Silylacetic acids

Si-transfer from oxygen to carbonIn the presence of a trialkylaluminum, particularly (CH3),AI, silyl ketene acetals rearrange at —78° to 20° to esters of trialkyl-silylacetic acid. This 1,3-rearrangemcnt of Si from oxygen to carbon is the reverse of the well-known Brook thermal rearrangement2 of Si from carbon to oxygen. But R AI does not rearrange trimethylsilyl enol ethers. 

Reactions of O-silyl O-alkyl keteneacetals Silylacetic acids by 0->C-silyl migration... 

Silylacetic acids undergo a thermal rearrangement to afford the silyl ester (eq 3). The same migration of the silyl group is observed at lower temperatures when base catalysis is employed. ... 

The isolation of the sily lacetal ketene is required when the reaction is catalyzed by a chiral Lewis acid. However, due to its instability, isolation yields of the compounds are very low. Reaction of difluoroketene silylacetal with electrophiles affords adducts with excellent ee (Figure 2.28). ° The instability of these difluoroketene silylacetals is due to the facile migration of the silyl group from oxygen to carbon, affording... 

Photocycloaddition reactions of aromatic aldehydes with cyclic ketene silylacetates have been investigated by Abe and coworkers [61]. Regio- and diastereoselective formation of the bicyclic 2-alkoxyoxetanes 69 was observed in high yields. Hydrolysis of these acid-labile cycloadducts with neutral water efficiently gave aldol-type adducts 70 with high threo-selectivity (Sch. 18). 

Oxazolidine 134 is a masked trifluoroacetaldehyde imine that generates in situ the corresponding imine 135 under Lewis acid catalysis conditions. Lewis acid-catalyzed reactions of 134 with TMS-cyanide and ketene silylacetal provide adducts 136 in high yields with good diastereoselectivities (see Scheme 9.29) [56]. Conventional chemical transformation of 137 produces 3-amino-4,4,4-trifluorobutanoic acid 138. Similarly, tri-fluoroacetone oxazolidine 139 is used for the synthesis of 2-trifluoromethylalanine 142... 

The reactions of aldehydes with enoxysilanes of ketones usually exhibit poor stereoselectivity. Enoxysilanes derived from esters and thiolesters, on the other hand, give good results. Chirality has been introduced either on the Lews acid or by using an ester of an enantiopure alcohol. As in the case of enolate reactions, one or two new stereocenters may be created. All these reactions take place at low temperatures, and they are sometimes limited by the instability of certain ketene silylacetals. 

Chiral boranes have been recommended as Lewis acids catalysts by Reetz [689], Yamamoto [787, 788], Kiyooka [795, 1302], Masamune and their coworicers [796, 797], These groups used, respectively, boranes 2.61, 3.9 (R = H, R = /-Pr), 3.10 (R = i-Pr or tert-Bu, R = H) and derivatives of 3.12 and 3.13. These boranes are very efficient catalysts in asymmetric additions of symmetrically substituted ketene silylacetals 6.113 to aldehydes (Figure 6.94). Similar reactions can also be conducted with enoxysilanes derived from methylketones or from tert-Bu thiolacetate [787, 794, 796], Oxazaborolidine 3.10 derived from tryptophan 3.11 is also a very potent catalyst [794],... 

Aminations which afford 6-aminocarboxylates are of interest in relation to monobactam antibiotics. A secondary aminomethyl group can be introduced at the -position of carboxylic esters by reaction of hexahydro-1,3,5-triazines with ketene silyl acetals in the presence of a catalytic quantity of trifluoromethanesulphonic acid (Scheme 25). The triazine (10) is considered to be converted into an N-silylated methyleneiminium salt which undergoes addition of the ketene silylacetal (11). 

Titanium complexes are often encountered in Lewis acid-catalysed reactions. This is certainly true for catalysed aldol reactions. Mikami and Matsukawa demonstrated that titanium/BINOL complexes e.g. complex (7.20) afforded high yield and enantioselectivity in the aldol reactions of thioester ketene silylacetals with a variety of aldehydes. In contrast to some of the aldol reactions described above, the stereochemistry of the adducts is dependant on the geometry of the enol ether. Thus, reaction of the (B)-enol ether (7.21) with aldehyde (7.22) yields the sy -aldol adduct (7.23) predominantly while the (Z)-e.no ether (7.24) results in isolation of the anti-adduct (7.25) as the major product. The authors invoke a closed silatropic ene transition state (structure (7.26) for syn-transition state), substantiated by suitable crossover experiments, to explain the diastereoselectivities... 

In the presence of a Lewis acid catalyst, ketene silylacetals have been found to undergo aldol reaction preferentially with ketones, rather than aldehydes. ... 

Scheme 3.56 Diastereocontrol in Lewis acid-catalyzed Michael reactions of 4-siloxy cyclopentenone with ketene silylacetals stereoelectronic versus steric effects.

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