Studies toward the Total Synthesis of the Oxindole Alkaloid Gelsedine: An Efficient Allene-Terminated N-Acyliminium Ion Cyclization

This paper reports the synthesis of the advanced intermediate 26 in a projected synthesis of enantiopure ent -gelsedine ( 5 ). The route starts from ( S )-malic acid and features the creation of four new stereocenters with complete control of stereochemistry. The key step is a novel allene-terminated N -acyliminium ion cyclization that leads to the required 7-azabicyclo[4.2.1]nonane-4,8-dione skeleton. Additional functionalities including the C-8 ethyl and the C-9 hydroxymethyl are introduced in an efficient manner.