Synthesis and Conformational Analysis of Naphthoxazine-Fused Phenanthrene Derivatives

Abstract

The synthesis of new phenanthr[9,10-e][1,3]oxazines was achieved by the direct coupling of 9-phenanthrol with cyclic imines in the modified aza-Friedel–Crafts reaction followed by the ring closure of the resulting bifunctional aminophenanthrols with formaldehyde. Aminophenanthrol-type Mannich bases were synthesised and transformed to phenanthr[9,10-e][1,3]oxazines via [4 + 2] cycloaddition. Detailed NMR structural analyses of the new polyheterocycles as well as conformational studies including Density Functional Theory (DFT) modelling were performed. The relative stability of ortho-quinone methides (o-QMs) was calculated, the geometries obtained were compared with the experimentally determined NMR structures, and thereby, the regioselectivity of the reactions has been assigned.

Structural and Conformational Aspects in the Chemistry of Heterocycles

“…In the paper by Hungarian and German researchers [ 5 ], a synthetically challenging aminophenanthrol motif was successfully prepared on the example of 10-morpholinobenzyl-9-phenanthrol by the three-component reaction of 9-phenanthrol, benzaldehyde, and morpholine. The product was then involved in reactions with cyclic imines—3,4-dihydroisoquinoline, 6,7-dihydrothieno[3,2-c] pyridine, or 4,9-dihydro-β-carboline—and the structure of the formed products was determined by the detailed NMR analysis.…”