F.L. van Delft

Preventing thiol-yne addition improves the specificity of strain-promoted azide-alkyne cycloaddition

The 1,3-dipolar cycloaddition of azides with ring-strained alkynes is one of the few bioorthogonal reactions suitable for specific biomolecule labeling in complex biological systems. Nevertheless, azide-independent labeling of proteins by strained alkynes can occur to a varying extent, thereby limiting the sensitivity of assays based on strain-promoted azide-alkyne cycloaddition (SPAAC). In this study, a subset of three cyclooctynes, dibenzocyclooctyne (DIBO), azadibenzocyclooctyne (DIBAC), and bicyclo[6.1.0]nonyne (BCN), was used to evaluate the azide-independent labeling of proteins in vitro. For all three cyclooctynes, we show that thiol-yne addition with reduced peptidylcysteines is responsible for most of the azide-independent polypeptide labeling. The identity of the reaction product was confirmed by LC-MS and NMR analysis. Moreover, we show that undesired thiol-yne reactions can be prevented by alkylating peptidylcysteine thiols with iodoacetamide (IAM). Since IAM is compatible with SPAAC, a more specific azide-dependent labeling is achieved by preincubating proteins containing reduced cysteines with IAM.

Application of Metal‐Free Triazole Formation in the Synthesis of Cyclic RGD–DTPA Conjugates

The tandem 1,3‐dipolar cycloaddition‐retro‐Diels–Alder (tandem crDA) reaction is presented as a versatile method for metal‐free chemoselective conjugation of a DTPA radiolabel to N‐δ‐azido‐cyclo(‐Arg‐Gly‐Asp‐d‐Phe‐Orn‐) via oxanorbornadiene derivatives. To this end, the behavior of several trifluoromethyl‐substituted oxanorbornadiene derivatives in the 1,3‐dipolar cycloaddition was studied and optimized to give a clean and efficient method for bio‐orthogonal ligation in an aqueous environment. After radioisotope treatment, the resulting 111In‐labeled c(RGD)‐CF3‐triazole‐DTPA conjugate was subjected to preliminary biological evaluation and showed high affinity for αvβ3 (IC50=192 nM) and favorable pharmacokinetics.

Targeting Aberrant Sialylation in Cancer Cells Using a Fluorinated Sialic Acid Analog Impairs Adhesion, Migration, and In Vivo Tumor Growth

Cancer cells decorate their surface with a dense layer of sialylated glycans by upregulating the expression of sialyltransferases and other glycogenes. Although sialic acids play a vital role in many biologic processes, hypersialylation in particular has been shown to contribute to cancer cell progression and metastasis. Accordingly, selective strategies to interfere with sialic acid synthesis might offer a powerful approach in cancer therapy. In the present study, we assessed the potential of a recently developed fluorinated sialic acid analogue (P-3Fax-Neu5Ac) to block the synthesis of sialoglycans in murine melanoma cells and the consequences on cell adhesion, migration, and in vivo growth. The results showed that P-3Fax-Neu5Ac readily caused depletion of α2,3-/α2,6-linked sialic acids in B16F10 cells for several days. Long-term inhibition of sialylation for 28 days was feasible without affecting cell viability or proliferation. Moreover, P-3Fax-Neu5Ac proved to be a highly potent inhibitor of sialylation even at high concentrations of competing sialyltransferase substrates. P-3Fax-Neu5Ac–treated cancer cells exhibited impaired binding to poly-l-lysine, type I collagen, and fibronectin and diminished migratory capacity. Finally, blocking sialylation of B16F10 tumor cells with this novel sialic acid analogue reduced their growth in vivo. These results indicate that P-3Fax-Neu5Ac is a powerful glycomimetic capable of inhibiting aberrant sialylation that can potentially be used for anticancer therapy. Mol Cancer Ther; 12(10); 1935–46. ©2013 AACR.

