Macrobicyclic polyethers with bridgehead carbon atoms
Syntheses of the carbon-bridgehead cryptands (5)–(9) have been achieved in a number of different ways from the ditosylate (12) of diethylene glycol and either pentaerythritol (10) or 1,1,1-tris(hydroxymethyl)ethane (11). The cryptands (5)–(9) all exist as their ‘out, out’ isomers and form complexes in solution with alkali metal cations. Variable temperature 1H n.m.r. spectroscopy indicates that the free energy of activation for the dissociation of the potassium ion complexes with the bisbenzyloxymethyl-(5) and dimethyl-(9)cryptands in deuteriochloroform is ca. 12 kcal mol–1. Stability constant measurements on the complexes of these  cryptands [i.e.(5) and (9)] and the bishydroxymethylcryptand (6) with sodium, potassium, rubidium, and caesium chlorides in methanolic solution indicate, surprisingly, that the complexes are extremely weak (K′ca. 100). All three -cryptands [i.e.(5), (6), and (9)] exhibit a slight selectivity for binding postassium ions.