Pd-Catalyzed Reductive Double Cyclization of Yne-enynals to Furans

Key words

furans - palladium catalysis - cycloreduction

Significance

A palladium-catalyzed reductive double cyclization of conjugated enynals bearing an alkyne tether leading to 2-(2-methylenecyclo­alkyl)-furans in moderate to excellent yields is reported. Hydropalladation of the internal triple bond with an in situ generated PdOCOH species is proposed as a mechanistic rationale. This is followed by carbonyl oxygen attack of the palladium nucleus and release of CO₂ during a hydride transfer reaction.

Comment

Due to their widespread occurrence in nature and their biological activities, the synthesis of furans has received increasing synthetic attention recently (see review below). Attractive transition-metal-catalyzed furan syntheses have been developed using, e.g., copper(I) salts (N. T. Patil, H. Wu, Y. Yamamoto J. Org. Chem. 2005, 70, 4531-4534) and gold(III) catalysis (T. Yao, X. Zhang, R. C. Larock J. Am. Chem. Soc. 2004, 126, 11164-11165). The herein reported method highlights an interesting double C-C bond-forming process which furnishes condensed furan systems, thus offering a reaction from simple precursors to systems of considerable complexity. Further investigation of scope and limitations would be of interest.

Review

M. Maier Organic Synthesis Highlights II; H. Waldmann, Ed.; VCH Weinheim: Germany, 1995, 231-242.