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 fur an diene and the allenyl ether dienophile. Reaction of the propargyl e ther (2) with t-BuOK in t-BuOH at 85-degrees-C for 3 h gave a mixture of the cycloadducts (3) and (4) in 90% yield, which were transferred t o the benzo derivatives (5), (6) and (7). Treatment of the propargyl e ther (8) with t-BuoK in t-BuOH at 85-degrees-C for 5 h gave the produc ts (9) and (10) in a ratio of 8:1 in 65% yield, no detectable amount o f the cycloadduct (8b) was obtained. Refluxing of the propargyl ether (13) with t-BuOK in t-BuOH at 85-degrees-C for 4 h gave the allenyl et her (14). Heating 14 in DMSO at 15-degrees-C for 12 h still did not gi ve 15.