Heterogeneous Diazo-Transfer Reaction: A Facile Unmasking of Azide Groups on Amine-Functionalized Insoluble Supports for Solid-Phase Synthesis

CuAAC: An Efficient Click Chemistry Reaction on Solid Phase

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.

Synthesis and in-depth characterization of reactive, uniform, crosslinked microparticles based on free radical copolymerization of 4-vinylbenzyl azide

A direct seed-swelling copolymerization formulation affords well-defined azide-functional porous or hollow microparticles amenable to click-modification.

Design and structure activity relationship of tumor-homing histone deacetylase inhibitors conjugated to folic and pteroic acids

Histone deacetylase (HDAC) inhibition has recently emerged as a novel therapeutic approach for the treatment of various pathological conditions including cancer. Currently, two HDAC inhibitors (HDACi) – Vorinostat and Romidepsin – have been approved for the treatment of cutaneous T-cell lymphoma. However, HDACi remain ineffective against solid tumors and are associated with adverse events including cardiotoxicity. Targeted delivery may enhance the therapeutic indices of HDACi and enable them to be efficacious against solid tumors. We showed herein that morphing of folic and pteroic acids into the surface recognition group of HDACi results in hydroxamate and benzamide HDACi which derived tumor homing by targeting folate receptor (FR), a receptor commonly overexpressed in solid tumors. We observed a correlation between the potency of HDAC1 inhibition and cytotoxicity as only the potent pteroate hydroxamates, 11d and 11e, displayed antiproliferative activity against two representative FR-expression cancer cells. Our observation further supports the previous results which suggest that for a drug to be successfully targeted using the FR, it must be extremely potent against its primary target as the FR has a low delivery efficiency.

1,4-linked 1,2,3-Triazole des-peptidic analogues of PNA (TzNA): Synthesis of TzNA oligomers by "click" reaction on solid phase and stabilization of derived triplexes with DNA

1,2,3-triazole analogues of PNA (TzNA) in which the amide link in backbone is replaced by triazole ring is synthesized on solid phase by 'click' chemistry and such triazolothymine PNA chimeric oligomers are shown to significantly stabilize the derived PNA(2):DNA triplexes. With increasing number of triazole units in the backbone, single stranded PNA oligomers exhibit enhanced self-ordering.

Macroporous uniform azide- and alkyne-functional polymer microspheres with tuneable surface area: synthesis, in-depth characterization and click-modification

With surface areas of up to 467 m² g⁻¹, crosslinked azide- and alkyne-functional poly(styrene-co-divinylbenzene) microparticles enable versatile modification throughout the particle.

Multivalency amplifies the selection and affinity of bradykinin-derived peptides for lipid nanovesicles

The trimer of a bradykinin derivative displayed a more than five-fold increase in binding affinity for phosphatidylserine-enriched nanovesicles as compared to its monomeric precursor. The nanovesicle selection is directly correlated with multivalency, which amplifies the electrostatic attraction. This strategy may lead to the development of novel molecular probes for detecting highly curved membrane bilayers.

Imidazole-1-sulfonyl azide-based diazo-transfer reaction for the preparation of azido solid supports for solid-phase synthesis

An efficient, standard, mild, and copper-free imidazole-1-sulfonyl azide hydrochloride-based diazo-transfer method was implemented in a set of four resins that cover a broad range of hydrophobicity. The imidazole-1-sulfonyl azide hydrochloride is easily prepared/commercially available, stable upon storage at 4 °C, and proved to be a suitable alternative to triflyl azide for diazo-transfer reactions in amine functionalized resins. We have successfully applied the azido resins for the conjugation of a TFA-labile Wang-type linker using Click Chemistry.

