Saari et al.
746 Journal of Medicinal Chemistry, 1978, Vol. 21, No. 8 (8) E. L. Stephen, J. W. Dominik, J. B. Moe, and J. S. Walker, Antimicrob. Agents Chemother., 10, 549 (1976). (9) E. W. Larson, E. L. Stephen, and J. S. Walker, Antirnicrob. Agents Chemother., 10, 770 (1976). (10) J. N. Dowling, B. Postic, and L. 0. Guevarra, Antimicrob. Agents Chemother., 10, 809 (1976). (11) E. DeClercq, M. Luczak, D. Shugar, P. F. Torrence, J. A. Waters, and B. Witkop, Proc. Sac. Exp. Biol. Med., 151,487 (1976). (12) J. B. Arensman, J. W. Dominik, and D. E. Hilmas, Antimicrob. Agents Chemother., 12, 40 (1977). (13) H. Renis, Arch. Virol., 54, 85 (1977). (14) G. A. Galegov, N. L. Pushkarskaya, N. P. Ohrosova-Serova, and V. M. Zhdanov, Experientia, 33, 906 (1977). (15) L. Dudycz, D. Shugar, E. DeClercq, and J. Descamps, J . Med. Chem., 20, 1355 (1977). (16) C. B. Zuniga, C. deAlmeida, A. C. L. Iervoline, I. 0. Castro. and P. A. A. Galvao, Rev. Assoc. Med. Bras., 20,386 (1974). (17) P. A. A. Galvao and I. 0. Castro, Rev. Bras. Clin. Ter., 3, 221 (1975). (18) P. A. A. Galvao and I. 0. Castro, Ann. A'. Y. Acad. Sci., 284, 278 (1977). (19) D. Huggins and G. J. M. Pereira. Rec. Bras. Med., 34,733 (1977). (20) F. Salido-Rengell, H. Nassar-Quinones, and B. BrisenoGracia, Ann. N.Y. Acad. Sci., 284: 272 (1977). (21) C. R. Magnussen, R. G. Douglas, Jr.?R. F. Betts, F. K. Roth. and M. P. Meagher, Antimicrob. Agents Chemothw., 12, 498 (1977). (22) H. F. Zertuche and R. D. Perches, Ann. N.Y. Acad. Sei.. 284, 284 (1977). 123) R. Lorenco, M. J. F. Camargo, and I. 0. Castro, Re[,.Bras. Med., 33, 401 (1977). (24) S. 0. Esper Dib, I. L. Scholz, and C. A. .Arroyo, Sem. Med. Mexico, 92, 245 (1977).
(25) D. G. Streeter, L. N. Simon, R. K. Rohins, and J. P. Miller, Biochemistry, 13,4543 (1974). (26) R. C. Willis, D. A. Carson, and J. E. Seegmiller, Proc. Natl. Acad. Sci., U.S.A., in press. (27) J. P. Miller, L. J. Kigwana, D. G. Streeter, L. N. Simon, and J. Roboz, Ann. N.Y. Acad. Sci., 284, 211 (1977). (28) D. G. Streeter, J. T. Witkowski, G. P. Khare, R. W. Sidwell, R. J . Bauer, R. K. Robins, and L. M. Simon, Proc. Natl. Acad. Sci. U.S.A., 70, 1174 (1973). (29) M. Smith, G. I. Drummond, and H. G. Khorana, J . Am. Chem. SOC.,83, 698 (1961). (30) T. IJeda and I. Kawai, Chem. Pharm. Bull., 18,2303 (1970). (31) J. G. Moffatt and H. G. Khorana, J . Am. Chem. Soc.. 83, 649 (1961). (32) R. W. Sidwell and J . H. Huffman, Appl. Microbiol., 22,797 (1971). (33) E'. Scheffler, D. Hagchenas, and Wigand, Acta Virol., 19, 106 (1975). (34) J . H. Huffman, R. W. Sidwell, G. P. Khare, J. T. Witkowski, I,. B. Allen, and R. K. Robins, Antimicrob. Agents Chemother., 3, 236 (1973). (35) G. P. Khare, R. W. Sidwell, J. T. Witkowski. L. N. Simon, and R. K. Robins, Antimicrob. Agents Chemother., 3,517 (1973). (36) R. W. Sidwell, J . H. Huffman, N. Campbell. and I,. B. Allen, Ann. N.Y.Acad. Sci., 284, 239 (1977). (37) C. Sholtissek, Arch. Virol., 50, 349 (1976). (38) W. E. G. Muller, A. Maidhof, H. Taschner, and R. K. Zahn, Biochern. Pharmacol., 26, 1071 (1977). (39) B. Eriksson, E. Helgstrand, N. G. Johansson, A. Larson, A. Misiorny, J. 0. Noren, L. Philipson, K. Stenberg, G. Stening, S. Stridh, and B. Oberg, An,timicrob. Agpnts Chemother.. 11, 946 (1977). (40) B. Oberg and E. Helgstrand, Proc. Int. Congr. Chemother., loth, 1977, 1, 332-334 (1978).
