PROCESS FOR MAKING AMINO ALKYLPHENYL CARBAMATES AND INTERMEDIATES THEREFOR

[0001] The present invention provides an improved process for making [3-[α- (Dimethylamino)ethyl]phenyl]-N-methylethylcarbamate, a useful pharmaceutically active agent.

PRIOR ART

[0002] Rivastigmine is a generic name for the compound (S)-[3-[α- (Dimethylamino)-ethyl]phenyl]-N-methylethylcarbamate of the formula (Ia):

[0003] More general, rivastigmine is the (S)-enantiomer of a compound of general formula (I), wherein the dotted line indicates that this carbon-carbon bond is attached to the asymmetric carbon and the compound of formula (I) may exist as a racemate or as an enantiomer.

[0004] Rivastigmine (Ia) is a pharmaceutically active compound that acts as a reversible, brain-selective acetylcholinesterase inhibitor. In existing medicinal products, it is marketed as a salt with L-tartaric acid - rivastigmine hydrogentartrate. Rivastigmine hydrogentartrate is indicated for the symptomatic treatment of mild to moderately severe Alzheimer disease.

[0005] The compound of formula (I) has been disclosed e.g. in EPB 193926 and the USP 4948807. Rivastigmine hydrogentartrate was specifically disclosed in US 5602176.

[0006] The compound of formula (I) is a phenylcarbamate, i.e. its molecule is formed by a phenolic moiety A linked by a carbonyl bridge C to an aminic moiety B as shown on the schematic figure below:

[0007] Accordingly, the known processes for making compounds of formula (I), including rivastigmine, employ the aminophenol compound (II) and ethylmethylamine (III) as starting materials:

with various ways of constructing the carbonyl linking bridge.

[0008] As in the compound (I), also the compound (II) has one asymmetric carbon (illustrated by the dotted line in the figure (II) ) and may be employed in the synthesis as the racemate (Ha) or as a single enantiomer, preferably the S-enantiomer (lib).

[0009] In the first case, racemic compound (I-R) is produced, which may resolve into enantiomers to yield rivastigmine. In the latter case, rivastigmine is produced directly,

however the compound (lib) is generally obtainable by the resolution of the compound (Ha) into enantiomers. Schematically, the whole process may be depicted as follows:

bridging with bridging with

(πi) (Hi)

(I-R) (Ia)

[0010] There are two synthetic concepts known how to build-up the bridge between the compounds (II) and (III) to form the desired carbamoyl ester.

[0011] In essence, the first concept is based on the activation of the amine (III) before the reaction with the phenol. The activated intermediate may be isolated and then it reacts with the aminophenol (II).

[0012] In EPB 193926, the activated intermediate ( the bridge-forming reagent) is a carbamoyl halide of the formula (IV).

(IV) formed by the reaction of (III) with a phosgene reagent. [0013] The condensation with (II) needs an excess of this carcinogenic substance and requires the presence of sodium hydride as a base.

[0014] The original process employed the racemic compound (Ha), yielding the racemate (I-R). The resolution of the (I-R) into enantiomers as well as the isolation of the (S)-enantiomer as a hydrogentartrate salt has been disclosed in GB 2203040.

[0015] This process has been improved in WO 2004-037771, wherein it was found that the racemic (Ha) may be replaced by the enantiomerically pure compound, particularly the (S)-enantiomer (lib). No racemization occurs during the subsequent bridge-forming reaction and the desired rivastigmine (Ia) may thus be prepared without the resolution in the last step. Similar process has been disclosed in CN 1486973.

[0016] In WO 03-101917, the bridge-forming reagent is a carbamoyl ester, particularly the p-nitro phenyl carbamate of the formula (V).

(V)

[0017] The reported advantage of it is that it does not require the use of NaH for the condensation with (II), but more common bases may be used. Another advantage is that the bridge-forming reagent is less hazardous and more user- friendly. However, the reaction requires high temperatures and long reaction time (100°C, 35-40 hours). The carbamate (V) must be synthetized in an extra reaction step from the ethylmethyl amine

(III) and p-nitrophenylchloroformate (VI).

( _VI)

[0018] In another possibility, the amine (III) may be converted into an isocyanate, which then reacts.

[0019] The second concept is based on the activation of the phenol group in (II) by an activation agent, followed by the reaction of the intermediate of general formula

(VII),

wherein L is a suitable leaving group, with the amine (III). This concept was generally discussed in US 2005/0096387 and was actually used for the synthesis of rivastigmine in WO 2006-048720, wherein the activation agent is preferably carbonyldiimidazole (VIII).

