Polypropionate fragments, aldol

Additionally, Evans et al. developed the aldol chemistry to join the polyacetate/polypropionate fragments in the stereoselective manner, which has made possible the convergent synthesis of polyketide chains vide irtfra). 

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. ... 

Yamamoto has also investigated the use of 1 and 2 with racemic aldehydes such as 5 in a series of addition reactions (Eq. (8.2)). The chiral enolate participates in aldol additions to afford a mixture of anti- and yn-products 6 and 7 in which the aldehyde facial selectivity has been determined in large part by the overriding bias of the auxiliary. Thus the process offers great promise for the construction of stereochemically complex, densely functionalized fragments common in numerous polypropionate-derived natural products. 

A synthesis of both the polypropionate and spiroacetal fragments of rutamycin B have been described by Panek and Jain [65]. The majority of stereocenters were introduced via asymmetric allylation and crotylation reactions however, an aldol... 

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