One-Step Synthesis of Substituted alpha-Pyrones from Cyclobutenediones and Lithiated O-Silyl Cyanohydrins

Base-Catalyzed Cascade Cyclization of 2-Nitrochalcones and Isocyanides to Access Pyrano[3,4-b]indol-1(9H)-one Frameworks

An unexpected cascade reaction of 2-nitrochalcones with isocyanoacetates has been reported for the efficient synthesis of indole carboxylic esters and pyranoindoles. The conversion was achieved by KOH-catalyzed cyclization and elimination of the nitro group with final decarbonylation-aromatization. The method was used to synthesize a series of potentially biologically active indole derivatives (49 examples) in 67-85% yields under transition-metal-free catalytic conditions.

Emergence of 2-Pyrone and Its Derivatives, from Synthesis to Biological Perspective: An Overview and Current Status

Pyrone moieties are present in natural products and can be synthesized by a diverse range of synthetic methods, resulting in the formation of various derivatives through chemical modifications. Many pyrone-based derivatives are commercially available and are biocompatible. They are building blocks of various intermediates in organic synthesis. They possess remarkable biological properties including antimicrobial, antiviral, cytotoxic, and antitumor activity. These characteristics have made them valuable for the development of drugs. We have summarized recent developments in the synthesis of 2-pyrone and its derivatives and their potential applications. With regard to synthetic approaches, the focus has been on metal-free and transition metal-catalyzed reactions.

Alkynyl triazenes enable divergent syntheses of 2-pyrones

The 2-pyrone motif occurs frequently in bioactive natural products and is appreciated as synthetic intermediates. However, only few methods allow for diversifying functional group modifications on this relevant heterocycle. The distinct properties of 1-alkynyl triazenes promote a smooth addition of propiolic acids across the triple bond. Addition of catalytic amounts of silver salt induces cyclization to 2-pyrones. Depending on the reaction temperature, either 6-triazenyl or 5-triazenyl 2-pyrones are selectively formed. The triazenyl unit is subsequently replaced by a variety of valuable groups in a one-pot process yielding for instance 2-fluoro pyrones. The substitution occurs with an intriguing 1,5-carbonyl transposition. Moreover, the triazenyl group serves as traceless activating group for subsequent Diels-Alder cycloadditions and as a constituting unit for rare fused aminopyrazole pyrone heterocycles.

Recent advances in the synthesis of 2-pyrones

The present review summarizes the recent progresses in the synthesis of 2-pyrones and the application to the synthesis of marine natural products. Especially, much attention was placed on the transition metal catalyzed synthetic methodologies in this review.

Reactive iron carbonyl reagents via reaction of metal alkoxides with Fe(CO)5 or Fe2(CO)9: synthesis of cyclobutenediones via double carbonylation of alkynes

Alkoxy bases such as t-BuOK react with Fe(CO)(5) to give reactive iron carbonyl intermediates that in turn react with alkynes at 70 °C in THF to give 1,2-cyclobutenediones in 70-93% yields after CuCl(2)·2H(2)O oxidation. A novel 1,2-diacyloxyferrole derivative was isolated in the reaction of diphenylacetylene with Fe(CO)(5)/t-BuOK in the presence of acetyl chloride in contrast to the formation of a 1,4-diacyloxyferrole complex formed in the reaction using Fe(CO)(5)/Me(3)NO. The Fe(2)(CO)(9)/t-BuOK reagent system also converts the alkynes to corresponding cyclobutenediones in 63-90% yields under similar reaction conditions.

Pd-catalyzed, Cu(I)-mediated cross-couplings of bisarylthiocyclobutenediones with boronic acids and organostannanes

Bisarylthiocyclobutenedione 7 reacted smoothly with a variety of both organostannanes and (hetero)arylboronic acids in the presence of a catalytic amount of Pd and a stoichiometric amount of CuTC to produce symmetrical disubstituted cyclobutenediones in yields that range from 37 to 94% (18 examples).

Selective Cross-Couplings. Sequential Stille−Liebeskind/Srogl Reactions of 3-Chloro-4-arylthiocyclobutene-1,2-dione

The synthesis and initial reactivity studies of 2 are described. It was found that it participates in Stille couplings exclusively at the C-Cl site with a number of organostannanes (58-71% yield) in the absence of Cu(I). Then, these new derivatives were functionalized at the C-S site with boronic acids by switching to the Liebeskind-Srogl reaction conditions (in the presence of a Cu(I) carboxylate) to yield the bifunctionalized cyclobutenediones (44-90% yield).

Applications of the squarate ester cascade to the expeditious synthesis of hypnophilin, coriolin, and ceratopicanol

The first applications of the squarate ester cascade to natural products synthesis have been realized. Only 10 laboratory steps mediate the conversion of diisopropyl squarate to (+/-)-hypnophilin (8). Key reactions include a combination of chlorination, reduction, dehydration, and oxidation maneuvers in the proper sequence. A penultimate precursor to 8 has previously been converted into coriolin (9), thereby allowing a formal synthesis of racemic 9 also to be claimed. A rather different strategy was employed to arrive at (+/-)-ceratopicanol (10). Of the seven steps involved, three consisted of the use of lithium in liquid ammonia. The three divergent synthetic objectives realized in these experiments involved (a) generation of an extended enolate anion and its regioselective C-methylation at the gamma-carbon; (b) unprecedented reductive cleavage of a beta-isopropoxy group in a 2,3-diisopropoxy-2-cyclopentenone setting; and (c) conventional conversion of an alpha-alkoxy ketone to the parent carbonyl system. Thus, the appreciable enhancement in structural complexity offered by the squarate cascade holds considerable potential for the concise synthesis of constitutionally intricate targets.