Indoloquinolizidine ketones

The first example of a successful iminium ion-alkyne cyclization appeared in 1977. The 4-alkynyl-amine (88) was cyclized in acidified aqueous formaldehyde to a mixture of indoloquinolizidine ketones which could be equilibrated in base to provide the more stable epimer (89) in 49% yield. Not surprisingly, the six-membered ring was formed exclusively. 

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