Process for stereoselective preparation of trans-azetidinones

Technical Field

The present invention relates to a process for stereoselective preparation of (3S,4S)-3-{ [(l'R)-l '-hydroxyethyl]-4-alkoxycarbonyl-2- azetidinone (I) (here-in-after, abbreviated as "trans- azetidinone") which is a useful intermediate for preparing carbaphenem and phenem type β - lactam antibiotics, starting from L-threonine, a kind of a -amino acids, which can be abundantly supplied from nature.

( I )

In the formula, Ri represents a Cι-4 lower alkyl group, and R ² , as a protective group for β -lactam ring, represents aryl group or substituted benzyl, particularly 4-methoxyphenyl group or 2,4-dimethoxy benzyl.

Background Art

The objective compounds of the present invention, trans- azetidinones of general formula (I) are known compounds, of which the process for preparation has been reported by Shiozaki et al. [Tetrahedron, Vol. 40, pi 795].

According to the disclosure, as shown in Scheme 1 , (2S,3R)-2- bromo-3-hydroxy-butyric acid (Formula A) synthesized starting from L- threonine was reacted with alkyl-N-(aryl or substituted benzyl) glycinate in the presence of a coupling agent (such as 1,3-

dicyclohexylcarbodiimide; DCC) to give hydroxybromoamide compound of general formula (B). Then the compound (B) was reacted with equivalent amount of alkali metallic strong base (such as lithium hexamethyldisilazide; LiHMDS) to obtain the epoxyamide of general formula (IV), which was then reacted with equivalent amount of alkali metallic strong base to give trans-azetidinone of general formula (I) by β -lactam ring formation. If required, the free hydroxy group of the trans-azetidinone (I) thus obtained can be protected by t- butyldimethylchlorosilane in the presence of a tertiary amine to give silyl ester azetidinone represented by general formula (C).

Scheme 1

L-threonine ( A) (B)

LiHMDS

(0 ( I ) ( IV)

In the formula, R ¹ and R ² are defined as above, and TBDMS represents t-butyldimethylsilyl group.

The present inventors have performed the reproducibility test on the basis of the process mentioned above, and found some problems in the process : 1> The byproduct(DCU, etc) generated during the condensation

reaction using 1 ,3-dicyclohexylcarbodiimide could not be readily removed, and the yield of the reaction was lower than the reported yield from epoxidation of compound (B) by using an equivalent amount of lithium hexamethyldisilazide. 2> In the C3-C β -lactam ring formation, a significant amount of stereoisomer was produced, which could hardly separated. 3> The synthetic process is troublesome and complicated and a serious side-reaction occurs in the process of synthesizing compound (I) to severely lower the yield. Thus, the known process needs to be further improved.

Disclosure of the Invention

The present inventors have performed intensive studies to overcome the problem as mentioned above, and, as a result, developed a novel process for stereoselective preparation of trans-azetidinone (I) to complete the present invention.

The object of the present invention is to provide a novel process for stereoselective preparation of the objective compound, trans-azetidinones (I), starting from L-threonine, an (2 -amino acid which can be abundantly supplied from nature, the process having higher overall yield than that of conventional process, without problems of the conventional process.

According to the present invention, (2R,3R)-epoxybutyric acid of general formula (II) was firstly obtained from L-threonine; and the compound (II) was reacted with N-arylalkylglycinate (III) synthesized by reacting arylamine with alkyl haloacetate, to obtain (2R,3R)- N- (aIkyloxycarbonyl)methyl-N-aryl-2,3-epoxybutyric amide of general formula (IV) (here-in-after, abbreviated as "epoxyamide"); then the compound (IV) was subjected to a stereoselective azetidinone ring formation to obtain the trans-azetidinone of general formula (I) [Scheme 2).

Scheme 2

/.-threon i ne ( I I ) ( I N )

( I V)

step 4

( I )

In the formula, R ¹ and R ² are defined as above.

