Site-Selective Conversion of Azido Groups at Carbonyl α-Positions to Diazo Groups in Diazido and Triazido Compounds

Iterative click reactions using trivalent platforms for sequential molecular assembly

A facile synthesis of multi(triazole)s by iterative click reactions is disclosed. Good functional group tolerance of sequential click assembly by sulfur-fluoride exchange (SuFEx), copper-catalyzed azide-alkyne cycloaddition (CuAAC), and thia-Michael reaction realizes the iterative click reactions. Diverse multi(triazole)-type mid-molecules can be synthesized easily from readily available modules through good chemoselective reactions without functional group transformation steps.

Amphos-Mediated Conversion of Alkyl Azides to Diazo Compounds and One-Pot Azide-Site Selective Transient Protection, Click Conjugation, and Deprotective Transformation

A one-pot conversion of alkyl azides to diazo compounds is outlined. After the reaction of α-azidocarbonyl compounds with Amphos, treatment of the resulting phosphazides with silica gel in a wet solvent afforded α-diazo carbonyl products. Through the azido group protection property of Amphos, inter- and intramolecular azide-site selective reactions of azido group protection, click functionalization, and deprotection of the diazo group have been demonstrated in one pot.

Fluorine-Assisted Rearrangement of Geminal Azidofluorides to Imidoyl Fluorides

Organoazide rearrangement constitutes versatile synthetic strategies but typically requires an extremely strong acid and/or a high reaction temperature. Our group recently discovered the remarkable accelerating effect of the geminal fluorine substituent that enables the facile rearrangement of azides into imidoyl fluorides without the aid of acid under much milder reaction conditions. The role of geminal fluorine was elucidated by both experimental and computational investigations. This new reactivity led to the development of a practical one-step tandem preparative method for potentially useful and bench-stable imidoyl fluorides from a wide range of structurally diverse geminal chlorofluorides. Our additional efforts to expand the reaction scope regarding the migrating group, halogen, and carbonyl function are described, and the synthetic utility of the imidoyl fluoride products was demonstrated in hopes of promoting the use of this under-appreciated functional group in the synthetic organic community.

Neighboring Nitrogen Atom-Induced Reactions of Azidoacetyl Hydrazides, including Unexpected Nitrogen-Nitrogen Bond Cleavage of the Hydrazide

We studied the hydrazide compounds of the α-azidoacetyl group, which showed specific click reactivity by the intramolecular hydrogen bonding between the azido group and the N-H of the hydrazide moiety. In the competitive click reactions with a general alkyl azide, both traceless and non-traceless Staudinger-Bertozzi ligation occurred azide-site-selectively by the acceleration effect of the hydrogen bonding. However, the product obtained from the traceless reaction was further transformed into heterocyclic compounds. In addition, in an attempt at a synthesis of naphthalimide-possessing azidoacetyl hydrazide, nitrogen-nitrogen bond cleavage of the azidoacetyl hydrazides occurred to give the reduced amine product. These unexpected results could help design molecules for the successful Staudinger-Bertozzi ligation of the hydrazide compounds and develop a new nitrogen-nitrogen bond cleavage method.

Recent advances in pyrazole synthesis employing diazo compounds and synthetic analogues

This review summarizes the recent developments in the construction of pyrazoles using diazo compounds, nitrile imines and their synthetic equivalents.

Modular synthesis of triazoles from 2-azidoacrylamides having a nucleophilic amino group

Assembling methods using 2-azidoacrylamides having a nucleophilic amino group are disclosed. Divergent transformations of the amine-type trivalent platform were accomplished with a wide variety of electrophiles to provide a broad...

Sulfonium ion-promoted traceless Schmidt reaction of alkyl azides

Schmidt reaction by sulfonium ions is described. General primary, secondary, and tertiary alkyl azides were converted to the corresponding carbonyl or imine compounds without any trace of the activators. This bond scission reaction through 1,2-migration of C-H and C-C bonds was accessible to the one-pot substitution reaction.

