# Cyclic Octamer Petroids: Simplified Isosters of Bioactive Fungal Cyclodepsipeptides
## 2 Results and Discussion
### Chemistry and Conformational Studies
In particular, relatively low values for C\(\alpha\)-H \(N\)-side chains' or small \(\Delta\delta\) for diastereotopic C\(\alpha\)-H\({}_{2}\) testified for _trans_ peptoid junctions; higher values of chemical shifts or larger \(\Delta\delta\) were indicative of _cis_ amide bonds [12]. The NMR data identified two possible alternative structures for symmetric **5** and **7**. In these congeners two of the four isobutyl/isopropyl and benzyl  
Figure 2: \({}^{1}\)H NMR spectra of cyclic peptoids **5–8**. 5.0–10.0 mM solutions in CDCl\({}_{3}\) (400 MHz). Residual solvent peaks are labelled with a red asterisk. H\({}_{2}\)O and further CDCl\({}_{3}\) impurities are labelled with a black asterisk. side chains were located on _cis_ amide bonds and the other two were on _trans_ peptoid junctions (**5a/5b** and **7a/7b**, Figure 3, see also Figure S2 of Supplementary Material). Accurate analysis of the \({}^{1}\)H NMR resonances showed no symmetry elements for compound **6** (to the best of our knowledge, this is the first case of symmetry breaking in the case of a conformationally stable, symmetrically substituted, cyclic octamer peptoid) (see experimental part and Figure S2 of Supplementary Material). The presence of two \(N\)-benzyl and two \(N\)-isobutyl side chains on _cis_ amide bonds, plus two \(N\)-isobutyl and two \(N\)-methyl side chains on _trans_ peptoid junctions testified for the asymmetric arrangement of cyclic oligomer **6**. \(C_{2}\)-symmetric _ccttcctt_-conformational diastereoisomers **5a/b** and **7a/b** were modelled by DFT (Density Functional Theory) studies and minimum energy structures were located in order to find the plausible most stable conformations. Free energies in CHCl\({}_{3}\) have been reported in Figure 4 (see S.M. for computational details). Isomers **5a** and **7b** were found to be more stable than the corresponding conformational diastereoisomers **5b** and **7a** of 2.2 and 0.8 kcal/mol, respectively. The energy differences are mainly due to steric interactions among the \(N\)-substituents and the ring. More in detail, as reported in Figure 4, **5b** shows isobutyl-ring distances shorter than **5a**, whereas interactions among the phenyl groups and the ring is responsible for the higher energy of **7a** with respect to **7b**. Furthermore, the higher energy difference between **5a** and **5b** (2.2 kcal/mol) could explain why the NMR spectrum of **5** shows only one conformational isomer, whereas the \({}^{1}\)H NMR spectrum of **7** shows a small amount of the other isomer (Figure 2c). These data are also concordant with the VT-experiments (the coalescent temperature and therefore the conformational interconversion barrier, is higher for cyclopeptoid **5**). Figure 3: Schematic structure of the possible conformational diastereoisomers for cyclic peptoids **5** and **7**. It is interesting to note that the removal of two phenyl groups in compounds **6** (where two \(N\)-methyl groups take the place of two \(N\)-benzyl side chains in **7**) causes a profound effect on the intrinsic stability of the cyclopeptidoid framework and breaks the symmetry of the octameric species. Another important aspect regarding the conformational isomerism of rigid cyclic octamer peptioids is related to their conformational chirality [10]. The presence of a \(C_{1}\)- or \(C_{2}\)-symmetry axis and the relatively high barrier for conformational inversion of the cyclooligomeric species (\(\Delta G^{\ddagger}\geq 10\) kcal/mol) makes cyclic peptoid **5-7** chiral  
Figure 4: Top and side view of minimum energy structures of conformational diastereisomers **5a/b** and **7a/b**. For the sake of clarity isobutyl groups are reported in green, whereas isopropyl and benzyl groups in yellow. Free energies were calculated in CHCl\({}_{3}\) and reported in kcal/mol. Distances are reported in Å.  
and observable as conformational enantiomers in solution by NMR spectroscopy (which, at room temperature, reveals species with lifetimes exceeding 10\({}^{-2}\) s) [10].