Total synthesis of (−)-indolizidine 167B via an unusual Wolff rearrangement from an α,β-unsaturated diazoketone

A concise synthesis of the (−)-indolizidine alkaloid 167B and two formal syntheses of (−)-indolizidine 209D and (−)-coniceine are described in just three steps from an α,β-unsaturated diazoketone, via an unusual photochemical Wolff rearrangement. Preparation of the unsaturated diazoketone is straightforward from N-Cbz-prolinal and a 3-diazo-2-oxopropylphosphonate, employing a Horner–Wadsworth–Emmons reaction. The strategy should be feasible and easily adaptable to the synthesis of other indolizidine alkaloids and analogues.