Access to site-specific Fc-cRGD peptide conjugates through streamlined expressed protein ligation.

An ideal drug should be highly effective, non-toxic and be delivered by a convenient and painless single dose. We are still far from such optimal treatment but peptides, with their high target selectivity and low toxicity profiles, provide a very attractive platform from which to strive towards it. One of the major limitations of peptide drugs is their high clearance rates, which limit dosage regimen options. Conjugation to antibody Fc domains is a viable strategy to improve peptide stability by increasing their hydrodynamic radius and hijacking the Fc recycling pathway. We report the use of a split-intein based semi-synthetic approach to site-specifically conjugate a synthetic integrin binding peptide to an Fc domain. The strategy described here allows conjugating synthetic peptides to Fc domains, which is not possible via genetic methods, fully maintaining the ability of both the Fc domain and the bioactive peptide to interact with their binding partners.

Comparative investigation of ruthenium-based metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands.

Exchange of one PCy3 unit of the classical Grubbs catalyst 1 by N-heterocyclic carbene (NHC) ligands leads to "second-generation" metathesis catalysts of superior reactivity and increased stability. Several complexes of this type have been prepared and fully characterized, six of them by X-ray crystallography. These include the unique chelate complexes 13 and 14 in which the NHC- and the Ru-CR entities are tethered to form a metallacycle. A particularly favorable design feature is that the reactivity of such catalysts can be easily adjusted by changing the electronic and steric properties of the NHC ligands. The catalytic activity also strongly depends on the solvent used; NMR investigations provide a tentative explanation of this effect. Applications of the "second-generation" catalysts to ring closing alkene metathesis and intramolecular enyne cycloisomerization reactions provide insights into their catalytic performance. From these comparative studies it is deduced that no single catalyst is optimal for different types of applications. The search for the most reactive catalyst for a specific transformation is facilitated by IR thermography allowing a rapid and semi-quantitative ranking among a given set of catalysts.

Exploiting the reversibility of olefin metathesis. Syntheses of macrocyclic trisubstituted alkenes and (R,R)-(-)-pyrenophorin.

[figure: see text] The formation of the trisubstituted cycloalkene 7 by RCM of diene 5 proceeds via the acyclic dimer 6, thus demonstrating the ready reversibility of olefin metathesis if catalyzed by "second generation" ruthenium carbene complexes such as 2-4. When applied to acrylate 11, these catalysts trigger a cyclooligomerization process that evolves with time and serves as key step en route to the lactide antibiotic (-)-pyrenophorin 8.

Total synthesis of (-)-salicylihalamide.

A concise total synthesis of the potent cytotoxic marine natural products salicylihalamide A and B (la, b) is reported. Key steps of our approach were the asymmetric hydrogenation reactions of beta-keto esters 18 and 32 catalyzed by [((S)-BINAP)Ru-Cl2]2. NEt3 and the cyclization of the macrolide core by ring closing olefin metathesis (RCM) using the "second-generation" ruthenium carbene complex 24 as the catalyst which bears an imidazol-2-ylidene ligand. The EIZ ratio obtained in this macrocyclization reaction was determined by the protecting groups at the remote phenolic OH group of the cyclization precursor. The elaboration of the resulting cycloalkene 37 into the final target involved a CrCl2-mediated synthesis of vinyliodide 49 which, after deprotection, did undergo a copper-catalyzed cross-coupling process with the (Z,Z)-configurated carboxamide 42 to form the labile enamide moiety of 1. Compound 42 was derived from a palladium-catalyzed Negishi coupling between butynylzinc chloride and 3-iodoacrylate 39 followed by a Lindlar reduction of enyne 40 thus obtained and a final aminolysis of the ester group.

Total syntheses of (S)-(-)-zearalenone and lasiodiplodin reveal superior metathesis activity of ruthenium carbene complexes with imidazol-2-ylidene ligands.

Total syntheses of the bioactive orsellinic acid derivatives zearalenone 3 and lasiodiplodin 1 are reported based on a ring-closing metathesis (RCM) reaction of styrene precursors as the key steps. These and closely related macrocyclizations are catalyzed with high efficiency by the "second generation" ruthenium carbene catalyst 5 bearing a N-heterocyclic carbene ligand, whereas the standard Grubbs carbene 4 fails to afford any cyclized product. Only the (E)-isomer of the macrocyclic cycloalkene is formed in all cases. The substrates for RCM can be obtained either via a Stille cross-coupling reaction of tributylvinylstannane or, even more efficiently, by Heck reactions of the aryl triflate precursors with pressurized ethene. Furthermore, the synthesis of 1 via RCM is compared with an alternative approach employing a low-valent titanium-induced McMurry coupling of dialdehyde 47 for the formation of the large ring. This direct comparison clearly ends in favor of metathesis which turned out to be superior in all preparatively relevant respects.

Asymmetric synthesis of the fully functional macrolide core of salicylihalamide: remote control of olefin geometry during RCM.

A catalysis-based approach to the core region 24 of the antitumor agents salicylihalamides A and B is reported. Key steps are two asymmetric hydrogenations of beta-keto esters 13 and 16 catalyzed by [(R)-BINAP.RuCl(2)](2).NEt(3) and an RCM-based macrocyclization effected by the NHC-containing ruthenium carbene 21. The stereochemical outcome of the latter reaction is controlled by remote substituents on the phenolic OH group of the cyclization precursor 23.

Amination of aromatic olefins with anilines: a new domino synthesis of quinolines.

A new catalytic amination of aromatic olefins with anilines is presented. In a domino reaction, substituted quinoline derivatives are obtained in the presence of cationic rhodium complexes, such as [Rh(cod)2]BF4, and PPh3. Ethylbenzene is formed as a by-product in this new oxidative reaction. The first transition metal catalyzed anti-Markovnikov hydroamination of styrene with anilines occurs as a side reaction. Mechanistic investigations strongly support the regioselective oxidative amination of styrene as the key reaction step.

Formal total synthesis of (-)-balanol: concise approach to the hexahydroazepine segment based on RCM.

A concise synthesis of the hexahydroazepine moiety 13 of (-)-balanol 1 is described that comprises only eight steps and is distinctly shorter than all previous reported approaches to this particular compound. Sharpless epoxidation of divinylcarbinol 4 and ring closing alkene metathesis (RCM) reaction for the formation of the heterocyclic scaffold 9 constitute the key transformations of this sequence. The latter reaction is best achieved with catalytic amounts of the ruthenium indenylidene complex 18 recently reported. Furthermore, it is demonstrated that RCM can be successfully carried out even in the presence of an azido function provided that Schrock's molybdenum alkylidene complex Mo(=NAr)(=CHCMe2Ph)[OC(Me)(CF3)2]2 (Ar = 2,6-diisopropylphenyl) is used as precatalyst.