One-Pot Synthesis ofE-Alkylidene Derivatives of 3H-Furan-2-ones from 4-Pentynoic Acid and Aldehyde

Pentynoic acid reacts with aldehyde under the co-cata- lyst system of Wilkinson's complex and 2-aminopyridine to give E- alkylidene derivatives of 3 H-furan-2-one through the transition metal-catalyzed cyclization of 4-pentynoic acid followed by con- densation of the resulting lactone with aldehyde. Interest in the chemistry of 5-substituted E-alkylidene de- rivatives of 3H-furan-2-one has recently emerged because of its potential antifungal uses such as preserving agricul- tural crops and wooden products. 1 However, only a few synthetic methods for these compounds have been report- ed, 2-4 and many individual reaction steps 2 or high pressure reaction conditions 3 are required to obtain these com- pounds. Recently, we developed an efficient co-catalytic system of Wilkinson's complex and 2-amino-3-picoline for the in- termolecular hydroacylation of alkenes or alkynes with al- dehydes giving ketones or a,b-enones. 5,6 During the course of our studies on this intermolecular hydroacyla- tion of various functionalized alkynes, we found that the reaction of benzaldehyde (1a) with 4-pentynoic acid (2) in the presence of Wilkinson's complex (3) and 2-amino-3- picoline (4a) resulted in ( E)-3-benzylidene-3H-furan-2- one (5a) exclusively instead of the expected hydroacyla- tion product, a,b-enone 6 (Equation 1). Herein, we report an unprecedented one-pot synthesis of E-alkylidene de- rivatives of 3 H-furan-2-one derived from 4-pentynoic acid and aldehyde using a co-catalyst system of Wilkin- son's complex and 2-aminopyridine. The reaction of benzaldehyde (1a) and 4-pentynoic acid (2) in the presence of (PPh 3)3RhCl (3, 5 mol%) and 2-ami- no-3-picoline (4a, 40 mol%) was performed at 100 °C in toluene for 3 hours to afford ( E)-3-benzylidene-3H-furan- 2-one (5a) 7 in a 74% isolated yield (82% yield by GC) af-