Aldol 2-trimethylsilyloxyfuran with

In the aldol reaction of 2-trimethylsilyloxyfuran with aldehydes catalyzed by (BINOL)2Ti complex, a significant impact of the product on the enantioselectivity of the catalysis was observed [124]. As shown in Scheme 14.44, the addition of 5 mol% of the product (82% ee) in the catalyst can enhance the enantiomeric excess of the product from 70% to 96%. Therefore, an asymmetric autoinduction might be involved in the catalytic system. On the basis of this observation. 

Silver(I) complexes with Tol-BINAP (270) were used by Yamamoto and coworkers for mediating diastereoselective and enantioselective Mukaiyama aldol additions. According to the authors conclusion, the mechanism does not involve transmetallation to silver enolates but follows the usual carbonyl group activation [135]. Hoveyda and coworkers used silver(II) fluoride in the presence of a dipeptide-type ligand for enantioselective additions of silyl enol ethers to a-keto esters [136]. The reaction of 2-trimethylsilyloxyfuran with aromatic and aliphatic aldehydes was catalyzed with chromium salen complex in the presence of protic additives like isopropanol [137]. Various protocols of enantioselective Mukaiyama aldol reactions that use water as cosolvent have been elaborated ... 

The less well-studied 3-silyloxyfuran was shown to react with aldehydes in an aldol addition manner under Lewis acidic conditions. High 5yn-diastereoselectivity was obtained with bulky aldehydes <05OL387>. 4-Alkoxy-3-lithio-2-silyloxyfurans reacted with a variety of electrophiles to form 3-substituted tetronates after acidic hydrolysis <05SL2735>. Furans and 2-trimethylsilyloxyfuran are effective nucleophiles in the organocatalytic tandem... 

The chemistry of Scheme 1 has been used to prepare the D-gfycero-D-to/o-heptonolactam 9. The initial aldol-type condensation was stereoselective for the D-arabino-pioduct 8, but it was found that treatment of 8 with EtsN-DMAP caused smooth isomerization at the asterisked carbon to give the D-ri6o-lactam.i Similar work by the same group using 2-trimethylsilyloxyfuran has been mentioned in earlier Volumes (Vol. 24, p. 174-5, and earlier). 

Trimethylsilyloxyfuran 338 has shown promise as a masked butenolide fragment To fuUy exploit these qualities, the threo versus erythro (339 vs 340) diastereoselectivity in aldol-type additions has to be controlled. In fact it has been shown that this is easily achieved by appropriate reaction conditions. Applying Mukaiyama conditions (i.e., using the silyl enol ether as the donor in the presence of a Lewis acid such as TESOTf to generate oxonium species) leads to threo preference for 339, presumably via an open transition state, whereas desilylation with TBAF generates the erythro-diastereomer 340, this time via a closed Diels-Alder (or Zimmerman-Traxler)-like transition state. In both cases, chelating effects can be ruled out... 

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