Synthesis and Antihypertensive Activity of Some Ester Progenitors of Methyldopa W a l f r e d S. Saari,* M a r k €3. F r e e d m a n , R i c h a r d D. H a r t m a n , Stella W. King, Andrew W. R a a b , William C. R a n d a l l , Edward L. E n g e l h a r d t , R a l p h H i r s c h m a n n ,
Merck Sharp & Dohme Research Laboratories, West Point, Pennsyli,vmia 19486 Avery Rosegay,
Merc?2 Sharp & Dohme Research Laboratories, Rahu,ay, N r u Jersey 07065 C a r l T. L u d d e n , a n d Alexander Scriahine
Merck Institute for Therapeutic Research. West Point, Pennsyluanin 19486. Receiced Januar?, 19. 1978 A variety of esters of methyldopa was synthesized with the objective of obtaining derivatives that would be more efficiently absorbed from the gastrointestinal tract than the free amino acid and would undergo conversion to methyldopa readily in the blood or target tissues. Two of the esters, tu-pivaloyloxyethyl( 4 4 and cu-succinimidoethyl (4w),were found to be more potent antihypertensive agents than methyldopa in animal models and were selected for further study in man. The amino esters were prepared by three different methods?including direct esterification of methyldopa without the use of N-or 0-protecting groups.
M e t h y l d o p a (1) is an effective, orally active a n t i h y pertensive a g e n t which has f o u n d wide use for t h e reduction of blood pressure in hypertensive subjects. In an a t t e m p t t o e n h a n c e oral absorption and thereby improve a n t i h y p e r t e n s i v e potency, a variety of m e t h y l d o p a derivatives was synthesized with t h e objective of obtaining compounds that would be more efficiently absorbed from t h e gastrointestinal t r a c t than t h e free a m i n o acid a n d would u n d e r g o conversion to m e t h y l d o p a readily in t h e blood or target tissues. Of' these derivatives, several novel esters of methyldopa were found to have antihypertensive activity in t h e spontaneously hypertensive (SH) rat. T w o O@22-2623/78;1821-0;46$01.@@'0
esters, 4u a n d 4w, which exhibited good oral a b s o r p t i o n a n d were particularly p o t e n t in h y p e r t e n s i v e a n i m a l models, were selected for further study in man. T h i s report describes t h e synthesis of s o m e m e t h y l d o p a esters a n d their evaluation as antihypertensive agents in t h e SH rat. Chemistry. T h e acid 2b, in which the catechol a n d a m i n o functions of m e t h y l d o p a are p r o t e c t e d as the diphenyl ketal' and the benzylcarbamate, respectively, could b e p r e p a r e d easily f r o m 1 in 52% overall yield. Initial a t t e m p t s t o esterify 2b t h r o u g h carboxyl activation, e.g., w i t h dicyclohexylcarbodiimide, and s u b s e q u e n t reaction with alcohols were generally unsuccessful. T h i s is probably
F 1978 American Chemical Society
Ester Progenitors of Methyldopa
Journal of Medicinal Chemistry, 1978, Vol. 21, No. 8 747 H
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due to the a-methyl and N-Cbz groups which provide sufficient steric hindrance to discourage reaction a t the carboxyl carbonyl. However, 2b could be esterified to 3 by alkylation of its triethylamine salt in DMF with suitable halides. In this method, reaction occurs a t a position one atom removed from the carboxyl carbon and therefore is less subject to steric hindrance. This esterification procedure, developed originally by Schwyzer,2has been applied recently to the synthesis of esters of hindered carboxylic acidse3Removal of the amino and catechol blocking groups of 3 under catalytic hydrogenation conditions led to the esters 4 in satisfactory overall yields (Tables I and 11). Alternatively, direct alkylation of the unprotected amino acid, presumably as the dipolar ion 1, in a polar, aprotic solvent such as DMF, Me2S0, or HMPA proved to be a more convenient route to the a-methylamino ester^.