Transition structure also Zimmerman-Traxler

Zimmerman-Traxler transition structure, as shown on the lower left [79], however the open structure shown in the lower right, which does not require coordination of the bulky silyloxy group to titanium, should also be considered. The aldehyde may be oriented to avoid the large tert-butyldimethylsilyl (TBS) group as shown, with the R group away from the TBS. Both of these models have the aldehyde approaching the enol ether from the front face, opposite the side that is shielded by the sulfonamide. Note also that the siloxy group is oriented downward, to avoid the sulfonamide. An anti-selective addition (92% ds) was also reported for the reaction of the E(0)-eno ether of this auxiliary with isobutyraldehyde [79]. 

The diagrams below continue the story. The aldehyde has to attack the front face of the auxiliary, but it also has to do so through what we termed in Chapter 33 a Zimmerman-Traxler transition state —a six-membered, chair-like cyclic structure which allows the enolate to attack the aldehyde while simultaneously transferring the metal (here the boron) from the enolate oxygen to the new hydroxyl group. 

The additions of allyl-, crotyl-, and prenylborane or -boronate reagents to aldehydes are among the most widely studied, well developed, and powerful reactions in stereoselective synthesis. The additions not only display excellent levels of absolute induction in enantioselective synthesis, but also exhibit superb levels of reagent control in diastereoselective additions. The additions of ( )- or (Z)-crotyl pinacol boronates to aldehydes have been observed to give predominantly 1,2-anti- and 1,2-syn-substituted products, respectively (Scheme 5.3) [31, 50]. The inherent stereospecificity of the reaction is consistent with a closed, cyclic Zimmerman-Traxler transition state structure [51], In the accepted model, coordination of the aldehyde to the allylation reagent results in synergistic activation of both the electrophile and the nucleophile... 

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