jo6b02878_si_002.cif (1.51 MB)
Construction of the Bacteriochlorin Macrocycle with Concomitant Nazarov Cyclization To Form the Annulated Isocyclic Ring: Analogues of Bacteriochlorophyll a
dataset
posted on 2017-02-22, 14:05 authored by Shaofei Zhang, Jonathan S. LindseyBacteriochlorophylls
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 132 carboalkoxy 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).