General and Efficient Approaches to Fused [l,2-a]Pyrroles and [1,2-a]Indoles

Abstract

2-[(Benzotriazol-l-yl)methyl]pyrroles 4a-c, and 15a,b and -indoles 23, prepared as previously reported,1-2 were converted into the corresponding fused [1,2-a]pyrroles 8a-c, and 18a,b and fused [1,2-a]indoles 25, 27 by intramolecular cyclizations; the formations of 25 and 27 involve [3 + 2] and [3 + 3] annulations. The benzotriazolyl-CH-ring moieties of intermediates 8a,b, 18a,b, 25, and 27 were further elaborated: (i) acylation of 8a and subsequent reduction and elimination promoted by LiAlH4 to give product 6; (ii) nucleophilic displacements of the benzotriazolyl moiety with NaCN, NaSPh, and Grignard reagents to afford compounds 9, 11, 12, 20, 29, and 32, respectively; (iii) base-assisted eliminations of benzotriazole from 8b and 18a to form the unsaturated fused [1,2-a]pyrroles 10 and 17; (iv) reactions of 18b and 25 with α,β-unsaturated ketone and aldehyde followed by acid-assisted elimination of the benzotriazolyl group to yield tricyclic compound 21 and product 31; and (v) Lewis acid promoted dimerization of 27 to form the fused indolo[3,2-6]carbazole 33.