Abstract
A new series of crown compounds crownopaddlanes 3a– c bearing three cyclobutane rings were prepared by means of intramolecular [2+2] photocycloaddition of styrene derivatives. The yield of crownopaddlane 3b possessing five ethereal oxygen atoms was remarkably high 52% with the addition of sodium borofluoride in the photoreaction system. As this template effect suggests, 3b showed extraordinarily high Na +-selectivity with high efficiency on the liquid–liquid extraction of alkali metal picrates, though 3a having four ethereal oxygen atoms did not extracted any cations in this system. The high Na +-selectivity of 3b was further clarified by the equilibrium stability constants (log K a) for Na + (5.85) and K + (2.91) in acetonitrile solution. The complexation of 3b to Na + cation was also examined by X-ray crystallography. Crownopaddlane 3c bearing six ethereal oxygen atoms also efficiently and selectively extracted alkali metal cations, compared with conventional 18-crown-6 derivatives.
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