Richard H. Blaauw

Palladium-Catalyzed Cyclization Reactions of Acetylene-Containing Amino Acids

Enantiomerically pure acetylene-containing - amino acids were used as versatile starting materials for the synthesis of a variety of heterocycles via Pd-mediated cyclization reactions. Depending on the protecting group strategy, both the carboxylate and the amine function of the amino acids could participate in the cyclizations, thus giving rise to oxygen heterocycles (-aminolactones) and nitrogen heterocycles (cyclic -amino acid derivatives), respectively. Beside the straightforward cyclization, cyclization/cross- coupling reactions were also successfully carried out to provide the corresponding substituted cyclic amino acid derivatives.

Discovery of Small Molecule Vanin Inhibitors: New Tools To Study Metabolism and Disease

Vanins are enzymes with pantetheinase activity and are presumed to play a role in the recycling of pantothenic acid (vitamin B5) from pantetheine. Pantothenic acid is an essential nutrient required to synthesize coenzyme A, a cofactor involved in many biological processes such as fatty acid synthesis and oxidation of pyruvate to fuel the citric acid cycle. Hydrolysis of pantetheine also liberates cysteamine, a known antioxidant. Vanin-1 is highly expressed in liver and is under transcriptional control of PPAR-α and nutritional status, suggesting a role in energy metabolism. The lack of potent and specific inhibitors of vanins has hampered detailed investigation of their function. We hereby report the design, synthesis, and characterization of a novel pantetheine analogue, RR6, that acts as a selective, reversible, and competitive vanin inhibitor at nanomolar concentration. Oral administration of RR6 in rats completely inhibited plasma vanin activity and caused alterations of plasma lipid concentrations upon fasting, thereby illustrating its potential use in chemical biology research.

Synthesis of the Right‐Hand Substructure of Solanoeclepin A

A synthesis of the right-hand substructure 7 of solanoeclepin A (1), the most active natural hatching agent of potatocyst nematodes, was approached by an intramolecular [2+2] photocycloaddition. The construction of the strained bicyclo[2.1.1]hexane skeleton was achieved in five steps from dioxenone 9. A Simmons−Smith cyclopropanation enabled the installation of the required cyclopropane. Finally, a TPAP oxidation allowed the smooth formation of the strained cyclobutanone moiety.

Studies towards the total synthesis of solanoeclepin A: synthesis and potato cyst nematode hatching activity of analogues containing the tetracyclic left-hand substructureElectronic supplementary information (ESI) available: further experimental details. See http://www.rsc.org/suppdata/p1/b2/b202020n/

In our studies towards the total synthesis of solanoeclepin A, a natural hatching agent of potato cyst nematodes, three analogues containing the tetracyclic left-handed substructure have been synthesised. First, the synthesis of the parent tetracycle 2 in enantiopure form is reported. Key steps are (1) chromium-mediated coupling of aldehyde 5 (see preceding paper in this issue) and vinyl triflate 6 to furnish an α,β-unsaturated lactone, which was transformed into triene 4 in six-steps, (2) ring-closing metathesis of 4 to tetracycle 3 and (3) oxidative functionalisation of the least substituted double bond of 3 to provide the fully functionalised tetracyclic left-handed substructure of solanoeclepin A. The methodology developed was successfully applied in the synthesis of two more elaborate solanoeclepin A analogues 9 and 11. Both compounds, prepared as mixtures of diastereomers, showed promising biological activity in hatching activity tests.

Combination of Pantothenamides with Vanin Inhibitors as a Novel Antibiotic Strategy against Gram-Positive Bacteria

The emergence of resistance against current antibiotics calls for the development of new compounds to treat infectious diseases. Synthetic pantothenamides are pantothenate analogs that possess broad-spectrum antibacterial activity in vitro in minimal media. Pantothenamides were shown to be substrates of the bacterial coenzyme A (CoA) biosynthetic pathway, causing cellular CoA depletion and interference with fatty acid synthesis. In spite of their potential use and selectivity for bacterial metabolic routes, these compounds have never made it to the clinic. In the present study, we show that pantothenamides are not active as antibiotics in the presence of serum, and we found that they were hydrolyzed by ubiquitous pantetheinases of the vanin family. To address this further, we synthesized a series of pantetheinase inhibitors based on a pantothenate scaffold that inhibited serum pantetheinase activity in the nanomolar range. Mass spectrometric analysis showed that addition of these pantetheinase inhibitors prevented hydrolysis of pantothenamides by serum. We found that combinations of these novel pantetheinase inhibitors and prototypic pantothenamides like N5-Pan and N7-Pan exerted antimicrobial activity in vitro, particularly against Gram-positive bacteria (Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pneumoniae, and Streptococcus pyogenes) even in the presence of serum. These results indicate that pantothenamides, when protected against degradation by host pantetheinases, are potentially useful antimicrobial agents.

