The present invention relates to a process to prepare esters of amino carboxylic acids and esters obtainable by such process.

A process to prepare esters of aminocarboxylic acids is disclosed in “Esters of 6- aminohexanoic acid as skin permeation enhancers: The effect of branching in the alkanol moiety”, A Habralek et al, Journal of Pharmaceutical Sciences, Vol. 94, 1494- 1499, (2005). The process disclosed in this document involves in a first step a conversion of the amino carboxylic acid to an amino acyl chloride that next reacts with an alkanol to give an ammonium functional ester that next is deprotonated to give the aminocarboxylic acid ester.

JP S49-082624 discloses a process to prepare esters of aminocarboxylic acids in which an aminocarboxylic acid or a cyclic amide thereof is first reacted with a mineral acid catalyst which will convert this aminocarboxylic acid or cyclic amide into its aminocarboxylic acid salt, and next this aminocarboxylic acid salt is esterified with an alkanol to give the ester of the alkanol and the aminocarboxylic acid. This process is hence an esterification (or condensation) reaction, in embodiments wherein the cyclic amide precursor is employed, preceded by a de-esterification (or hydrolysis) reaction. The process is done at a temperature of 150 deg C and the alkanol reactant is 20% overdosed on cyclic amide molar amount.

J Klimentova, et al, in “Transkarbams with terminal branching as transdermal permeation enhancers", Bioorganic & Medicinal Chemistry Letter 18 (2008), 1712-1715 disclose also an esterification reaction of the salt of an aminocarboxylic acid with an alkanol in the presence of thionylchloride. The molar ratio of aminocarboxylic acid salt to alkanol in this document is 6.16: 5.54 which corresponds with 1.1 1 :1.00. US 4,216,227 discloses a process to prepare omega amino acid esters by reacting omega amino acid salts with an excess of alkanol in the presence of dry HCI gas. The details of the process are not disclosed, other than that a temperature of 150 deg C is employed for 3 hours, and neither is the use of cyclic amide instead of amino acid salt disclosed. EP 847987 also discloses a process to prepare esters of aminocarboxylic esters. The process disclosed in EP 987 involves reacting an aminocarboxylic acid or a cyclic amide thereof with an alkanol in the presence of a divalent or trivalent inorganic acid, wherein the alkanol is dosed in high molar excess relative to the amount of the aminocarboxylic acid and later distilled off. The process is said to be performed under reflux conditions for several hours.

US 3,211 ,781 like EP 847987, discloses a process to prepare esters of amino carboxylic acids wherein a cyclic amide of an amino carboxylic acid is reacted with an alkanol in the presence of a Bronsted-Lowry acid under reflux conditions. Also in this document the alkanol is dosed in a high molar excess compared to the molar amount of cyclic amide.

It has been found that a process to prepare esters of aminocarboxylic acids can also be performed by dosing the alkanol and the cyclic amide of aminocarboxylic acid in similar molar amounts so that the alkanol does not have to be distilled off and furthermore that the transesterification reaction when performing the process in this way can be catalyzed by many Bronsted-Lowry acids, also monovalent acids. The advantage of performing a transesterification reaction of a cyclic amide of an amino carboxylic acid instead of an esterification of an amino carboxylic acid or salt thereof is that it is much easier to control the amount of water in the process. When doing a transesterification reaction there is no net formation of water. When performing an esterification reaction, more water will form and such increased water content may give rise to side reactions, such as in particular under an acidic pH, a hydrolysis reaction wherein formed product falls apart in the starting compounds again. Also, when attempting to remove water from the reaction mixture, the reactants, most notably the alkanols, might form an azeotrope with the water and get lost as well. Furthermore, it has been found that the side reaction wherein the cyclic amide self- polymerizes that is suppressed in state of the art processes by diluting the reaction mixture with an excess of alkanol reactant was found to be similarly suppressed in the process of the invention, wherein the reactants cyclic amide and alkanol are dosed equimolar, or close to equimolar, especially in embodiments wherein the process is done under somewhat more moderate temperatures, such as temperatures under 150 deg C. Quite unexpectedly, in the process of the invention the (trans)esterification reaction takes similarly long as in the prior art processes wherein the alkanol is clearly overdosed and still very similar yields of desired product are obtained. Finally, it is an advantage that the excess of alkanol does not need to be removed from the reaction mixture.

