One-Pot Organocatalytic Tandem Aldol/Polycyclization Reactions between 1,3-Dicarbonyl Compounds and α,β,γ,δ-Unsaturated Aldehydes for the Straightforward Assembly of Cyclopenta[b]furan-Type Derivatives: New Insight into the Knoevenagel Reaction

Biomimetic Domino Knoevenagel/Cycloisomerization Strategy for the Synthesis of Citridone A and Derivatives

Studies on Knoevenagel condensations between conjugated dienals and 4-hydroxy-2-pyridone/quinolone-type 1,3-dicarbonyl equivalents led to the development of a simple one-pot strategy to access citridone A and related synthetic cyclopenta[b]furopyridones/quinolones. The present work highlights the power of domino cascades in the synthesis of natural product frameworks and may help promote future studies on this promising new class of pyridone alkaloids.

Oxa-[3+3] annulation of MBH-carbonates of propiolaldehydes with α-nitro/bromo ketones to access 2H-pyrans

A novel alkyne-assisted annulation reaction of MBH-carbonates of propiolaldehydes with α-nitro/bromo ketones is reported, providing a facile synthesis of substituted 2H-pyrans in good yields. This reaction divulges the inimitable reactivity of the MBH-carbonates of propiolaldehydes as C3-synthons wherein the alkyne functionality promoted the reaction without participating in the oxa-[3+3] annulation. The obtained products, having alkyne and ester functionalities, allowed further annulations to generate diverse pyrano[3,4-c]pyran-1-ones.

Iodine-Catalyzed Iso-Nazarov Cyclization of Conjugated Dienals for the Synthesis of 2-Cyclopentenones

Molecular iodine was identified as an efficient catalyst for the cycloisomerization of conjugated dienals to substituted 2-cyclopentenones. DFT calculations suggested an unexpected concerted character for this cyclization.

Chiral Brønsted Acid Catalyzed Enantioselective Dehydrative Nazarov-Type Electrocyclization of Aryl and 2-Thienyl Vinyl Alcohols

An efficient chiral Brønsted acid-catalyzed enantioselective dehydrative Nazarov-type electrocyclization (DNE) of electron-rich aryl- and 2-thienyl-β-amino-2-en-1-ols is described. The 4π conrotatory electrocyclization reaction affords access to a wide variety of the corresponding 1 H-indenes and 4 H-cyclopenta[ b]thiophenes in excellent yields of up to 99% and enantiomeric excess (ee) values of up to 99%. Experimental and computational studies based on a proposed intimate contact ion-pair species that is further assisted by hydrogen bonding between the amino group of the substrate cation and chiral catalyst anion provide insight into the observed product enantioselectivities.

NMR and experimental reinvestigation of the condensation reaction between γ-methylene-α,β-unsaturated aldehydes and propargyl aldehydes

An experimental and computational study of the reaction between γ-methylene enals and propargyl aldehydes uncovered a deconjugative aldol condensation.

Preparation of Substituted 2H-Pyrans via a Cascade Reaction from Methyl Coumalate and Activated Methylene Nucleophiles

The reaction of methyl coumalate with a wide range of methylene active compounds, such as keto-esters or keto-sulfones and cyclic or acyclic diketones, afforded more than 30 2,3,5,6-tetrasubstituted 2H-pyrans. The reaction proceeds via a cascade reaction involving a Michael addition-6π-electrocyclic ring opening-proton transfer and 6π electrocyclization, in which a variety of functional groups were tolerated.

The iso-Nazarov reaction

The acid-promoted cycloisomerization of conjugated dienals and linearly-conjugated dienones for the synthesis of five-membered ring systems is reviewed.

Hypervalent Iodine(III)-Induced Domino Oxidative Cyclization for the Synthesis of Cyclopenta[b]furans

A new strategy for cyclopenta[b]furan synthesis mediated by hypervalent iodine(III) has been described. The approach employs diacetoxyiodobenzene-induced initial dehydrogenation to a putative trienone intermediate and triggered sequential cycloisomerization to form the cyclo-penta[b]furan targets.