Synthesis of calixfuran macrocycles and evidence for gas-phase ammonium ion complexation

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Abstract
Furan and 2-hydroxymethylfuran reacted under Lewis acidic conditions to yield oligomeric furylmethane compounds 5 and 6. Difurylmethane 5 was also obtained via the reaction of 2-furyllithium with bromochloromethane or reaction with 2-furaldehyde, followed by in situ reduction of the resulting lithium alkoxide with sodium boranuide–trifluoroacetic acid mixture. The trimeric furan compound 6a could also be prepared from 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 2b, hexamer 2c and octamer 2e. Traces of cyclic heptamer 2d could also be detected under certain conditions. Chemical ionisation mass spectrometry using ammonia gas demonstrates that the calixfurans and their acyclic precursors are capable of chelating hydrogen and ammonium ions in the gas phase.

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