Multiply charged anions from molecules with extended π-systems

Diagnosing Ring Current(s) in Figure-Eight Skeletons: A 3D Through-Space Conjugation in the Two-Loops Crossing

The macrocyclic structures with local conjugation readily undergo a redox-triggered change in the diatropic character, leading to a global current–density pathway of the doubly charged systems. The figure-eight geometry of the neutral dimer does not significantly change upon oxidation according to the spectroscopic and computational data. The oxidation leads to 3D cross-conjugation at the intersection of the two ethylene bridges resulting in a global ring current.

Figure-eight tetrathiaoctaphyrin and dihydrotetrathiaoctaphyrin

The acid-catalyzed condensation of pyrrole and 2,5-bis(p-tolylhydroxymethyl)thiophene yields two novel giant heteroporphyrins: 5,10,15,20,25,30, 35,40-octa(p-tolyl)-41,43,45,47-tetrathia[36]octaphyrin(1.1.1.1.1.1.1.1) (S4OP) and 5,10,15,20,25,30,35,40-octa(p-tolyl)-dihydro-41,43,45,47-tetrathia[38]octaphyrin(1.1.1.1.1.1.1.1) (S4OPH2). These tetrathiaoctaphyrins are interconvertible through dihydrogenation or dehydrogenation processes. Both compounds possess helical figure-eight geometry with two thiophene rings for S4OP or two pyrrole rings for S4OPH2 in the ribbon-crossing center as shown by NOESY experiments. The synthesis of the figure-eight tetrathiaoctaphyrin implies a prearrangement process to form helical M or P dithiatetrapyrromethanes. The P-P or M-M condensation leads to the figure-eight molecule in competition with an intramolecular ring closure. Two dithiaporphyrin-like pockets of S4OP or S4OPH2 behave as independent proton acceptors. The stepwise process yields symmetric or asymmetric cationic species that depends on whether an even or odd number of NH protons are added. Dihydrotetrathiaoctaphyrin contains 38 pi electrons in its conjugation pathway, which corresponds to the formal [4n + 2] Hückel type pi-electron formulation that is consistent with a modest diatropic ring current effect in their 1H NMR spectra. The formal 4n type pi-electron formulation for tetrathiaoctaphyrin accounts for the residual paratropic shifts. A figure-eight conveyor-belt-like movement of the whole macrocyclic ring without a racemization step is proposed to account for the dynamic properties of S4OP. The molecule shuttles between two degenerate configurations. A S4OP-S4OPH2 couple may be considered as a molecular element which, while preserving the overall figure-eight geometry, "chooses" pyrrole or thiophene rings as spacers as a function of the macrocyclic oxidation state.

Double-helical cyclic peptides: design, synthesis, and crystal structure of figure-eight mirror-image conformers of adamantane-constrained cystine-containing cyclic peptide cyclo (Adm-Cyst)(3)

A large number of macrocycles containing alternating repeats of cystine diOMe(-NH-CH(CO(2)Me)-CH(2)-S-)(2) and either a conformationally rigid aromatic/alicyclic moiety or a flexible polymethylene unit (X) in the cyclic backbone with ring size varying from 13- to 78-membered have been examined by spectral ((1)H NMR, FT-IR, CD) and X-ray crystallography studies for unusual conformational preferences. While (1)H NMR measurements indicated a turnlike conformation for all macrocycles, stabilized by intramolecular NH.CO hydrogen bonding, as also supported by FT-IR spectra in chloroform, convincing proof for beta-turn structures was provided by circular dichroism studies. Single-crystal X-ray studies on 39-membered cyclo (Adm-L-Cyst)(3) revealed a double-helical fold (figure-eight motif) for the macrocycle. Only a right-handed double helix was seen in the macrocycle constructed from L-cystine. The mirror-image macrocycle made up of D-cystine units exhibited a double helix with exactly the opposite screw sense, as expected. The enantiomeric figure-eights were stabilized by two intramolecular NH. CO hydrogen bonds and exhibited identical (1) H NMR and FT-IR spectra. The CD spectra of both isomers had a mirror-image relationship. The present results have clearly brought out the importance of cystine residues in inducing turn conformation that may be an important deciding factor for the adoption of topologically important structures by macrocycles containing multiple S-S linkages.