E and Z Secondary Amides in Supramolecular Nanotubes and Nanospheres - Molecular Self-Assembly: Advances and Applications - page 44

in the stacking process, replacing them by weaker non-covalent
interactions. This goal can be reached by replacing selected amides by
isosteric -alkenes. Accordingly, each of the three Gly-Gly dipeptides

of cyclo-Gly

E

was changed for the corresponding dipeptide surrogate,

6

δ

-alkene
(dubbedGly//Gly)[63].Thisresultedinourfirsttargetcyclo-(Gly//
Gly)

the -amino acyl residue NHCH CH=CHCH CO bearing an E 2 2

) only capable of developing three hydrogen bonds at the
most with each ring of the stack [64]. Several methods of synthesis
were developed for this

( 1 3

symmetric compound relying on direct
cyclization of a reactive trimeric precursor or cyclooligomerization
of an activated monomer [65,66].
The material was easily crystallized by difusion in the
conventional solvents ethanol and ether as a proof of its improved
solubility. In the crystal state cyclo-(Gly//Gly)

C

3

adopts the same
geometry as one of the four conformers found in crystals of isosteric
cyclohexaglycine(Fig.2.7).Itsshapeisrectangular,beingstabilized
by an intramolecular

3

β

-turn-like hydrogen bond. Two amides out
ofthreeareinvolvedinthestackingleadingtoparallelflattened
endless tubes with no inter-stack hydrogen bonds.

Figure 2.7 Crystal structure of ( ). (a) Stacking as rectangular tubes

and, (b) their packing with no inter-tubes hydrogen bonds.

1