Polypropionate fragments reactions

Scheme 20. Stereoselective synthesis of polypropionate fragments by a three-component pericyclic domino reaction...

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). 

In another report, Panek and Jain used chiral allylsilane methodology for construction of C1-C17 polypropionate fragment of ru-tamycin B and oligomycin C. The aldehyde partner used for the sequence consisted of a diol protected as a di-f-butylsilylene 18. The reaction proceeded in the presence of TiCU in excellent diastereoselectivity and yield (eq 9). 

Polypropionate fragment 24 is obtained upon desulfurization (Raney nickel) of the corresponding thiane. It is interesting to note that addition of nucleophile to aldehyde ( )-21 shows exclusive Felkin diastereoface selectivity. One can assume that the transition state of the aforementioned transformation equals that predicted for proline-catalyzed aldol reactions between cyclohexanone... 

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. ... 

The additions of chiral nonracemic allenylmetal reagents to chiral a-methyl propanal derivatives have been proven useful for the assembly of polypropionate fragments. These reagents rely on allene chirality to favor one of the two possible diastereomeric transition states in the addition and, thus, differ in a fundamental way from the aforementioned methods in which a chiral auxiliary or catalyst provides the control element. For example, a chiral allenylstannane 246 is added to a chiral aldehyde (S)-230, derived from the Roche ester, in the presence of various Lewis acid promoters to afford any of the four diastereo-mers with excellent diastereo- and enantioselectivity, depending on the reaction conditions. Representative results are depicted in Scheme 10.48. From the stereocontrol point of view, these transformations follow Cram-fike open transition state models without or with chelation, respectively. If InBr3, SnCLi, BuaSnCl, or other additives... 

SCHEME 10.51. Application of asymmetric hetero-Diels-Alder reaction in synthesis of polypropionate fragments. 

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