Simple approach to "locked" chlorins.

A novel synthetic approach to diversely functionalized “locked” chlorins is described. A suitably substituted 2,5-diformylpyrrole undergoes the macrocyclization reaction with tripyrranes, thereby generating porphyrins. Upon the reaction with 1,3-dipoles these porphyrins regioselectively furnish pyrrolidine-fused chlorins, which cannot oxidize to the corresponding porphyrins. In the process involving just six steps from commercially available and cheap materials we are able to obtain ∼200 mg of pure stable chlorins (the overall yield is 1.5−2.8%).