Synthesis and Evaluation of Polycyclic Pyrazolo[3,4-d]pyrimidines as PDE1 and PDE5 cGMP Phosphodiesterase Inhibitors Yan Xia,* Samuel Chackalamannil, Michael Czarniecki, Hsingan Tsai, Henry Vaccaro, Renee Cleven, John Cook, Ahmad Fawzi, Robert Watkins, and Hongtao Zhang Schering-Plough Research Institute, 2015 Galloping Hill Road, Kenilworth, New Jersey 07033 Received July 29, 1997 X Polycyclic pyrazolo[3,4-d]pyrimidines (represented by 3 and 4) were synthesized as analogues of the recently reported polycyclic guanine phosphodiesterase (PDE) inhibitors. From the structure-activity relationship (SAR) development of a series of compounds, it was discovered that C-3 benzyl and N-2 methyl disubstitution on the pyrazole ring gave the best combination of potency and selectivity for PDE1 and PDE5 cGMP PDEs as represented by compound 4c: PDE1, IC 50 ) 60 nM; PDE3, IC 50 ) 55 000 nM; PDE5, IC 50 ) 75 nM. These compounds were also evaluated in vivo and found to be good orally active antihypertensives in laboratory animal models. Finally, comparisons were made of the in vitro and in vivo profiles of the pyrazolo- [3,4-d]pyrimidine compound 4c with those of two representative guanine compounds. Introduction The emerging field of cyclic nucleotide level regulation by the isozyme family of phosphodiesterases (PDE) has brought renewed interest in PDE inhibition as a target for therapeutic intervention. 1 Recently the role of cGMP levels and the potential beneficial effects of inhibition of the cGMP PDE for the treatment of cardiovascular diseases have been reviewed. 2 The considerable interest in this area has been the focus of a large number of programs directed at the identification of new chemical classes of inhibitors of cGMP PDE. 3 We have recently disclosed our discovery of a new class of polycyclic guanine PDE inhibitors which are illustrated in Chart 1. 4 These compounds are potent inhibitors of PDE1 and PDE5 cGMP PDEs in vitro and potent oral antihyper- tensives in vivo. The desirable properties of the poly- cyclic guanine compounds prompted us to investigate analogues of these guanines to discover other related classes of PDE inhibitors. In particular, we were interested in pyrazolo[3,4-d]pyrimidine analogues (such as 3 and 4 in Chart 2) where an isomeric pyrazole ring replaces the imidazole ring in the polycyclic guanine structure. 5 Prior to our work, there was a brief patent description of 5-hexyl-7-methylpyrazolo[3,4-d]pyrimi- dine-4,6-dione as a PDE inhibitor. 6 Since the initiation of our work, there have been several recent reports of pyrazolo[3,4-d]pyrimidines and pyrazolo[4,3-d]pyrim- idines as potent and selective PDE5 inhibitors. 7 Here we describe the synthesis of polycyclic pyrazolo- [3,4-d]pyrimidines and their in vitro and in vivo evalu- ations as PDE1 and PDE5 cGMP PDE inhibitors. From the structure-activity relationship (SAR) development of this series of compounds, a new class of potent PDE1 and PDE5 cGMP PDE inhibitors with oral antihyper- tensive activity was discovered. Synthesis The syntheses of the polycyclic pyrazolo[3,4-d]pyri- midine compounds are outlined in Schemes 1-3. Scheme 1 shows the preparation of starting materials 5b-e. Condensation of ethyl cyanoacetate with orthoacetate gave 5b. 8 Condensation of ethyl cyanoacetate with phenylacetyl chloride gave an enol intermediate (5c) which was O-alkylated to give 5d or converted to the chloro intermediate 5e by treatment with POCl 3 and tributylamine. 9 The pyrazole compounds of type 3 were prepared as outlined in Scheme 2. Intermediates 7b-e were pre- pared based on a literature synthesis of pyrazolo[3,4- d]pyrimidine 7a from 5a via the pyrazole intermediate 6a. 10 Condensation of 5a,b,d with methylhydrazine or benzylhydrazine gave pyrazoles 6b-e. Treatment with X Abstract published in Advance ACS Abstracts, December 1, 1997. Chart 1. Polycyclic Guanine PDE Inhibitors Chart 2. Pyrazolo[3,4-d]pyrimidine Analogues Scheme 1 a a Reagents: (i) CH3C(OEt)3, Ac2O, reflux, 23%; (ii) BnCOCl, NaH, THF; (iii) Ag2CO3, EtI, 41% (from ethyl cyanoacetate); (iv) POCl3, n-Bu3N, 40%. 4372 J. Med. Chem. 1997, 40, 4372-4377 S0022-2623(97)00495-0 CCC: $14.00 © 1997 American Chemical Society