Non-iterative asymmetric synthesis of C₁₅ polyketide spiroketals

The 2,2′-methylenebis[furan] (1) was converted to 1-{(4R,6S))-6-[(2R)-2,4-dihydroxybutyl]-2,2-dimethyl-1,3-dioxan-4-yl}-3-[(2R,4R) -tetrahydro-4,6-dihydroxy-2H-pyran-2-yl)propan-2-one ((+)-18) and its (4S)-epimer (-)-19 with high stereo- and enantioselectivity (Schemes 1-3). Under acidic methanolysis, (+)-18 yielded a single spiroketal, (3R)-4-((1R,3S, 4′R,5R,6′S,7R)-3′,4′,5′,6′-tetrahydro- 4′-hydroxy-7-methoxyspiro[2,6-dioxabicyclo[3.3.1]nonane-3,2′-[2H] pyran]-6′-yl}butane-1,3-diol ((-)-20), in which both O-atoms at the spiro center reside in equatorial positions, this being due to the tricyclic nature of (-)-20 (methyl pyranoside formation). Compound (-)-19 was converted similarly into the (4′S)-epimeric tricyclic spiroketal (-)-21 that also adopts a similar (3S)-configuration and conformation. Spiroketals (-)-20, (-)-21 and analog (-)-23, i.e., (1R,3S,4′R,5R,6′R)-3′,4′,5′, 6′-tetrahydro-6′-[(2S)-2-hydroxybut-3-enyl]-7-methoxyspiro[2, 6-dioxabicyclo[3.3.1]-nonane-3,2′-[2H]pyran]-4′-ol, derived from (-)-20, were assayed for their cytotoxicity toward murine P388 lymphocytic leukemia and six human cancer cell lines. Only racemic (±)-21 showed evidence of cancer-cell-growth inhibition (P388, ED₅₀: 6.9 μg/ml).