The renin-angiotensin-aldosterone system plays an important role in the pathogenesis of hypertension in man. A component of this system, angiotensin I-converting enzyme (a peptidyldipeptide hydrolase, EC 3.4.15.1), has been known which converts the relatively inactive decapeptide angiotensin Ⅰ to the potent vasoconstrictor octapeptide angiotensin Ⅱ. Angiotensin I-converting enzyme also has been designated as kininase Ⅱ or bradykininase, because it inactivates bradykinin, a nonapeptide hormone of blood plasma that has vasodilating effect.
Competitive inhibition of angiotensin I-converting enzyme has received considerable attention for the treatment of hypertension in mankind. Recently M. A. Ondetti, D. W. Cushman and their colleagues have reported the development of catopril (SQ 14,225), an orally active competitive inhibitor of angiotensin I-converting enzyme. However, the chemical synthetic method of captopril developed by M. A. Ondetti $\underline{et al}$. shows a very low yield and also has fault that $\underline{L}$-proline $\underline{tert}$-butyl ester, which we have difficulties in preparing, was used. Such a synthetic scheme also requires the tedious column operation in order to separate and remove acetate derivatives or side products produced during the ammonolysis of acetyl group used to protect the 3-mercapto function.
So as to overcome this drawbacks, new chemical synthesis of captopril was approached in this research, and contained the following scheme :
(1) the conjugate (or 1,4) addition of anhydrous hydrogen chloride to the methacrylic acid
(2) the preparation of acyl chloride by means of thionyl chloride in the presence of $\underline{N}$, $\underline{N}$-dimethylformamide as a catalyst
(3) the direct $\underline{N}$-acylation reaction of $\underline{L}$-proline by the Schotten-Baumann procedure
(4) the resolution of diastereomeric acyl $\underline{L}$-proline by using of dicyclohexylamine
(5) the substitution of chloride by sodium hydrogen sulfide or sodium thiosulfate/$H^+ (H_2O)$
As expected, the first step of synthetic route followed the anti-Markovnikov's rule or more rigorously Michael-type addition, and showed an almost quantitative yield. The preparation of $\underline{DL}$-3-chloro-2-methylpropanoyl chloride was carried out by using only about 10 percent of the stoichiometric quantity of$\underline{N}$, $\underline{N}$-dimethylformamide in thionyl chloride (yield : 95%). 1-($\underline{DL}$-3-chloro-2-methylpropanoyl)- $\underline{L}$-proline synthesized by the Schotten-Baumann procedure was prepared in a high yield. The dicyclohexylammonium salt of 1-($\underline{DL}$-3-chloro-2-methylpropanoyl)- $\underline{L}$- proline was obtained as a white crystal in 90% yield. The diastereomeric acyl $\underline{L}$-proline was resolved by the following solvent system :
(1) acetonitrile - dichlomethane (1:1)
(2) 2-propanol
(3) ethyl acetate - hexane (1:1)
The final step is to substitute sulfhydryl function for chloride, and in progress. The overall yield of 1-($\underline{D}$-3-chloro-2-methylpropanoyl)- $\underline{L}$-proline was 33 percent.
Angiotensin I 전환 효소의 강력한 경쟁적 억제 작용을 기전으로 하는 고혈압 치료제로서 최근에 개발된 captopril (SQ 14,225) 의 새로운 화학적 합성에 관해 연구하였다. 1977년 M. A. Ondetti, D. W. Cushman 등에 의하여 보고된 합성방법은 제조하기 어렵고 수율도 낮은 $\underline{L}$-proline $\underline{tert}$-butyl ester를 사용하였고 따라 $\underline{N}$,$\underline{N}$-disubstituted amide bond 존재 하에서 선택적으로 $\underline{tert}$-butyl group 을 deprotection해야되는 결점을 가지고 있다. 또한 acetyl group 을 sulfhydryl function 의 보호기로 사용하므로써 deprotection 할 때 생성되는 acetate 유도체를 분리 제거하기 위해 column operation을 요한다. 이와 같은 문제점을 제거하기 위해 먼저 $\underline{L}$-proline tertbutyl ester를 이용하지 않고 Schotten-Baumann 반응에 의한 직접적인 $\underline{L}$-proline 의 $\underline{N}$-acylation 을 시도해 보았고 좋은 수율을 얻을 수 있었다. 또한 protective group의 사용을 피하기 위하여 합성 route 의 최종단계에서 sulfhydryl 기의 치환을 생각해 보았다. 위와 같은 점을 고려하여 다음과 같은 합성 방법을 시도 하였다.
1. Methacrylic acid 에 대한 염화수소의 공액 (혹은 1,4) 부가
2. DMF 를 촉매로 하여 thionyl chloride 에 의한 acyl chloride의 제조.
3. Schotten-Baumann 반응에 따라 alkali 존재하에서 acyl chloride에 의한 $\underline{L}$-proline 의 $\underline{N}$-acylation.
4. Dicyclohexylamine 을 사용한 diastereomer resolution.
5. Sodium hydrogen sulfide에 의한 sulfhydryl function 도입.
예상되어지는 바와 같이 methocrylic acid 에 대한 염화수소의 공액부가는 anti-Markovnikov's rule 혹은 보다 엄밀히 Michael-type addition 을 따랐고, 거의 정량적인 수율을 나타냈다.
Acyl chloride 의 제조는 DMF 를 촉매로한 thionyl chloride 의 사용에 의해 95% 의 높은 수율을 얻었다. Schotten-Baumann 반응에 의한 $\underline{L}$-proline 의 $\underline{N}$-acylation 반응도 대단히 좋은 시도였고 백색의 dicyclohexylammonium salt 로서 90% 의 수율을 얻을수 있었다.
여러가지 용매계를 이용하여 diastereomeric acyl $\underline{L}$-proline을 분리했으며, 43% 회수율을 보였다. 황산 수소 칼륨 수용액으로 처리하고 ethyl acetate 로 추출, 탈수, 용매 제거, 결정화 하여 90% 의 회수율로 1-($\underline{D}$-3-chloro-2-methylpropanoyl)-$\underline{L}$-proline 의 백색 결정을 얻었다. 이 합성 route 의 마지막 단계인 sulfhydryl function 치환 반응은 진행 중이다.
1-($\underline{D}-3-chloro-2-methylpropanoyl)-$\underline{L}$-proline의 전반적인 수율은 33% 였다.
