Stereoselective tetrahydropyrido[2,1-a]isoindolone synthesis via carbanionic and radical intermediates: a model study for the Tacaman alkaloid D/E ring fusion

Stereoselective cyclisation of malic acid-derived α-sulfanyllactam (1) via radical and carbanionic intermediates affords stereo-defined tetrahydropyrido[2,1-a]isoindolones as model compounds for the D/E cis-ring fusion of the Tacaman indole alkaloid skeleton. A transition-state model to explain the stereoselectivity is presented.