Enol silanes reaction with aldehydes

With aldehydes 73 and 74 in hand, two alternatives were considered for the completion of the synthesis of preswinholide A, i.e. carrying out the butanone aldol reaction on either the methyl or the ethyl side first (see Scheme 9-24). Initially, the former option was investigated. While the reaction of the kinetic boron enolate of butanone with aldehyde 73 did not favour the desired Felkin adduct 83, the addition of allyl silane 84 (a masked butanone equivalent) proved selective in the desired sense (Scheme 9-27). This change in selectivity indicates the stereochemical reversal possible when switching from a cyclic to an acyclic transition state. 

Additions of enol silanes to p-alkoxy aldehyde (85 equation 25) are reported in Table 17. High selectivity (chelation control) was obtained with TiCU via complex (78 entries 1, 2). The same preference for isomers (86) and (87) was obtained with BF3 via complex (80), which simulates chelation. The influence of chelation on simple stereoselection is also evident in the reactions of achiral aldehydes (90) and (92) with silyl enol ethers (Z)-(91) and ( )-(93), which are usually moderately anti selective in their reactions with aldehydes incapable of chelation high syn selectivity was obtained irrespective of the enol ether geometry (equations 26 and 27). - ... 

Titanium-based Lewis acids are widely used in the Mukaiyama reaction, i. e. the condensation of an enol silane and an aldehyde, giving 3-hydroxycarbonyl derivatives. In this context, BINOL-derived chiral titanium complexes were used in the condensation of thioester-derived ketene silyl acetals with a large range of aldehydes, in high yields and enantioselectivities (Scheme 7.39). ... 

Kobayashi and coworkers reported addition of enol silanes (72) to aldehydes (71) catalyzed by Cu(OTf)2/(52) (Scheme 17.14) [19]. Moderate to good enantioselectivities could be obtained with low syn/antiselectivity. The reduced enantioselectivity relative to bidentate acceptors employed may be attributed to single-point coordination to the Lewis acid. This point is noteworthy, as acceptors restricted to single-point coordination continue to be challenging substrates in copper-catalyzed aldol reactions. 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

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