Pyrazole fused systems

Alkyl-3-chloro-4-cyano-l -hydrazino-5,6,7,8-tetrahydro-2,7-naphthal-pyridines react with nitrous acid to form the azides, which are in equilibrium with the tetrazoles (Scheme 29). Unlike other examples of this equilibrium, these appear to exist primarily as the azides. As such, they have a number of handles (such as the chloride and cyano), which allow them to be transformed into other fused systems including two new tet-racycles. Reaction of the azides with amines proceeded via nucleophilic substitution of the chloride at C-3.With more forcing conditions of higher temperature and an excess of the amine, the azido can be replaced by nucleophilic substitution, which is rare for these compounds. When the azides were treated with hydrazine, a pyrazole-fused system is formed.With ethyl mercaptoacetate, a thiazole-fused system is formed (14T8648). 

Bielectrophiles have found appreciable applications in the synthesis of ring-fused systems, especially those involving [5,6] fused systems. The following serve to illustrate these applications. Reaction of pyrazole with (chlorocarbonyl)phenyl ketene (214) (Type 1, Scheme 6) readily formed the zwitterionic pyrazolo[l,2-a]pyrazole derivative (215) (80JA3971). With l-methylimidazole-2-thione (216), anhydro-2-hydroxy-8-methyl-4-oxo-3-phenyl-4//-imidazo[2,l-6][l,3]thiazinium hydroxide (217) was obtained (80JOC2474). 

Phenyl-l,2-dithiolylium salt (483) with hydrazine, methylhydrazine or phenylhydrazine yielded the corresponding pyrazole (485) via the intermediates (484a-c). The ring-fused system (486) is a convenient source of the ring-fused pyrazole (487) when treated with hydrazine (see Chapter 4.31). 

Ring contraction and intramolecular cyclization constitute a convenient route to ring-fused systems that would be difficult to synthesize in other ways. H- 1,2-Diazepines (538) undergo electrocyclic ring closure to the fused pyrazole system (539) (71CC1022). Azepines also undergo similar valence bond isomerizations. 

The pyrazole analogues of anthranilic acids or anthranilonitriles are a convenient source of [5.6] fused systems (for a general review see (80T2359)). Thus 5-amino-4-cyanopyrazoles (in some examples an ester or a hydrazido group replaced the cyano group) have been transformed into pyrazolo[3,4-d]pyrimidines (552) and into pyrazolo[2,3-e]diazepinones (553), and 4-amino-5-methoxycarbonylpyrazoles have been converted into pyrazolo[4,3-d]pyrimidines (554). 

Pyrazoles can be prepared by ring opening reactions of fused systems already containing the pyrazole nucleus. Thus several [5.5], [5.6] and [5.7] fused heterocycles have been opened to substituted pyrazoles, usually in basic medium. In general, the method has little preparative interest since another pyrazole derivative has usually been used to build the ring-fused system. However, due to the unexpected structures obtained, two publications are worthy of notice. 6//-Cyclopropa[5a,6a]pyrazolo[l,5-a]pyrimidine (638) was readily obtained from the corresponding pyrazolopyrimidine by the action of diazomethane at room temperature (Scheme 59) (81H(15)265). When (638) was treated with potassium hydroxide, the pyrazole (640) was formed, probably via the diazepine (639). 

A series of pyrazolo[3,4-, pyridazinones 430 and analogues, potentially useful as peripheral vasodilators, were synthesized and evaluated as inhibitors of PDE5 extracted from human platelets. Several of them showed ICso values in the range 0.14-1.4 pM. A good activity and selectivity profile versus PDE6 was found for compound 430 (6-benzyl-3-methyl-l-isopropyl-4-phenylpyrazolo[3,4-r/]pyridazin-7(6/7)-one). Structure-activity relationship studies demonstrated the essential role played by the benzyl group at position 6 of the pyrazolopyridazine system. Other types of pyridazinones fused with five- and six-membered heterocycles (pyrrole, isoxazole, pyridine, and dihydropyridine), as well as some open-chain models were prepared and evaluated. Besides the pyrazole, the best of the fused systems proved to be isoxazole and pyridine <2002MI227>. 

Crystallographic studies show that pyrazole-fused derivative 136 forms staircase-like 7t-stacked rods <2003TL9161>. AMI calculations have been reported for the pentacyclic system 137 <1996H(43)1991>. 

Most syntheses of imidazo-fused systems involve cyclization of amino substituted heterocycles (or their tautomers), suitably substituted at an adjacent ring position. This provides a convenient route to imidazo[2,1 -ft]thiazoles, imidazo[1,2- ]imidazoles (Scheme 4), imidazo[l,2-6]pyrazoles, imidazo[l,5- Jimidazoles and imidazo[l,5-c]thiazoles. 

The molecule contains a sulfonamide and a benzene ring as well as the part that interests us most—a bicydic aromatic heterocyclic system of a pyrazole fused to a pyrimidine. We shall discuss in detail how Pfizer made this part of the molecule and just sketch in the rest. The sulfonamide can be made from the sulfonic acid that can be added to the benzene ring by electrophilic aromatic sulfonation (Chapter 22). 

The conditions used for nucleophilic attack on pyridine and the diazines are probably too severe and would cause decomposition of the fused systems considered here. Further, the 6-fused pyrazoles would clearly be deactivated to nucleophilic attack and in the c-fused pyrazoles, electron withdrawal by the pyrazole ring would make attack on the six-membered ring difficult. However, displacement of a suitably positioned halide or other good leaving group is often possible. 

Representative examples for various therapeutic areas show how pyrazole or fused pyrazole ring systems can have varied applications in drug discovery efforts. Only nonfused pyrazole ring examples will be shown in the examples below fused pyrazole rings systems are presented in Volumes 9 and 10. 

The cheletropic extrusion of SO2 from heterocyclic sulfones is now the most widely used and flexible route to the r)-quinodimethanes. The sulfones are stable, easily handled and readily synthesised. The ease of this thermal elimination depends on the bond order of the sulfolene 3,4-bond. It is normally in the range 150-200 C for a typical aromatic heterocyclic fused system but can be much higher for 2,3-naphtho fused and other systems, e.g. 11, where this bond order is reduced by bond fixation. The extrusion temperature for the 2-substituted pyrazoles 29 is significantly higher than for the 1-substituted isomers 30 and, in contrast to the... 

Many methods for the preparation of pyrazole-fused ring systems were published. The different structural types are listed in the table below. 

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