TECHNICAL FIELD

The present invention relates to a novel stereoselective process for preparing the ( 2R,3S)-β- pheny lisoserine derivative represented by the following formula (1) which can be used as an useful intermediate for synthesizing the terpene taxan derivatives paclitaxel and taxotere :

in which

Ri represents hydrogen, benzoyl or t-butoxycarbonyl, and 2 represents hydrogen, C1-C4 alkyl, Cι-C ₄ alkenyl, C1-C4 alkynyl or benzyl.

BACKGROUND ART

US Patent Nos. 5,420,337 and 5,256,803, WO 94/07876

(PCT FR93/00965), WO 94/12482(PCT/FR93/01133) and French Patent No. 2,698,361 disclose that the (2R,3S)- β -phenylisoserine derivative of formula (1) can be used as an intermediate for producing the terpene taxan derivatives paclitaxel and taxotere represented by the following formula (2), which are well known chemotherapeutic agents used for treatment or control of cancer. Compound (2) is paclitaxel when R and R' are benzoyl and Ac, respectively, and taxotere when R and R' are t-butoxycarbonyl and hydrogen, respectively.

The terpene taxan derivatives of formula (2) can be prepared by a condensation reaction of the (2R,3S)- β -phenylisoserine derivative of formula (1) with the compound represented by the following formula (3), which is 7-triethylsilyl baccatin when R" is triethylsilyl.

in which R" represents hydrogen or -Si(Et>3.

Many of the processes disclosed in the prior art for preparing the (2R,3S)- β -phenylisoserine derivative (1) cannot readily be employed on an industrial scale, because they utilize and produce substances of low stereospecif icity .

In contrast to these processes, the prior art process represented by the following reaction scheme 1 utilizes highly stereospecific (S) -pheny lglycine (4) to produce the compound (7), a derivative of the compound (1), with high stereospecificity but with low yield. The process of reaction scheme 1 employs benzoyl as the amine- protecting group, because benzoyl is the corresponding moiety in paclitaxel. The yield of this process is approximately 30% because the oxidation reaction

from alcohol to aldehyde is largely determined by the amine-protecting group.

Reaction Scheme 1:

on

Furthermore, since the process of reaction scheme 1 is carried out under anhydrous conditions using vinyl magnesium bromide, extremely complicated reaction conditions would need to be satisfied to utilize this process on an industrial scale. The process is also problematic due to the low conversion yield of the double bond of the vinyl group into carboxylic acid.

DISCLOSURE OF INVENTION

The present invention relates to the process represented by the following reaction scheme 2 for preparing the (2R,3S)- β -phenylisoserine derivative (1), in which compound (11) is hydrolyzed in the presence of an acid catalyst to provide a compound (12), which is in turn hydrolyzed in the presence or absence of an alcohol to provide a compound (13), which is then deprotected, and optionally the resulting product wherein Ri

is hydrogen is reacted with benzoyl chloride or di-t-butyl dicarbonate

Reaction Scheme 2-

in which

Ri represents hydrogen, benzoyl or t-butoxycarbonyl,

R2 represents hydrogen, Cι-C ₄ alkyl, 0^04 alkenyl, C1-C4 alkynyl or benzyl, and R3 represents hydrogen or trimethylsilyl.

If the (2R,3S)- β -phenylisoserine derivative (1) is prepared according to the process of reaction scheme 2, it is obtained in a yield of more than 60% as well as in a high stereospecificity(99%, ee). Thus, the process of the present invention is more economical than the prior art processes.

BEST MODE FOR CARRYING OUT THE INVENTION

Reaction scheme 2 is explained below in more detail.

The compound of formula (11) having a high stereoselectively is

used as a starting substance in the reaction scheme 2. This compound (11) can be prepared by introducing a cyano group into a compound of the following formula (10) in the presence of a Lewis acid and a cyanide, as represented by the following reaction scheme 3.

Reaction Scheme 3:

N(Bn) ₂ N(Bn) ₂

(10) (1 1)

in which, R3 is hydrogen or trimethylsilyl.

Lewis acids suitable for the above cyanogation reaction include anhydrous magnesium bromide, titanium tetrachloride, and tin tetrachloride. The desired (2R,3S)-isomer (11) can be obtained in a stereospecificity of more than 90% when either anhydrous magnesium bromide or titanium tetrachloride is the Lewis acid used. The cyanide which is appropriate for the above reaction is one selected from a group consisting of trimethylsilyl cyanide, sodium cyanide, and potassium cyanide.

