1,7-Electrocyclisation reactions of 2-azaheptatrienyl lithium compounds: Synthesis of 1-acyl-2,3-dihydroazepines

Article abstract of DOI:10.1016/0040-4020(96)00938-6

Deprotonation of the 2-aza-2,4,6-heptatriene 14 at -78°C using lithium diisopropylamide as base yields the 5-azaheptatrienyl lithium compound 15. During warming up to 40°C 15 undergoes a 1,7-electrocyclisation to afford the 1-azacycloheptadienyl lithium compound 16, as predicted by quantum chemical ab initio calculations. This cyclisation reaction was monitored in detail by ¹H and ¹³C NMR spectroscopy. Subsequent treatment with various acylating agents provides access to several 1-acyl-2,3-dihydroazepines 17. A X-ray diffraction structure determination of derivative 17a shows a trans arrangement of the two aromatic substituents in 2- and 3-position, indicating a conrotatory mode of the ring closure reaction. The rotational barrier for the amide relation in 17b was determined to 15.4 kcal/mol.