Stereoselective synthesis of 3,7- and 2,6-dioxabicyclo[3.3.0]octane systems by iodoetherification of diene diols

Abstract

Perhydrofurofuran (2,6and 3,7-dioxabicyclo[Y3.0]octane) systems are typical structural fragments of lignans I and metabolites of some red algae. 2,3 However, until recently the synthesis of such systems remained an untrivial problem. We have developed a simple way to construct both 2,6and 3,7-dioxabicyclo[3.3.0]octane compounds by intramolecular electrophilic cyclization of diene diols 1 and 2 containing double bonds in the ~,,6-position relative to each hydroxyl. We have found that cyclization 4,s of 2,3-divinyl-l,4diols ~ 1 under the action of 12 NaHCO 3 in an ether water system proceeds stereospecifically to form only one isomer of the corresponding 2,2,6,6-tetrasubstituted cisfused 4,8-di-exo(iodomethyl)3,7-dioxabicyclo[ 3.3.0]octane 3 in 60--80% yield (Scheme 1). Under similar conditions 3,4-dimethylene1,6-diols 2 prepared by allylborylation of carbonyl compounds with 2 ,3-b i s (d ipropylbory lmethyl ) 1 ,3-butadiene 7 yield 3 ,3 ,7 ,7te t rasubst i tu ted 1,5-cis-di(iodomethyl)-2,6dioxabicyclo[3.3.0]octanes (4) along with an unsaturated bicyclic diether 5 (2,2,6,6-tetrasubstituted 3,7-dioxa1,2,3,4,5,6,7,8-octahydronaphthalene) containing no iodine. It should be noted that only 5 (R = Ph) is formed in 75% yield from the tetraphenyl derivative 2c. Compound 5 is likely to be formed due to two intramolecular processes: 1,4-conjugated addition of iodine and hydroxyl oxygen to the diene system followed by nucleophilic substitution of iodine by the oxygen atom of the second hydroxyl group. The structure of the compounds obtained was established on the basis of IH and 13C NMR spectral data, high resolution mass-spectra, and elemental analysis. Compound 3a, m.p. 98-101~ (hexane). Found,%: C 33.00; H 5.02; I 56.74. C12H2oI20 2 Calculated,%: C 32.02; H 4.48; I 56.39. lH NMR spectrum (6,