One-pot multicomponent diastereoselective synthesis of novel dihydro-1H-furo[2,3-c]pyrazoles

Recent developments in the synthesis and applications of furopyrazoles

This review aims to describe the different strategies developed so far for the synthesis of furopyrazoles and their biological and pharmacological properties.

Synthesis of 2H-furo[2,3-c]pyrazole ring systems through silver(I) ion-mediated ring-closure reaction

Fused pyrazole ring systems are common structural motifs of numerous pharmaceutically important compounds. Nevertheless, access to derivatives of the aromatic 2H-furo[2,3-c]pyrazole ring system is still quite limited, and their chemistry and functional properties remain largely underexplored. The current study investigates routes to construct this system from easily accessible starting materials using metal-catalyzed reactions. A simple and efficient procedure to access the 2H-furo[2,3-c]pyrazole ring system was developed by employing the silver(I) ion-mediated ring-closure reaction of 4-alkynyl-3-hydroxy-1-phenyl-1H-pyrazoles as a key step. The required intermediate hydroxyalkynyl substrates for this reaction were prepared by a Pd-catalyzed coupling of 4-iodo-1-phenyl-1H-pyrazol-3-ol with ethyne derivatives. The structures of the obtained target compounds were unequivocally confirmed by detailed ¹H, ¹³C and ¹⁵N NMR spectroscopic experiments, HRMS and a single-crystal X-ray diffraction analyses. This silver(I)-mediated 5-endo-dig cyclization of readily available 4-alkynyl-3-hydroxy-1H-pyrazoles can be used as an efficient method to access many novel 2,5-disubstituted 2H-furo[2,3-c]pyrazoles.

Microwave Domino Diastereoselective Synthesis of Novel Trans-4,5-Dihydro-1H-Furo[2,3-c]Pyrazoles Using Pyridinium Salts in an Aqueous Medium

Novel fused 4,5-dihydro-1 H-furo[2,3- c]pyrazole derivatives containing both biologically active pyrazole and furan templates are synthesised by a one-pot two-step four-component domino reaction involving hydrazine hydrate, a β-keto ester, an aromatic aldehyde and a pyridinium salt catalysed by DABCO with high diastereoselectivity in H2O under microwave irradiation. To minimise the formation of byproducts, the hydrazine hydrate and ethyl acetoacetate were first irradiated until a pyrazolone was formed. Next, the aryl aldehyde, the pyridinium salt and DABCO were added and the reaction could be completed in good to excellent yields. The salient features of this eco-friendly methodology are highlighted by its short reaction time (10–12 min), high yields, high atom-economy, efficiency of producing five new bonds (2C–C, C=N, C–N and one C–O), two new rings and two stereocentres in a single operation, absence of any tedious work-up or purification and avoidance of separation of intermediates.