Syn-periplanarity the second best choice

In those cases where the developing anion cannot attain antiperiplanarity to a good acceptor, lesser but still significant stabilization can be provided by the syn-periplanar arrangement. 

The synperiplanar stabilization of the incipient anion can overcome the stereoelectronically disfavored endo selectivity 

In summary, this chapter has discussed stereoelectronic effects on reactivity and stability which benefit from the rigid preorganization and closer spatial orbital proximity imposed by the presence of a double bond between the donor and acceptor orbitals. 

The following chapters will provide further examples of reactivity controlled by stereoeleclronics. Our goal in the next chapter is to evaluate the evolution of stabilizing orbital interactions throughout a reaction path and show how it can be used for accelerating or slowing down chemical reactions. 

Alabugin, I. V., Zeidan, T. A. (2002). Stereoelectronic Effects and General Trends in Hyperconjugative Acceptor Ability of s Bonds. Journal of the American Chemical Society, 124, 3175-3185. 

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