfonic acid, montmorillonite clay, zeolites, 4-dodecylbenzenesulfonic acid, cation exchange resins, and a mixture thereof.
[00049] In another embodiment of the present invention, water formed during the reaction is simultaneously removed from the reaction mixture by techniques known in the art like distillation, azeotropic distillation or absorption over a drying agent like molecular sieves or silica.
[00050] In another embodiment of the present invention, the molar ratio of acid compound of Formula (A) to the alcohol compound of Formula (B) ranges from 1: 1 to 1: 10, preferably 1: 1 to 1: 4.
[00051] In another embodiment of the present invention, the alcohol compound of Formula (B) is reacted with acid compound of Formula (A) at elevated temperature of greater than 50°C or preferably greater than 100°C with a simultaneous removal of by-products formed. The reaction monitoring is done by techniques like Titrimetry, Thin Layer Chromatography, High Performance Liquid Chromatography or Gas Chromatography as known in the art. After completion of the reaction, optionally a base selected from sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate sodium bicarbonate and the like is charged and the separated solids are filtered to get the desired ester. Alternately the aqueous solution of base can be used wherein the desired ester is obtained by separation of aqueous layer from the ester layer. Optionally the ester is dissolved in a solvent like toluene, n-heptane, benzene, n-hexane, xylene, chlorinated hydrocarbon like methylene dichloride, ethylene dichloride, or 1,2-dichlorobenzene and the resultant solution is washed with an aqueous solution of base followed by water. The ester is isolated by removal of solvent by distillation.
[00052] In yet another embodiment of the present invention, the acid of formula (A) can be reacted with the alcohol of formula (B) at elevated temperature in a stirred tank reactor in a batch mode or in a flow reactor in a continuous mode or in combination of flow reactor and stirred tank reactor in a semi-batch mode.
[00053] In another embodiment of the present invention, the representative compounds of formula (I) prepared according to the process of the present invention are given below in Table-3.

Table-3
Compound number R R1 X Y
C01 X(C6H4)- -CH3 p-OCH3 ----
C02 -C2H5 p-OCH3 ----
C03 -n-C3H7 p-OCH3 ----
C04 -CH(CH3)2 p-OCH3 ----
C05 (X)(Y)(C6H3)OCH2- -CH3 o-CH3 p-Cl
C06 -C2H5 o-CH3 p-Cl
C07 -CH(CH3)2 o-CH3 p-Cl
C08 -CH2(CH2)2CH3 o-CH3 p-Cl
C09 -CH2CH(C2H5)-(CH2)3-CH3 o-CH3 p-Cl
C10 -CH3 o-Cl p-Cl
C11 -C2H5 o-Cl p-Cl
C12 -CH(CH3)2 o-Cl p-Cl
C13 -CH2CH(C2H5)-(CH2)3-CH3 o-Cl p-Cl
C14 -CH2CH(CH3)2 o-Cl p-Cl
C15 -CH2(CH2)2CH3 o-Cl p-Cl
C16 CH3(CH2)4- -CH3 ---- ----
C17 -C2H5 ---- ----
C18 -CH2CH=CH2 ---- ----
C19 CH3(CH2)5- -CH3 ---- ----
C20 -C2H5 ---- ----
C21 -CH2CH=CH2 ---- ----