The present invention now provides an improved process to prepare esters of an amino carboxylic acid of the formula (I)

wherein R is an alkyl group containing 5 to 16 carbon atoms that may be branched or linear, k is a value of 1 to 3, m is an integer from 0 to 25, each A independently is - CH2-CH2- or -CH2CH(CH3)- or -CH2-CH(CH2-CH3)-, n is an integer of at least 3 and at most 8, each R1 is a hydrogen atom, each R2 is independently a hydrogen atom or methyl or ethyl group, preferably each R2 is a hydrogen atom, and wherein X is an anion derivable from deprotonating a Bronsted-Lowry acid comprising the steps of reacting a cyclic amide of the formula II

and an alkanol of the formula R-(0-A)mOH in the presence of a Bronsted-Lowry acid at a temperature of between 60 and 200 degrees C wherein the total molar amount of cyclic amide to the molar amount of the alkanol is between 1 :0.8 and 1 :1.5 and wherein the Bronsted-Lowry acid is not added to the reaction mixture until at least 50% of the total of the alkanol and cyclic amide are dosed. The invention furthermore provides esters of amino carboxylic acids of the formula (I) as obtainable by the process of the invention.

More preferred esters include

6-aminobutanoic acid 2-ethylhexan-1-yl ester sulphate, 6-aminobutanoic acid 2- ethylhexan-1-yl ester chloride, 6-aminobutanoic acid 2-ethylhexan-1-yl ester hydrogen sulphate, 6-aminobutanoic acid 2-ethylhexan-1-yl ester phosphate,

6-aminobutanoic acid isodecan-1 -yl ester sulphate, 6-aminobutanoic acid isodecan-1 - yl ester chloride, 6-aminobutanoic acid isodecan-1 -yl ester hydrogen sulphate, 6- aminobutanoic acid isodecan-1 -yl ester phosphate,

6-aminohexanoic acid isodecan-1 -yl ester sulphate, 6-aminohexanoic acid isodecan-1 - yl ester chloride, 6-aminohexanoic acid isodecan-1 -yl ester hydrogen sulphate, 6- aminohexanoic acid isodecan-1 -yl ester phosphate,

6-aminohexanoic acid 2-ethylhexan-1-yl ester sulphate, 6-aminohexanoic acid 2- ethylhexan-1-yl ester chloride, 6-aminohexanoic acid 2-ethylhexan-1-yl ester hydrogen sulphate, and 6-aminohexanoic acid 2-ethylhexan-1-yl ester phosphate.

The compounds of formula (I) were determined to be readily biodegradable, which adds to the environmental profile of any application in which they are used.

In the process of the invention the cyclic amide derivative of formula (II), and the alkanol are preferably employed in a molar ratio of total cyclic amide of formula (II) to alkanol that is preferably between 1 :0.8 and 1 :1.3, more preferably between 1 :0.9 and 1 :1.1 , most preferably both compounds are used in a substantially equimolar amount.

Unexpectedly the reaction can be performed under relatively mild conditions such as a milder reaction temperature, and provide high yields of the ester if the alkanol is used in the ratio of the present invention. Effectively the reaction was found to progress substantially towards the claimed ester product in which there is only a limited remaining amount of unreacted alkanol and cyclic amide and in which the inventors have only found trace amounts of only one side product that is the reaction product of alkanol and Bronsted-Lowry acid. In applications wherein a considerable amount of alkanol or side products caused by not being able to control the amount of water, create a problem but a minor amount of unreacted alkanol or cyclic amide are not an issue, the product of the process of the invention can be used without intermediate steps to remove impurities such as for example unreacted starting materials.

As indicated, the Bronsted-Lowry acid is not added to the reaction mixture until at least 50% of the total of the alkanol and at least 50% of the total of the cyclic amide of the aminocarboxylic acid are added. In a preferred embodiment at least 75% of the total of the alkanol and at least 75% of the cyclic amide are dosed before the Bronsted-Lowry acid is added to the reaction mixture, even more preferably at least 90%. In a preferred embodiment, at least part of the alkanol is first dosed to the reactor and next at least part of the cyclic amide, after which the Bronsted-Lowry acid is added. It however is also possible to dose the cyclic amide partially or completely before the alkanol is dosed to the reactor or to simultaneously dose the cyclic amide and the alkanol after which the Bronsted- Lowry acid is added.

The temperature is preferably increased to at least 60 deg C and to up to 200 deg C after the reactants and the Bronsted-Lowry acid have been dosed. It is beneficial to first increase the temperature to between between 60 and 100 deg C and next to a temperature between 100 and 150 deg C, or even more preferably 110 and 140 deg C.

