(S)-Atenolol

(S)-atenolol

• Description
  Selective β₁ adrenoceptor antagonist
• Biological descriptionSelective β₁ adrenoceptor antagonist. Orally active. Limited ability to cross the blood-brain barrier. Antihypertensive activity in vivo.

Properties

• Chemical name(S)-(-)-4-[2-Hydroxy-3-[(1-methylethyl)amino]propoxy]benzeneacetamide
• Molecular Weight266.34
• Chemical structure
• Molecular formulaC₁₄H₂₂N₂O₃
• CAS Number93379-54-5

The compound (R,S)-atenolol (4-[2-hydroxy-3-[(1-methylethyl)amino]propoxy]-benzeneacetamide) is useful as a β-adrenegic blocker for the treatment of angina pectoris, arrhythmia and hypertension. It is known that atenolol is a 1-aryloxy-3-aminopropane-2-ol derivative wherein the hydroxy bearing carbon is an asymmetric carbon and hence exists as R- and S-isomers. It is also known that the S-isomer is particularly useful as a β-adrenegic blocker in view of its superior pharmacological activities. It is reported that S-atenolol has hypotensive activity and activity on brachycardia (A. A. Pearson, T. E. Gaffney, T. Walle, P. J. Privitera; J. Pharmacol. Exp. Ther., 250(3), 759, 1989).

In prior art, the optical resolution of racemic atenolol has been studied to obtain the desired optically active atenolol, however, any practical method has not been reported so far. It is also reported that the diastereomers of atenolol having high purity is obtained from racemic mixture by using (R,R)-O,O-di-toluoyltartaric acid anhydride (M. J. Wilson et al., J. Chromatogr. (NLD) 431 (1), 222–227, 1988). However, this method is not suitable for large scale production of optically active atenolol as it requires a large volume of solvent and further it is technically very troublesome to recycle (R,R)-O,O-di-toluoyltartaric acid anhydride.

Another method of preparing optically active atenolol has been proposed in JP-A-50-77331 and DE-A-2453324:

Wherein Z is halogen atom or sulphonyloxy group, and * means asymmetric carbon.

However, this process has some disadvantages as this process requires several steps for obtaining optically active S-atenolol stating from D-manitol; moreover the yield of S-atenolol by this process is less than 50% and the optical purity is just about 80% ee.

Another method for the preparation of S-atenolol has been reported in U.S. Pat. No. 5,223,646 which consists of reacting sodium salt of 4-carbamoylmethylphenol with R-epichlorohydrin at 0° to 35° C. to obtain an intermediate—an optically active glycidyl ether and then reacting the optically active intermediate glycidyl ether with isopropylamine to obtain S-atenolol (see also EP-435068 A2; EP-605384; JP 03077856 A2).

It has also been reported that the above procedure gives optically active glycidyl ether and atenolol of 90–96% ee optical purity. According to this report, the optical purity of atenolol may be enhanced to 98% or higher, if the intermediate optically active glycidyl ether is repeatedly recrystallised from a suitable solvent.

It has also been reported that the optically active atenolol in an optical purity of 98% or higher can be produced from atenolol of lower optical purity by converting it to its salt with Bronsted’s acid (K. Kazuhiro; T. Yosikazu; F. Yoshiro; Y. Hiroshi; O. Junzo, Chem. Pharm. Bull., 46(3), 505–507, 1998).

The separation of the atenolol salt having higher optical purity (>98% ee) is carried out by dissolving the atenolol salt having lower optical purity in a solvent, precipitating solid materials having a high content of racemic atenolol salt, and then isolating the desired atenolol salt having higher optical purity (>98% ee) by solid-liquid separation method. The optically active salt having high optical purity is then subjected to removal of acid moiety to isolate the desired optically active atenolol in free form. Though this process yields atenolol of higher optical purity, it involves salt formation and tedious separation of racemic salt from an optically active salt, which leads to the lower yields of desired optically active atenolol. Further, the salt has to be converted to free atenolol either by neutralisation or using ion exchange resins. Thus, this process gives lower overall yield of the desired optically active atenolol is low.

