Coupling Reactions of Hydralazine with Amino Acids and their Adducts for Antihypertensive Activities. Sherine N. Khattab [a], Abdel Moneim M. El Massry [a], Ayman El-Faham [a], Adnan A. Bekhit [b] and Adel Amer [a][1] [a] Department of Chemistry, Faculty of Science, Alexandria University, Egypt [b] Department of Pharmaceutical Chemistry, College of Pharmacy, Alexandria University, Egypt Received October 13, 2003 Dedicated to the memory of Prof. Hans Zimmer J. Heterocyclic Chem., 41, 387 (2004). Despite the fact that hydralazine (phthalazin-1-yl- hydrazine - Apresoline) 1 was introduced in hyperten- sion therapy for more than 50 years, the mechanism of its action is still obscure [2]. The hydrazine group being a reactive pharmacophore might actively react with any of the C=O of the biological receptors to exert its effect [3,4]. It is notable that the so called N-acetyl derivative of 1 which was found to be among the major metabolites, was reassigned as the hydrate form of the stable triazolophtha- lazine system, namely, 3-methyl-s-triazolo[3,4-a]phthalazine (3-MeTP) [5-7]. Consequently, an extensive acylation of 1 was studied using a variety of acids and derivatives and attempts were made to fuse six, seven and eight-membered rings to the phthalazine ring system for better understanding of this molecule chemical behavior [8-18]. Furthermore, up to our knowledge no trial was carried out to deliver hydralazine supported on amino acids or peptide moiety as a pro-drug concept for improving its bioavailability and decreasing its undesirable side effects. Spurred by the afore- mentioned finding, it became interesting enough to assign a synthetic program to obtain such triazolophthalazines sup- ported on amino acid or peptide moiety. The open-chain counter-parts were also not far from our attention. Results and Discussion. Chemistry. The reaction of phthalazin-1-yl-hydrazine hydrochloride 1 . H C l with different N-Boc-L-amino acids 2 a - 2 e i n dimethylformamide in the presence of diisopropylethy- lamine (DIEA) as a base and N-[(Dimethylamino)-1 H - 1,2,3-triazolo[4,5-b]pyridin-1-ylmethylene]- N- m e t h y l- methanaminium hexafluorophsphate N-oxide (HATU) as coupling reagent at 0 °C gave smoothly the triazolophtha- lazine derivatives 3a-3e. The structures of compounds 3a- 3e were established by mean of ir, 1 H nmr, 13 C nmr and elemental analyses. The infrared spectra of these com- pounds showed absorption peak of the urethane carbonyls in the range 1690-1711 cm -1 and the absorption peak of the N-H in the range 3234-3359 cm -1 . The 1 H nmr showed only one exchangeable signal for NH and the structure of 3 was secured by 13 C nmr where a peak corresponding to the amide carbonyl was detected and only the urethane car- bonyls were observed at 148.10-155.94 ppm. On the other hand, molecular modeling supported the idea that the introduction of a bulky R group might provide an avenue to stabilize the open intermediate structure by keeping distance between the two active sites (N-2 of the phthalazine ring and the amide carbonyl side chain), with the hope that such designed structure comply with the enhanced characters or pro-drug requirements. Indeed it was the case. Thus, utilizing the N-Boc-L-(O-benzyl)-tyro- sine 2f (R= CH 2 -[4-(OBn)C 6 H 4 ]) afforded compound in the open structure and this was also supported by ir, 1 H nmr, 13 C nmr and elemental analysis. The infrared spec- trum of 4 showed an absorption peak for the amide car- bonyl at 1663 cm -1 , an urethane carbonyl at 1710 cm -1 and the absorption peak of the N-H at 3442 cm -1 . The 13 C nmr showed also a peak corresponding to the amide carbonyl at 172.16 ppm in addition to the urethane carbonyl peak at 166.88 ppm. Sterric hindrance may prevented ring closure in com- pound 4 as the distance between the N-2 nitrogen atom and the amide carbonyl carbon atom is larger. The N-C dis- tance was compared for the intermediate open structure 4 w i t h d i fferent R group utilizing PM3/MOPAC [19] (Table 1, Figure 2 and 3). Table 1 The N-C Bond Distance of Compound 4 R N-C Bond Distance CH 3 3.460 Aº CH(CH 3 ) 2 3.503 Aº CH(CH 3 )CH 2 -CH 3 3.546 Aº CH 2 -OBn 3.461 Aº CH 2 -CH 2 -COOBn 3.376 Aº CH 2 -[4-(OBn)C 6 H 4 ] 4.035 Aº Unfortunately, it was found that this intermediate was very sensitive to acidic medium. Thus, deprotection of the Boc group by passing HCl gas into a solution of com- pounds 3a-3e and compound 4 as well as in methylene chloride led to the formation of the corresponding tria- zolophthalazine 5 as stable white hydrochloride salts in good yield. Deprotection of the Boc group from com- pounds 3a-3e and 4 was carried out with p - t o l u e n e s u l- phonic acid (2 equivalent) in acetonitrile, and gave the May-Jun 2004 387