A (1,3) Strain in cis‐ and trans‐5, 6‐dihydro‐4, 6‐dimethyl‐4H, 8H‐pyrido [3,2,1‐de]phenanthridin‐8‐ones

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Abstract
The stereochemistry of trans‐ and cis‐2, 4‐dimethyl‐tetrahydroquinolines, 6 and 7 were derived from 1H‐NMR. studies. These were converted respectively into trans‐ and cis‐5, 6‐dihydro‐4, 6‐dimethyl‐4H, 8H‐pyrido [3, 2, 1‐de]phenanthridin‐8‐ones 18 and 20 by a Pschorr reaction on the anthranilamides 10 and 15. Bromophenanthridones 19 and 21 were similarly prepared from bromoanthranilamides 12 and 17. Detailed 1H‐NMR. studies on 18 and 20 indicated axial disposition of the methyl groups at C(2) in both compounds in contrast to the situation in 6 and 7. This is presumably to avoid adverse CH3CO group interaction of the A (1, 3) type. The severity of this is gauged by the preference of 20 for a normally forbidding 1, 3‐diaxial orientation of two methyl groups. X‐ray crystallographic studies on 19 and 20 confirm the stereochemical assignments.