Tert-Butyldiphenylsilyl group

Akiyama et al. (180) overcame this problem by employing ch/ro-inositol derivatives as chiral auxiliaries for the acrylic ester, which afforded dipolar cycloadducts with a high degree of stereoselectivity (Scheme 6.34). Formation of the major products [(55)-isoxazoline-5-esters] was suggested to arise from the s-cis conformer of acrylate 27, the minor product being derived from the s-trans conformer 28. The bulky protective group (in this case tert-butyldiphenylsilyl) would effectively shield the Re face of the olefinic double bond and destabilize the s-trans conformer 28. 

Aldol reactions using catalyst 12 were also performed with water without any organic solvent (Table 2.9) [15]. In these examples, it is assumed that the reaction occurred in the organic phase composed of reactants and the catalyst separated from the water phase. A large excess of ketone was used in these reactions. Catalysts possessing a tert-butyldimethylsilyl (TBS) or a triisopropylsilyl (TIPS) group instead of the tert-butyldiphenylsilyl (TBDPS) group on 12 also effectively catalyzed the reaction with water, but hydroxyproline 2 did not catalyze the reaction under the same conditions this indicated that the hydrophobic substituents of... 

Adoption of the Diels-Alder approach depicted requires that the eventual C(9) configuration be inverted at some point in the synthesis and that the six-member B ring undergo ring contraction. Prior to addressing these two issues, the hydroxyl group in (302) was protected [TBDPSC1, imidazole, DMF, 70%] as its tert-butyldiphenylsilyl ether (303). 

The addition, alteration, or removal of protecting groups will not be discussed here Ac = Acetyl, Ar = 2,4,6-triisopropylphenyl. Bn = benzyl, Bz = benzoyl, BOC = rerf-butoxycar-bonyl, BOM = benzyloxymethyl, TBPS = tert-butyldiphenylsilyl, TBS = fert-butyldime-thylsilyl, TES = triethylsilyl, TIPS = triisopro-pylsilyl, TMS = trimethylsilyl. Actual yields of the first published procedures are given, conversions and loss to formation of isomers were included in the calculations where necessary. For precursors the yields were taken from the references given by the authors and used to... 

The Cram chelation model (sec. 4.7.B) is an example where the chelation effects of the heteroatom influence the rotamer population and, thereby, the selectivity of the reduction. Zinc borohydride [Zn(BH4)2], effectively chelates the carbonyl oxygen and alcohol oxygen atoms in the reduction of 42 and leads to intermediate 43. Transfer of hydride to the carbonyl gave primarily the anti diastereomer, 45 (4 96, 44/45). When the chelating hydroxyl group was blocked as a tert-butyldiphenylsilyl ether (in 46 - sec. 7.3.A.i), reduction with Red-Al (sec. 4.3) led to a reversal in selectivity (96 4, 47/48).The ability to chelate a heteroatom varies with the reagent used. Lithium aluminum hydride shows less selectivity, due in part to poorer coordination with the heteroatom and reduction of 42 gave a 27 73 mixture of 44 and 45,... 

Cleavage of Protecting Groups. THP ethers of primary, secondary, and phenolic alcohols can be conveniently de-protected at room temperature by treatment with 1.2 equiv of TMSOTf in methylene chloride (eq 96). Deprotection of tert-butyldimethylsilyl ethers in the presence of a tert-butyldiphenylsilyl ether has been smoothly accomplished using TMSOTf at —78°C, as one of the steps of the total s)mthesis of marine macrolide ulapualide A. ... 

Scheme 70). Different functional end-groups were attached by transformation of the bromide or tert-butyldiphenylsilyl end-groups of the PS and PEO arms, respectively. 

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