Acetylated and methylated β-cyclodextrins as viable soluble supports for the synthesis of short 2′-oligodeoxyribo-nucleotides in solution

Novel soluble supports for oligonucleotide synthesis 11a–c have been prepared by immobilizing a 5′-O-protected 3′-O-(hex-5-ynoyl)thymidine (6 or 7) to peracetylated or permethylated 6-deoxy-6-azido-β-cyclodextrins 10a or 10b by Cu(I)-promoted 1,3-dipolar cycloaddition. The applicability of the supports to oligonucleotide synthesis by the phosphoramidite strategy has been demonstrated by assembling a 3′-TTT-5′ trimer from commercially available 5′-O-(4,4′-dimethoxytrityl)thymidine 3′-phosphoramidite. To simplify the coupling cycle, the 5′-O-(4,4′-dimethoxytrityl) protecting group has been replaced with an acetal that upon acidolytic removal yields volatile products. For this purpose, 5′-O-(1-methoxy-1-methylethyl)-protected 3′-(2-cyanoethyl-N,N-diisopropyl-phosphoramidite)s of thymidine (5a), N⁴-benzoyl-2′-deoxycytidine (5b) and N⁶-benzoyl-2′-deoxyadenosine (5c) have been synthesized and utilized in synthesis of a pentameric oligonucleotide 3′-TTCAT-5′ on the permethylated cyclodextrin support 11c.

Synthesis of Farnesol Analogues Containing Triazoles in Place of Isoprenes through 'Click Chemistry'

A solid-phase three-component Huisgen reaction has been used to generate polar farnesol and farnesyl diphosphate analogues. The Cu(I)-catalyzed 1,3-cycloadditions of various azides with solid supported (E)-3-methylhept-2-en-6-yn-1-ol provided only the 1,4-disubstituted 1,2,3-triazole regioisomers. The organic azides were generated in situ to minimize handling of potentially explosive azides. We have employed this powerful 'click chemistry' to make farnesol analogues where both β- and γ-isoprenes were replaced by triazole and substituted aromatic rings, respectively.

Synthesis and biological properties of C-2 triazolylinosine derivatives

O(6)-(Benzotriazol-1H-yl)guanosine and its 2'-deoxy analogue are readily converted to the O(6)-allyl derivatives that upon diazotization with t-BuONO and TMS-N(3) yield the C-2 azido derivatives. We have previously analyzed the solvent-dependent azide·tetrazole equilibrium of C-6 azidopurine nucleosides, and in contrast to these, the O(6)-allyl C-2 azido nucleosides appear to exist predominantly in the azido form, relatively independent of solvent polarity. In the presently described cases, the tetrazole appears to be very minor. Consistent with the presence of the azido functionality, each neat C-2 azide displayed a prominent IR band at 2126-2130 cm(-1). A screen of conditions for the ligation of the azido nucleosides with alkynes showed that CuCl in t-BuOH/H(2)O is optimal, yielding C-2 1,2,3-triazolyl nucleosides in 70-82% yields. Removal of the silyl groups with Et(3)N·3HF followed by deallylation with PhSO(2)Na/Pd(PPh(3))(4) gave the C-2 triazolylinosine nucleosides. In a continued demonstration of the versatility of O(6)-(benzotriazol-1H-yl)purine nucleosides, one C-2 triazolylinosine derivative was converted to two adenosine analogues via these intermediates, under mild conditions. Products were desilylated for biological assays. The two C-2 triazolyl adenosine analogues demonstrated pronounced antiproliferative activity in human ovarian and colorectal carcinoma cell cultures. When evaluated for antiviral activity against a broad spectrum of DNA and RNA viruses, some of the C-2 triazolylinosine derivatives showed modest inhibitory activity against cytomegalovirus.

Azide-tetrazole equilibrium of C-6 azidopurine nucleosides and their ligation reactions with alkynes

Facile syntheses of C-6 azidopurine ribonucleosides and 2'-deoxyribonucleosides have been developed. For silyl- and acetyl-protected as well as unprotected nucleosides, access to the azido derivatives could be readily attained via displacement of BtO(-) from the O(6)-(benzotriazol-1-yl)inosine nucleosides by azide anion. Use of diphenylphosphoryl azide/DBU as a simple route to the acetyl-protected azido nucleosides was also evaluated, but this proved to be inferior. Since these azido nucleosides can exist in an azide.tetrazole equilibrium, the effect of solvent polarity on this equilibrium was investigated. Subsequently, a detailed analysis of Cu-mediated azide-alkyne ("click") ligation was undertaken. Biphasic CH(2)Cl(2)/H(2)O medium proved to be best for the ligation reactions, suppressing the undesired azide reduction that was competing. Interestingly, although the tetrazolyl isomer predominates (ca. 80%) in CD(2)Cl(2) and in CD(2)Cl(2)/D(2)O, the Cu-catalyzed click reactions proceed smoothly with the silyl-protected ribo- and 2'-deoxyribonucleosides, leading to the C-6 triazolyl products in good to excellent yields. Thus, depletion of the azido form from the reaction mixture shifts the azide.tetrazole equilibrium, eventually resulting in complete consumption of azide and tetrazole. In several cases, major and minor azide-alkyne ligation products were observed, and characterization data are provided for both. In order to confirm the regiochemistry leading to the major isomer, one product was crystallized and evaluated by X-ray crystallography. The Cu-catalyzed azide-alkyne ligation is clearly efficient and significantly superior to thermal reactions, which were slow. Biological evaluation showed low cytotoxicities for the agents, suggesting their usefulness as biological probes.