Now, the process according to the present invention is described step by step, in more detail.

In Step 1 , the (2 -amino group of L-threonine is subjected to diazotization by the nitrous acid generated during the reaction, and it is substituted by halogen atom without the inversion of stereo-orientation; the halohydrin is converted into an epoxide by using a strong base, and then acidified to provide (2R,3R)-epoxybutyric acid of general formula (II) by a one-pot reaction. While the details are reported by a patent filed by the present inventors et al. [Tae-Sub Hwang, Korea Patent Laid- Open No. 96-41 161 ], the process is modified in the present invention in

order to perform the reaction more effectively and economically. The reagents which can generate nitrous acid in the reaction mixture of Step 1 include sulfuric acid and sodium nitrite, sulfuric acid and potassium nitrite, hydrochloric acid and sodium nitrite, or hydrochloric acid and potassium nitrite, or the like. In particular, nitrous acid was effectively generated when using 2 to 10 equivalents of 1 - 10 N hydrochloric acid and 1 to 8 equivalents of sodium nitrite. Halogen atom, particularly, chlorine atom is supplied by chloride ion (Cl ) which is generated by hydrochloric acid and functions as a nucleophilic agent, so that (2S,3R)- 2-chloro-3-hydroxybutyric acid can be obtained without additional supply of halogenating agent. Epoxidation readily occurs upon treating with 1 - 10 equivalents of sodium hydroxide in situ. At this time, the reaction temperature is preferably controlled at 0 ^° C to room temperature to inhibit initial generation of heat. The reaction mixture is then acidified by excess acid such as hydrochloric acid or sulfuric acid, and extracted from common inert organic solvent. After removing the organic solvent by evaporation under reduced pressure, relatively pure (2R,3R)-epoxybutyric acid (II), which can be directly used in the next step, is obtained in a high yield. In Step 2 of the present invention, an aryl amine and an alkylhaloacetate is reacted with a dehalogenating agent in the presence or absence of an organic solvent to prepare N-arylalkylglycinate of general formula (III). As an aryl amine, aniline, p-anisidine, 2,4- dimethoxyaniline or 3,4-dimethoxyaniline may be used. Among these, p-anisidine is preferably used in the present invention. As an alkyl haloacetate, methyl chloroacetate, methyl bromoacetate, ethyl chloroacetate, ethyl bromoacetate, ethyl iodoacetate, n- propylchloroacetate, n-propylbromoacetate, n-butyl chloroacetate, n- butyl bromoacetate, isopropyl chloroacetate, isopropyl bromoacetate, t- butyl chloroacetate or t-butylbromoacetate may be used. Among these, ethyl chloroacetate is preferably used in the present invention. The inert

organic solvents used in the step include any organic solvent which can dissolve every compounds involved in the reaction, without participating the reaction under the given reaction condition or lowering the reactivity, and minimize the side reaction. Preferable solvents include hydrocarbons such as hexane and benzene; ethers such as diethyl ether and tetrahydrofuran (THF); halogenated hydrocarbons such as dichloromethane, carbon tetrachloride, 1 ,2-dichloroethane and chloroform; esters such as methyl acetate and ethyl acetate; lower alcohols such as methanol and ethanol; and other solvents such as acetonitrile, toluene, N,N-dimethylformamide (DMF), and so on. The dehalogenating agents, that is bases, which can be used in this step include alkali metallic bases such as n-butyl lithium, lithium amide, sodium amide, sodium hydride and potassium hydride; organic tertiary amines such as triethylamine, pyridine, DBN and DBU; and alkali metal hydroxides such as sodium hydroxide, potassium hydroxide and ammonium hydroxide. Best result has been obtained when using triethyl amine as both an organic solvent and an dehalogenating agent, without using inert organic solvent. It is preferable to use triethylamine in an amount of 2 to 6 equivalents. The reaction temperature is not strictly restricted, but properly selected between room temperature and the reflux temperature.