There is therefore a need to provide a process whereby S-atenolol may be obtained in high yield and high optical purity.

US6982349

Emcure Pharmaceuticals Limited

http://www.google.com/patents/US6982349

Satish Ramanlal Mehta, Baburao Manikroa Bhawal, Vishnu Hari Deshpande, Mukund Keshav Gurjar

Accordingly, the present invention provides a process for the preparation of (S)-atenolol (1), which comprises the steps of:

• • a) reacting a phenol of formula 2:

    with an (R)-epichlorohydrin of formula (3):

    in presence of an alkali metal hydroxide and a quaternary ammonium salt as phase transfer catalyst (PTC) in an aqueous solution at a temperature in a range of −10° C. to 0° C. to obtain optically active intermediate glycidyl ether of formula 4:
  • b) reacting the optically active intermediate glycidyl ether (4) with isopropylamine at 10° to 40° C. to obtain (S)-atenolol of the formula 1:

    in good chemical yield and high optical purity (>99 ee).

One major advantage of this process is that S-atenolol may be obtained directly without going through the cumbersome step of recrystallization or additional salt formation step, as in the prior art.

The aqueous alkali metal hydroxide used in the process is selected from sodium hydroxide or potassium hydroxide and is used as aqueous solution in 1 to 1.5 moles to 1 mole of the phenol 2. The (R)-epichlorohydrin (3) used in the process is preferably of high optical purity and used in an amount of 1 to 3 moles, more preferably 1 to 1.6 moles, to 1 mole of phenol (2).

The quaternary ammonium salt has the formula:
R¹R²R³R⁴N⁺X⁻

Wherein R¹, R², R³ and R⁴ are same or different, each an alkyl group having 1 to 16 carbon atoms (e.g. methyl, ethyl, propyl butyl etc), phenyl or benzyl, X is chlorine, bromine, iodine, hydrogen sulphate or hydroxyl group. The amount of quaternary ammonium salt used is 0.001 to 2% by weight of phenol (2).

The Applicant studied the reaction temperature extensively and found that it plays an important role in deciding optical purity of (S)-atenolol (1) formed via optically active glycidyl ether. When the reaction of phenol (2) and (R)-epichlorohydrin is carried out at 5° C. or at any other higher temperature, (S)-atenolol (1) of a lower optical purity was obtained via optically active glycidyl ether, as for example in EP 435068.

The Applicant, after studying the prior art processes found that during the course of these reactions, the phenoxide (or phenol) attacks the C-1 carbon atom of (R)-epichlorohydrin with the expulsion of chloride to yield (R)-glycidyl ether, which on reaction with isopropyl amine gives (R)-atenolol. The original epoxide ring remains unchanged in the reaction.

Thus, the reaction of phenol (2) at carbon centre C-1 of (R)-epichlorohydrin by nucleophilic displacement of chlorine leads to the formation of undesired (R)-atenolol via optically active (R)-glycidyl ether as a side product, which accounts for the low yield of optically active S-atenolol in the prior art.

The Applicant then conducted this reaction at a lower temperature and found to their surprise that S-atenolol could be obtained in high yield. The reason is that during the course of reaction, the phenoxide (or phenol) ion attacks the C-3 carbon atom of (R)-epichlorohydrin and opens the epoxide ring. The new epoxide ring formation takes place by the attack of O⁻ on C-3 carbon with expulsion of chloride to give (S)-glycidyl ether, which on reaction with isopropyl amine gives (S)-atenolol. Thus, the reaction of phenol (2) at carbon centre C-3 of (R)-epichlorohydrin leads to the formation desired (S)-atenolol (1) as a major product via optically active glycidyl ether (4).

The lower optical purity in (S)-atenolol formation in the prior art may therefore be on account of the slow reaction rate at carbon atom 1 and the high yield of S-atenolol obtained by the process of the present invention may be due to the reaction at carbon atom 3 of (R)-epichlorohydrin (3). Both these reactions occurring on different atoms are shown as path ‘a’ and path ‘b’ in the following scheme herebelow.

Path ‘a’ is the process of the present invention whereas path ‘b’ is the process of the prior art.