Formation of 1,10-disubstituted benzo[c]cinnolines. Synthesis and molecular structure of 1-amino-10-propylthiobenzo[c]cinnoline and cyclization to 4-propylcinnolino[5,4,3][c,d,e][1,2]benzothiazine

The first 1,10-heterodisubstituted benzo[c]cinnoline derivative I was prepared from the trinitrobiphenyl 2. Investigation of the mechanism of ring closure in 2, 5, and 8 revealed a complex reduction-oxidation-cyclization sequence. The mechanism is discussed in light of the stereoelectronic demands of the substituent functionalities. Benzo[c]cinnoline derivative 1 [C₁₅H₁₅N₃S, monoclinic, P2₁/c: a = 7.4063(3) Å, b = 10.3739(5) Å, c = 16.7642(8) Å, β = 91.816(1)°, Z = 4] and its 5-N-oxide 7(N5) [C₁₈H₁₈N₃OS, triclinic, P(1̄): a = 8.1510(7) Å, b = 8.6106(7) Å, c = 12.102(1) Å, α = 86.262(1)°, β = 83.364(1)°, γ = 74.711(1)°, Z = 4] were structurally characterized and showed a significant helical distortion of the heterocyclic ring. Oxidation of 1 with NCS or triamine 12 with PhI(OAc)₂ led to a new heterocyclic ring system, ylide 13. Both benzo[c]cinnoline 1 and ylide 13 were characterized spectroscopically and the absorption spectra were correlated with the results of ZINDO calculations.