Pyridazine derivatives 32: Stille-based approaches in the synthesis of 5-substituted-6-phenyl-3(2H)-pyridazinones

A series of 6-phenyl-3(2H)-pyridazinones bearing different substituents in the 5-position of the pyridazinone ring were prepared using Stille-based approaches in the search for new platelet-aggregation inhibitors.     

Pyridazine derivatives. Part 38: Efficient Heck alkenylation at position 5 of the 6-phenyl-3(2H)-pyridazinone system

Several 6-phenyl-3(2H)-pyridazinones bearing different alkenyl groups at position 5 have been prepared in the search for novel antiplatelet agents. The target compounds were synthesised by a palladium-catalysed Heck cross-coupling reaction. Variable amounts of 4-phenyl-6-substituted-2-phthalazinones were isolated as by-products during these experiments. The crucial issues for successful Heck coupling in these systems concern the protection of position 2 of the heterocyclic ring and the use of tris(o-tolyl)phosphine as a ligand.     

Pyridazine derivatives. Part 40: Reactivity of 5-alkynylpyridazin-3(2H)-ones toward hydrochloric acid

5-Alkynylpyridazin-3(2H)-ones or 5-(2-chloroalkenyl)pyridazin-3(2H)-ones were isolated during the cleavage of the methoxymethyl group in a series of 5-alkynyl-2-methoxymethylpyridazin-3(2H)-ones by treatment with hydrochloric acid. The efficient and selective cleavage of the methoxymethyl group in these compounds can be performed under mild conditions by employing aluminium chloride.     

Pyridazines. Part 34: Retro-ene-assisted palladium-catalyzed synthesis of 4,5-disubstituted-3(2H)-pyridazinones

The efficient one-pot bis-functionalization of the 4,5-positions of the 3-pyridazinone ring has been performed using Suzuki, Sonogashira and Stille cross-coupling reactions assisted by a retro-ene fragmentation. This route allows access in a shorter synthetic sequence to several pharmacologically useful 3(2H)-pyridazinones.     

Facile synthesis of 4,5-disubstituted-3(2H)-pyridazinones

4,5-Disubstituted-3(2H)-pyridazinones were initially synthesized via sodium alkoxide additions to an advanced bromide intermediate. A small parallel chemistry effort resulted in a poor success rate, and we thus increased the reactivity of the reaction partner by performing a copper-catalyzed Finkelstein reaction. Copper-catalyzed coupling of a diverse set of alcohols with the resulting iodide resulted in a more successful effort. A number of alternative syntheses of this series of compounds are also described and these methods proved to be versatile, efficient, and amenable to parallel synthesis.     

6-Chloro-2(2H)-pyranone: a new 2(2H)-pyranone synthon

6-Chloro-2(2H)-pyranone, which can be prepared in high yield from commercially available trans-glutaconic acid, undergoes facile Pd/Cu-catalyzed reaction with various 1-alkynes to give rise to the corresponding 6-(1-alkynyl)-2(2H)-pyranones in moderate to good yields. These last hitherto unknown compounds have been used as direct precursors to 6-alkyl- and 6-[(Z)-1-alkenyl]-2(2H)-pyranones.     

Convenient synthesis of 5-methylene-4-substituted-2(5H)-furanones

A synthesis of novel 4-(substituted)benzyl-5-methylene-2(5H)-furanones involving Stobbe condensation of substituted aldehydes with ethyl levulinate followed by treatment with acetic anhydride in the presence of sodium acetate, has been developed.     

First synthesis of 4,5-dihydro-3(2H)-pyridazinones via Zn-mediated hydrohydrazination

The hydrohydrazination of 4-pentynoic acid with different arylhydrazines proceeds smoothly in the presence of zinc chloride. The domino amination-amidation sequence leads to aryl-substituted 4,5-dihydro-3(2H)-pyridazinones.     

Regioselective synthesis of 6-benzylthiazolo[3,2-b]1,2,4-triazoles during Sonogashira coupling

The reaction of 3-mercaptopropargyl-1,2,4-triazoles with various iodobenzenes catalyzed by Pd-Cu leads to the regioselective formation of 6-benzylthiazolo[3,2-b]1,2,4-triazoles 4. The structure of 4d was confirmed by X-ray analysis.     

Novel efficient synthesis of 3,4-dihydro-6-substituted-3-phenylpyrimidin-2(1H)-ones

A new attractive and convenient strategy for the synthesis of 3,4-dihydro-6-substituted-3-phenylpyrimidin-2(1H)-ones is described. Photolysis (λ = 254 nm) of 4-allyl-tetrazolones in alcoholic solutions produces the corresponding pyrimidinones as the sole product in nearly quantitative yields, with simultaneous extrusion of molecular nitrogen. Work-up procedures consist in the simple evaporation of the solvent under reduced pressure, in mild conditions. Heats of reaction for the photocleavage process of 4-allyltetrazolones were calculated, indicating high stability of the resulting products.     

Pyridazines. Part 31: synthesis and antiplatelet activity of 4,5-disubstituted-6-phenyl-3(2H)-pyridazinones

This paper describes the synthesis and the antiplatelet activity of a series of 4,5-disubstituted-6-phenyl-3(2H)-pyridazinones. Some of these compounds show a dose-dependent activity and were found to be more active than their 5-substituted analogues.     

