Polypropionate fragments approach

Sevin, A-F, Vogel, P, A new stereoselective and convergent approach to the synthesis of long-chain polypropionate fragments, J. Org. Chem., 59, 5820-5926, 1994. 

Vogel and co-workers have used 1 as double chain elongation synthon in their synthesis of long chain polypropionate fragments (eq 48). Their approach is based on an one-pot reaction cascade which involves hetero-Diels-Alder addition of electron-rich l-alkoxy-3-acyloxy-l,2-dienes (e.g., 25) to sulfur dioxide, subsequent ionization into zwitterionic intermediates (e.g., 26), then reaction of the latter with allylsilanes. The final step of this sequence is the retro-ene desulfinylation of an intermediate allylsilyl sulfinate (e.g., 27) to produce useful synthetic intermediates containing two more stereogenic centers (e.g., 28). 

Class II aldolase mimics (Scheme 10.4) were the first small-molecule catalysts that were reported for the direct intermolecular aldol reaction. These catalysts are characterized as bimetallic complexes that contain both Lewis acidic and Brpnsted basic sites. Shibasaki et al. first reported on the use of such a catalyst in the aldol reaction in 1997, demonstrating its potential with the reaction of various acetophenones 52 and aldehydes 53 (Scheme 10.13). Aldols 55 were obtained in good yields and enantioselectivities. A similar approach was used in the direct catalytic asymmetric aldol-Tishchenko reaction.Nevertheless, for the moment, this method does not provide access to true polypropionate fragments. ... 

SCHEME 10.50. Diastereoselective hetero-Diels-Alder approach to polypropionate fragments. 

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