As observed by NMR, iodopyrroles 1a and 1b (ethyl and methyl 3, 5-dimethyl-4-[(1′-iodo- 2′, 2′-dimethyl) propyl] pyrrole-2-carboxylate) and a variety of related derivatives with iodine replaced by methoxy 2, thiomethyl 3, acetic acid esters 4, propionic acid ester 5 or malonic esters 6 exhibit restricted rotation about the C (4) C (1′) bond due to the bulky tert- butyl group and an ortho effect from the sterically crowded 3, 5-dimethylpyrrole. Most of ...