A laccase/(2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO) mediated oxidation was combined with an aqueous, enantioselective copper-catalyzed Michael addition reaction of water in one pot. The copper catalyst was also immobilized onto DNA to induce enantioselectivity in the reaction. Low conversions were observed when the reactions were performed simultaneously, caused by an undesired reaction of an oxidised TEMPO intermediate. We increased the conversions by using a stepwise approach. Thus, after completion of the oxidation, the first reaction was stopped by inhibiting the enzyme with HCO₂K and reducing the reactive TEMPO intermediate. Next, the Michael addition reaction was started by adding the Cu catalyst. By applying this strategy, an efficient two-step one-pot sequence, proceeding with 20 % ee, was realized. The yield and ee of the second reaction were not affected by the oxidation reaction.

The commercially available steroid dehydroepiandrosterone 3-acetate (DHEA) was converted into new, highly functionalized spiro derivatives by modification of the D-ring. The transformation proceeded through conversion of the C-17 carbonyl group into an electron-deficient alkene, followed by either [2+2] or [4+2] cycloaddition. The cycloaddition reactions were successful for alkylidene malononitriles and 2-cyano acrylates. Application of high pressure (15 kbar) was essential for good conversions due to the high steric hindrance on position C-17. The cycloadducts formed from the reaction of 2-cyano acrylates and Danishefsky's diene have high potential for further functionalization.

Here we demonstrate that deracemization of isoindolinones using Viedma ripening is possible starting from a racemic mixture of conglomerate crystals. Crystals of the enantiopure isoindolinones lose their chiral identity upon dissolution even without the need for a catalyst. This enabled complete deracemization of the reported isoindolinones without a catalyst.

The conversion of oxygen-containing compounds is often achieved by the use of phosphorus reagents. The newly formed phosphine oxide bond delivers the enthalpic gain that drives reactions, such as the Wittig, Mitsunobu, and Appel reaction, to completion. However, phosphine oxides are recognized as undesirable waste products and in the past decade several methods have emerged that address this issue by in situ regeneration of the phosphorus reagent. This Minireview outlines the two distinct strategies and underpinning research that led to these advances. The potential of the emerging field of phosphorus catalysis in chemistry is shown and new developments that may stimulate further research are described.

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-X_AA-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.

Enzyme-specific activation and the substrate mimetics strategy are effective ways to circumvent the limited substrate recognition often encountered in protease-catalyzed peptide synthesis. A key structural element in both approaches is the guanidinophenyl (OGp) ester, which enables important interactions for affinity and recognition by the enzyme—at least, this is usually the explanation given for its successful application. In this study we show that leaving group ability is of equal or even greater importance. To this end we used both experimental and computational methods: 1) synthesis of close analogues of OGp, and their evaluation in a dipeptide synthesis assay with trypsin, 2) molecular docking studies to provide insights into the binding mode, and 3) ab initio calculations to evaluate their electronic properties.

The synthesis of a series of peptides containing C-terminal 7-amino-4-methylcoumarin (AMC) for use in the thrombin generation test (TGT) is described. The lead structure in this project was H-Gly-Gly-Arg-AMC, of which the water solubility and kinetic parameters (K_M and k_cat) are greatly improved over those of the substrate in current use in the TGT: Cbz-Gly-Gly-Arg-AMC. A series of N-terminally substituted Gly-Gly-Arg-AMC derivatives were synthesized, as well as implementation of structural changes at either the P₂ or P₃ position of the peptide backbone. Furthermore, two substrates were synthesized that have structural similarities to the chromogenic thrombin substrate SQ68 or that contain a 1,2,3-triazole moiety in the peptide chain, mimicking an amide bond. To determine the applicability of newly synthesized fluorogenic substrates for monitoring continuous thrombin generation, the K_M and k_cat values of the conversion of these fluorogenic substrates by thrombin (FIIa) and factor Xa (FXa) were quantified. An initial selection was made on basis of these data, and suitable substrates were further evaluated as substrates in the thrombin generation assay. Assessment of the acquired data showed that several substrates, including the SQ68 derivative Et-malonate-Gly-Arg-AMC and N-functionalized Gly-Gly-Arg-AMC derivatives, are suitable candidates for replacement of the substrate currently in use.

