Rearrangement reactions stereoselective deprotonation

Asymmetric Pummerer rearrangement is a very attractive reaction as previously described. In particular, the reactions induced by SKA work well, and may be synthetically exploited in many cases. The results described here demonstrate that the stereoselective a-deprotonation of the sulfoxide is a prerequisite process for asymmetric induction in the Pummerer reaction. Since many kinds of synthetic and enzymatic preparative methods of optically pure sulfoxides have been developed, the present Pummerer-type reaction will be applicable to many other chiral sulfoxides with one a-substituent, chiral vinylsulfoxides and chiral co-carbamoylsulfox-ides, thus leading to enantioselective syntheses of many new bioactive compounds in the near future. 

Other Enantioselective Reactions. Enantioselective epoxide elimination by chiral bases has been demonstrated. More recently, the enantioselective [2,3]-Wittig rearrangement of a 13-membered propargylic ally lie ether has been performed using the lithium amide of (f ,f )-(l) as the base for deprotonation (eq 15). For this particular substrate, THF is a better solvent than ether, although pentane produces better results in a related transformation (eq 16). In fact, a change in solvent in this type of reaction has been shown to lead to a reversal of the stereoselectivity of the transformation. ... 

In a preliminary chapter, entitled Introduction, the underlying principles of physical organic chemistry, as applied to stereoselective reactions, are succintly recalled. The three subsequent chapters describe the chiral auxiliaries, reagents, catalysts and ligands that are most commonly used in asymmetric synthesis. The remaining chapters are devoted to the description and delineation of the scope of the main classes of asymmetric organic reactions. These indude protonations and deprotonations alkylations and related reactions additions to C=0, C=N and C=C double bonds cycloadditions rearrangements and transition metal-catalyzed reactions. 

The same stereoselectivity due to a boat-hke transition state has been observed by Lallemand [16] during an approach in the synthesis of antifeedent compound clerodine. Interestingly, a chemical correlation has been done with the products resulting from a Claisen-Ireland rearrangement in the open chain series. Accordingly, the E-ester enolate obtained after deprotonation and silylation of ester 64 [17] afforded, via a chair-like transition state, a compound which was correlated via 63 with the cyclic orthoester product 62 which resulted from a necessarily Z-ketene acetal. Consistently the E-ester enolate gave rise to a diastereomer 65 after the same sequence of reactions (Scheme 6.8). 

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