Use of the Nascent Isocyclic Ring to Anchor Assembly of the Full Skeleton of Model Chlorophylls

The chlorophyll skeleton contains a chlorin macrocycle
and an annulated
fifth (or isocyclic) ring bearing 131-oxo and 132-carbomethoxy substituents. The isocyclic ring has traditionally
been constructed by annulation of an intact tetrapyrrole macrocycle.
Here, a complementary route employs reaction of a gem-dimethyl-substituted
dihydrodipyrrin–carboxaldehyde (AD half) and a dipyrromethane
bearing a 3-methoxy-1,3-dioxopropyl group (BC half). A McMurry-like
reaction of a 2-(2-nitro-5-oxohexyl)­pyrrole was employed to construct
the second pyrrole ring in one of three BC halves, whereas the other
two were prepared by known routes. An AD half and a BC half were joined
by Knoevenagel condensation at room temperature, affording the AD,BC-substituted
2-methoxycarbonyl-2-propenone. The subsequent reaction of three AD,
BC-propenones (mixture of Z,E-isomers)
in CH3CN containing InCl3 and In­(OTf)3 at 80 °C afforded the chlorophyll skeleton as the chloroindium­(III)
chelate; the reaction proceeds via Nazarov cyclization (to form the
isocyclic ring), SEAr (to construct the macrocycle), and
2e–,2H+ oxidation (to give the aromatic
chromophore). The absorption spectra of the complexes closely resemble
that of chlorophyll a. The present work exploits
the nascent isocyclic ring as an anchor for directed assembly of the
AD and BC halves, forming both the chlorin macrocycle and the isocyclic
ring in a single-flask transformation.