[0020] Either the racemic compound (Ha) or the single enantiomer, particularly the S-enantiomer (lib), may be employed.

[0021] The reaction intermediate may be produced in situ and may react with the amine (III) without a need of its isolation, which is an advantage over the first concept.

[0022] However, the reported conditions of the reaction with carbonyldiimidazole are quite disadvantageous (long reaction time, high temperatures) and there is a need of an improvement.

[0023] The WO '720 provides also for an alternative suggestion within this second concept, where the activation agent may be p-nitrophenylchloroformate (VI) (see pg. 6, line 29-30). Apparently, the active intermediate (VII) is then the compound (VII-I).

[0024] The present inventors have found out that the reaction conditions for making the compound (VII-I) are disadvantageous as well as very low temperatures (less than -40°C) are necessary to obtain the product with sufficient yield and purity otherwise (e.g., if working at about O ⁰ C, i.e. under conditions disclosed in Ex3 of US 2005/0096387) the overall yield of the product is very low - see Comparative example 1.

[0025] Accordingly, the present invention provides for an improved process for making the compound of formula (I), particularly rivastigmine of the formula (Ia) and/or its racemic analogue (I-R).

BRIEF DESCRIPTION QF THE INVENTION

[0026] The present invention provides for a process for making the compound of general formula (I), wherein the dotted line indicates a carbon-carbon bond on the asymmetric carbon, which process comprises the reaction of the compound of formula (II), wherein the dotted line has the same meaning, with bis(p-nitrophenyl)carbonate of the formula (IX)

preferably at a temperature between -20 to 50°C and in an inert solvent, followed by the reaction of the so formed intermediate of the formula (VII-I) with ethylmethylamine of

the formula (III), whereupon the formed compound of formula (I) may be optionally isolated from the reaction mixture and/or, provided that the compound of formula (I) is a racemate, resolved into enantiomers and/or isolated as an acid addition salt, most preferably as a hydrogentartrate salt. [0027] In particular, the above process provides the rivastigmine of formula (Ia) as a free base or as an acid addition salt, preferably the hydrogentartrate salt.

[0028] In particular, the starting material is the compound of formula (Ha).

[0029] In a specific aspect, the invention provides for a compound of formula (VII-I a), and particularly for the S-enantiomer of the formula (VII-Ib), in an isolated state.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The starting material of the invented process is a compound of general formula (II), which actually comprises both the racemic compound of the formula (Ha) and any single enantiomer thereof, particularly the (S) enantiomer of the formula (lib), or a mixture of both. Any of the specific compounds of the general formula (II) are equally useful. They may be obtained by methods and processes known in the art and discussed above. [0031] The process for making the compound (I) involves two subsequent chemical reactions. Accordingly, it will be described as a two step process, although it will become apparent that actually it may be performed in a one pot arrangement as well.

[0032] In the first step of the process of the invention, the compound of formula (II), particularly the racemate (Ha) and/or the (S)- enantiomer of the formula (lib), reacts with bis(p-nitrophenyl)carbonate of the formula (IX). The reaction conditions are not particularly limited but typically comprise reacting both compounds at a temperature of between -20 to 50°C, most preferably between 0 and 30°C, in an organic solvent and, advantageously but not necessarily, under the presence of a base. Concerning the temperatures, higher temperatures then those as above are less suitable as side products may be formed in higher extent, while lower temperatures are less suitable as specific cooling equipment and extra energy costs are generally necessary for industrial production and the reaction time is quite long. Concerning the organic solvent, any inert solvent may be employed in principal, but solvents with higher solubility for the reactants and the products are preferred because of better control of the reaction. For instance, the solvent may be an Cl-ClO ether ( e.g. methyl tert. butyl ether), a C5-C8 cyclic ether (e.g. tetrahydrofuran) a C2-C10 aliphatic ester (e.g. ethyl acetate), a C2-C8 aliphatic nitrile (e.g. acetonitrile), an amide (e.g. dimethylformamide), a sulfoxide (e.g. dimethyl sulfoxide), a ketone (e.g. acetone), a C5-C10 hydrocarbon (e.g. hexane or toluene), a Cl- C8 chlorinated hydrocarbon (e.g. chloroform or dichloromethane) and/or mixtures thereof. Protic solvents such as alcohols or basic solvents such as pyridine are not suitable as they may react and therefore they are not inert. Concerning the base, suitable is an organic base, preferably a tertiary amine, e.g. triethylamine or pyridine. More than one molar equivalent of the base is generally used, if a base is used at all.