Step 3 is a process for reacting (2R,3R)-epoxybutyric acid (II) from <Step 1> and the N-arylalkyl glycinate (III) from <Step 2> by using an amide bond coupling agent to synthesize the epoxyamide compound of general formula (IV). As the amide coupling process in this step, an acid halide process, a mixed anhydride process and an active ester process can be generally applicable. Among them, the mixed anhydride process is preferable as it minimizes the side reactions and increases the reaction yield under a mild reaction condition. The activating agents for the mixed anhydride process include ethyl chloroformate, isopropylchloroformate, and isobutyl chloroformate. Best result has

been obtained when using ethylchloroformate in an amount of 1 to 3 equivalents. While the inert organic solvents mentioned above may be used in this step, preferable is dichloromethane, chloroform, or ethyl acetate. As a compound (so-called scavenger) for removing hydrochloric acid (HC1) which is generated during this step, tertiary amines such as triethylamine, pyridine, N,N-dimethylaminopyridine, N- methylmorpholine, or bicyclic amines (e.g. DBN, DBU) may be used. It is preferable to use 1 to 5 equivalents of triethylamine or N- methylmorpholine. The reaction temperature is preferably selected between -40 ^° C and room temperature.

Though the point of Step 4 has common features of the known art reported by Shiozaki et al, it is an enhanced synthetic process having excellence from the view point of process economics and industrialization. In the step, the epoxyamide (IV) is reacted by using a catalytic amount of an alkali metal amide and a catalytic amount of a Lewis acid, or an alkali metal amide and a catalytic amount of a secondary amine to synthesize the trans-azetidinone of general formula (I) in a high yield. As an alkali metal amine, lithium amide, sodium amide, lithium hexamethyldisilazide, lithium diisopropylamide or lithium dicyclohexylamide may be used. As a Lewis acid, a halide of an amphoteric element or a transition element such as zinc, manganese, tin, titanium, aluminum or boron may be used. Among these, better results have been shown by using ZnCb or ZnBn. As a secondary amine, dimethyl amine, diethyl amine, dicyclopentyl amine, dicyclohexyl amine or hexamethyldisilazane may be used. Among these, hexamethyldisilazane is preferably used as a catalyst. The catalytic amount may be in the range of 0.01 to 0.9 equivalent. As an inert organic solvent used in Step 4, dichloromethane or THF may be used. The reaction is preferably performed at a temperature between -20 °C and reflux temperature. The step 4 according to the present invention have several advantages as compared to the prior art : 1 > The yield (84%) is

highly increased by using an equivalent of LiHMDS and a catalytic amount of ZnBn, while the prior art gives only 61% yield by using an equivalent of expensive lithium hexamethyldisilazide (LiHMDS). In addition, as a main base, lithium amide (LiNH2) which has low cost and is commonly used in the industry is used, while hexamethyldisilazane is used only in a catalytic amount, so that the step is very advantageous from the view point of economy. Finally, from the aspect of reaction mechanism, the process of the present invention performs kinetic control in the presence of dichloromethane solvent to basically inhibit the production of isomers, while the prior art performs thermodynamic control in the presence THF solvent and thus produces significant amount of isomers.

Best Mode for Carrying out the Invention The present invention is described in more detail with reference to the Examples. It should be noted that the scope of the present invention is not restricted to those Examples.

Example 1 : (2R,3R)-Epoxybutyric acid L-Threonine (20g, 0.15 mol) was added to cooled 7.5N - HC1 (90 ml) to be dissolved completely. As maintaining the reaction temperature at 5-10 ^° C , NaNθ2 (18.2 g) was added in small portions over 5 hours. The internal temperature of the reactor was chilled to 0 ^° C , and 40% NaOH solution was slowly added dropwise thereto. After stirring the mixture at room temperature for 15 hours, the mixture was acidified (pH 2.0) by adding 6N-HC1 while prohibiting the raise of the reaction temperature. The mixture was extracted from ethyl acetate (400 ml x 2), and the combined organic layer was dried over anhydrous sodium sulfate, and concentrated under reduced pressure to give relatively pure title compound (14.3 g, 90%). The product was used in the next step without further purification.

