Elaboration of the carbocyclic ring systems in plumarellide and rameswaralide using a coordinated intramolecular cycloaddition approach, based on a common biosynthesis model

Treatment of the furanbutenolide 18a with trifluoroacetic acid in dichloromethane results in hydrolysis and rearrangement leading to the cycloheptene ring-containing system 19a found in rameswaralide (5). Similar treatment of the diastereoisomeric furanbutenolide 17 produces the cyclohexene ring-containing system 23 present in plumarellide (1), together with the C-7 epimer 24 of 19a. The acid-catalysed conversions 18a → 19a and 17 → 23 are rationalised implicating [6+4] and [4+2] cycloaddition pathways and/or two-step carbocation cyclisation sequences.