[0033] The reaction time depends in part on the temperature and is generally less than 10 hours. In an advantageous mode, the course of the reaction is monitored by a suitable analytical method, for instance by HPLC and/or by TLC, and the next step of the

reaction process does not start earlier than the reaction is essentially completed, i.e. more than 95%, preferably more than 98% of the starting material has been converted.

[0034] The product of the reaction of the first reaction step is the carbonate intermediate of the formula (VII-I). Dependent on the starting material, one may produce the racemate of the formula (VII -Ia), the single enantiomer, preferably the S-enantiomer (VII-I b), or a mixture of both. The optical purity of the intermediate (VII-I) is essentially identical with that of the starting material (II). Thus, the racemic (Ha) provides the racemate (VII-Ia), the S-enantiomer (lib) provides the S- enantiomer ( VII-Ib) etc.

[0035] In essence, there is no need to isolate the reaction product (VII-I) from the reaction mixture. Nevertheless, the product is isolatable and may be isolated from the reaction mixture, e.g., with the aim to remove the rest of the unreacted products and to enhance the quality of the final product. Suitable mode of isolation is an extraction process.

[0036] In the second step of the process of the invention, the product of the first step, either in the isolated or in the non-isolated state, reacts with the ethylmethylamine of the formula (III). This compound is commercially available.

[0037] The reaction conditions typically comprise contacting both reagents in a suitable inert organic solvent at a temperature of between -20 and 50°C, preferably between 0 and 30°C. [0038] As stated above, it is possible to use the reaction mixture from the preceded step as the starting material for the reaction with the compound (III). Accordingly, the compound (III) is just added, portion wise or at once, as such or dissolved in a solvent, to the reaction mixture and the reaction proceeds, typically with stirring the reaction mixture, at the selected temperature. More than 1 equivalent of the amine (III), and preferably more than 2 molar equivalents should be added.

[0039] If the isolated intermediate from the first step is used, the reaction with (III) proceeds in the same kind of solvent listed above as useful for the first step. A base, preferably an organic base may be accordingly added to the reaction mixture.

[0040] Preferably, the course of the reaction is monitored by a suitable analytical method, e.g. by HPLC and the reaction is terminated in the proper time.

[0041] The product of the second reaction step is the compound of formula (I). Similarly as above, the actual conformation of the product depends on the conformation on the starting material. It was found out that no racemization occurs within the second step, so that a single enantiomer of the formula (Ha) provides for the single enantiomer of the compound (I), the compound of formula (Ia), i.e. rivastigmine.

[0042] The reaction product from the second step is preferably isolated from the reaction mixture. Suitably, the reaction mixture is first treated with water and the product is isolated from the organic phase. Before isolation, the organic phase may be optionally further purified, e.g. by chromatography or by extraction. The extraction process is preferred. In a suitable mode, the extraction process comprises the extraction of the product (which is an amine) from the organic solvent by acidified water, followed by adjusting the aqueous extract to an alkaline pH and re-extraction of the alkaline aqueous solution by an organic solvent. As to the acid, no particular limitations exist and, e.g., hydrochloric acid may be used for the acidification of water. As to the alkali, e.g. sodium hydroxide may be used for adjustment the pH to alkaline values. Concerning the organic solvent, any water immiscible organic solvent may be used.

[0043] The product may be isolated from the solution as the base, e.g. by evaporation of the solvent. In such a case, the product generally appears an oil. Advantageously however, the product may be isolated as an acid addition salt, whereby it may appear in a solid state, by adding the suitable acid to the reaction mixture or to the

isolated base and precipitation of the salt. The suitable acid is e.g. hydrochloric acid, or, the most preferably, L-tartaric acid. In the later case, the commercially most useful form, the hydrogentartrate salt of the product of formula (I) is obtained.

[0044] If the starting compound (II) is the racemate, the product of the process, the compound of formula (I) is a racemate as well. As stated above, the pharmaceutically preferable form of the compound of formula (I) is the S- enantiomer, the rivastigmine of formula (Ia) and particularly the rivastigmine hydrogentartrate. Therefore, if the rivastigmine is to be obtained, the formed racemate (I-R) has to be resolved into enantiomers and the desired S- enantiomer has to be isolated. The resolution processes are known in the art and employ the treatment of the racemate with a chiral acid. A diastereomeric pair of salts is formed by such treatment, whereby the conditions of formation may be so selected that one of the salts of this pair precipitate from the reaction mixture and the second remains in the solution.