Synthesis and conformational analysis of a cyclic peptide obtained via i to i+4 intramolecular side-chain to side-chain azide-alkyne 1,3-dipolar cycloaddition

Intramolecular side-chain to side-chain cyclization is an established approach to achieve stabilization of specific conformations and a recognized strategy to improve resistance toward proteolytic degradation. To this end, cyclizations, which are bioisosteric to the lactam-type side-chain to side-chain modification and do not require orthogonal protection schemes, are of great interest. Herein, we report the employment of Cu(I)-catalyzed 1,3-dipolar cycloaddition of side chains modified with azido and alkynyl functions and explore alternative synthetic routes to efficiently generate 1,4-disubstituted [1,2,3]triazolyl-containing cyclopeptides. The solid-phase assembly of the linear precursor including epsilon-azido norleucine and the propargylglycine (Pra) in positions i and i+4, respectively, was accomplished by either subjecting the resin-bound peptide to selective on-resin diazo transformation of a Lys into the Nle(epsilon-N3) or the incorporation of Fmoc-Nle(epsilon-N3)-OH during the stepwise build-up of the resin-bound peptide 1b. Solution-phase Cu(I)-catalyzed 1,3-dipolar cycloaddition converts the linear precursor Ac-Lys-Gly-Nle(epsilon-N3)-Ser-Ile-Gln-Pra-Leu-Arg-NH2 (2) into the 1,4-disubstituted [1,2,3]triazolyl-containing cyclopeptide [Ac-Lys-Gly-Xaa(&(1))-Ser-Ile-Gln-Yaa(&(2))-Leu-Arg-NH2][(&(1)(CH2)4-1,4-[1,2,3]triazolyl-CH2&(2))] (3). The conformational preferences of the model cyclopeptide 3 (III), which is derived from the sequence of a highly helical and potent i to i+4 side-chain to side-chain lactam-containing antagonist of parathyroid hormone-related peptide (PTHrP), are compared to the corresponding lactam analogue Ac[Lys(13)(&(1)),Asp(17)(&(2))]hPTHrP(11-19)NH2 (II). CD and NMR studies of 3 and II in water/hexafluoroacetone (HFA) (50:50, v/v) revealed a high prevalence of turn-helical structures involving in particular the cyclic regions of the molecule. Despite a slight difference of the backbone arrangement, the side-chains of Ser, Gln, and Ile located at the i+1 to i+3 of the ring-forming sequences share the same spatial orientation. Both cyclopeptides differ regarding the location of the turn-helical segment, which in II involves noncyclized residues while in 3 it overlaps with residues involved in the cyclic structure. Therefore, the synthetic accessibility and conformational similarity of i to i+4 side-chain to side-chain cyclopeptide containing the 1,4-disubstituted [1,2,3]triazolyl moiety to the lactam-type one may result in similar bioactivities.

Direct diazo-transfer reaction on beta-lactam: synthesis and preliminary biological activities of 6-triazolylpenicillanic acids

In this study we report the first example of a direct diazo-transfer reaction on readily available 6-aminopenicillanates to give 6-azidopenicillanates in high yield. Subsequent Cu(I)-catalyzed Huisgen cycloaddition between these 6-azidopenicillanates and assorted terminal alkynes facilely furnished 6-triazolylpenicillanic acids. Preliminary biological screening indicates that these triazolylpenicillanic acids possess low to moderate antibacterial activities.