'H-NMR(300MHz, CDCh) δ ; 1.44(3H, d, J=5.33Hz), 3.38(1H, m),

3.57(1 H, d, J=4.72Hz), 9 ~ 10(acid-H, brs) ppm. [ c ]D = -10.2 (c=0.5, MeOH)

Example 2-6 : (2R,3R)-Epoxybutyric acid

The procedure of Example was repeated by using 1 equivalent of L- threonine as a starting material, to give the title compound with the yield as listed in the following Table 1.

Table 1

Example Reaction conditions ield(%)

2 1.25M-H2S04(2 eq.), NaN02(1.6 eq.), KBr(3.5 eq.) 70

3 2N-HC1(5 eq.), NaN02(2.4 eq.) 70

4 6.5N-HC1(5 eq.), NaN02(2.4 eq.) 78

5 7.5N-HC1(5 eq.), NaN02(2.4 eq.) 90

6 8.5N-HC1(5 eq.), NaN02(2.4 eq.) 65

Example 7: Ethyl N-p-methoxyphenyl glycinate p-Anisidine (20 g, 0.16 mol) was dissolved with heating in triethylamine (100 ml). While maintaining the interior temperature of the reactor at 50 ^° C , ethyl chloroacetate (23.3 ml, 0.22 mol) was added thereto. The resultant mixture was stirred under reflux for 30 minutes. After the completion of the reaction, the interior temperature of the reactor was slowly lowered. Water/methanol (2/1 ) solution (500 ml) was added thereto, and the resulting mixture stirred vigorously to obtain pale yellow crystals. The solid was filtered under reduced pressure and dried in vacuo to give yellow brown pure title compound.

Η-NMR(300MHz, CDCh) δ ; 1.26(3H, t, J=7.2Hz), 3.74(3H, s), 3.86(2H, s), 4.23(2H, m), 6.6(2H, d, J=10Hz), 6.78(2H, d, J=10Hz)ppm.

Example 8-12: Alkyl N-p-methoxyphenyl glycinate

By using 1 equivalent of p-anisidine and 1.33 equivalents of alkyl haloacetate, the title compounds are obtained in various reaction conditions as follows :

Table 2

Example R X Reaction conditions Yield(%)

8 Et Br Et3N / cat. DMAP, THF, r.t., 6 hr 62

9 Et Br /?-anisidine(excess), THF, reflux, 2 hr 76

10 Et Cl Et3N, CH2C12, reflux, 18 hr 77

1 1 Me Cl Et3N(excess), reflux, 30 min 90

12 t-Bu Cl Et3N(excess), reflux, 30 min 90

Example 13: (2R,3JR)-N-(Ethoxycarbonyl)methyl-N-p- methoxyphenyl-2,3-epoxybutyric amide

(2R,3R)-Epoxybutyric acid (14.3 g, 0.1 mol) was dissolved in chloroform (300 ml), and the solution chilled to -30 ^° C . N- methylmorpholine (20 ml, 0.18 mol) was added, and ethylchloro formate (17.4 ml, 0.18 mol) was slowly added dropwise thereto. The mixture was stirred vigorously for 30 minutes. To the reaction mixture, Ethyl N-p-methoxyphenyl glycinate (29.2 g, 0.14 mol prepared in Example 7 was added, and the reaction temperature was raised to room temperature. Then the reaction mixture was stirred for 10 hours to complete the reaction. After the reaction, the organic layer was washed with an appropriate amount of dilute hydrochloric acid, 5% NaHC03 solution and saturated brine, sequentially, dried over anhydrous sodium sulfate, and concentrated under reduced pressure to give crude title compound (45 g). The crude product was purified by a column chromatography (eluent: EA/Hex = 1/2) to obtain pure pale-yellow title compound (34.5 g, yield: 84%).

