N-Morpholinomethyl-5-lithiotetrazole: A Reagent for the One-Pot Synthesis of 5-(1-Hydroxyalkyl)tetrazoles

Expanding the bioorthogonal chemistry toolbox: innovative synthetic strategies for cyclooctynes

Cyclooctyne derivatives represent an important class of compounds that play a key role in bioorthogonal chemistry. The presence of an endocyclic triple bond endows these molecules with the necessary reactivity for strain-promoted azide-alkyne cycloaddition (SPAAC); however, stability issues may hamper their use in biological systems. Many research groups, with the aid of computational studies, are devoting their efforts to finding ideal cyclooctyne candidates that strike a delicate balance between reactivity and stability. In this context, providing reliable and general synthetic procedures for accessing such chemical scaffolds is of critical importance. This review covers the recent synthetic strategies found in the literature to achieve this goal. Specifically, six main methodologies are discussed, highlighting the synthetic pathways, the key precursors for each, the applicability to a wide range of cyclooctyne derivatives and the challenges encountered in fulfilling this target.

Oxa-azabenzobenzocyclooctynes (O-ABCs): heterobiarylcyclooctynes bearing an endocyclic heteroatom

We report the synthesis of heterobiarylcyclooctynes bearing an endocyclic heteroatom, oxa-azabenzobenzocyclooctynes (O-ABCs). The integration of design strategies for accelerating strain-promoted azide-alkyne cycloadditions results in reactivity with organic azides that surpasses all cyclooctyne reagents reported to date. O-ABCs and related compounds provide insights into the effects of structural modifications on reactivity that can aid in the design of new reagents for click and bioorthogonal chemistry.

Functionalization of Tetrazoles Bearing the Electrochemically Cleavable 1N-(6-Methylpyridyl-2-methyl) Protecting Group

6-Methylpyridyl-2-methyl protected tetrazoles can be C-H deprotonated using the turbo-Grignard reagent and involved in the reactions with aldehydes and ketones. The protecting group can be cleaved under reductive electrochemical conditions using Pb bronze as a cathode and Zn as a sacrificial anode.

Functionalization of 1N-Protected Tetrazoles by Deprotonation with the Turbo Grignard Reagent

1N-PMB-protected tetrazole undergoes C-H deprotonation with the turbo Grignard reagent, providing a metalated intermediate with increased stability. This can be used for the reaction with electrophiles such as aldehydes, ketones, Weinreb amides, and iodine. C-H deprotonation with the turbo Grignard reagent is compatible with the PMB-protecting group at the tetrazole, which can be cleaved using oxidative hydrogenolysis and acidic conditions. The method enables the tetrazole functionalization at the fifth position by overcoming the difficulties associated with retro [2 + 3] cycloaddition of the metalated intermediates.

Acceleration of 1,3-Dipolar Cycloadditions by Integration of Strain and Electronic Tuning

The 1,3-dipolar cycloaddition between azides and alkynes provides new means to probe and control biological processes. A major challenge is to achieve high reaction rates with stable reagents. The optimization of alkynyl reagents has relied on two strategies: increasing strain and tuning electronics. We report on the integration of these strategies. A computational analysis suggested that a CH → N aryl substitution in dibenzocyclooctyne (DIBO) could be beneficial. In transition states, the nitrogen of 2-azabenzo-benzocyclooctyne (ABC) engages in an n→π* interaction with the C=O of α-azidoacetamides and forms a hydrogen bond with the N-H of α-diazoacetamides. These dipole-specific interactions act cooperatively with electronic activation of the strained π-bond to increase reactivity. We found that ABC does indeed react more quickly with α-azidoacetamides and α-diazoacetamides than its constitutional isomer, dibenzoazacyclooctyne (DIBAC). ABC and DIBAC have comparable chemical stability in a biomimetic solution. Both ABC and DIBO are accessible in three steps by the alkylidene carbene-mediated ring expansion of commercial cycloheptanones. Our findings enhance the accessibility and utility of 1,3-dipolar cycloadditions and encourage further innovation.

An Ugi Reaction/Intramolecular Cyclization/Oxidation Cascade towards Tetrazole-Linked Dibenzoxazepines

A series of tetrazole-linked dibenzo[b,f][1,4]oxazepines is synthesized through a short sequence involving an Ugi tetrazole reaction. The intermediate tetrazole undergoes a potassium carbonate mediated SNAr cyclization, followed by oxidation to afford the target tricyclic heterocyclic scaffold. The optimization, scope and limitations of this two-step and efficient methodology are investigated. A 1000-member library of tetrazole-linked dibenzo[b,f][1,4]oxazepines is generated and the physicochemical properties are analyzed. great