Epoxidation of both terminal and non-terminal olefins with peroxy acids is a well-established and powerful tool in a wide variety of chemical processes. In an additional step, the epoxide can be readily converted into the corresponding trans-diol. Batch-wise scale-up, however, is often troublesome because of the thermal instability and explosive character of the peroxy acids involved. This article describes the design and semi-automated optimization of a continuous flow process and subsequent scale-up to preparative production volumes in an intrinsically safe manner.

The substrate mimetics approach is a versatile method for small-scale enzymatic peptide-bond synthesis in aqueous systems. The protease-recognized amino acid side chain is incorporated in an ester leaving group, the substrate mimetic. This shift of the specific moiety enables the acceptance of amino acids and peptide sequences that are normally not recognized by the enzyme. The guanidinophenyl group (OGp), a known substrate mimetic for the serine proteases trypsin and chymotrypsin, has now been applied for the first time in combination with papain, a cheap and commercially available cysteine protease. To provide insight in the binding mode of various Z-X_AA-OGp esters, computational docking studies were performed. The results strongly point at enzyme-specific activation of the OGp esters in papain through a novel mode of action, rather than their functioning as mimetics. Furthermore, the scope of a model dipeptide synthesis was investigated with respect to both the amino acid donor and the nucleophile. Molecular dynamics simulations were carried out to prioritize 22 natural and unnatural amino acid donors for synthesis. Experimental results correlate well with the predicted ranking and show that nearly all amino acids are accepted by papain.

The piperidine ring system is one of the most common structural subunits and is found in many natural products. Moreover, piperidine alkaloids and derivatives thereof are of great interest for the pharmaceutical industry since they exhibit a wide range of biological activities. Consequently, short and valuable routes to highly functionalized building blocks for this important ring system are of general interest. A ring-closing metathesis (RCM)-mediated approach based on linear (substituted) dehydroamino esters was developed to provide cyclic dehydroamino esters. These structures represent valuable intermediates en route to highly substituted pipecolic acids as well as more elaborate heterocycles.

The cover picture shows an artistic impression of the interaction of different types of 3-hydroxypiperidine-containing natural products with an imaginary receptor protein. The 3-hydroxypiperidine motif represents a privileged structural element that occurs in a large variety of biologically relevant compounds displaying a range of activities. Examples include the natural products 3-hydroxypipecolic acid, prosopinine, morusimic acid D, cassine and febrifugine. Enantioselective synthetic strategies to these compounds are presented in the Microreview by F. P. J. T. Rutjes et al. on p. 2831 ff. Cover picture design by Dr. Sander S. van Berkel.

The 3-hydroxypiperidine moiety is a privileged scaffold that is encountered in many bioactive compounds and natural products. This review summarizes the investigations ofvarious research groups concerning the synthesis of natural products containing this scaffold.

A comprehensive study on the synthesis and ring-closing metathesis (RCM) of α,β-dehydroamino acids is described. This sequence has led to the formation of a range of biologically relevant functionalized nitrogen heterocycles. The incorporation of chiral building blocks in the RCM precursors eventually resulted in the formation of optically active 4-substituted cyclic dehydroamino acids. In addition, olefin isomerization under metathesis conditions was observed for a number of compounds, which could be successfully inhibited either by the introduction of allylic substituents or by the addition of a ruthenium hydride scavenger.

The rhodium-catalyzed asymmetric hydrogenation of several β-substituted itaconic acid monoesters, using a library of monodentate phosphoramidite and phosphite ligands is described. Two β-alkyl-substituted substrates were readily hydrogenated by the rhodium complex Rh(COD)₂BF₄ in combination with (S)-PipPhos as a ligand resulting in ees of 99 %. In contrast, the corresponding more hindered β-aryl-substituted substrates did not exhibit acceptable enantioselectivities under these conditions. However, the use of a 48-membered ligand library led to the identification of several suitable ligands for these substrates, resulting in ees of 89–99 %. The resulting optically active succinic acid derivatives are potentially useful building blocks for more elaborate compounds, because of the ability to differentiate between the carboxylic acid and the ester groups on either side of the molecule.

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 α_vβ₃ (IC₅₀=192 nM) and favorable pharmacokinetics.