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In this work we report the synthesis of new isosteric heterotricyclic derivatives, presenting the nucleus pyrazolo[3,4-b]thieno[3,2-e]pyridine 3a, pyrazolo[3,4-ö]pyrrolo[3,2-e]pyridine 3b and furo[2,3fc]pyrazolo[4,3-e]pyridine 3c. Compounds 3a-c were obtained in good overall yields, exploring the 5cartxjethoxy-6-h^roxy-3-methyl-1-phenyl-pyrazolo[3,4-6]pyridine 5 as key-intermediate. In a research program aiming the development of new bioactive compounds, we have reported previously the synthesis of new functionalized heterotricyclic compounds, belonging to pyrazolo[3,4b]thieno[2,3-d]pyridine la and their pyrrole lb and furane 1c isosteres (1), starting from chloroester derivative 2. These compounds were useful in the synthesis of new antithrombotic PAF antagonists, presenting the thienyl-containing heterocycle la as the core nucleus (2). In this context, this paper describes the synthesis of novel functional ized heterotricyclic derivatives, corresponding to pyrazolo[3,4-b]thieno[3,2e]pyridine 3a, pyrazolo{3,4-i)]pyiTolo{3,2-e]pyridine 3b and furo[2,3-b]pyrazolo[4,3-e]pyridine 3c, structurally planned as position isomers of 1a-c. which are important synthons to access a new class of bioactive compounds (3) (Figure 1). Considering our interest in construct the terminal five-member heterocycle (a, Figure 1) of desired compounds 3a-c. i.e. thiophene, pyrrole and furane rings, by applying a similar synthetic strategy to that used in preparation of 1a-c. i.e. nucleophilic displacement of chlorine atom of 2, followed by an one-pot Dieckmann cyclization, we elected the regbisomer of 2, ethyl 6-chloro-3-methyl-1-phenyl-1/-/-pyrazolo[3,4b]pyridine-5-carboxylate 4 derivative as a key intermediate (Figure 1). In order to obtain the chloro-ester derivative 4, we identify the corresponding hydroxy-ester derivative 5, as a suitable starting compound for our proposed synthesis (Figure 1). Compound 5 was prepared in 62 % overall yield, following a procedure previously described by Ahluwalia and Goyal (4), by exploring the modified Friedländer reaction of the appropriated o-amino-aldehyde-pyrazole 6, as an efficient key step to construction of these condensed bicyclic heterocycles, with unequivocal control of annelation direction by adequate location of functional groups in 6.

heterocyclic communications

SYNTHESIS OF NEW ISOSTERIC HETEROTRICYCLIC DERIVATIVES: PYRAZOLO[3,4-b]THIENO[3,2-e]PYRIDINE, PYRAZOLO[3,4-b]PYRROLO[3,2-e]PYRIDINE AND FURO[2,3-b]PYRAZOLO[4,3-e]PYRIDINE