The synthesis of non-racemic natural products

Steven M. Allin, Stephen J. Shuttleworth and Philip C. Bulman Page 

These tetrahedral structures are capable of interconversion by pyramidal inversion and C—S bond rotation. Williams suggests that the relative reactivity of the above a-lithio sulfinyl carbanion diastereoisomers with aldehydes (and therefore the stereoselectivity of the reaction) is governed by the steric environment of the process. 

The absence of the (R) alcohol product can be explained from the greater steric hindrance observed in transition states C and D, which would lead to the formation of the (R) product. Transition states C and D are again related to each other through attack of the aldehyde from the opposite apex of the p-orbital. Transition state C is related to A by attack at the opposite face of the prochiral carbonyl moiety (as B is to D). 

Natural (+)-elaeokanine A (42) was obtained in 93% yield through similar manipulation of (38a) or (38b). 

The above synthetic schemes represent the first asymmetric synthesis of these alkaloids as a result, Hua was able to establish the absolute configurations of the natural elaeokanine alkaloids. 

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