[0045] When using the L-tartaric acid as the chiral acid, and, e.g., acetone as the solvent, then rivastigmine L-hydrogentartrate, the most desired salt of rivastigmine, directly precipitates.

[0046] If starting with the compound (Ha), rivastigmine (Ia) and particularly rivastigmine L-hydrogentartrate may be obtained in a simple process, which does not require special equipment and uncommon reaction conditions. In essence, the advantage of the process of the invention is that it does not require extremely high or extremely low reaction temperatures as do the similar processes of the prior art. It provides the desired product in high yield and high purity without racemization.

Examples

Comparative example 1 Reaction scheme :

[0047] 1 g 3-(l-Dimethylamino)ethyl) phenol was dissolved in 20 ml dichloromethane (dried on CaCl ₂ ). The solution was cooled to -78 ⁰ C.

[0048] 0.92 g Triethylamine was added. A solution of 1.22 g 4-nitrophenyl chloro formate in 5 ml dichloromethane (dried on CaCl ₂ ) was added dropwise over 5 minutes. The solution was stirred at -78 ⁰ C. Reaction progress was monitored by HPLC. After 3 hours at -78 ⁰ C, HPLC showed that there was still starting material present. A second addition of 0.61 g 4-nitrophenyl chloroformate in 2 ml dichloromethane (dried on CaCl ₂ ) was made. After 30 min at -78°, the starting material was completely converted (ace. to HPLC). [0049] 715 mg N-Ethylmethylamine was added and the yellow reaction mixture was allowed to warm slowly to O ⁰ C. The reaction mixture was stirred for 40 hours at 4 ⁰ C. The reaction mixture was allowed to warm to room temperature and was then washed with 2x20 ml water. The organic phase was concentrated in vacuo. To the resulting yellow oil, 20 ml diethyl ether and 20 ml of a solution of hydrochloric acid (2 M) were added. The 2- phase system was stirred for 15 minutes. The acidic aqueous layer was washed with 20 ml diethyl ether.

[0050] 20 ml dichloromethane was added to the aqueous layer and a solution of sodium hydroxide (2 M) was added until pH~10-l 1 was reached. The organic layer was washed with 2x20 ml water, dried (Na ₂ SO ₄ ), filtered and concentrated in vacuo.

Isolated yield: 1.4 g (93%), yellow oil

HPLC : 97.6% purity

H-NMR: confirmed the expected structure

[0051] The Comparative example 1 was repeated with the following solvents and reaction temperatures with the following results: Table 1

*) Reaction was stopped after first step, because of bad result obtained. Example 1 Synthesis and isolation of compound VII-Ia

-78 ⁰ C

[0052] 1 g 3-(l-Dimethylamino)ethyl) phenol was dissolved in 20 ml dichloromethane (dried on CaCl ₂ ). The solution was cooled to -78°C.

[0053] 0.92 g triethylamine was added. A solution of 1.22 g4-nitrophenyl chloro formate in 5 mldichloromethane (dried on CaCl ₂ ) was added dropwise over 5 minutes. The mixture was stirred at -78 ⁰ C. Reaction progress was monitored with HPLC. After 3 hours at -78°C, the reaction mixture was allowed to warm to ambient temperature and was then washed with 2x20 ml water, dried (Na ₂ SO ₄ ), filtered and concentrated in vacuo.

Isolated yield: 2.1 g, yellow oil

HPLC : 83% purity

¹ H- and ¹³ C-NMR: confirmed the expected structure

Example 2 Synthesis and isolation of the compound ( VII-Ib) Reaction scheme:

[0054] 0.5 g (S)-3-(l-(Dimethylamino)ethyl) phenol was dissolved (not completely) in 10 ml dichloromethane (distilled from CaCl ₂ ).

[0055] 1. Ig bis(4-nitrophenyl)carbonate was added. The yellow solution was stirred at ambient temperature. Reaction progress was monitored with HPLC.

[0056] The reaction mixture was concentrated in vacuo. To the resulting yellow oil, 20 ml diethyl ether and 20 ml of a solution of hydrochloric acid (2 M) were added.

The 2-phase system was stirred for 15 minutes. The acidic aqueous layer was washed with 20 ml diethyl ether.

