The synthesis of porphyrinic macrocycles and related compounds requires the ability to carry out reactions at the pyrrolic α- and α′-positions (2- and 5-positions, respectively). The controlled introduction of a single substituent via electrophilic substitution can necessitate the use of a blocking group, particularly when the newly introduced substituent activates the pyrrole to further substitution. In the synthesis of naturally occurring porphyrins, which typically entails the use of 3,4-disubstituted pyrroles (e.g., A), an ester (or carboxylic acid) suffices to block the 2-position: substitution occurs at the 5-position, which contains the only open carbon in the pyrrolic nucleus (eq 1). Removal of the blocking carboxy moiety typically requires treatment at high temperature with a strong base, a strong acid, and/or a halogen reagent. Use of the halogen reagent affords the 2-halopyrrolic species, which is converted to the pyrrole with the open 2-position by catalytic hydrogenation (Paine, J. B., III. In The Porphyrins; Dolphin, D. Ed.; Academic Press: New York, 1978; Vol. I, pp 101-234). In general, the use of a carboxylate (or other electron-withdrawing group) to block the 2-position in a pyrrole lacking substituents at the 3- and 4-positions (e.g., B, eq 2) is expected to present three problems: (1) sluggish reaction, (2) diminished selectivity for the direction of the incoming group to the 5-position vs the 4-position, and (3) harsh conditions for removal of the carboxylate group.

To our knowledge, only one β-unsubstituted dipyrromethane has been prepared via this approach (Setsune, J.-i. et al., J. Chem. Soc., Chem. Commun. 1994, 657-658; Setsune, J.-i. et al., Tetrahedron 1998, 54, 1407-1424). On the other hand, the incoming group could be directed rapidly to the 5-position with an α-blocking group that is not deactivating, but such a simple protective group for pyrroles has heretofore not been developed.
The absence of a suitable α-blocking group for unsubstituted pyrroles has substantially affected a number of synthetic transformations. For example, the synthesis of β-unsubstituted dipyrromethanes is typically carried out by reaction of an aldehyde with excess pyrrole (up to 100 mol equiv), resulting in 1, N-confused dipyrromethane 2, tripyrrane 3, and oligomeric byproducts (Scheme i).
The presence of excess pyrrole is required to trap the initially formed pyrrole-carbinol and thereby suppress the competitive self-oligomerization of the pyrrole-carbinol. Although considerable refinement has gone into streamlining the conditions for carrying out this reaction and purifying the product (Lee, C. H.; Lindsey, J. S. Tetrahedron 1994, 50, 11427-11440; Littler, B. J. et al., J. Org. Chem. 1999, 64, 1391-1396; Laha, J. K. et al., Org. Process Res. Dev. 2003, 7, 799-812), the use of such a large excess of pyrrole remains an inherent disadvantage. The availability of a blocking group that is not deactivating would enable the use of a stoichiometric amount of the protected pyrrole (2 mol equiv) and the aldehyde. Thompson and co-workers have recently reported a sulfonyl or 2,4-dinitrophenylsulfinyl group for protecting the α-pyrrole position, but again, both groups are deactivating (Thompson, A. et al., J. Org. Chem. 2005, 70, 3753-3756).
USPTO Backgrounds
USPTO Backgrounds