Ring Expansion of Thiolactams via Imide Intermediates: An Amino Acid Insertion Strategy

Cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles

This Feature Article discusses recent advances in the development of cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles. Cascade ring expansion reactions have much potential for use in the synthesis of biologically important medium-sized rings and macrocycles, most notably as they don't require high dilution conditions, which are commonly used in established end-to-end macrocyclisation methods. Operation by cascade ring expansion method can allow large ring products to be accessed via rearrangements that proceed exclusively by normal-sized ring cyclisation steps. Ensuring that there is adequate thermodynamic driving force for ring expansion is a key challenge when designing such methods, especially for the expansion of normal-sized rings into medium-sized rings. This Article is predominantly focused on methods developed in our own laboratory, with selected works by other groups also discussed. Thermodynamic considerations, mechanism, reaction design, route planning and future perspective for this field are all covered.

Expanding the scope of the successive ring expansion strategy for macrocycle and medium-sized ring synthesis: unreactive and reactive lactams

New methods are described that expand the scope of the Successive Ring Expansion (SuRE) with respect to synthetically challenging lactams. A protocol has been developed for use with 'unreactive' lactams, enabling SuRE reactions to be performed on subsrates that fail under previously established conditions. Ring expansion is also demonstarted on 'reactive' lactams derived from iminosugars for the first time. The new SuRE methods were used to prepare a diverse array of medium-sized and macrocyclic lactams and lactones, which were evaluted in an anti-bacterial assay against E. coli BW25113WT.

Divergent Cascade Ring-Expansion Reactions of Acryloyl Imides

Macrocyclic and medium-sized ring ketones, lactones and lactams can all be made from common acryloyl imide starting materials through divergent, one-pot cascade ring-expansion reactions. Following either conjugate addition with an amine or nitromethane, or osmium(VIII)-catalysed dihydoxylation, rearrangement through a four-atom ring expansion takes place spontaneously to form the ring expanded products. A second ring expansion can also be performed following a second iteration of imide formation and alkene functionalisation/ring expansion. In the dihydroxylation series, three- or four-atom ring expansion can be performed selectively, depending on whether the reaction is under kinetic or thermodynamic control.

Amide Skeletal Elongation via Amino Acid Insertion

Amide derivatization is useful to access valuable organic compounds considering the ready availability of molecules containing amide functionality. Current methods to derivatize amide mainly focus on the synthesis of carbonyl-containing compounds and amines. Incorporating both parts of the initial amide into the new derivatives is rare. Herein, we describe a simple and practical amide derivatization through amino acid insertion to prepare more complex amides. This insertion is applicable to a wide range of amino acids and more importantly, the chiral information is completely conserved during the insertion. Comparison of this insertion strategy with conventional amide synthesis demonstrates the synthetic advantages of this new protocol.

Ring Expansion Strategies for the Synthesis of Medium Sized Ring and Macrocyclic Sulfonamides

Two new ring expansion strategies are reported for the synthesis of medium sized ring and macrocyclic sulfonamides. Both methods can be performed without using classical protecting groups, with the key ring expansion step initiated by nitro reduction and amine conjugate addition respectively. Each method can be used to make diversely functionalised cyclic sulfonamides in good to excellent yields, in a range of ring sizes. The ring size dependency of the synthetic reactions is in good agreement with the outcomes modelled by Density Functional Theory calculations.

A Lewis Acid-Promoted Michael Addition and Ring-Expansion Cascade for the Construction of Nitrogen-Containing Medium-Sized Rings

A Lewis acid-promoted annulation of azadienes and cyclobutamines was developed. This reaction proceeded through Michael addition and ring-expansion cascade, affording the corresponding nitrogen-containing medium-sized rings with a broad scope in moderate to high yields. The catalytic asymmetric version of this reaction has also been explored using a chiral base.

Single Atom Ring Contraction of Peptide Macrocycles Using Cornforth Rearrangement

Site-selective transformations of densely functionalized scaffolds have been a topic of intense interest in chemical synthesis. Herein we have repurposed the rarely used Cornforth rearrangement as a tool to effect a single-atom ring contraction in cyclic peptide backbones. Investigations into the kinetics of the rearrangement were carried out to understand the impact of electronic factors, ring size, and linker type on the reaction efficiency. Conformational analysis was undertaken and showed how subtle differences in the peptide backbone result in substrate-dependent reaction profiles. This methodology can now be used to perform conformation-activity studies. The chemistry also offers an opportunity to install building blocks that are not compatible with traditional C-to-N iterative synthesis of macrocycle precursors.

Site Specific Preparation of N-Glycosylated Peptides: Thioamide-Directed Activation of Aspartate

A site-specific method for the preparation of N-glycosylated peptides is described. Incorporation of a peptide backbone thioamide linkage adjacent to an Asp residue facilitates a Ag^I -promoted, site-specific conversion to N-glycosylated Asn residues in peptides.

Chemo- and Site-Selective Replacement of N-Terminal Carbamates in Peptides

In peptide synthesis, it is important to distinguish the terminal amino group and carry out the selective transformation of only the N-terminal protecting group. We describe herein a reaction for the chemo- and site-selective replacement of carbamates with various other carbamates only at the N-terminus of peptides. We demonstrate the scope of carbamates and peptides and the introduction of fluorine into a peptide. This strategy is applicable to the late stage of peptide synthesis.

Backbone thioamide directed macrocyclisation: lactam stapling of peptides

A novel method for lactam stapling of Asp/Lys-containing peptides has been developed that does not require coupling agents. A backbone thioamide is incorporated at the N-terminal side of the aspartate residue. Ag(I)-promoted activation of the thioamide in the vicinity of the Asp carboxylate generates a cyclic isoimide intermediate that is trapped by the Lys amine to generate the macrolactam. This method is suitable for generation of i,i+2, i,i+3, and i,i+4-spaced lactam-bridged peptides.

Synthesis of medium-ring lactams and macrocyclic peptide mimetics via conjugate addition/ring expansion cascade reactions

A novel conjugate addition/ring expansion (CARE) cascade reaction sequence is reported that enables medium-sized ring and macrocyclic bis-lactams to be prepared from primary amines and cyclic imides. The reactions are simple to perform, generally high yielding, and very broad in scope, especially with respect to the primary amine component. CARE reactions can also be performed iteratively, enabling β-peptoid-based macrocyclic peptide mimetics to be ‘grown’ via well controlled, sequential 4-atom ring expansion reactions, with the incorporation of varied functionalised amines during each iteration.

A conjugate addition/ring expansion (CARE) cascade reaction sequence is reported that enables medium-sized ring and macrocyclic bis-lactams to be prepared from primary amines and cyclic imides.