Chiral Pyridine-Based Macrobicyclic Clefts: Synthesis and Enantiomeric Recognition of Ammonium Salts

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Abstract
An achiral (3) and two chiral pyridine-based macrobicyclic clefts (4 and 5) have been prepared by treating 2,6-bis[[2‘,6‘-bis(bromomethyl)-4‘-methylphenoxy]methyl]pyridine (2) with the appropriate achiral and chiral glycols. Starting 2 was prepared by first treating 2,6-bis(hydroxymethyl)-4-methylphenol with 2,6-[(tosyloxy)methyl]pyridine followed by phosphorus tribromide. Achiral macrobicyclic cleft 3 formed a complex at 25 °C in 50% CH3OH/50% CHCl3 (v/v) with a primary ammonium salt (log K = 3.15) as evidenced by a significant change in the 1H NMR spectrum. Highly organized (S,S,S,S)-4, prepared by treating 2 with (1S,5S)-3-oxapentane-1,5-diol, exhibited recognition at 25 °C in 20% C2H5OH/80% 1,2-C2H4Cl2 (v/v) for the (S)-enantiomer of α-(1-naphthyl)ethylammonium perchlorate (NapEt) over its (R)-form (Δ log K = 0.85). This high recognition factor probably reflects an increase in molecular rigidity by the introduction of a second macro ring on the monocyclic pyridinocrown ligand.

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