[00054] In accordance with the present invention, the compound of Formula (I) can be obtained in a yield of more than 90%, preferably more than 95%, with respect to the acid compound of Formula (A).
[00055] In various embodiments of the present invention, the compound of Formula (I) produced in accordance with the process disclosed herein has a purity of greater than 95%, preferably more than 98% by GC or HPLC.
EXAMPLES
[00056] The present invention is further explained in the form of following examples. However, it is to be understood that the following examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.
EXAMPLE 1: Preparation of Compound-C09
[00057] Compound-A02 (628.5kg) and 2-ethylhexanol (612 kg) were charged to the reactor fitted with reflux distillation set up. The reaction mass was heated to 150±3°C over a period of 2-3hrs. Simultaneously collect the distilled water azeotropically. Stir the reaction mass at 150±3°C for 6-8 h and after completion of the reaction, distill out the unreacted 2-ethyl-1-hexanol. Cool the reaction mass to 25-30°C to give 930.2 kg of the title compound (Purity 99.00 %; Yield 97 %).
EXAMPLE 2: Preparation of Compound-C11
[00058] Compound-A03 (7000g) and ethanol (786g) were charged in a reactor fitted with a downward distillation set up. The reaction mass was heated to 135±3°C and added ethanol (3908g) slowly in 24±1 h to the reaction mass at 135±3°C. At the end of addition, the reaction mass was cooled to 120±3°C and added aqueous sodium carbonate solution (72g, 20%) slowly under stirring and maintain for 1h. Remove water by vacuum distillation. The reaction mass was further cooled to 25-30°C and filtered the solids obtained. The filtrate obtained was collected to give 7700g of the title compound (Purity 99.0%; Yield 98%).
EXAMPLE 3: Preparation of Compound-C13
[00059] Compound-A03 (3536g) and 2-ethylhexanol (2704g) were charged to the reactor fitted with reflux distillation set up. The reaction mass was heated to 150±3°C over a period of 2-3hrs. Simultaneously collect the distilled water azeotropically. Stir the reaction mass at 150±3°C for 8-10 h and after completion of the reaction, distill out the unreacted 2-ethylhexanol. Cool the reaction mass to 25-30°C and filter the product. The filtrate obtained was collected to give 5127g of the title compound (Purity 99.00 %; Yield 97 %).
EXAMPLE 4: Preparation of Compound-C14
[00060] Compound-A03 (1105g) and isobutyl alcohol (482g) were charged to the reactor fitted with reflux distillation set up. The reaction mass was heated to 150±3°C over a period of 2-3hrs. Simultaneously collect the distilled water azeotropically. Stir the reaction mass at 150±3°C for 15 h and after completion of the reaction, distill out the unreacted isobutyl alcohol. Charge toluene (325g) followed by washing with 20% aqueous soda ash solution (260g). Separated the layers and distilled off organic layer under vacuum. Cool the reaction mass to 25-30°C and filter the product. The filtrate obtained was collected to give 1370g of the title compound (Purity 99.00 %; Yield 98 %).
EXAMPLE 5: Preparation of Compound-C01
[00061] Compound-A01 (2.38g) and methanol (5g) were charged in a reactor. The reaction mass was heated at a temperature of 150-175°C for 8-10 hours. After completion of the reaction, the reaction mass was cooled to 30-40°C. Filtered the solids and distilled out the filtrate under reduced pressure to obtain the title compound (Purity 98.4%; Yield 85.8%).

EXAMPLE 6: Preparation of Compound-C18
[00062] Compound-A04 (150g) and allyl alcohol (150g) were charged in a reactor. The reaction mass was heated at a temperature of 100±10°C for 7 to 9 hours. Checked acidity by titration after ~5h and after completion of the reaction, the reaction mass was cooled to 30-40°C. Filtered the reaction mass and distilled out the filtrate under reduced pressure to obtain the title compound (Purity 98.4%; Yield 85.8%).
EXAMPLE 7: Preparation of Compound-C11
[00063] Compound-A03 (221g), ethanol (24g), p-toluene sulfonic acid (1.2 g) was charged in a reactor fitted with a downward distillation set up. The reaction mass was heated to 130-140°C and added ethanol (124g) slowly in 24-25h to the reaction mass at 130-140°C and maintain for 2-3h. Then the reaction mass was cooled to 30-40°C and added sodium carbonate solution (25g, 20%) under stirring then heat the reaction mass upto 70°C and stop stirring and settle the reaction mass separate the 2,4-DEE layer and emulsion layer. The reaction mass was further cooled to 25-30°C and filtered. The filtrate obtained was collected as title compound with 98.7% purity.
[00064] A skilled artisan will appreciate that the quantity and type of each ingredient can be used in different combinations or singly. All such variations and combinations would be falling within the scope of present disclosure. The foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.
[00065] The applicant has developed an improved process for the synthesis of some ester compounds using environment friendly and economic reaction conditions by reacting corresponding acids with alcohols at elevated temperature optionally in the presence of catalyst. Developed process is also taking in consideration the industrial need to operate in mild conditions and with easy available and easy to handle reagents.