Design, Synthesis, and Utility of Defined Molecular Scaffolds

A growing theme in chemistry is the joining of multiple organic molecular building blocks to create functional molecules. Diverse derivatizable structures—here termed “scaffolds” comprised of “hubs”—provide the foundation for systematic covalent organization of a rich variety of building blocks. This review encompasses 30 tri- or tetra-armed molecular hubs (e.g., triazine, lysine, arenes, dyes) that are used directly or in combination to give linear, cyclic, or branched scaffolds. Each scaffold is categorized by graph theory into one of 31 trees to express the molecular connectivity and overall architecture. Rational chemistry with exacting numbers of derivatizable sites is emphasized. The incorporation of water-solubilization motifs, robust or self-immolative linkers, enzymatically cleavable groups and functional appendages affords immense (and often late-stage) diversification of the scaffolds. Altogether, 107 target molecules are reviewed along with 19 syntheses to illustrate the distinctive chemistries for creating and derivatizing scaffolds. The review covers the history of the field up through 2020, briefly touching on statistically derivatized carriers employed in immunology as counterpoints to the rationally assembled and derivatized scaffolds here, although most citations are from the past two decades. The scaffolds are used widely in fields ranging from pure chemistry to artificial photosynthesis and biomedical sciences.

Assembly of four modules onto a tetraazide platform by consecutive 1,2,3-triazole formations

Efficient consecutive 1,2,3-triazole formations using multiazide platforms are disclosed. On the basis of unique clickability of the 1-adamantyl azido group, a four-step synthesis of tetrakis(triazole)s was achieved from a tetraazide platform molecule. This method was applied to a convergent synthesis of tetrafunctionalized probes in a modular synthetic manner.

Taming the reactivity of alkyl azides by intramolecular hydrogen bonding: site-selective conjugation of unhindered diazides

The intramolecular hydrogen bonding in the α-azido secondary acetamides (α-AzSAs) enabled site-selective integration onto the diazide modular hubs even without steric hindrance.

2-Azidoacrylamides as compact platforms for efficient modular synthesis

Efficient methods to assemble modules with compact platform molecules by triazole formations and Michael reactions are disclosed. The good electrophilicity of 2-triazolylacrylamides realized Michael additions using various nucleophiles. An iterative synthesis of a tetrakis(triazole) was accomplished by orthogonal triazole formations and Michael reactions.

Triazole formation of phosphinyl alkynes with azides through transient protection of phosphine by copper

An efficient preparation method of functionalized phosphines by copper-catalyzed azide-alkyne cycloaddition (CuAAC) through the transient protection of phosphine from the Staudinger reaction is disclosed. Diverse phosphines were prepared from phosphinyl alkynes and azides by the click reaction at the ethynyl group without damaging the phosphinyl group. Double- and triple-click assemblies of azides were accomplished by triazole formations and robust azaylide formation.

Sequential conjugation methods based on triazole formation and related reactions using azides

The recent remarkable progress in azide chemistry has realized sequential conjugation methods with selective 1,2,3-triazole formation. On the basis of the diverse reactivities of azides and azidophiles, including terminal alkynes and cyclooctynes, various selective reactions to furnish triazoles and a wide range of platform molecules, such as diynes, diazides, triynes, and triazides, have been developed so far for bis- and tris(triazole) syntheses. This review highlights recent transformations involving selective triazole formation, allowing the efficient preparation of unsymmetric bis- and tris(triazole)s using diverse platform molecules.

Facile assembly of three cycloalkyne-modules onto a platform compound bearing thiophene S,S-dioxide moiety and two azido groups

An efficient method to assemble three cycloalkyne-modules onto a platform bearing a thiophene S,S-dioxide moiety and two azido groups has been developed. The sequential reactions without catalysis or additives enabled the facile preparation of trifunctional molecules by a simple procedure.

Copper-Catalyzed Cascade Cyclization Reactions of Diazo Compounds with tert-Butyl Nitrite and Alkynes: One-Pot Synthesis of Isoxazoles

A novel copper-catalyzed [3 + 2] cycloaddition reaction of alkynes with nitrile oxides generated in situ from the coupling reaction of copper carbene and nitroso radical has been developed. The three-component reaction provides a simple and efficient method for the construction of isoxazoles in a highly regioselective manner in a single step. On the basis of the experimental results and density functional theory calculations, a catalytic cycle (Cu^I-Cu^II-Cu⁰-Cu^I) for this cascade cyclization reaction is proposed.

A facile preparation of functional cycloalkynes via an azide-to-cycloalkyne switching approach

Terminal alkyne-selective click conjugation of diynes bearing strained and terminal alkyne moieties with functional azides has been achieved by transient protection of strained alkynes via complexation with copper to easily afford various functional cycloalkynes.

Site-selective conversion of azido groups at carbonyl α-positions into oxime groups leading triazide to a triple click conjugation scaffold

This paper reports the selective conversion of alkyl azido groups at the carbonyl α-position into oximes, and one-pot triple click conjugation is demonstrated.