The multiply oxygenated ABC-ring system of the dihydro-β-agarofurans was synthesized by employing two highly stereoselective reactions. The quinidine-catalyzed Diels-Alder reaction between a chiral dienophile and 3-hydroxy-4-methyl-2-pyrone simultaneously installed the C2-stereogenic center and two contiguous tetrasubstituted carbon centers (C5 and C10) of the A-ring. After 12 additional transformations, the aldol reaction of the resulting spiral AC-ring cyclized the B-ring with stereoselective introduction of the C7- and C8-centers.
A synthetic route to the septahydroxylated ABC-ring system of dihydro-β-agarofurans was established. The B-ring was formed by a base-promoted diastereoselective Diels-Alder reaction between 3-hydroxy-2-pyrone and a d-glyceraldehyde-derived dienophile, while the C-ring was cyclized by PhSeCl-mediated etherification. The remaining A-ring was constructed via a 6-exo-dig radical reaction. Selective transformations gave rise to the ABC-ring system 1 with nine contiguous stereocenters. The thus obtained 1 corresponded to the enantiomer of the densely oxygenated core structure of dihydro-β-agarofurans.
Concise Synthesis of the Multiply Oxygenated ABC-Ring System of the Dihydro-−β-agarofurans. -Diels-Alder reactions of the lactones (I) and (II) is optimized with respect to selective production of the exo-adduct (III) thus establishing the C5and C10-tetrasubstituted and C2-alkoxy-substituted stereocenter for the conversion to the ABC-ring fragment (IV*) of dihydro-β-agarofurans. -(ISHIYAMA, T.; URABE, D.; FUJISAWA, H.; INOUE*, M.; Org. Lett. 15 (2013) 17, 4488-4491, http://dx.
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