An intramolecular [4+3]-cycloaddition approach to rameswaralide inspired by biosynthesis speculation

A concise synthesis of the polycycle 27, which incorporates the 5,5,7-tricyclic ring core of rameswaralide 1, using a biogenetically inspired acid-catalysed [4+3]-cycloaddition approach starting from the furanobutenolide 26 is described, namely, 26→28/29→27. Under thermal conditions, the same furanobutenolide 26 gives the alternative polycyclic compound 35, resulting from a [4+2]-cycloaddition involving the furan as dienophile.