The process can be performed in a solvent or without a solvent. The process is preferably performed with an amount of solvent that is 0 to 10 wt% based on total reactants. If a solvent is used it is preferably a non-alcoholic organic solvent with a boiling point of at least 100 deg C, or water.

In a preferred embodiment the molar amount of water on total moles of cyclic amide is between 0 and 10 mole%, more preferably 0.01 and 5 mole%, most preferably 0.1 and 2 mole%.

After the reaction is completed, optionally (to decrease viscosity) the product can be diluted with organic solvent. The preferred organic solvents are glycolic solvents, such as propylene glycol, triethylene glycol, ethylene glycol, 2-methoxyethanol, glycerol, or isopropanol.

The Bronsted-Lowry acid is preferably used as a concentrated (i.e. 20 to 100 wt%) aqueous solution and dosed to the reaction mixture containing all the cyclic amide and the alkanol in portions to control the exothermal effect of the reaction.

The Bronsted-Lowry acid is not an aminocarboxylic acid. The Bronsted-Lowry acid is preferably an acid with a pKa of between -10 and 3. The Bronsted-Lowry acid is even more preferably an inorganic acid, even more preferably it is sulfuric acid, phosphoric acid or a hydrohalic acid and most preferably it is sulfuric acid as hydrohalic acids such as HCI in embodiments result in solid products and phosphoric acid results in lower conversions. Moreover both HCI and H3P04 are available as aqueous solutions (HCI 37% cone, H3P04 85% cone) while H2S04 is available in substantially water-free form (such as 95-98% concentrate). The use of H2S04 therefore allows to get products of high conversion (>90%) in a liquid form, and moreover no need to evaporate water exists.

X is in a preferred embodiment an anion derived from an inorganic Bronsted- Lowry acid, more preferred a halogenide, sulphate, hydrogen sulfate, hydrogen phosphate, dihydrogen phosphate, or phosphate anion

The process can be done under reduced pressure but is preferably performed at a pressure that is atmospheric.

The product can optionally be neutralized by a base to a pH from 4-7. The product can be purified by methods available to someone skilled in the art. However, because of the low level of side products, it can also be used without further processing or purification steps, such as as a surfactant.

In a preferred embodiment R is an alkyl group of 6 to 16 carbon atoms. In a more preferred embodiment R is an alkyl group that contains 8 to 13 carbon atoms. In another preferred embodiment R is branched on the carbon atom beta from the oxygen atom. In further embodiments R can contain more than a single branched carbon atom. The process is very favorable for alkanols that have a boiling point up to 220 deg C, or, for alkanols that can form an azeotrope with water having a boiling point lower than 220 deg C. Compared to the state of the art processes, in the process of the invention when using such alkanols, they will not be stripped from the reaction mixture when reaction water is removed, as in the process of the invention no such water is formed and hence does not need to be removed. Many alkanols that can be used in the process of the invention have a boiling point in this range (especially the alkanols that are smaller or that contain some branching in their structure).

It should be noted that a mixture of alkanols can also be employed in the process.

It is furthermore preferred that n is 4, 5 or 6.

The process is preferably done at a temperature of between 60 and 150 deg C, more preferably between 80 and 145 deg C, most preferably 110 and 140 deg C.

Examples

Compounds used

e-Caprolactam (ex Acros Organics)

pyrrolidone (ex Acros Organics)

2-ethylhexanol (ex Perstorp)

Isodecanol (ex Exxon)

HCI (ex Merck)

H3P04 (85%) (ex Fisher scientific)

H2S04 (95%) (ex VWR)

Example 1 - preparation of 6-aminohexanoic acid ethylhexan-1-yl ester salt 2-Ethylhexanol was added to a round bottom flask and dried on a rotavapor at 70 °C for 3 hours (water content 0.049%). Caprolactam (1 14 g; 1.01 mol) and pre dried 2- Ethylhexanol (131.18 g; 1.01 mol) were added to a 250-mL glass reactor fitted with a thermometer, a heating mantel and a mechanical stirrer. The reaction mixture was heated to 60-65 °C for a homogeneous reaction mixture. Concentrated aqueous hydrochloric acid (HCL (cone 37%, 99 g; 1 mol)) was added dropwise to the reaction mixture and showed a small exotherm. The temperature was increased to 140 °C. The reaction was stopped after 12 hours. The product was isolated and analyzed by ¹ H NMR. 70% conversion of the aminocarboxylic acid and alkanol to the desired alkyl-6-aminohexanoate was established.

