Ketal, cyclohexylidene diol protection

These problems were circumvented by protecting the C(4),C(5) diol prior to Wittig olefination step (Figure 3). Thus, treatment of 10b (a mixture of pyranose and furanose anomers prepared by hydrolysis of 8 with aqueous trifluoroacetic acid) with excess EtSH and concentrated HCI (as solvent) at provided dithioacetal 9 in 50% yield, along with 25% of a mixture of thiopyranosides and thiofuranosides that was recycled to 10b in high yield by treatment with HgCIa and CaCOa in aqueous CH3CN. Finally, the diol unit was protected as a cyclohexylidene ketal, and then the thioacetal was hydrolyzed under oxidative conditions to arrive at the key aldehyde intermediate 3. 

Higher homolog 1,3-polyols are prepared by an iterative procedure (Scheme 88). Acetonide 601 is first protected as the cyclohexylidene ketal 604. Selective deprotection of the acetonide affords diol 605 in 42% yield, along with unreacted 604 (yield = 89% based on consumed starting material). Conversion of 605 to epoxide 606 followed by reaction of 606 with lithiated 598 in a procedure similar to the sequence outlined in Scheme 87 ultimately produces polyol 607 with 95 5 syn selectivity. 

Perhaps classical for the protection of 1,2- and 1,3-cis-diols is the isopropyl-idene (acetonide) function. This acid labile ketal may be formed by reaction of a ribonucleoside with acetone in the presence of an acid (HCl, p-toluene-sulfonic, H2SO4) and water scavenger (2,2-dimethoxypropane, ethyl orthoformate). Other similar protecting groups include the benzylidene and cyclohexylidene functions. 

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