[0057] 20 ml dichloromethane was added to the aqueous layer and a solution of sodium hydroxide (2 M) was added until pH~12 was reached. The organic layer was washed with 2x20 ml water, dried (Na ₂ SO ₄ ), filtered and concentrated in vacuo. Isolated yield: 0.57 g (57%), yellow oil

HPLC: 84.2% purity

Example 3 Synthesis of Rivastigmine from the compound (VII-Ib) Reaction scheme:

[0058] 0.32 g Crude carbonate compound from Example 2 was dissolved in 5 ml dichloromethane (distilled from CaCl ₂ ).

[0059] 86 mg JV-Ethylmethylamine was added dropwise. The yellow solution was stirred at ambient temperature. Reaction progress was monitored with HPLC.

[0060] After 3 hours, the reaction mixture was concentrated in vacuo. To the resulting yellow oil, 20 ml diethyl ether and 20 ml of a solution of hydrochloric acid (2 M) were added. The 2-phase system was stirred for 15 minutes. The acidic aqueous layer was washed with 20 ml diethyl ether.

[0061] 20 ml dichloromethane was added to the aqueous layer and a solution of sodium hydroxide (2 M) was added until pH~12 was reached. The organic layer was washed with 2x20 ml water, dried (Na ₂ SO ₄ ), filtered and concentrated in vacuo. Isolated yield: 0.15 g (62%), yellow oil

HPLC: 96.9% purity; 99. l%e.e.

Example 4 Synthesis of the compound (I-R)

Reaction scheme:

Process:

1 g 3-(l-(Dimethylamino)ethyl) phenol was dissolved in 20 ml dichloromethane (distilled from CaCl ₂ ). The solution was cooled with an ice-water bath (0-5 ⁰ C).

0.92 g triethylamine was added.

2.21 g bis(4-nitrophenyl)carbonate was added. The yellow solution was stirred in an ice-water bath.

[0062] Reaction progress was monitored by HPLC. After 4 hours, 715 mg N- ethylmethylamine was added and the yellow solution was stirred for 17 hours at 4°C. The reaction mixture was allowed to warm to room temperature and was then washed with 2x20 ml water. The organic phase was concentrated in vacuo. To the resulting yellow oil, 20 ml diethyl ether and 20 ml of a solution of hydrochloric acid (2 M) were added. The 2- phase system was stirred for 15 minutes. The acidic aqueous layer was washed with 20 ml diethyl ether.

[0063] 20 ml dichloromethane was added to the aqueous layer and a solution of sodium hydroxide (2 M) was added until pH~12 was reached. The organic layer was washed with 2x20 ml water, dried (Na ₂ SO ₄ ), filtered and concentrated in vacuo.

Isolated yield: 1.37 g (90%), yellow oil

HPLC: 98% purity

¹ H-NMR: confirmed the expected structure

Example 5 Synthesis of the compound (I-R) [0064] 1 g 3-(l-(Dimethylamino)ethyl) phenol was dissolved in 20 ml dichloromethane (distilled from CaCl ₂ ).

[0065] 0.92 g triethylamine was added.

[0066] 2.21 g bis(4-nitrophenyl)carbonate was added. The yellow solution was stirred at ambient temperature. [0067] Reaction progress was monitored by HPLC. After 2.5 hours, 715 mg N- ethylmethylamine was added dropwise and the yellow solution was stirred for 1.5 hours at ambient temperature. The reaction mixture was washed with 2x20 ml water. The organic phase was concentrated in vacuo. To the resulting yellow oil, 20 ml diethyl ether and 20 ml of a solution of hydrochloric acid (2 M) were added. The 2-phase system was stirred for 15 minutes. The acidic aqueous layer was washed with 20 ml diethyl ether.

[0068] 20 ml dichloromethane was added to the aqueous layer and a solution of sodium hydroxide (2 M) was added until pH~12 was reached. The organic layer was washed with 2x20 ml water, dried (Na ₂ SO ₄ ), filtered and concentrated in vacuo.

Isolated yield: 1.36 g (90%), yellow oil HPLC: 97% purity

¹ H-NMR, ¹³ CNMR: confirmed the expected structure Examples 6-9

[0069] The Example 5 was repeated in the following solvents and with the following results:

Table 2

Example 10 Synthesis of the compound (I-R)

[0070] 0.5 g 3-(l-(Dimethylamino)ethyl) phenol was dissolved in 10 ml dichloromethane (distilled from CaCl ₂ ).

