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

Klötgen S, Fröhlich R, Würthwein EU

Abstract

Deprotonation of the 2-aza-2,4,6-heptatriene 14 at -78°C using lithium diisopropylamide as base yields the 2-azaheptatrienyl lithium compound 15. During warming up to 40°C 15 undergoes a 1,7-elec-trocyclisation 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 rotation in 17b was determined to 15.4 kcal/mol.