Intramolecular Diels-Alder Reaction of Furans with Allenyl Ethers: High Effect of the Chain Length on the Structure and Reactivity of the Cycloadducts

Abstract

The structure and reactivity of the cycloadducts of the title reaction were found to be highly dependent on the chain length between the furan diene and the allenyl ether dienophile. Reaction of the propargyl ether (2) with t-BuOK in t-BuOH at 85°C for 3 h gave a mixture of the cycloadducts (3) and (4) in 90% yield, which were transferred to the benzo derivatives (5), (6) and (7). Treatment of the propargyl ether (8) with t-BuOK in t-BuOH at 85°C for 5 h gave the products (9) and (10) in a ratio of 8:1 in 65% yield, no detectable amount of the cycloadduct (8b) was obtained. Refluxing of the propargyl ether (13) with t-BuOK in t-BuOH at 85°C for 4 h gave the allenyl ether (14). Heating 14 in DMSO at 150°C for 12 h still did not give 15