Study on the reaction mechanism of the base-catalyzed intramolecular Diels-Alder reaction of furfuryl propargyl ethers

Abstract

There is considerable current interest in the intramolecular Diels-Alder reaction, and it has been applied to a number of synthetic objectives with notable success.1 The vast majority of the work reported in this area has dealt with reactions utilizing ethylenic and acetylenic dienophiles. On the other hand, the intramolecular DielsAlder reaction of allene has received much less attention.2 A decade ago, Kanematsu et al. demonstrated that the allene unit is a versatile synthon as a dienophile in the intramolecular cycloaddition due to the absence of unfavorable nonbonded interactions in the transition state.3 Afterward, they developed a furan ring transfer reaction via the intramolecular Diels-Alder reaction of furan diene and allenyl ether dienophile and applied this reaction to the synthesis of natural product^,^ Scheme 1. For the purpose of furan ring transfer reaction, in all of their cases there were only one carbon atom and one oxygen atom connection between the furan diene and the allene dienophile for the cycloaddition. In these cases, the cycloadducts were not isolated under the reaction conditions but were further transferred to the isobenzofuran precursors via ring opening of the bridged oxygen ring of the cycloadducts. Recently, we accomplished the intramolecular Diels-Alder reactions of furans with allenyl ethers by varying the chain length and found the