In the next step, the compound of formula (11) is hydrolyzed in the presence of an acid catalyst, for example, a strong acid such as hydrochloric acid, sulfuric acid or nitric acid, to produce a white solid, which is then recrystallized to obtain the amide compound of formula (12) having a high stereospecificity. Then, a mixture of cone, hydrochloric acid and acetic acid is added dropwise to the amide compound of formula (12) and refluxed while heating in the presence or absence of an alcohol to prepare a white solid in a high yield, which is again recrystallized to produce (2R,3S)-N,N-dibenzyl- ? -phenylisoserine of formula (13) with a high stereospecificity(99% ee). In this step, cone, hydrochloric acid and

acetic acid are mixed in a ratio of 2:1 — 1:2, preferably 1:1 by volume. The compound of formula (13) wherein R2 is hydrogen is obtained when the preparation process is carried out in the absence of an alcohol, while the compound(13) of ester form is obtained in the presence of an alcohol. In the later steps for preparing the terpene taxan derivatives of formula (2), the ester form of the compound (13) is preferred. Alcohols which can be used for preparing the ester compound includes C1-C4 alkanol, C1-C4 alkenyl alcohol, C1-C4 alkynyl alcohol and benzyl alcohol.

(2R,3S)- β -phenylisoserine of formula (1) can be prepared by eliminating the benzyl moiety as an amine-protecting group from the compound of formula (13). Specifically, the compound of formula (1) can be obtained in a high yield by refluxing N,N-dibenzyl- β -phenylisoserine together with ammonium formate in methanol in the presence of an activated Pd/C, or by pressurizing hydrogen gas into the reaction solution in the presence of an activated Pd/C. The former is preferable to the latter on an industrial scale.

In addition, the compound of formula (10) used as a starting material in the reaction scheme 3 can be prepared according to the procedure as depicted in the following reaction scheme 4. As can be seen from the reaction scheme 4, the compound (10) is prepared by reacting a compound having the following formula (4) with benzyl halide to prepare a compound having the following formula (8), reducing the compound (8) to obtain a compound having the following formula (9) and carrying out a Swern oxidation reaction on the compound (9).

Reaction Scheme 4-

NH ₂ N(Bn) ₂ N(Bn) ₂ N(Bn) ₂

(4) (8) (9) (10)

The process of reaction scheme 4 can be explained more specifically as follows. (S)-phenyl glycine (4) having high stereospecificity is used as a starting material in the preparation of the aldehyde compound of formula (10). That is, (S)-phenyl glycine is reacted with benzyl halide in aqueous solution in order to protect the amino group therein by benzyl, however through this reaction a white solid of formula (8) in which both the amino and carboxylic groups are protected by benzyl is obtained stoichiometrically. In this case, selective and quantitative protection wherein 2 equivalents of benzyl group are introduced into the amino group only is difficult to accomplish. Also, if benzoyl instead of benzyl is used as an amino -protecting group, the Swern oxidation reaction in the later step may not be easily carried out. The preparation of the compound of formula (8) is carried out without any difficulties because water is used as a solvent in this step.

The alcohol compound of formula (9) can be prepared by reducing the benzyl ester group of the compound (8) with lithium aluminum hydride, a strong reducing agent, in an organic solvent, then the alcohol compound thus obtained is used in the subsequent reaction in a crude state not purified. Suitable organic solvents for this reaction include tetrahydrofuran, toluene, diethylether, methylene chloride, benzene arid xylene.

Finally, compound (10) is prepared through out the well known Swern oxidation reaction by reacting compound (9) with oxalyl chloride and dimethylsulfoxide in solvent dichloromethane.

Conversion yield from (S)-phenyl glycine (4) to compound (1) is

61%, which is more than twice the yield (30%) of prior art reaction scheme 1. Therefore, the process of the present invention is economically superior to the earlier processes and yields the desired compound in higher stereospecificity than process of the prior art.

The present invention will be more specifically explained by the following examples. However, it should be understood that the following

examples are intended to illustrate the present invention and not to limit the scope of the present invention in any manner.

EXAMPLE 1 : Synthesis of N.N-dibenzyl-(S)-2-Dhenylglvcine benzyl este

70g (0.463mol) of (S)-2-phenylglycine(4) and 320g (2.32mol) of potassium carbonate were added to 700m£ of water, 267m£ (2.32mol) of benzyl chloride was added thereto and then the whole mixture was refluxed for 5 hours. After the reaction was completed, the reaction temperature was cooled down to room temperature. 300m£ of water was further added thereto and the resulting mixture was stirred until solid was formed. The solid thus produced was filtered, washed sufficiently with water and then dried under reduced pressure to obtain 195g (Yield 100%) of the title compound.

m.p. : 71 to 72 ^° C IR(KBr, cm ^"1 ) : 1728 [ α] ² ^ : +100 (ethanol, Cl) 1H NMR(80MHz, CDCla) : δ (ppm) 3.75(s,6H), 5.22(s,lH), 7.1~7.3(m,20H)

EXAMPLE 2 : Synthesis of N.N-dibenzyl-(S)-2-Dhenylglvcinol(9)

35.2g (0.93mol) of lithium aluminum hydride was dissolved in 1 £ of tetrahydrofuran, to which were slowly added dropwise 195g (0.463mol) of N,N-dibenzyl-(S)-2-phenylglycine benzyl ester(8) prepared in Example 1 and 0.2 I of tetrahydrofuran. This is an exothermic reaction which generates hydrogen gas. After the reaction was completed, 150m£ of water was slowly added dropwise in order to scavenge the remaining lithium aluminum hydride. After filtration, tetrahydrofuran was removed under reduced pressure from the filtrate, and then benzyl alcohol was also eliminated under reduced pressure (oil temperature of 150 to 180 ^° C, 5mmHg). According to this, 124.8g (Yield 85%) of the title compound was obtained as a liquid not distilled off but remained.

