Intramolecular [2+2] photocycloadditions as an approach towards the right-hand side of solanoeclepin A

A racemic synthesis of the bicyclo[2.1.1]hexane substructure of solanoeclepin A (1), the most active natural hatching agent of potato cyst nematodes, was approached via an intramolecular [2+2] photocycloaddition of 6-unsubstituted dioxenones with variously substituted pendent alkenes. The synthesis of the cyclisation precursors involved a very efficient iodide-magnesium exchange reaction with iododioxenone 6, which allowed facile allylation at C-5 of the dioxenone. Photochemistry with dioxenones 12 and 17 led to novel bicyclo[2.2.0]hexanes 24 and 26. The use of the more rigid lactone precursor 14 led to bicyclo[2.1.1]hexane 25, and allowed the stereoselective synthesis of the complex tricyclic core of solanoeclepin A. The structure of 25 was unequivocally proven by X-ray crystal structure determination.