Stereochemistry thermodynamic control over

The other half was made from 172 by formation of the protected keto-aldehyde 183 followed by successive stereoselective Wittig-Horner and Wittig reactions to put in the two alkenes with control over stereochemistry. The Wittig-Horner is under thermodynamic control giving the trisubstituted E-alkene 185 while the Wittig is under kinetic control giving the Z-iodoalkene 175 R = Ac (chapter 15). 

On the other hand, when equihbration exists between the two species B and ent-B, and one of the two stereoisomers exists preferentially over the other (in the presence of a chiral Hgand) and then stereospecifically rearranges, the reaction provides the enantioenriched product C or ent-C in which the stereochemistry was determined at the post-lithiation step by a thermodynamic control mechanism (Scheme 2). Alternatively, if the equilibration occurs rapidly, and either lithio species rearranges much faster than the other species, either C or ent-... 

In sharp contrast, the ylide (325) rearranges spontaneously above -5 C to give a mixture of cis- and frartj-sulfides (326) and (327) in a 25 1 ratio. The fact that the stereochemistry of the reaction products is under kinetic control was demonstrated by the base-catalyzed epimerization of (326) and (327) to a thermodynamic 1 19 ratio. Ammonium ylides also undergo ring contraction but with complete cis dia-stereoselectivity (Scheme 84). Base-catalyzed equilibration affords some trans isomer. The high dia-stereoselectivities displayed in these reactions contrasts with the poor and rather unpredictable ones observed for acyclic ylide rearrangements (c/. Section 3.10.5.4). In all of the cases examined, the endo transition state endo-(32S) is highly favored over the exo transition state exo-(32S) (Scheme 85). A... 

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