Thermal rearrangement of spirans (14) and (31) yields the xanthones (16,34)
and diphenylrnethane (15, 32), respectively. The structure for xanthone (16),
isolated from thermolysis, has been established by reduction to xanthene (20)
followed by reoxidation to give xanthone (16). Alternatively, methylation of the
diphenylmethane (15) followed by heating with selenium dioxide also yields
xanthone (16). The transformation of the diphenylrnethane (15) into xanthone
(16) occurs by heating with selenium dioxide either with or without solvent.