On the Stacking of β ‐Rings: The solution self‐association behavior of two partially N‐methylated cyclo(hexaleucines)

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Abstract
Cyclo(‐D‐Leu‐L‐MeLeu‐D‐Leu‐L‐MeLeu‐D‐Leu‐L‐MeLeu‐) (1) and cyclo(‐L‐Leu‐D‐Leu‐L‐MeLeu‐D‐Leu‐L‐Leu‐D‐Leu‐) (2) were synthesized and used for a study of the interaction of β ‐rings in solution. In appropriate solvents, 1 and 2 formed dimers consisting of two β ‐rings connected through six interannular H‐bonds and having the N ‐Me group(s) on the solvent‐exposed face. The study also afforded indications that 2 formed different dimers of this kind, differing in the relative orientation of the two β ‐rings. These experimental observations provide strong support to the idea that unsubstituted D,L‐alternating cyclooligopeptides, such as cyclo(hexaleucine), can self‐assemble and give rise to long tubular stacks of β ‐rings.