Diols monosilylated, reactions

The successive reaction of (dibromomethyl)silanes with LDA (hthium diisopropyl-amide) and two equivalents of benzaldehyde gives 1,3-diol monosilyl ethers in good yield (Scheme 10.221) [574]. This tandem reaction would proceed via anionic 1,3-silyl migration of /l-lithioxyalkylsilane intermediate 152 and addition of the resulting lithium carbenoid to benzaldehyde. Thus, internal activation of the silicon-lithium alkoxide promotes nucleophilic addition of a-haloalkylsilanes. Similar tandem reactions of 2-trimethylsilyl-l,3-dithiane with aldehydes [575] and epoxides [576] have been reported. 

Recently, Schaumann et al. 153,154 an(j Bienz et tf/.155,156 have developed dependable routes for the resolution of racemic functionalized organosilanes with Si-centered chirality using chiral auxiliaries, such as binaphthol (BINOL), 2-aminobutanol, and phenylethane-l,2-diol (Scheme 2). For instance, the successive reaction of BINOL with butyllithium and the chiral triorganochlorosilanes RPhMeSiCl (R = /-Pr, -Bu, /-Bu) affords the BINOL monosilyl ethers 9-11, which can be resolved into the pure enantiomers (A)-9-ll and (7 )-9-11, respectively. Reduction with LiAlFF produces the enantiomerically pure triorgano-H-silanes (A)- and (R)-RPhMeSiH (12, R = /-Pr 13, -Bu 14, /-Bu), respectively (Scheme 2). Tamao et al. have used chiral amines to prepare optically active organosilanes.157... 

To a solution of diol 8 in dichloromethane (0.3 mL) was added pyridine (0.5 mL) and the reaction mixture, cooled to —18 °C, was treated dropwise with 1.2 equiv. of TIPSOTf. During the addition the reaction temperature was carefully controlled. When the starting material was consumed (checked by TLC), the solution was evaporated to dryness to give a residue which was processed by column chromatography on silica gel (petroleum ether/EtOAc, 3 1). Monosilyl ether 9 (146 mg, 95%) was obtained as a glassy solid. 

The above kinetically controlled regioselectivity is also reflected in the selective monosilylation of butane-1,2,4-triol. In this case, the sole product obtained is 4-r-butyldimethylsiloxy-butane-l,2-diol with 99% yield (eq 28). The authors rationalized that, with the bulky TBDMSCl, the dibutylstannylene acetal rapidly migrates between the 1,2-diol and 1,3-diols and affords the product of kinetic control. This sequence of reactions was also used to selectively block the equatorial alcohol of a cw-diol on the pyranoside ring... 

Finally, a very recent work described the synthesis of spacer-linked neodisaccharides of L-daunosamine [84], In a glycosylation reaction on a monosilylated 1,4-butanediol the main product obtained was the monoglycosylated unprotected diol. This may suggest either a desilylation due to the acidic conditions or a higher reactivity of the silylated oxygen of the acceptor. 

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