Podophyllum lignans synthesis

In connection with the synthesis of podophyllum lignans, ester (62) was deprotonated and the resulting enolate condensed with 3,4,5-trimethoxybenzaldehyde to give a 1 1 mixture of diastereomeric aldols (equation 68). The structure of (63) was established by X-ray analysis the other diastereomer was assigned the 2,3-anti relative stereochemistry (64) on circumstantial evidence. It was suggested that the 1 1 mixture of isomeric products results from a 1 1 mixture of the ( )- and (Z)-enolate, each of which shows complete simple and diastereofacial selectivity in its reactions with 3,4,5-trimethoxybenzaldehyde. For this to be true, it is also necessary that the ( )-enolate reacts through a non-Zimmerman , boat-like transition state, whereas the (Z)-enolate reacts through the normal chair-like transition state. 

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