,CLAIMS:1. A process for preparation of an ester of formula (I),

wherein,
R is substituted phenyl, substituted phenoxy alkyl, or linear or branched alkyl group having C2 to C8 carbon atoms; and
R1 is linear or branched alkyl group having C1 to C8 carbon atoms, or linear or branched alkenyl group having C2 to C5 carbon atoms;
and wherein the process comprises reacting an acid of formula (A) with an alcohol of formula (B),

wherein R and R1 are as defined above,
at a temperature in the range of 50°C to 250°C and optionally in presence of a catalyst with simultaneous removal of water formed during the reaction.
2. The process as claimed in claim 1, wherein R and R1 in formula (I) are selected from the group consisting of:
R R1
X(C6H4)-
(wherein X = p-OCH3)
-CH3
-C2H5
-n-C3H7
-CH(CH3)2
(X)(Y)(C6H3)OCH2-
(wherein X = o-CH3 and
Y= p-Cl)
-CH3
-C2H5
-CH(CH3)2
-CH2(CH2)2CH3
-CH2CH(C2H5)-(CH2)3-CH3
(X)(Y)(C6H3)OCH2-
(wherein X = o-Cl and
Y= p-Cl)
-CH3
-C2H5
-CH(CH3)2
-CH2CH(C2H5)-(CH2)3-CH3
-CH2CH(CH3)2
-CH2(CH2)2CH3
CH3(CH2)4- -CH3
-C2H5
-CH2CH=CH2
CH3(CH2)5- -CH3
-C2H5
-CH2CH=CH2.

3. The process as claimed in claim 1, wherein the acid of formula (A) is reacted with the alcohol of formula (B) at a temperature of from 100°C to 250°C.
4. The process as claimed in claim 1, wherein water formed during the reaction is simultaneously removed by distillation, azeotropic distillation or absorption over a drying agent.
5. The process as claimed in claim 1, wherein R is substituted phenyl, wherein the substitution is at least one moiety selected from the group consisting of halogen, alkyl, hydroxy and lower alkoxy, and further wherein the substitution is positioned at any position on the phenyl ring.
6. The process as claimed in claim 1, wherein R is substituted phenoxy alkyl, wherein the substitution is at least one moiety selected from the group consisting of halogen, alkyl, hydroxy and lower alkoxy, and further wherein the substitution is positioned at any position on the phenyl ring of the phenoxy alkyl.
7. The process as claimed in claim 6, wherein the alkyl of the substituted phenoxy alkyl group is of from 1 to 4 carbon atoms.
8. The process as claimed in claim 1, wherein the acid of formula (A) is 4-methoxybenzoic acid, 2-methyl-4-chlorophenoxyacetic acid, 2,4-dichlorophenoxyacetic acid, hexanoic acid, or heptanoic acid.
9. The process as claimed in claim 1, wherein the alcohol of formula (B) is a linear or branched, saturated or unsaturated, aliphatic alcohol having 1 to 8 carbon atoms; optionally wherein the alcohol of formula (B) is selected from the group consisting of methanol, ethanol, isobutanol, 2-ethylhexanol and allyl alcohol.
10. The process as claimed in claim 1, wherein the catalyst is selected from the group consisting of phosphoric acid, methanesulfonic acid, p-toluenesulfonic acid, montmorillonite clay, zeolites, 4-dodecylbenzenesulfonic acid, cation exchange resins and mineral acid.
11. The process as claimed in claim 1, wherein the molar ratio of the acid of formula (A) to the alcohol of formula (B) ranges from 1: 1 to 1: 10, preferably 1: 1 to 1: 4.
12. The process as claimed in claim 1, wherein the reaction is carried out in a batch mode using a stirred-tank reactor, in a continuous mode using a flow reactor, or in a semi-batch mode using a combination of flow reactor and stirred-tank reactor.