1,2-O-Isopropylidenefuranose Templates for the Synthesis of Complex Cyclic Ethers via Neighboring Group Participation by the Acetal Ring Oxygen
作者:Weifang Shan、Phyllis Wilson、Wei Liang、David R. Mootoo
DOI:10.1021/jo00105a012
日期:1994.12
Novel strategies are described for the direct transformation of C5-allylated-1,2-O-isopropylidene-furanoses into complex cyclic ethers. Reaction of these substrates with iodonium ion under aqueous conditions resulted in the facile formation of 2,5-disubstituted tetrahydrofurans (THF's) containing highly substituted branches, in good to high yields, albeit with modest cis stereoselectivity. A similar result was obtained for the one case of the analogous tetrahydropyran (THP) system that was examined. These reactions presumably proceed via neighboring group participation by the acetal ring oxygen on an iodonium ion-alkene charge transfer complex to give a cyclic oxonium ion, thence a cyclic ether-oxocarbenium ion which undergoes intermolecular capture by water. Under anhydrous conditions, substrates which contained a 3-0-(2'-methyl-2'-buten-4'-yl) substituent led to high yields of adjacently connected THF-THP or THP-THP structures. These products are explainable in terms of a two-step process in which a cyclic ether-oxocarbenium ion, resulting from the initial participation reaction, undergoes nucleophilic capture by the activated alkene. A notable feature of this reaction was that only a single THP isomer was obtained in the second ring forming reaction, presumably a consequence of the cyclic nature of the oxocarbenium ion derived from the 1,2-O-isopropylidene acetal residue.