An Efficient Approach to Substituted 1,5,7,8,9-Pentahydrocyclopenta- [h]-2-Benzopyran-3-one Derivatives by a Palladium-Catalyzed Tandem Reaction of 2,7-Alkadiynylic Carbonates with 2,3-Allenoic Acids

摘要

Transition-metal-catalyzed cyclization reactions of function-alized allenes in the presence of organic halides have become powerful tools for the synthesis of cyclic compounds. Based on this approach, we envisioned that a 1,3,4-trienyl palladium intermediate I, which is formed from the oxidative addition and cyclic carbopalladation of 2,7-alkadiynylic carbonates, might act similarly to the aryl/alkenyl palladium intermediates formed from the oxidative addition of organic halides with palladium(O) to trigger the cyclization of 2,3-allenoic acids 2 to afford butenolides with a 1,3,4-trienyl unit at the p position (Scheme 1). However, our preliminary study showed that the reaction of 2,7-alkadiynylic carbonate 1a and 2,3-allenoic acid 2a in the presence of [Pd(PPh3)4] (5 mol %) in acetonitrile at 60 °C for one hour failed to afford the II-type product. Instead, an interesting and unexpected new product was formed and isolated cleanly (Table 1, entry 1). The structure of this product was unambiguously established as the tricyclic product 3 a by X-ray diffraction analysis (Figure 1). Herein, we report our recent observations in this area.