5- pyrazolo pyridines

Scheme 72 Solvent-free synthesis of pyrazolo-pyridines...

Reproduoed with permission A Novei Method for the Synthesis of Isoxazolo and Pyrazolo Pyridines Using Hypervaient iodine Reagent A. C. S. Reddy ef al. 1997 Tayior Francis Ltd. 

Substitution of two carbons in position 2,3 of the pentadienyl anion (see Section 6.1) creates a type 2,3 system. Such molecules are the dihydropyrazolo-pyridines (DHPP) 42.50 51 The 1,3 dipolar cycloaddition reaction, which is so effective as a method for synthesizing five-membered rings, can also be used to prepare spiroheterocyclic compounds of the 2,3 type. If a suitable bromo-substituted dibenzopentafulvene is reacted with pyridine the pyrazolo-pyridine 42 is formed in 30-90% yields (Scheme 18). 

Cryptands based on a macrobicycles tetralactam scaffold bearing either a BP unit 55 or a bis-pyrazolo-pyridine unit 56 as chromophore have been described [146, 147] (Fig. 9). They showed encouraging physicochemical and photophysical properties, but to the best of our knowledge they were not further studied. 

Formation of Complex Heterocycles. The C—H activation of pyridine (generally used in excess amount) has enabled the synthesis of more complex heterocycles such as imidazo and pyrazolo pyridines as well as indolizines. Copper metal salt... 

The reaction of 1-aminopyridinium iodide (429) with dimethyl chlorofumarate in ethanol/K2C03 to form, ultimately, a pyrazolo[l,5-n]pyridine also occurs via a 1,5-dipolar mechanism. The initially formed 1 1 adduct (430), stabilized by delocalization of the negative charge, underwent disrotatory ring closure as shown to give (431) in which the 3... 

When the 1-position is substituted, 3- and 5-aminopyrazoles react at the C-4 carbon atom, the reactivity of which is enhanced by the amino group. Thus pyrazolo[3,4-Z ]pyridines (545) are obtained either by the Skraup synthesis or from 1,3-diifunctional compounds. Here also aminopyrazolinones have been used instead of aminopyrazoles to prepare (545 R = OH). If 1,4-ketoesters (succinic acid derivatives) are used instead of /3-ketoesters, pyrazolo[3,4-Z ]azepinones (546) are obtained. 

Pyrazolo[3,4-6]pyridine (545) 68AHC(6)347, 67AG981, 68CB3265, 70JHC247,... 

Pyrazolopyridines isomeric to those described previously have been obtained by other methods. Thus, the derivative (558) was formed by Raney nickel reduction of the 4-nitrosopyrazole (557) (7UHC1035), and the pyrazolo[3,4-c]pyridine derivative (560) was prepared from the azide (559) (79CC627). 

Pyrazolo[ 1,5-n]pyridine electron density, 5, 306 (71JA1887, 72CB388, 74JHC223, 65JHC410)... 

Pyrazolo[4,3-c]pyridine, 2-benzyl- H NMR, 5, 308 <73X441) Pyrazolo[3,4-b]pyridin-3-one, 6-metbyl-4-pbenyl-IR, 5, 332 <69M1250)... 

Pyrazolo[3,4-b]pyridin-4-one, 1,3,6-trimetbyl- C NMR, 5, 332 <75JHC517) Pyrazolo[3,4-b]pyridin-6-one, 1,3,4-trimetbyl- C NMR, 5, 332 <75JHC517) Pyrazolo[l,5-a]pyrimidine electron density, 5, 306 <75CJC119) Pyrazolo[3,4-d]pyrimidine electron density, 5, 306 <58JCS2973, 69CJC1129) Pyrazolo[3,4-d]pyrimidine, 4-amino- C NMR, 5, 310 <58JCS2973) Pyrazolo[4,3- /]pyrimidine electron density, 5, 306 <58JCS2973)... 

Pyrazolo[ 1,5-n Jpyridine, 2-hydroxy-tautomerism, 5, 309 Pyrazolo[3,4-6]pyridine, 3-bromo-synthesis, 5, 315... 

Pyrazolo[l,5-a]pyridine-2,3-dicarboxylic acid dimethyl ester synthesis, 5, 153 Pyrazolopyridines acylation, 5, 311 6-fused... 

Ring-methylated 1-ethynylpyrazoles were similarly obtained as minor produets in the pyrolysis of 3,5-dimethyl- and a mixture of 3- and 5-methyl- 1-propynoylpyrazoles. Pyrolysis of the 3-methyl derivative gave only pyrazolo[l,5-a]pyridin-5-ol... 

The Diels-Alder adduct 4 of dimethyl acetylenedicarboxylate and the triazepine 3 undergoes rearrangement and elimination of methancthiol to give the pyrazolo[3,4-/ ]pyridine 5.342... 

The mechanism of attack of 1,3-dipolar reagents on fluoroalkenes can be considered to be either stepwise or concerted. Heteroaromatic N-imines react by a stepwise 1,3 addition to perfluoroalkenes and -alkynes to give fluorinated pyrazolo[l,5-a]pyridines [82JCS(P1)1593]. Pyridinium /-butoxycarbonylmethylide with fluoroalkenes gave pyrrolo[l,2-a]pyri-dines [86JCS(P 1) 1769] and indolizines (22) are obtained with pyridinium phenacylide [91JFC(51)407]. 

Conversions of diazonium salts into halogenated derivatives have been reported for compounds of type 128 and 129 [73JCS(P1)2901 80JCS(P 1)2398], Although some 7-substituted pyrazolo[ 1,5-u]pyridines have been made from the lithiated precursors, no halogeno products have been reported (92JOC5538). 

The first microwave-assisted hetero-Diels-Alder cycloaddition reaction was described by Diaz-Ortiz and co-workers in 1998 between 2-azadiene 198 and the same electron-poor dienophiles as for the preparation of pyrazolo[3,4-b]pyridines 200 (Scheme 72) [127]. These dienes reacted with... 

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