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Synthesis and equilibration of bis-spiroketals. Reagents and conditions: a) 2,2,6,6-tetramethylpiperidine, nBuLi, THF then 9; b) TPAP, NMO, CH2Cl2, M.S., 92% over two steps; c) Na/Hg, Na2HPO4, THF, H2O, 86%; d) PPTS, THF/H2O (4:1), 18 h; e) CSA, tBuOH/PhMe (1:1), 18 h. M.S. = molecular sieves; CSA = camphorsulfonic acid.