^ Esters 4a,p,s,vwere prepared by this one-step procedure in 37-6170 yields. That only modest yields were obtained in this reaction is probably a consequence of the fact that in these solvents the starting acid anion, 1, is generally a stronger base than the resulting amino ester. As a result, the esterification reaction does not proceed to completion because the remaining amino acid anion becomes protonated as the reaction proceeds. Two esters, the 2-acetamidoethyl (4f) and the 3-acetamidopropyl (4j), were prepared from methyldopa and the corresponding alcohol with SOCl2 in low yields. Since the presence of a quarternary chiral center in (S)(or L-)a-methyldopa precludes inversion or racemization during preparation of the esters in this series, the amino acid portion of these esters retains the S configuration. When the alcohol portion contains an additional chiral center, e.g., the a-succinimidoethyl (4a) and the a-pivaloyloxyethyl (4t) esters, two diastereomeric isomers are possible. In these cases, the pure diastereomers, 4u, 4w, and 4x, arbitrarily designated a and p, were obtained either through fractional crystallization of the isomeric mixture or, in the case of 4a, by removing the protecting group of the purified a and 6 isomers of 3. The configuration of the second chiral center of 4u, 4w, and 4x was not determined. The required alkylating agents 3-chloromethyl-lmethylhydantoin (6) and N-chloromethylglutarimide were synthesized by hydroxymethylation of 5 or glutarimide with formaldehyde, followed by reaction with S0Cl2. Addition of HC1 to N-propenyl- (sa) or N-vinyl- (8b) succinimides provided 9a and 9b in good yield. NPropenylsuccinimide in turn was prepared by Fe(CO)5 catalyzed isomerization of N-allylsuccinimide (7)by the procedure of Barolo and R o s ~ i .a-Chloroethyl ~ pivalate was obtained by reaction of pivaloyl chloride with acetaldehyde in the presence of ZnC126. Biological Results and Discussion. The relative
antihypertensive potencies following oral administration to the S H rat of some of the more potent esters from the series are compared with methyldopa and its ethyl ester in Tables I and 11. Except for the nicotinamidoethyl ester 4b, all esters having a heteroatom separated from the carboxylate oxygen by two or more carbon atoms proved to be less active than methyldopa. Of those esters having a one carbon atom separation of heteroatoms, the pivaloyloxy and succinimidoalkyl derivatives, 4a and 4s-x, were found to be more potent than methyldopa. It is interesting to note that ring expansion of the succinimide group to glutarimide 4k, conversion to the hydantoin 4m, or lengthening the alkyl chain to a-ethyl, 4c, led to a decrease in antihypertensive activity. On the basis of the antihypertensive data in Tables I and 11, the crystalline esters 4u and 4w were selected for further pharmacological study. The pivaloyloxyethyl ester 4u was tested in S H rats a t 6,18, and 54 mg/kg ip and at 6,18,54, and 162 mg/kg PO, with a minimum of six animals in each treatment group. Antihypertensive activity was observed at 18 mg/kg ip or PO and higher doses, while no significant activity was seen a t 6 mg/kg ip or PO. The maximal antihypertensive effect was reached by either route at 2-8 h after treatment, with a duration of action exceeding 1 2 h and in some experiments even 24 h. The effect ranged from a 17 mmHg fall in mean arterial pressure a t 18 mg/kg PO to 56 mmHg at 162 mg/kg PO. The succinimidoethyl ester 4w was tested in SH rats at 9.5, 19, and 38 mg/kg ip and 4, 9.5, 12, 19, 36, and 38 mg/kg PO, with six animals in each treatment group. In contrast to 4u, this ester exhibited antihypertensive activity a t all doses used. The maximal effect was usually observed a t 2-4 h following ip and at 4-12 h following PO administration. As with 4u, the duration of action usually exceeded 12 h and in some experiments even 24 h. The antihypertensive effect of 4w ranged from 7 mmHg a t 4 mg/kg PO to 30 mmHg at 38 mg/kg PO. The duration of antihypertensive action of either 4u or 4w a t the highest doses used was longer than that of methyldopa a t 90 mg/kg PO (maximal dose used in our experiments). At this dose, the duration of action of methyldopa exceeded 8 but not 12 h. At doses considerably higher than those used in SH rats (50-240 mg/kg PO),esters 4t and 4w also lowered systolic arterial pressure in conscious renal hypertensive African green monkeys. The antihypertensive effect in this species did not exceed 20 mmHg with either compound a t any of the doses used. Hydrolysis of the esters was studied at 37 "C over a wide range of pH values in order to explore the possibility that antihypertensive activity might correlate with the rate of ester hydrolysis. In these studies, liberated methyldopa was determined fluorometrically following reaction with an alkaline aqueous solution of o-~hthaldehyde.~ Solutions of esters capable of forming aldehydes upon hydrolysis,
748 Journal of Medicinal Chemistry, 1978, Vol. 21, No. 8
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