Studies towards the total synthesis of solanoeclepin A: synthesis of the 7-oxabicyclo[2.2.1]heptane moiety and attempted seven-membered ring formation

This paper details studies towards the total synthesis of solanoeclepin A (1), the most active natural hatching agent of potato cyst nematodes. The first goal was the preparation of the tetracyclic left-handed substructure 2 in enantiopure form. The 7-oxabicyclo[2.2.1]heptane moiety was obtained via a diastereoselective intramolecular Diels–Alder strategy by using (R)-phenylglycinol as a chiral auxiliary as pioneered by Mukaiyama. A chromium-mediated nickel-catalysed coupling of aldehyde 5 with vinyl triflate 6 gave α,β-unsaturated lactone 18 as a single stereoisomer. The seven-membered ring was expected to arise from a McMurry coupling of dialdehyde 4. Surprisingly, oxidation of diol 24 did not lead to the desired dialdehyde 4, but to the eight-membered ring lactone 25.

Intramolecular [2+2] photocycloadditions as an approach towards the bicyclo[2.1.1]hexane substructure of solanoeclepin A

The synthesis of a tricyclic substructure of solanoeclepin A is described. The key step involves an intramolecular [2+2] photocycloaddition between a dioxinone and a tetrasubstituted bicyclic alkene providing the strained bicyclo[2.1.1]hexane moiety.

Novel pantothenate derivatives for anti-malarial chemotherapy

BackgroundA number of synthetic pantothenate derivatives, such as pantothenamides, are known to inhibit the growth of the human malaria parasite Plasmodium falciparum, by interfering with the parasite Coenzyme A (CoA) biosynthetic pathway. The clinical use of pantothenamides is limited by their sensitivity to breakdown by ubiquitous human pantetheinases of the vanin family.MethodsA number of pantothenate derivatives (pantothenones) with potent and specific inhibitory activity against mammalian vanins were tested in a proliferation assay of asexual P. falciparum blood stages alone, and in combination with pantothenamides.ResultsThe vanin inhibitors were found to protect pantothenamides against breakdown by plasma vanins, thereby preserving the in vitro anti-malarial activity. Moreover, some of the vanin inhibitors showed in vitro anti-malarial activity in the low micromolar range. The most potent antimalarial in this series of compounds (RR8), was found to compete with pantothenate in a combination proliferation assay. No correlation, however, was found between anti-vanin and anti-malarial activity, nor was pantetheinase activity detected in P. falciparum extracts.ConclusionsGrowth inhibition is most likely due to competition with pantothenate, rather than pantetheinase inhibition. As vanin inhibitors of the pantothenone class are stable in biological fluids and are non-toxic to mammalian cells, they may represent novel pantothenate-based anti-malarials, either on their own or in combination with pantothenamides.

Intramolecular [2+2] photocycloadditions as an approach towards the right-hand side of solanoeclepin A

A racemic synthesis of the bicyclo[2.1.1]hexane substructure of solanoeclepin A (1), the most active natural hatching agent of potato cyst nematodes, was approached via an intramolecular [2+2] photocycloaddition of 6-unsubstituted dioxenones with variously substituted pendent alkenes. The synthesis of the cyclisation precursors involved a very efficient iodide–magnesium exchange reaction with iododioxenone 6, which allowed facile allylation at C-5 of the dioxenone. Photochemistry with dioxenones 12 and 17 led to novel bicyclo[2.2.0]hexanes 24 and 26. The use of the more rigid lactone precursor 14 led to bicyclo[2.1.1]hexane 25, and allowed the stereoselective synthesis of the complex tricyclic core of solanoeclepin A. The structure of 25 was unequivocally proven by X-ray crystal structure determination.

Chemo-enzymatic synthesis of chiral fluorine-containing building blocks

Two complementary strategies for the synthesis of optically active fluorine-containing building blocks have been probed. The first strategy involves either the enzymatic resolution of fluorinated alpha,alpha-disubstituted -alpha-amino acid amides, or the asymmetric hydrogenation of fluorinated clehydroamino acids. The second strategy involves the transition metal-catalyzed introduction of fluorine-containing substituents onto olefin- or acetylene-containing alpha-H-alpha-amino acids. These amino acids in turn are made optically active by enzymatic resolution of the corresponding amides.