Reductive cleavage acetals, diisobutylaluminum hydride

Perlmutter used an oxymercuration/demercuration of a y-hydroxy alkene as the key transformation in an enantioselective synthesis of the C(8 ) epimeric smaller fragment of lb (and many more pamamycin homologs cf. Fig. 1) [36]. Preparation of substrate 164 for the crucial cyclization event commenced with silylation and reduction of hydroxy ester 158 (85-89% ee) [37] to give aldehyde 159, which was converted to alkenal 162 by (Z)-selective olefination with ylide 160 (dr=89 l 1) and another diisobutylaluminum hydride reduction (Scheme 22). An Oppolzer aldol reaction with boron enolate 163 then provided 164 as the major product. Upon successive treatment of 164 with mercury(II) acetate and sodium chloride, organomercurial compound 165 and a second minor diastereomer (dr=6 l) were formed, which could be easily separated. Reductive demercuration, hydrolytic cleavage of the chiral auxiliary, methyl ester formation, and desilylation eventually led to 166, the C(8 ) epimer of the... 

Diisobutylaluminum hydride (DIBAL-H) hydrogenolyzes simple ortho esters to acetals at room temperature and reduces acetals and ketals to ethers at 70-80 °C (equation 6) benzyl ethers are cleaved at still higher temperatures. This reagent shows good selectivity and considerable versatility. It has been used to reduce acetals of formaldehyde (equation 7), ° which few other reagents can accomplish. With catechol ketals a single reductive cleavage occurs at room temperature (equation 8). ... 

The BPS group is more stable toward acidic hydrolysis than TBS or the TIPS groups. It survives DIBAL (diisobutylaluminum hydride) reductions, cleavage of THP with aqueous AcOH, and cleavage of isopropylidene and benzylidene acetals with aqueous CF3CO2H. BPS-Cl has been used to selectively protect primary OH groups in inositol (hexahydroxycyclohexane). ... 

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