The synthesis of pillar [4]anthracene (3) was also attempted (Figure ). The addition of another benzyne precursor, 3-(trimethylsilyl)-2-naphthyl trifluoromethanesulfonate (8g), to 1 under the same condition detailed above resulted in clean formation of calix [4](1,4-epoxyanthracene) (11), as evident in its MALDI-TOF (Figure ) and high-resolution (HR) APCI-MSspectrum (Figure ). Unlike 4a with the concomitant formation of multiple conformers, HPLC analysis of 11 displayed a much less complicated chromatogram (Figure ). Despite that, clean separation of these species failed, and thus this mixture was used directly for the next step. Deoxygenation of 11 proved to be challenging, presumably due to the enhanced steric protecting the epoxy group. Under the same reaction condition as 4a, albeit low yield, 11 was successfully converted to 3. The mother ion signal [M-2] + (m/z = 758.2972) (theoretical mass = 758.2968) was unambiguously observed by HR-MALDI-TOF spectroscopy (Figure ). We have also evaluated other deoxygenation protocols reported in literature (Table ), but they did not give any detectable amount of 3. Finally, the host-guest chemistry of pillar [4]naphthalene host was evaluated. The complexation of alkali metal cations and 2d were investigated in the gas phase by electrospray ionization mass spectrometry (ESI-MS). To our surprise, 2d exhibits superior molecular recognition property towards NH4 + over other alkali metal ions. For example, when a CH3CN solution containing 2d (3.0 × 10 -4 M) and a mixture of alkali metal triflates (3.0 × 10 -4 M for NH4OTf; and 3.0 × 10 -2 M for LiOTf and NaOTf) in a 1:1:100:100 of 2d:NH4 + :Li + :Na + was directly injected into a mass spectrometer with an ESI source. The major signal belongs to that of 2d ⊃ NH4 + (Figure ). When a 1:1 mixture of NH4OTf and KOTf was added to 1 equiv. of 2d, ESI signal assigned to 2d ⊃ NH4 + is almost 20 times more intense than that of 2d ⊃ K + (Figure ), demonstrating uniquely selective ammonium recognition property in pillar [4]naphthalene host.