Reserpine diastereoselection

Waldmann used (R) and (5>aminoacid methyl esters and chiral amines as chiral auxiliaries in analogous aza-Diels-Alder reactions with cyclodienes.111 The diastereoselectivity of these reactions ranged from moderate to excellent and the open-chain dienes reacted similarly. Recently, the aza-Diels-Alder reaction was used by Waldmann in the asymmetric synthesis of highly functionalized tetracyclic indole derivatives (Eq. 12.45), which is useful for the synthesis of yohimbine- and reserpine-type alkaloids.112... 

A stereoselective electrocyclization developed for the synthesis of reserpine results from a stereocentre six atoms away from the newly fornoing chiral centre that is responsible for the diastereoselectivity of the ring closure. The presence of allylic strain in the disfavoured transition state results in the torquoselective ring closure (Scheme 38). 

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