Design of new anti-Alzheimer drugs: Ring-expansion synthesis and synchrotron X-ray diffraction study of dimethyl 4-ethyl-11-fluoro-1,4,5,6,7,8-hexahydroazonino[5,6-b]indole-2,3-dicarboxylate

The title compound, C20H23FN2O4, is the product of a ring-expansion reaction from a seven-membered fluorinated hexahydroazepine to a nine-membered azonine. The nine-membered azonine ring of the molecule adopts a chair-boat conformation. The C=C and C - N bond lengths [1.366 (3) and 1.407 (3) A, respectively] indicate the presence of conjugation within the enamine CH2 - C=C - N - CH2 fragment. The substituent planes at the C=C double bond of this fragment are twisted by 16.0 (3)° as a result of steric effects. The amine N(Et) N atom has a trigonal-pyramidal configuration (sum of the bond angles = 346.3°). The interplanar angle between the two carboxylate substituents is 60.39 (8)°. In the crystal, molecules form zigzag chains along [010] by intermolecular N - H⋯O hydrogen-bonding interactions, which are further packed in stacks toward [100]. The title azoninoindole might be considered as a candidate for the design of new Alzheimer drugs.