Synthesis of ld-Hepp and KDO containing di- and tetrasaccharide derivatives of Neisseria meningitidis inner-core region via iodonium ion promoted glycosidations

Abstract The synthesis of methyl(ethyl) 2-thio-KDO i.e. 1, 2, 5, 7 and 10 and ethyl 1-thio- ld Hepp (i.e. 26 and 43 derivatives will be described. The latter derivatives proved to be suitable donors in iodonium ion (NIS/TfOH) promoted glycosidation reactions. The usefulness of the glycosidation approach was illustrated by the successful conclusion of a spacer containing dimer l -α- d -Hepp(1→5)-α-KDO-2-O-(CH2)3NH2 (37) and tetramer β- d -Galp-(1→4)- β- d Glcp-(1→4)- l -α- d -Hepp(1→5)-α-KDO-2-O-(CH2)3NH2 (47).

Synthetic applications of aliphatic unsaturated alpha-H-alpha-amino acids

This article provides an overview of the literature concerning synthetic applications of unsaturated aliphatic amino acids in the period May 2000 to December 2004.

N,N-Acetals as N-acyliminium ion precursors: Synthesis and absolute stereochemistry of epiquinamide

A stereoselective synthesis of (+)-epiquinamide is presented in combination with determination of the absolute configuration of the natural product. Key steps in the sequence involved chemoenzymatic formation of an enantiomerically pure cyanohydrin, reductive cyclization to the corresponding cyclic N,N-acetal, and subsequent conversion into a suitable N-acyliminium ion precursor to enable construction of the second ring.

Strain-Promoted Oxidation-Controlled Cyclooctyne–1,2-Quinone Cycloaddition (SPOCQ) for Fast and Activatable Protein Conjugation

A main challenge in the area of bioconjugation is to devise reactions that are both activatable and fast. Here, we introduce a temporally controlled reaction between cyclooctynes and 1,2-quinones, induced by facile oxidation of 1,2-catechols. This so-called strain-promoted oxidation-controlled cyclooctyne-1,2-quinone cycloaddition (SPOCQ) shows a remarkably high reaction rate when performed with bicyclononyne (BCN), outcompeting the well-known cycloaddition of azides and BCN by 3 orders of magnitude, thereby allowing a new level of orthogonality in protein conjugation.

Enantioselective chemoenzymatic synthesis of cis- and trans-2,5-disubstituted morpholines.

A versatile synthesis of enantiomerically pure cis- and trans-2,5-disubstituted morpholines is described. Hydroxynitrile lyase-mediated cyanide addition onto aldehydes provided cyanohydrins in virtually quantitative yield and excellent enantioselectivity. Subsequent formation of diastereomerically pure amino esters via a three-step, one-pot reduction-transimination-reduction sequence followed by reduction and simultaneous protection provided cyclization precursors. Finally, cyclization and SmI(2)-mediated reductive detosylation completed the synthesis of cis- and trans-2,5-disubstituted morpholines in good yields and excellent diastereoselectivities.

Synthesis of novel acetylene-containing amino acids

Summary. Novel synthetic procedures for the modification of non-proteinogenic acetylene-containing amino acids have been developed. The functionalization either proceeds via zinc/copper-mediated introduction of alkyl substituents, or via tungsten-catalyzed ring-closing alkyne metathesis reactions.

Papain-Specific Activating Esters in Aqueous Dipeptide Synthesis

Enzymatic peptide synthesis has the potential to be a viable alternative for chemical peptide synthesis. Because of the increasing commercial interest in peptides, new and improved enzymatic synthesis methods are desirable. In recently developed enzymatic strategies such as substrate mimetic approaches and enzyme‐specific activation, use of the guanidinophenyl ester (OGp) group has been shown to suffer from some drawbacks. OGp esters are sensitive to spontaneous chemical hydrolysis and the group is expensive to synthesize and therefore not suitable for large‐scale applications. On the basis of earlier computational studies, we hypothesized that OGp might be replaceable by simpler ester groups to make the enzyme‐specific activation approach to peptide bond formation more accessible. To this end, a set of potential activating esters (Z‐Gly‐Act) was designed, synthesized, and evaluated. Both the benzyl (OBn) and the dimethylaminophenyl (ODmap) esters gave promising results. For these esters, the scope of a model dipeptide synthesis reaction under aqueous conditions was investigated by varying the amino acid donor. The results were compared with those obtained from a previous study of Z‐XAA‐OGp esters. Computational docking analysis of the set of esters was performed in order to provide insight into the differences in the reactivities of all the potential activating esters. Finally, selected ODmap‐ and OBn‐activated amino acids were applied in the synthesis of two biologically active dipeptides on preparative scales.