Diels‐alder reactions of 3‐(2‐nitrovinyl)indoles: Formation of carbazoles and bridged carbazoles

The Diels‐Alder reactions of 1‐substituted‐3‐(2‐nitrovinyl)indoles 3 with quinones and acetylenes give aromatized 1:1 adducts (‐ nitrous acid) (1) or (‐ nitrous acid, ‐2 hydrogens) 2,5. Likewise, dimerization (‐2 nitrous acids) of 3 gives aromatized 2‐(3‐indolyl)carbazoles 4. In contrast, 3 reacts with maleimides 6 to give 1:2 adducts (‐ nitrous acid or ‐2 hydrogens) 10 and 11, respectively, along with smaller amounts of 1:1 adducts (‐ nitrous acid, ‐2 hydrogens; or ‐4 hydrogens) 12 and 13, respectively. A mechanism for formation of the nitro products 11 and 13 is discussed. A 1:2 adduct (‐2 hydrogens) 19 was also obtained from a Diels‐Alder reaction between maleimide and the vinylindole produced in situ by condensing 1‐methylindole with acetone. The stereochemisty of this 1:2 adduct has been determined by X‐ray crystallography.