Phosphine-Catalyzed Synthesis of 6-Substituted 2-Pyrones:  Manifestation of E/Z-Isomerism in the Zwitterionic Intermediate

Abstract

[reaction: see text] We report a one-step phosphine-catalyzed annulation between aldehydes and ethyl allenoate to form 6-substituted 2-pyrones. The mechanistic rationale for this reaction requires explicit discussion of the E/Z-isomerism of the zwitterionic intermediate formed by the addition of a phosphine to the allenoate. Sterically demanding trialkylphosphines facilitate the shift of equilibrium toward the E-isomeric zwitterion and lead to the formation of 6-substituted 2-pyrones. Various aromatic as well as aliphatic aldehydes undergo the transformation in moderate to excellent yield.

Lewis Base Catalysis in Organic Synthesis

“…[138] Interestingly, Kwon and co-workers showed that the phosphane-catalyzed reaction of allenoates and aldehydes does not give the expected formal [3+2] cycloaddition products, 2,5-dihydrofurans. Depending on the catalyst struc-ture and reaction conditions, 1,3-dioxan-4-ylidenes 91 [139] or 2-pyrones 92 [140] can be prepared with high selectivity (Scheme 32). This change in the reaction pathway is traced to the fate of the intermediate zwitterion XL.…”

Lewis‐Base‐Katalyse in der organischen Synthese

“…[136] Das Verfahren wurde erfolgreich in den Totalsynthesen von Hirsuten [137] und Hinesol genutzt. [138] Interessanterweise erhielten Kwon und Mitarbeiter bei der phosphankatalysierten Reaktion von Allenoaten mit Aldehyden nicht die erwarteten 2,5-Dihydrofurane als die formalen [3+2]-Cycloadditionsprodukte, sondern es entstanden -je nach Katalysatorstruktur und Reaktionsbedingungenhoch selektiv die 1,3-Dioxan-4-ylidene 91 [139] oder die 2-Pyrone 92 [140] (Schema 32). Diese Abweichung wird auf das intermediäre Zwitterion XL zurückgeführt.…”

Aliphatic aldehydes in the synthesis of carbo- and heterocycles: Part II. Synthesis of six- and seven-membered rings, bicyclic compounds, and macrocycles

“…Condensation of acetaldehyde (1f) with Scheme 34. Despite fairly large number of methods reported for the synthesis of pyran-2-ones, a simple convenient one-step economic procedure for the preparation of 6-alkyl(aryl)pyran-2-ones was proposed in [121]; compound 109 was thus synthesized by reaction of butyraldehyde (1b) with ethyl buta-2,3-dienoate (110) (Scheme 37). Cycloaddition of dienes to carbonyl compounds underlies a procedure for the synthesis of 5,6-dihydropyrans.…”