Synthesis of Isoxazolopyrrolo[2,1-a]isoquinoline, Isoxazolo[5′,4′: 1,2]indolizino[8,7-b]indole, and Isoxazolo-[5,4-a]thieno[2,3-g]indolizine Derivatives by Intramolecular Cyclization of Hydroxylactams Constituting a Fragment of the Pyrroloisoxazole System

Abstract

6-Hydroxy-5-[2-(naphthalen-1-yl)ethyl]-3a,5,6,6a-tetrahydro-4H-pyrrolo[3,4-d]isoxazol-4-ones in the presence of boron trifluoride–diethyl ether complex underwent cyclization to benzo[f]isoxazolo[5′,4′: 3,4]- pyrrolo[2,1-a]isoquinoline derivatives as mixtures of two diastereoisomers. Analogous cyclization of 6-hydroxy- 5-(naphthalen-1-ylmethyl)-3a,5,6,6a-tetrahydro-4H-pyrrolo[3,4-d]isoxazol-4-ones gave benzo[de]isoxazolo[ 5′,4′: 3,4]pyrrolo[2,1-a]isoquinolines as a single diastereoisomer. The cyclization of 6-hydroxy-5-[2-(1Hindol- 3-yl)ethyl]-6,6a-dihydro-3aH-pyrrolo[3,4-d]isoxazol-4(5H)-ones catalyzed by Sn(NTf₂)₄ afforded isoxazolo[ 5′,4′: 1,2]indolizino[8,7-b]indol-4(12H)-ones in moderate yields. 6-Hydroxy-5-[2-(thiophen-2-yl)ethyl]-6,6a-dihydro-3aH-pyrrolo[3,4-d]isoxazol-4(5H)-ones were converted to 4,5,10a,10b-tetrahydroisoxazolo- [5,4-a]thieno[2,3-g]indolizin-7(7aH)-one derivatives in the presence of BF3 · Et2O or Sn(NTf₂)₄.