A Rapid and Efficient Access to Diaryldibenzo[<italic>b</italic>,<italic>f</italic>][1,5]diazocines

2-Benzoylbenzoyl azides undergo facile cyclization under acidic conditions to give substituted dibenzo[b, f ][1,5]-diazocines in good yields. This approach shortens the synthetic steps toward these compounds as compared with conventional methods. The mechanism of the diazocine synthesis is assumed to proceed by an unprecedented intermolecular [2 þ 2] cyclization. Dibenzo[b,f][1,5]diazocines were first synthesized more than one century ago. The synthesis of this class of compounds has attracted more attention since the 1960s due to the discovery of the hormone-like activity of 2,8dichloro-6,12-diphenyldibenzo[b,f][1,5]diazocine. More recently, material scientists considered them a potential building block for artificial muscles due to their reversible conformational changes occurring during electrochemical redox processes. However, synthetic strategies toward diazocines are quite limited. The most common approach is the condensation reaction of 2-aminobenzophenones, as shown in Scheme1.This pathwayhas a fewdrawbacks: the synthesis of 2-aminobenzophenones either requires tedious protection/deprotection steps of the 2-aminobenzoic acid (route 1) which is atom inefficient or requires expensive 3-arylbenzo[c]isoxazoles (route 2). Furthermore, the condensation reaction generally requires a long reflux time and the yield varies dramatically with different substrates and reaction conditions. The third route involves a Friedel-Crafts acylation of an aromatic ring with 2-isocyanobenzoyl chloride, which further reacts with 2-aminobenzophenones to give unsymmetric 6,12-diaryl-dibenzo[b,f][1,5]diazocines (route 3), but in relatively lowyields.Weherein report a facile synthetic strategy toward dibenzodiazocines which follows a completely different mechanism compared with the reactions mentioned above. In our efforts to synthesize dibenzodiazocines as a building block for artificial muscles, we initially adopted the conventional 2-aminobenzophenone route. To avoid the protection/deprotection process in the preparation of 2-aminobenzophenone, we envisioned that this precursor could be prepared from 2-benzoylbenzoyl azide via a (1) Sondheimer, A. Chem. Ber. 1896, 29, 1272. (2) Duncan, G.W.; Lyster, S. C.;Wright, J. B.Proc. Soc. Exptl. Biol. Med. 1965, 120, 725. (3) Marsella, M. J. Acc. Chem. Res. 2002, 35, 944. (4) Suga, T.; Wi, S.; Long, T. E. Macromolecules 2009, 42, 1526. (5) Scheifele, H. J.; Detar, D. F. Org. Synth. 1952, 32, 8. (6) Metlesics, W.; Resnick, T.; Silverman, G.; Tavares, R.; Sternbach, L. H. J. Med. Chem. 1966, 9, 633. (7) Boruah, R. C.; Sandhu, J. S. J. Heterocycl. Chem. 1988, 25, 459. (8) Konwar, D.; Bouah, R. C.; Sandhu, J. S.; Baruah, J. N. Indian J. Chem., Sect. B 1982, 21B, 899. (9) Isagawa, K.; Ishiwaka, T.; Kawai, M.; Fushizak, Y. Bull. Chem. Soc. Jpn. 1969, 42, 2066. (10) Acharya, B. P.; Rao, Y. R. Synthesis 1986, 324.