CuAAC: An Efficient Click Chemistry Reaction on Solid Phase Vida Castro,* ,#,⊥ Hortensia Rodríguez,* ,#,⊥,§ and Fernando Albericio* ,#,⊥,◊,∥ # Institute for Research in Biomedicine (IRB Barcelona), The Barcelona Institute of Science and Technology 08028-Barcelona, Spain ⊥ CIBER-BBN, Networking Centre on Bioengineering, Biomaterials and Nanomedicine, 08028-Barcelona, Spain ◊ Department of Organic Chemistry, University of Barcelona, 08028-Barcelona, Spain § School of Chemistry, Yachay Tech, Yachay City of Knowledge, Urcuqui, Ecuador ∥ School of Chemistry & Physics, University of KwaZulu-Natal, 4001-Durban, South Africa ABSTRACT: Click chemistry is an approach that uses efficient and reliable reactions, such as Cu(I)-catalyzed azide−alkyne cycloaddition (CuAAC), to bind two molecular building blocks. CuAAC has broad applications in medicinal chemistry and other fields of chemistry. This review describes the general features and applications of CuAAC in solid-phase synthesis (CuAAC-SP), highlighting the suitability of this kind of reaction for peptides, nucleotides, small molecules, supramolecular structures, and polymers, among others. This versatile reaction is expected to become pivotal for meeting future challenges in solid-phase chemistry. KEYWORDS: Click Chemistry, CuAAC, solid-phase, azide, alkyne 1. INTRODUCTION In 1963, Merrifield 1 introduced the concept of solid-phase peptide synthesis (SPPS), reporting the first efficient production of a tetrapeptide on a solid matrix, wherein the peptide chain was grown by covalent attachment of one end to the functionalized support. Thanks to Merrifield’s pioneering work, this concept has become a fully established method in peptide synthesis; however, many other known organic reactions have also been applied on solid phase (SP) supports to address synthetic problems and generate new molecular entities. 2,3 Click chemistry 4,5 promotes the use of organic reactions that allow the connection of two molecular building blocks in a facile, selective, high-yield reaction under mild conditions with few or no byproducts. 4,5 Diels−Alder, Michael addition, pyridyl sulfide reaction, oxyme, thiolene, strain-promoted azide−alkyne cycloaaddition (SPAAC), and Cu(I)-catalyzed azide−alkyne cycloaddition (CuAAC) have all been reported as Click reactions. 6−24 In the past decade, the CuAAC 25−27 has emerged as an efficient alternative in SP to replace amide bonds in peptides 28 and to generate amino acid triazole derivatives, 29 cyclic peptides, 30 nucleotides, 31−33 and new resins. 34,35 Furthermore, this chemistry has the capacity to promote bioconjugation and peptide ligation, stemming from the properties of the triazole linkage as a peptide mimetic. This Review describes the general features and applications of CuAAC on SP (CuAAC-SP) and reveals the suitability of this kind of reaction for the modification of peptides, 36−38 nucleotides, 31−33 small molecules, 39 supramolecular struc- tures, 40,41 and polymers. 42 2. GENERAL CONSIDERATIONS REGARDING Cu(I)-CATALYZED AZIDE−ALKYNE CYCLOADDITION ON SOLID PHASE (CuAAC-SP) CuAAC is a type of Huisgen1,3-dipolar cycloaddition based on the formation of 1,4-disubstituted [1,2,3]-triazoles between a terminal alkyne and an aliphatic azide in the presence of copper 43,44 and is classified as a Click Chemistry reaction. 5 Click Chemistry was defined by Sharpless et al. 4,5 as any chemical reaction that allows high yields, generates no side- products or ones that are easily removed, is stereospecific, gives physiologically stable products, exhibits a large thermodynamic driving force, and has simple reaction conditions. Research into the synthesis of biomolecules via CuAAC-SP has emerged because of the stability of triazole scaffolds against metabolic degradation. 4,5 In this Review, we use the terms CuAAC and Click Chemistry interchangeably. In 2001, Meldal et al. 25,26 developed a method for preparing 1,4-disubtituted 1,2,3-triazoles using Cu(I) salts as a catalyst for the 1,3-dipolar cycloaddition of terminal alkynes to azides on SP at room temperature using organic solvents such as ACN, THF, DCM, toluene, and DMF. Shortly after and using protic polar solvents such as t-butyl alcohol, ethanol or water, Sharpless et al. 27 independently reported the same reaction in solution, naming it CuAAC (Scheme 1). The CuAAC reaction was a breakthrough in triazole chemistry. The reactions of organic azides with terminal alkynes were shown to be accelerated by copper ions and to proceed regioselectively under these conditions, giving the 1,4- disubstituted 1,2,3-triazole regioisomer exclusively. The for- Received: May 29, 2015 Revised: November 20, 2015 Published: December 11, 2015 Review pubs.acs.org/acscombsci © 2015 American Chemical Society 1 DOI: 10.1021/acscombsci.5b00087 ACS Comb. Sci. 2016, 18, 1−14