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

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 a t 85 OC 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 a t 85 OC for 5 h gave the products (9) and ( 10) in a ratio of 8:l in 65% yield, no detectable amount of the cycloadduct ( 8 b ) was obtained.Refluxing of the propargyl ether (13) with t-BuOK in t-BuOH a t 85 OC for 4 h gave the allenyl ether (14).Heating 14 in DMSO a t 150 OC for 12 h still1 did not give 16.