R-butyldimethylsilyl enol ether

Reaction of the r-butyldimethylsilyl enol ether of a protected dipeptide with xenon difluoride in a mixture of acetonitrile and 1,1,2-trichlorotrifluoroethane gave the fluoro ketone in 71 % yield as a 1 1 mixture of diasteroisomers, while further transformation to enol ether and fluorination affords the difluoro ketone81 (Scheme 23). 

Transsilylation. Several reagents have been recommended for preparation of t-butyldimethylsilyl ethers by transsilylation. These include allyl-r-butyldimethyl-silane and r-butyldimethylsilyl enol ethers of pentane-2,4-dione and methyl aceto-acetate,2 both prepared with r-butyldimethylchlorosilane and imidazole. Unlike the reaction of f-butyldimethylchlorosilane with alcohols, which requires a base catalyst, these new reagents convert alcohols to silyl ethers under slightly acidic conditions (TsOH) in good yield. The trimethylsilyl ethers of pentane-2,4-dione and methyl acetoacetate convert alcohols to trimethylsilyl ethers at room temperature even with no catalyst. The former reagent is also useful for silylation of nucleotides.3... 

Veiy few pericyclic reactions of carbene complexes have been studied that are not in the cycloaddition class. The two examples that are known involve ene reactions and Claisen rearrangements. Both of these reactions have been recently studied and thus future developments in this area are anticipated. Ene reactions have been observed in the the reactions of alkynyl carbene complexes and enol ethers, where a competition can exist with [2 + 2] cycloadditions.Ene products are the major components ftom the reaction of silyl enol ethers and [2 + 2] cycloadducts are normally the exclusive products with alkyl enol ethers (Section 9.2.2.1). As indicated in equation (7), methyl cyclohexenyl ether gives the [2 + 2] adduct (84a) as the major product along with a minor amount of the ene product (83a). The r-butyldimethylsilyl enol ether of cyclohexanone gives the ene product 9 1 over the [2 + 2] cycloadduct. The reason for this effect of silicon is not known at this time but if the reaction is stepwise, this result is one that would be expected on the basis of the silicon-stabilizing effect on the P-oxonium ion. 

Vinyl sulfoxides (221), which are aldehyde a-cation equivalents, and vinylthiolium ions (230), which are a.jj-unsaturated carbonyl 3-cation equivalents, are also suitable acceptors for silyl ketene acetals and enol silyl ethers (Scheme 36). Kita reports that the bulky r-butyldimethylsilyl ketene acetals and tri-methylsilyl ketene acetals form 1 1 adducts (224) and 1 2 adducts (225) with (221), respectively 91 mechanistically, these additions proceed via an initial Pummerer rearrangement The vinylthiolium ion additions are notable for their synthetic flexibility for example, additions to the ketene dithioacetal (229) proceed with higher diastereoselectivity than the corresponding enolate additions to a,3-unsaturated esters.9 lc,91d... 

Enol silyl ethers Bromomagnesium diiso-propylamide. f-Butyldimethylchlorosilane. r-Butyldimethylsilyl trifluoromethanesul-fonate. Chlorotrimethylsily 1-Sodium iodide. Iron. Lithium f-octyl-t-butylamide. 

Although alkylation of 3-hydFoxy ester dianions occurs with high diastereofacial selectivity, the aldol reaction of the dianion obtained from methyl 3-hydroxybutanoate with benzaldehyde gives all four dia-stereomeric aldols in a ratio of 43 34 14 9 (equation 117). On the other hand, dianions of 8-hydroxy esters show rather good diastereofacial preferences under the proper conditions. Deprotonation of t-butyl-5-hydroxyhexanoate with lithium diethylamide in the presence of lithium triflate gives an enolate that reacts with benzaldehyde to give aldols (196) and (197) in a ratio of 91 9 (equation 118). Use of the r-butyldimethylsilyl ether instead of the alcohol resulted in no facial preference. 

Zimmerman-Traxler transition structure, as shown on the lower left [79], however the open structure shown in the lower right, which does not require coordination of the bulky silyloxy group to titanium, should also be considered. The aldehyde may be oriented to avoid the large tert-butyldimethylsilyl (TBS) group as shown, with the R group away from the TBS. Both of these models have the aldehyde approaching the enol ether from the front face, opposite the side that is shielded by the sulfonamide. Note also that the siloxy group is oriented downward, to avoid the sulfonamide. An anti-selective addition (92% ds) was also reported for the reaction of the E(0)-eno ether of this auxiliary with isobutyraldehyde [79]. 

When the alkenyl component is an O-terf-butyldimethylsilyl (TBDMS) enol ether, another anomaly occurs independent of enol ether geometry, the anti product is favored (Scheme 6.8) [62]. With trimethylsilylpropargyl ethers, the anti selectivity is 95-98%, making this reaction an excellent route for the preparation of anti 1,2-diols. In these cases, transition structures similar to Figure 6.6c and d are operative, the dominant influence being mutual repulsion between the carbanion substituent, R, and the 0-silyl group. 

Silylation at nitrogen with r-butyldimethylsilyl triflate, generates pyridinium salts which, because of the size of the N-substitutent, react with Grignard reagents exclusively at C-4 montmorillonite-catalysed addition of silyl enol ethers to pyridines has a comparable effect in producing l-trimethylsilyl-1,4-dihydropyridines carrying an acylalkyl substituent at C-4. ... 

Three approaches to the problem of preparing t-butyldimethylsilyl derivatives have appeared this year, two of which make use of silyl enol ether derivatives. Both (135) and (136, R = CH3, OCHa) " give silylated products under mild... 

UButylmethoxyphenylsilyl ethers (r-BMPSi ethers). In DMF in the presence of NfCjHj), this bromosilane reacts with primary, secondary, and tertiary alcohols to form silyl ethers in good yield, and also with some enolizable ketones to form enol silyl acetals. Selective silylation of primary alcohols is possible by use of CHjClj as solvent. The hydrolytic stability of these ethers is intermediate between that of t-butyldimethylsilyl ethers and that of t-butyidiphenylsilyl ethers. The most useful feature of this new protecting group is the selective cleavage by fluoride ion in the presence of other silyl ethers. 

Scheme 8.80. A representation of the pathway from the f-butyldimethylsilyl ether of the enol of cyclohexanone to 2-methylcyclohexanone via the addition of methylene from the Simmons-Smith reagent and rearrangement of the cyclopropane formed (see Simmons, H. E. Smith, R. D. J. Am. Chem. Soc., 1959,81,4256).

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