Electrophilic substitution in verdins: deuteriation and nitration of 2,3,7,8,12,13,17,18-octaethylbilin-1,19(21H,24H)-dione (octaethylbilatriene-abc): X-ray crystal structure of the 5-nitro-derivative

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Abstract
Electrophilic substitution (deuteriation, nitration) of octaethylbilindione occurs preferentially at the outer bridge positions (C-5 and C-15), and there is evidence that meso-nitration occurs during the Gmelin reaction; X-ray analysis of the 5-nitro derivative shows that the C-4–C-5 bond has the Z-configuration, and that ring A is approximately at 90° to ring B, thus accounting for the unexpected absorption spectrum.