Solid phase synthesis of tetrahydropyrazine-2-ones by intramolecular Mitsunobu reactions

Intramolecular monomer-on-monomer (MoM) Mitsunobu cyclization for the synthesis of benzofused thiadiazepine-dioxides

The utilization of a monomer-on-monomer (MoM) intramolecular Mitsunobu cyclization reaction employing norbornenyl-tagged (Nb-tagged) reagents is reported for the synthesis of benzofused thiadiazepine-dioxides. Facile purification was achieved via ring-opening metathesis (ROM) polymerization initiated by one of three metathesis catalyst methods: (i) free metathesis catalyst, (ii) surface-initiated catalyst-armed silica, or (iii) surface-initiated catalyst-armed Co/C magnetic nanoparticles.

Selectively N-protected enantiopure 2,5-disubstituted piperazines: avoiding the pitfalls in solid-phase Fukuyama-Mitsunobu cyclizations

An efficient solid-phase route to ring-substituted piperazines from O-linked resin-bound (S)-aziridine-2-methanol is described. Regioselective microwave-assisted aminolysis followed by intramolecular Fukuyama-Mitsunobu cyclization constitute the key features of the protocol. Simple piperazines and diazepanes were readily obtained without preceding N-protection of the acyclic intermediate, whereas attempts to extend this protocol to chiral 2,5-disubstituted piperazines failed. Modifications encompassing N-carbamoylation prior to ring-closure were therefore investigated. However, standard carbamoylating agents, for example, Fmoc-Cl and Alloc-Cl tended to give bis-protected by-products. Thus, novel microwave-assisted solid-phase N-protection procedures were developed for efficient introduction of Fmoc, Boc and Alloc groups. The subsequent cyclization proceeded in moderate to excellent yields depending on the bulk of the side chain and type of N-protecting group. This protocol readily provided novel cis- and trans-2,5-disubstituted piperazines displaying a variety of N-protecting group patterns after further on-resin manipulations. Also, unexpected by-products obtained during these optimization studies were identified and characterized. This includes nosylated ureas arising from an alternative cyclization pathway. Finally, post-cleavage oxidation gave access to the Fmoc/Boc-protected alpha-amino acid as well as the corresponding aldehyde. The chiral piperazines described in this work will enable construction of combinatorial libraries with a higher chemical diversity compared to those containing simple N,N'-difunctionalized piperazines, often present in drug-like compounds.

Utilizing the intramolecular Fukuyama–Mitsunobu reaction for a flexible synthesis of novel heterocyclic scaffolds for peptidomimetic drug design

We report the synthesis of the novel scaffolds pyrazino[1,2-b]isoquinoline and pyrrolo[1,2-a]pyrazine displaying the somatostatin pharmacophores. Both classes of compounds contain a pyrazine heterocycle, which can be prepared in a straightforward manner utilizing an intramolecular Fukuyama-Mitsunobu reaction. As both the families derive from amino acids, they can be accessed in high optical purity.

Solid-phase synthesis of peptidomimetic inhibitors for the hepatitis C virus NS3 protease

The NS3 serine protease enzyme of the hepatitis C virus (HCV) is essential for viral replication. Short peptides mimicking the N-terminal substrate cleavage products of the NS3 protease are known to act as weak inhibitors of the enzyme and have been used as templates for the design of peptidomimetic inhibitors. Automated solid-phase synthesis of a small library of compounds based on such a peptidomimetic scaffold has led to the identification of potent and highly selective inhibitors of the NS3 protease enzyme.

Solid-Phase Traceless Synthesis of Selected Nitrogen-Containing Heterocyclic Compounds. The Encore Technique for Directed Sorting of Modular Solid Support

The acid lability of electron-rich N-benzylanilines has been exploited in a linker for the traceless solid-phase synthesis of benzimidazoles, 2-aminobenzimidazoles, quinoxalinones and tetrahydroquinoxalines. The target compound precursors were assembled on a solid-phase support derivatized with either a benzylamine or a benzhydrylamine linker. Exposure to an acidic reagent caused cleavage of the C(benzyl)-N(aniline) bond, releasing the product with only a hydrogen atom on the descending nitrogen. The Encore technique for directed sorting on SynPhase Lanterns has been developed and applied to combinatorial synthesis of generic drug discovery libraries.