Amino Acid-Viologen Hybrids: Synthesis, Cucurbituril Host-Guest Chemistry, and Implementation on the Production of Peptides

We present herein the development of a series of viologen–amino acid hybrids, obtained in good yields either by successive alkylations of 4,4′-bipyridine, or by Zincke reactions followed by a second alkylation step. The potential of the obtained amino acids has been exemplified, either as typical guests of the curcubituril family of hosts (particularly CB[7]/[8]) or as suitable building blocks for the solution/solid-phase synthesis of two model tripeptides with the viologen core inserted within their sequences.

An electrochemically controlled supramolecular zip tie based on host-guest chemistry of CB[8]

Three molecular threads incorporating two viologen units attached by an oligo-ethylene glycol chain of variable length and an electron rich naphthalene moiety were prepared. The internal viologen unit is connected to the naphthalene by a rigid linker that prevents the simultaneous complexation of these systems. The axle with the shortest chain (14+) in the presence of CB[8] placed the chain inside the macrocycle in aqueous media. In contrast, the longer chains in 24+ and 34+ allow locating the terminal viologen and the electron rich unit inside the cavity. Upon reduction of the bipyridinium salts, the system behaves like a zip tie relaxing the chain as a consequence of the insertion of both radical cation moieties within the CB[8] and making these switches as potential components for molecular machinery.

Electrocyclic Ring-Opening of 1,2,4-Oxadiazole[4,5-a]piridinium Chloride: a New Route to 1,2,4-Oxadiazole Dienamino Compounds

1,2,4‐Oxadiazole[4,5‐a]piridinium chloride adds nucleophiles to undergo electrocyclic ring opening affording 1,2,4‐oxadiazole dienamino derivatives. These pyridinium salts represent a special class of Zincke salts that are prone to rearrange when treated with primary amines or in the presence of bicarbonate to give the pyridones. The pivotal tuning of the experimental conditions leads to a straightforward synthesis of valuable 1,2,4‐oxadiazole dienamine derivatives. The mechanism is also discussed in the light of NMR experiments and theoretical calculations.