Guidelines for the application of the Scholl reaction were developed. Labeling experiments demonstrate that the Scholl reaction fails in small, unsubstituted oligophenylenes (e.g., o-terphenyl) due to oligomerization of the products (e.g., triphenylene). Incorporation of suitably placed blocking groups (e.g., t-butyl) suppresses oligomerization. The well-established directing group effects in electrophilic aromatic substitution predict the outcome of Scholl reactions of substituted substrates. Activating o,p-directing groups (e.g., MeO) direct bond formation o,p, either intramolecularly or intermolecularly. Deactivating o,p-directing groups (e.g., Br) also direct bond formation o,p but yields are lower. Deactivating m-directors (e.g., NO2) suppress reaction. MoCl5 and PhI(OOCCF3)2/BF3.Et2O are general and effective reagents for the Scholl oxidation. Calculations (B3LYP/6-31G(d)) predict the Scholl reaction in alkoxyarenes to proceed via arenium cations, not radical cations. Suzuki-Miyaura couplings were used to generate 12 substituted o-terphenyl derivatives.

Galanin is an endogenous neuropeptide that modulates seizures in the brain. Because this neuropeptide does not penetrate the blood-brain barrier, we designed truncated galanin analogues in which nonessential amino acid residues were replaced by cationic and/or lipoamino acid residues. The analogues prevented seizures in the 6 Hz mouse model of epilepsy following intraperitoneal administration. The most active analogue, Gal-B2 (NAX 5055), contained the -Lys-Lys-Lys(palmitoyl)-Lys-NH(2) motif and exhibited high affinity for galanin receptors (K(i) = 3.5 nM and 51.5 nM for GalR1 and GalR2, respectively), logD = 1.24, minimal helical conformation and improved metabolic stability. Structure-activity-relationship analysis suggested that cationization combined with position-specific lipidization was critical for improving the systemic activity of the analogues. Because the anticonvulsant activity of galanin is mediated by the receptors located in hippocampus and other limbic brain structures, our data suggest that these analogues penetrate into the brain. Gal-B2 may lead to development of first-in-class antiepileptic drugs.

A protease from ribosomal peptide biosynthesis macrocyclizes diverse substrates, including those resembling nonribosomal peptide and hybrid polyketide-peptide products. The proposed mechanism is analogous to thioesterase-catalyzed chemistry, but the substrates are amide bonds rather than thioesters.Macrocyclization is a common strategy to improve the rigidity and stability of bioactive metabolites. 1,2 In polyketide and nonribosomal peptide biosynthesis, macrocyclization via lactones or lactams is typically catalyzed by thioesterase (TE) domains, which contain a serine protease-like Asp-His-Ser catalytic triad (Figure 1). 1 The TE domain transfers the peptide/polyketide chain from a carrier protein to the active site serine, which is then displaced by a nucleophile, generating either a linear product, or more commonly a macrocycle, as in tyrocidine A (1).Quite interestingly, one of the major groups of macrocyclic ribosomal peptides, the cyanobactins, 3 is cyclized in a similar way: a subtilisin-like serine protease catalyzes cleavage of a C-terminal peptide sequence in tandem with N-C macrolactamization, leading to compounds such as patellamide C (2). 4-6 Among ribosomal peptides, both the cyanobactins and cyclotides are N-C cyclic, while other ribosomal peptides, such as capistruin, 7 microcin J25, 8 and the microviridins, 9 are cyclized via side-chain residues using ATP via wholly different biochemical mechanisms. There is indirect evidence that cyclotides are circularized in a similar fashion to cyanobactins, 10 and circular ribosomal peptides are common in diverse organisms. However, no definitive enzymatic or genetic studies of N-C macrocyclization have been performed on any ribosomal peptide system other than the cyanobactins. Previously, we have shown that the subtilisin-like protease, PatG, is solely responsible for catalyzing macrocyclization in the patellamide pathway. 6 Metagenomic and biochemical analyses of the patellamide pathway showed that PatG is a broad-substrate enzyme, processing 29 known precursor peptide sequences encoding macrocycles of 7-8 amino acids. 4,5 Every natural product residue is mutated at least once in this series, and PatG could also produce the unnatural compound eptidemnamide (3) both in vivo and in vitro ( Figure 2). Consequently, the cyanobactin macrocyclases exhibit exceptionally relaxed substrate specificity. Furthermore, unlike other ribosomal peptide natural product pathways, 11,12 the cyanobactin macrocyclases require only a C-terminal 4-5 amino acid recognition sequence, AYDG(E), which allowed us to employ short synthetic peptides as substrates.We proposed that PatG performs macrocyclization in a manner that is mechanistically analogous to TE domains, although the proteins and the substrates are quite different. In particular, while as their name implies TEs require activated thioesters (or esters) 13 to catalyze circularization, the PatG substrates are simple amides readily accessible through standard solid phase peptide synthesis. Based on this mechanistic...

Disulfide bridges, which stabilize the native conformation of conotoxins impose a challenge in the synthesis of smaller analogs. In this work, we describe the synthesis of a minimized analog of the analgesic μ-conotoxin KIIIA that blocks two sodium channel subtypes, the neuronal NaV1.2 and skeletal muscle NaV1.4. Three disulfide-deficient analogs of KIIIA were initially synthesized in which the native disulfide bridge formed between either C1-C9, C2-C15 or C4-C16 was removed. Deletion of the first bridge only slightly affected the peptide’s bioactivity. To further minimize this analog, the N-terminal residue was removed and two non-essential Ser residues were replaced by a single 5-amino-3-oxapentanoic acid residue. The resulting “polytide” analog retained the ability to block sodium channels and to produce analgesia. Until now, the peptidomimetic approach applied to conotoxins has progressed only modestly at best; thus, the disulfide-deficient analogs containing backbone spacers provide an alternative advance toward the development of conopeptide-based therapeutics.