Dihydropyrromethenones by Pd(0)-Mediated Coupling of Iodopyrroles and Acetylenic Amides. Synthesis of the A,B-Ring Segment of Phytochrome.

Dihydropyrromethenone derivative 32b, which constitutes the A,B-ring segment of phytochrome (6), has been prepared in enantiomerically pure form beginning with acetylenic amide 47b and iodopyrrole 27. The key steps involved the TBAF-catalyzed 5-exo-dig cyclization of the acetylenic pyrrole 48b, followed by thia-Mitsunobu inversion of the resulting alcohol derivative 31b.