title: Construction of the Bacteriochlorin Macrocycle with Concomitant Nazarov Cyclization To Form the Annulated Isocyclic Ring: Analogues of Bacteriochlorophyll a.
abstract: Bacteriochlorophylls contain a bacteriochlorin macrocycle bearing an annulated fifth ring. The fifth ring, termed the isocyclic ring or ring E, is equipped with 131-oxo and 132-carbomethoxy substituents. Herein, a general route to stable, synthetic bacteriochlorophyll analogues is described. Knoevenagel condensation (∼40 mM, rt, CH2Cl2, piperidine/AcOH/molecular sieves) of a dihydrodipyrrin-carboxaldehyde (AD half) and a dihydrodipyrrin substituted with a β-ketoester (BC half) forms a propenone bearing the two halves (a hydrobilin analogue). Subsequent treatment (0.2 mM) with acid (Yb(OTf)3, CH3CN, 80 °C) promotes a double ring-closure process: (i) condensation between the α-position of pyrrole ring A and the α-acetal unit attached to pyrroline ring B forms the bacteriochlorin macrocycle, and (ii) Nazarov cyclization of the β-(propenoyl)-substituted ring C forms the isocyclic ring (E). Five new bacteriochlorins bearing various substituents (alkyl/alkyl, aryl, and alkyl/ester) at positions 2 and 3 (β-pyrrole sites, ring A) and 132carboalkoxy groups (R = Me or Et) were constructed in 37-61% yield from the hydrobilin analogues. The BC half and AD half are available in five and eight steps, respectively, from the corresponding pyrrole-2-carboxaldehyde and unsaturated ketone. The bacteriochlorins exhibit absorption spectra typical of bacteriopheophytins (free base bacteriochlorophylls), with a strong near-infrared absorption band (707-751 nm).
authors.affiliation: Department of Chemistry, North Carolina State University , Raleigh, North Carolina 27695-8204, United States.
authors.forename: Shaofei
authors.initials: S
authors.lastname: Zhang
authors.affiliation: Department of Chemistry, North Carolina State University , Raleigh, North Carolina 27695-8204, United States.
authors.forename: Jonathan S
authors.identifier: 0000-0002-4872-2040
authors.initials: JS
authors.lastname: Lindsey
mesh_terms: D025541:Bacteriochlorophyll A / Q000737:chemistry*; D047028:Macrocyclic Compounds / Q000737:chemistry*; D011166:Porphyrins / Q000737:chemistry*; D013057:Spectrum Analysis / Q000379:methods
chemical_list: D025541:Bacteriochlorophyll A; D047028:Macrocyclic Compounds; D011166:Porphyrins; C452433:bacteriochlorin