Η-NMR(300MHz, CDCh) δ ; 1.27(3H, t, J=7.2Hz),

1.42(3H, d, J=5.4Hz), 3.05(1H, m), 3.30(1 H, d, J=4.5Hz), 2.83(3H, s), 4.14(1H, d, J= 17.1 Hz), 4.19(2H, q, J=7.2Hz), 3.66(1 H, d, J= 17.1 Hz), 6.95(2H, d, J=10Hz), 7.29(2H, d,J=10Hz)ppm.

Example 14-21: (2R,3R}-N-(Ethoxycarbonyl)methyl-N-p- methoxyphenyl-2,3-βpoxybutyric amide

On the bases of the procedure described in Example 13, the reaction was performed by using 1 equivalent of (2R,3R)-epoxybutyric acid and applying a variety of coupling method, to obtain the title compounds as follows :

Table 3

Example Coupling agent (eq.) Solvent & reaction Yield(%) temperature

14 DCC(1.5)/DMAP(0.2) THF, 0 ^° C 78

15 ClCOO--Pr(1.3)/TEA(1.6) THF,-30 ^° C→r.t. 54

16 ClCOO--Pr( 13)1 NMM( 1.3) EA,-30 ^° C→r.t. 66

17 ClCOO-t-Bu(1.3)/NMM(1.3) CHC13,-30 ^° C→r.t. 82

18 ClCOOEt(1.3)/TEA(1.8) EA, -30 ^° C→r.t. 75

19 ClCOOEt(1.3)/TEA(1.8) CHC13,-30 ^° C→r.t. 82

20 ClCOOEt( 1.3)/ NMM( 1.3) CH2C12,-30 ^° C→r.t. 63

21 ClCOOEt(1.3)/NMM(1.3) EA,-30 ^° C→r.t. 70

Example 22: (3S _> 4S)-3-[(l'R)-l ^, -Hydroxyethyl]-4-ethoxy carbonyl- 1 - p-methoxyphenyl-2-azetidinone

(Method A)

The epoxyamide (IV) (20 g, 68 mmol) was dissolved in THF (300 ml) under nitrogen atmosphere, and ZnBn (2.3 g, 10 mmol) was added

II

thereto. The reaction mixture was then chilled to - 10 ^° C , and 1 M lithium hexamethyldisilazide (80 ml, 80 mmol) was rapidly added dropwise thereto. After slowly raising the reaction temperature to 0 ^° C , the reaction was quenched by adding an appropriate amount of dilute hydrochloric acid, and the reaction mixture was diluted with dichloromethane (600 ml). The separated organic layer was washed with 10% NaHC03 solution and saturated brine, and concentrated under reduced pressure to give crude title compound. The residue was purified by a short column chromatography (CFLCh/acetone = 20/1 ) to obtain pure title compound (17.2 g, yield: 86%) as pale yellow needle crystals.

Η-NMR(300MHz, CDCh) δ ; 1.27(3H, t, J=7.2Hz), 1.39(3H, d, J=6.4Hz), 2.51(1 H, d, J=4.5Hz), 2.36(1 H, dd, J=2.5 and 4.0Hz), 3.77(3H, s), 4.27(2H, m), 3.35(1H, m), 4.57(1H, d, J=2.5Hz), 6.85(2H, d, J=10Hz),

7.23(2H, d, J=10Hz)ppm.

(Method B)

The epoxyamide (IV) (20 g, 68 mmol) was dissolved in dichloromethane (400 ml) under nitrogen atmosphere, and lithium amide (3.1 g, 136 mmol) and hexamethyldisilazane (1.6 ml, 7.48 mmol) were added thereto. At the point of starting reflux, ethanol (1 ml) was added thereto, and the mixture was heated under reflux for 5 hours. After completion of the reaction, the work-up procedures according to (Method A) were repeated to obtain pure title compound (14.4 g, yield: 84%)