IRdiquid phase, cm ^"1 ) : 3443

[cΛ : +40 (ethanol, Cl)

1H NMR (80MHz, CDC1 ₃ ) : δ (ppm) 3.0(d,3H), 4.1(s,lH), 7.1 ~7.3(m,15H)

EXAMPLE 3: Synthesis of ⁽ 2R.3S ⁾ -N.N-dibenzyl-B-nhenylisoserine(13)

51.5roβ (0.59mol) of oxalyl chloride was dissolved in 1.5 1 of dichloromethane and the solution was cooled to -78°C. A mixture of 55.8m£ (0.79mol) of dimethylsulfoxide and 80m£ of dichloromethane was slowly added dropwise to the above solution over 30 minutes and 124.8g (0.39mol) of N,N-dibenzyl-(S)-2-phenyl glycinolO) dissolved in 0.4 1 of dichloromethane was then added dropwise thereto over 15 minutes at -78 °C . Finally, 253m£ of triethylamine was slowly added dropwise to the resulting solution. After the reaction was completed, the solution was heated to 0°C and 100m£ of water was added thereto. The reaction solution was washed twice with 600m£ of 2N hydrochloric acid and then the water contained therein was removed over anhydrous magnesium sulfate. The filtrate thus obtained was introduced into a 3 £ round- bottomed flask and temperature was adjusted to -20 ^* C. Then, 72.5g (0.39mol) of anhydrous magnesium bromide and 105m£ (0.79mol) of trimethylsilylcyanide were added dropwise thereto in due order. After the reaction was completed, 500mϋ of water was added and the resulting solution was stirred vigorously for 10 minutes. Water was eliminated from the reaction solution and diethylether was added to the organic layer in the same amount with dichloromethane exhausted during the reaction, and then the organic layer was thoroughly washed with saturated aqueous sodium chloride solution. The water contained in the organic layer was eliminated over anhydrous magnesium sulfate and the organic solvent was removed under reduced pressure to obtain the cyano compound (11).

The cyano compound (11) thus obtained was added to 600m£ of 6N aqueous hydrochloric acid solution and the resulting solution was refluxed while being heated for 1 hour. After the reaction was completed, the solid produced was filtered at room temperature. The filtered solid was

recrystalhzed from a solvent mixture of hexane : ethylacetate (1:1, by volume) to obtain the (2R,3S)-amide compound(12) having the desired high stereo selectivity (99% ee on HPLC).

To a mixture of 375m£ of cone, hydrochloric acid and 375πv£ of acetic acid was added the amide compound(12) and the resulting solution was refluxed while being heated (wherein, the amount of acetic acid is adjusted in the ratio of amide(g)/acetic acid(m£)=l/3 and the cone, hydrochloric acid is used in the same amount with the acetic acid). If the solution is allowed to stand for more than 30 minutes after the solid is totally dissolved, the solid begins to appear. The reaction was completed after about 3 hours, and then the reaction solution was filtered at room temperature. The solid obtained after filtration was recrystallized from a solvent mixture consisting of acetone and ethyl acetate (1:2, by volume) to prepare 107.7g (Yield 76%, 99% ee on HPLC) of (2R,3S)-N,N-dibenzyl- β -phenylisoserine(13).

m.p. : 218 to 220°C IR(KBr, cm ^"1 ) : 3463, 1733 +73 (ethanol, Cl)

1H NMR (80MHz, CDC1 ) : δ (ppm) 3.3(s,lH), 4.05(d,2H), 4.65(d,2H), 4.7 (d,lH), 6.1(d,lH), 7.2~7.6(m,15H)

EXAMPLE 4: Synthesis of (2R.3S)-β-Dhenylisoserine (1)

107.7g (0.3mol) of (2R,3S)-N,N-dibenzyl-(j-phenylisoserine(13) prepared in Example 3 was thoroughly dissolved in 3 I of methanol. To this solution were added dropwise 10.8g of Pd(activated Pd/C containing water, Degussa type E101 NE/W) and 75.5g (1.2mol) of ammonium formate, and then the whole mixture was refluxed while being heated for 3 hours. After the reaction was completed, the reaction solution was cooled down to room temperature and then filtered. The solvent included in the filtrate was removed under reduced pressure and the residue was recrystallized from methanol to eliminate the remaining ammonium formate, and finally 50.9g (Yield 94%, 99% ee on HPLC) of

- li ¬

the title compound was obtained.

m.p. : 236 to 238 °C IR(KBr, cm ^"1 ) : 3452, 1638, 1571 1H NMR (80MHz, CDC1 ₃ ) = δ (ppm) 4.2(d,lH), 4.5(d,lH), 7.3~7.5(m,5H)