Synthesis and X-ray structure of a cyclopenta[d]furo[2,3-b]furan derivative from isomerisation of 3,4,4-triacetyl-2-methyl-cis-4,4a,5,7a-tetrahydrocyclopenta[b]pyran

TLDR: In this paper, a new heterocyclic system was definitively established by an X-ray analysis, whose structure was solved by direct methods and refined by least squares to a final R of 0.073 for 1 665 observed reflections.

Abstract: 3,4,4-Triacetyl-2-methyl-cis-4,4a,5,7a-tetrahydrocyclopenta[b]pyran (3), obtained by reaction of tetra-acetyl-ethylene (1) with cyclopentadiene, was converted into a mixture of the trans-isomer (4) and 3,3a-diacetyl-2,7a-dimethyl-3aα,4,6aα,7aα-tetrahydro-3bαH-cyclopenta[d]furo[2,3-b]furan (5), a new heterocyclic system, whose structure was definitively established by an X-ray analysis. Crystals are orthorhombic, a= 15.824(5), b= 15.465(5), c= 11.129(4)A, Z= 8, space group Pbca. The structure was solved by direct methods and refined by least squares to a final R of 0.073 for 1 665 observed reflections. Some aspects of the mechanism are discussed.