SYNTHESIS OF CALIXFURAN MACROCYCLES AND EVIDENCE FOR GAS-PHASE AMMONIUM ION COMPLEXATION

Furan and 2-hydroxymethylfuran reacted under Lewis acidic conditions t o yield oligomeric furylmethane compounds 5 and 6. Difurylmethane 5 wa s also obtained via the reaction of 2-furyllithium with bromochloromet hane or reaction with 2-furaldehyde, followed by in situ reduction of the resulting lithium alkoxide with sodium boranuide-trifluoroacetic a cid mixture. The trimeric furan compound 6a could also be prepared fro m furyllithium by a similar route. Oligomers 5 and 6 were utilised for a subsequent Lewis acid-catalysed cyclisation to afford calixfurans, i.e; cyclic tetramer 2a, and small quantities of the cyclic pentamer 2 b, hexamer 2c and octamer 2e. Traces of cyclic heptamer 2d could also be detected under certain conditions. Chemical ionisation mass spectro metry using ammonia gas demonstrates that the calixfurans and their ac yclic precursors are capable of chelating hydrogen and ammonium ions i n the gas phase.