[0071] 1.1 g bis(4-nitrophenyl)carbonate was added. The yellow solution was stirred at ambient temperature.

[0072] Reaction progress was monitored with HPLC. After 2 hours, 358 mg N- ethylmethylamine was added dropwise and the yellow solution was stirred at ambient temperature for 16 hours. The reaction mixture was concentrated in vacuo. To the resulting yellow oil, 20 ml diethyl ether and 20 ml of a solution of hydrochloric acid (2 M) were added. The 2-phase system was stirred for 15 minutes. The acidic aqueous layer was washed with 20 ml diethyl ether.

[0073] 20 ml dichloromethane was added to the aqueous layer and a solution of sodium hydroxide (2 M) was added until pH~12 was reached. The organic layer was washed with 2x20 ml water, dried (Na ₂ SO ₄ ), filtered and concentrated in vacuo. Isolated yield: 0.62 g (82%), yellow oil

HPLC: 97.4% purity

Example 11 Synthesis of the compound (I-R)

[0074] 1.0 g 3-(l-(Dimethylamino)ethyl) phenol was dissolved in 20 ml acetone.

[0075] 2.21 g bis(4-nitrophenyl)carbonate was added. The yellow solution was stirred at ambient temperature.

[0076] Reaction progress was monitored with HPLC. After 1.5 hour, 537 mg N- ethylmethylamine was added dropwise and the yellow solution was stirred at ambient temperature for 1.5 hour. The reaction mixture was concentrated in vacuo. To the resulting yellow oil, 20 ml t-butylmethyl ether and 20 ml of a solution of hydrochloric acid (2 M) were added. The 2-phase system was stirred for 15 minutes. The acidic aqueous layer was washed with 20 ml t-butylmethyl ether.

[0077] 20 ml t-Butylmethyl ether was added to the aqueous layer and a solution of sodium hydroxide (2 M) was added until pH~12 was reached. The organic layer was washed with 2x20 ml water, dried (Na ₂ SO ₄ ), filtered and concentrated in vacuo. Isolated yield: 1.34 g (88%), yellow oil

HPLC: 95.6% purity

Example 12 Synthesis of rivastigmine (Ia)

Reaction scheme:

L NH I

[0078] 0.5 g (S)-3-(l-(Dimethylamino)ethyl) phenol was dissolved in 10 ml dichloromethane (distilled from CaCl ₂ ).

[0079] 1.1 g bis(4-nitrophenyl)carbonate was added. The yellow solution was stirred at ambient temperature.

[0080] Reaction progress was monitored with HPLC. After 1.5 hour, 357 mg N- ethylmethylamine was added dropwise and the yellow solution was stirred at ambient temperature for 2.5 hours. The reaction mixture was concentrated in vacuo. To the resulting yellow oil, 10 ml diethyl ether and 10 ml of a solution of hydrochloric acid (2 M) were added. The 2-phase system was stirred for 10 minutes. The acidic aqueous layer was washed with 10 ml diethyl ether.

[0081] 10 ml dichloromethane was added to the aqueous layer and a solution of sodium hydroxide (2 M) was added until pH~12 was reached. The organic layer was washed with 2x10 ml water, dried (Na ₂ SO ₄ ), filtered and concentrated in vacuo.

Isolated yield: 0.58 g (76%), yellow oil

HPLC: 94 % purity;

Example 13 Synthesis of Rivastigmine [0082] 0.5 g (S)-3-(l-(Dimethylamino)ethyl) phenol was dissolved in 10 ml dichloromethane (distilled from CaCl ₂ ).

[0083] 1.11 g bis(4-nitrophenyl)carbonate was added. The yellow solution was stirred at ambient temperature.

[0084] Reaction progress was monitored by HPLC. After 1.5 hours, 357 mg N- ethylmethylamine was added dropwise and the yellow solution was stirred for 2.5 hours at

ambient temperature. The reaction mixture was concentrated in vacuo. To the resulting yellow oil, 10 ml diethyl ether and 10 ml of a solution of hydrochloric acid (2 M) were added. The 2-phase system was stirred for 10 minutes. The acidic aqueous layer was washed with 10 ml diethyl ether. [0085] 20 ml dichloromethane was added to the aqueous layer and a solution of sodium hydroxide (2 M) was added until pH~12 was reached. The organic layer was washed with 2x10 ml water, dried (Na ₂ SO ₄ ), filtered and concentrated in vacuo. Isolated yield: 0.58 g (76%), yellow oil

HPLC: 93.92% purity, 99% ee