A stereocontrolled synthesis of anti-and syn-β-isopropenyl alcohol moieties at the C(2)-C(3) positions of kallolide A and pinnatin A was accomplished employing the [2,3] Wittig rearrangement of (E)-and (Z)-cyclic furfuryl ethers. Both (5E)-and (5Z)-5,15-dimethyl-3,16-dioxabicyclo [11.2.1] hexadeca-1(15),5,13-trienes rearranged with high diastereoselectivity to give (2R^*,3R^*)-and (2R^*,3S^*)-3-isopropenyl-12-methyl-13-oxabicyclo[8.2.1]trideca-1(12), 10-dien-2-ols, respectively. Enantioselective Wittig rearrangement of (E)-and (Z)-furfuryl ethers using butyllithium and a chiral bis(oxazoline) was also examined to provide (2R, 3R)-homoallylic alcohol in up to 61% ee and (2R, 3S)-alcohol in up to 93% ee, respectively.
A stereoselective synthesis of the cyclopropylfuran unit in pinnatin A was carried out. Cyclopropanation of vinylfuroate gave desired cycloporpylfuroate, albeit in low yield. Suzuki cross-coupling of cyclopropylboronic acid with bromofuroate afforded the corresponding (1R, 2R, 2'S)-5-[2-(1,4-dioxaspiro[4.5]decan-2'-yl)-2-methylcyclopropyl]-3-methyl-2-furoic acid methyl ester in 77% yield. This compound was converted into aldehyde, a key intermediate for the synthesis of pinnatin A. Chiral synthesis of the core structure of bipinnatin J was investigated. Stille cross-coupling of tributylstannylfuran with (Z)-bromoalkene gave (S, Z)-1-(5-tert-butyldimethylsiloxymethyl-4-methyl-2-furyl)-2-methyl-4,5-O-(p-methoxybenzylidene)-1-pentene stereoselectively. Ru-catalyzed cyclic carbonylation of allenyl alcohol afforded γ-lactone, (S, 1E, 5Z, 9E)-1-bromo-11-tert-butyldimethylsiloxy-4-hydroxy-2,10-dimethylundeca-1,5,9-triene-6-carboxylic acid lactone.