Polycyclic ketones, conjugate addition

Biichi disclosed a sequence of cyclizations involving an indole that rapidly leads to structural complexity (Scheme 11.22) [121]. Exposure of 137 to BF2 OEt2 enabled the conjugate addition of the indole to the unsaturated ketone to produce iminium ion 138. This was in turn subject to attack by the enol of the methyl ketone to form 139 in 89 % yield. This process constituted a key transformation in the construction of the targeted polycyclic skeleton of the alkaloid vindoline (140). 

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