Cu-free click cycloaddition reactions in chemical biology

Abstract

Bioorthogonal chemical reactions are paving the way for new innovations in biology. These reactions possess extreme selectivity and biocompatibility, such that their participating reagents can form covalent bonds within richly functionalized biological systems--in some cases, living organisms. This tutorial review will summarize the history of this emerging field, as well as recent progress in the development and application of bioorthogonal copper-free click cycloaddition reactions.

Fig. 1

A general bioorthogonal reaction. The bioorthogonal functionality, oval with horizontal lines, reacts with its counterpart, oval with vertical lines, to label a biomolecule in live cells or organisms.

Fig. 2

The Cu-free reaction of azides and alkynes. (A) A comparison of bond angles between linear alkynes and triazoles. (B) A comparison of bond angles between a strained cyclooctyne and its corresponding triazole product. (C) Comparison of second-order rate constants of cyclooctynes in a model reaction with benzyl azide.

Fig. 3

Imaging cell-surface azidosugars with cyclooctyne probes. Azidosugars are metabolized by cells and incorporated into cell-surface glycans. The azide-labeled glycans are then reacted with a cyclooctyne-conjugated imaging probe.

Fig. 4

Applications of cyclooctynes in cells. (A) LplA-mediated addition of an azido-fatty acid to a target protein and imaging with a fluorinated cyclooctyne probe. (B) Use of OCT to select for E. coli strains that incorporate high levels of an azido amino acid into cell-surface proteins. (C) Imaging an OCT–phospholipid conjugate on live cells using an azide-functionalized probe.

Scheme 1

Reaction between a dibenzocyclooctyne and an azide.

Scheme 2

The two-step reaction of oxanorbornadienes with azides forms triazoles.

Scheme 3

Inverse-electron-demand Diels–Alder reactions of strained alkenes. (A) The reaction between trans-cyclooctene 10, with biaryltetrazine 11. (B) The reaction between norbornene 15 with aryltetrazine 14. (C) The reaction between cyclobutene 17 with biaryltetrazine 18.

Scheme 4

In situ generated dipoles as bioorthogonal reagents. (A) The photo-click reaction. (B) The generation and reaction of the nitrile-oxide.

Scheme 5

Synthesis of 1.

Scheme 6

Synthesis of a monofluorinated cyclooctyne (34).

Scheme 7

First synthesis of DIFO (35).

Scheme 8

More practical synthesis of DIFO analog (42).

Scheme 9

Synthesis of water-soluble cyclooctyne 47.

Scheme 10

Synthesis of dibenzocyclooctyne 50.

Scheme 11

Synthesis of oxanorbornadiene 7.

Scheme 12

Synthesis of reactive alkenes.

Scheme 13

Synthesis of various tetrazines.

Scheme 14

Synthesis of tetrazoles.

Scheme 15

Synthesis of nitrile oxides.