Pd-Cu catalyzed heterocyclization during Sonogashira coupling: synthesis of 2-benzylimidazo[1,2-a]pyridine

The reaction of 2-amino-1-(2-propynyl)pyridinium bromide with various iodobenzenes, catalyzed by Pd-Cu, leads to the formation of 2-benzylimidazo[1,2-a]pyridines.     

A convenient access to furo[3,2-c]pyridin-6(5H)-ones by the reaction of 5-iodo-4-methoxy-2-pyridones with terminal alkynes under microwave-enhanced Sonogashira conditions

N-Alkyl-3-aryl-5-iodo-4-methoxypyridin-2(1H)-ones readily undergo sequential acetylide cross-coupling, demethylation, and furan annulation under classical Sonogashira reaction conditions to furnish 7-arylfuro[3,2-c]pyridin-4(5H)-ones, a class of hitherto unknown compounds, in a one-pot operation. Microwave irradiation was found to significantly reduce reaction times and to allow lower catalysts and reagent loadings.     

Pyridazine derivatives. XIX: Functionalization studies at the 5 position in the 6-phenyl-3(2H)-pyridazinone system

A series of 6-phenyl-3(2H)-pyridazinones bearing different substitution at the 5 position of the pyridazinone ring were prepared in the search for new platelet aggregation inhibitors. The structure of the final compounds was determined on the basis of spectroscopics methods.     

Microwave assisted copper-free Sonogashira coupling/5-exo-dig cycloisomerization domino reaction: access to 3-(phenylmethylene)isoindolin-1-ones and related heterocycles

An efficient microwave assisted one-pot synthesis of substituted 3-(phenylmethylene)isoindolin-1-ones is reported via a copper-free Sonogashira coupling and a regioselective 5-exo-dig cycloisomerization. This domino reaction was also extended to other related heterocycles.     

Carbonylative Sonogashira annulation sequence: One-pot synthesis of 4-quinolone and 4H-chromen-4-one derivatives

Carbonylative Sonogashira annulation sequence for one pot synthesis of 4-quinolone and 4H-chromen-4-one has been developed in presence of Pd-NHC catalyst. Substituted 2-iodoaniline and 2-iodophenol independently underwent in the carbonylative Sonogashira annulation reaction with a variety of acetylenes to result in 4-quinolone and flavone derivatives respectively in good to excellent yield. Moreover, this protocol does not require toxic CO gas, high catalyst loading and any expensive salt/additive. Herein we, for the first time, are using Mo(CO)₆, as solid CO source for the one pot synthesis of flavone derivatives via carbonylative Sonogashira annulation reaction.     

Facile access to 4-(1-alkynyl)-2(5H)-furanones by Sonogashira coupling of terminal acetylenes with β-tetronic acid bromide: efficient synthesis of cleviolide

A mild and convenient synthesis of 4-(1-alkynyl)-2(5H)-furanones has been achieved by Sonogashira or Heck-type alkynylation of β-tetronic acid bromide. As an illustration of this methodology, the natural product cleviolide was prepared in two steps and 78% overall yield.     

Synthesis of 6-substituted-4,5-dihydro-3(2H)-pyridazinones

A convenient synthetic method for the pharmaceutically important 6-substituted-4,5-dihydro-3(2H)-pyridazinones is described. The synthetic strategy is based on Δ-soxazolines chemistry which were in turn unmasked by N? O bond cleavage and cyclized to the target compounds.     

Regioselective synthesis of cytotoxic 4-(1-alkynyl)-substituted 2-(5H)-furanones

4-(1-Alkynyl)-3-bromo- and 4-(1-alkynyl)-3-chloro-2(5H)-furanones have been regioselectively synthesized in moderate to good yields by a new version of the Pd/Cu-catalyzed Sonogashira reaction involving treatment of 1-alkynes with 3,4-dibromo- and 3,4-dichloro-2(5H)-furanone, respectively, in the presence of KF as a base. 4-(1-Alkynyl)-3-bromo-2(5H)-furanones have been found to be able to undergo Stille-type and Suzuki-type reactions with aryl(tributyl)tins and arylboronic acids, respectively, to give 4-(1-alkynyl)-3-aryl-2(5H)-furanones in modest to satisfactory yields. Some 4-(1-alkynyl)-substituted 2(5H)-furanones so prepared have been found to exhibit significant cytotoxic activities, especially against human leukemia cell lines.     

Pyridazine derivatives. Part 39: Reactivity of 5-iodopyridazin-3(2H)-ones in palladium-catalysed reactions

In the search for novel antiplatelet agents, convenient and efficient methods for the preparation of 2,5-disubstituted pyridazin-3(2H)-ones are reported that utilise palladium-catalysed cross-coupling reactions. A post-coupling base-promoted isomerisation has been observed during Sonogashira alkynylation of 5-iodopyridazin-3(2H)-ones (3) with 1-phenyl-2-propyn-1-ol. Variable amounts of phthalazinones were isolated as by-products during the Heck alkenylation of 3. The usefulness of the hydroxymethyl fragment as a protecting group during the synthesis of 5-substituted pyridazin-3(2H)-ones has been validated.