Example 2 - preparation of 6-aminohexanoic acid 2-ethylhexan-1-yl ester sulphate

The above Example 1 was repeated, however now 90 wt% aq. sulfuric acid (H2S04, 54.9 g; 0.504 mol) was used instead of hydrochloric acid.

The product was isolated and analyzed by NMR to have been obtained with 85% conversion.

Example 3 - Preparation of 6-aminohexanoic acid 2-ethylhexan-1-yl ester sulphate

The above Example 2 was repeated, however now 95 wt% aq. sulfuric acid (H2S04, 52 g; 0.504 mol) was used instead of 90 wt% aq. sulfuric acid.

The product was isolated and analyzed by NMR to have been obtained with 88% conversion. Example 4 - Preparation of 6-aminohexanoic acid 2-ethylhexan-1-yl ester sulphate

The above Example 3 was repeated, however now 10% excess of caprolactam (125.4g; 1.11 1 mol) and H2S04 (57.5g; 0.56mol) were used. The product was isolated and analyzed by NMR to have been obtained with 92% conversion.

Example 5 - Preparation of 6-aminohexanoic acid isodecan-1 -yl ester sulphate

The above Example 2 was repeated, however now instead of 2-ethylhexanol as the alkanol, isodecanol was used (159.43 g, 1.01 mol).

The product was isolated and analyzed by NMR to have been obtained with 86% conversion.

Example 6 - Preparation of 6-aminohexanoic acid 2-ethylhexan-1-yl ester phosphate

Example 1 was repeated however now instead of hydrochloric acid, phosphoric acid H3P04 (85%, 34.1 g; 0.296 mol) was used as the acid.

The product was obtained with 31 % conversion.

Example 7 - Preparation of 6-aminobutanoic acid 2-ethylhexan-1-yl ester sulphate The above Example 3 was repeated, however now pyrrolidin-2-one (85.96; 1.01 mol) was used instead of caprolactam.

The product was isolated and analyzed by NMR to have been obtained with 53% conversion.

Comparative Example 8 - Preparation of 6-aminohexanoic acid isodecan-1 -yl ester sulphate according to state of the art process

The procedure as described in JP S49-082624 was followed: A 500 ml. flanged glass reactor with overhead stirrer, thermometer and cooler was loaded with e-caprolactam (66 g, 0.58 mol) and hydrochloric acid (18% in water, 177 g, 0.88 mol). The mixture was heated to 110 deg C for three hours. Then, 2-ethylhexanol (91 g, 0.70 mol) was added and the mixture was heated to 150 deg C. The cooler was removed and a gentle nitrogen stream was applied to the system for eight hours. When leaving the vessel, the nitrogen stream was directed into a gas washing bottle with aq. NaOH (20%, 200 ml). Finally, the mixture was cooled to room temperature and analysed by NMR. The product was obtained as a white solid (81 g, 50%)

In the Examples 1 to 7 e-caprolactam becomes activated by the acid to initiate ring- opening by 2-ethylhexanol, the reaction in the Comparative Example 8 is based on a two-step mechanism. In the latter process, caprolactam becomes opened by water and acid producing 6-aminohexanoic acid in salt form, which then subsequently undergoes a condensation with the alcohol in a second step. The intermediate 6-aminohexanoic acid is formed virtually quantitative as verified by NMR. Afterwards, addition of the alcohol and heating to 150 deg C initiates a condensation reaction. In the Examples 2 to 4, e-caprolactam, 2-ethylhexanol and concentrated sulfuric acid were mixed at temperatures below 70 deg C.

The product formation in the Examples 1 to 7 is based on a ring-opening reaction by the alcohol in a closed vessel (i.e. a transesterification) rather than a condensation reaction (a hydrolysis followed by a condensation). The different mechanism has a tremendous influence on the outcome of the reaction. Especially, the last step in Comparative Example 8 requires high temperatures and water stripping which result in a lower yield, even more so when using alcohols with boiling points lower than 220 deg C that will be stripped off with the water. It was noticed, that large quantities of 2- ethylhexanol were stripped off at 150 deg C together with water through the nitrogen stream due to the boiling point of 2-ethylhexanol being 184 deg C, and some azeotrope formation. A total yield of 50% of product resulted, which is low compared to the Examples 1 to 7.