Synthesis of macrolide analogues of sanglifehrin by RCM: unique reactivity of a ruthenium carbene complex bearing an imidazol-2-ylidene ligand. Ring-closing metathesis

Non-Immunosuppressive Cyclophilin Inhibitors

Cyclophilins, enzymes with peptidyl-prolyl cis/trans isomerase activity, are relevant to a large variety of biological processes. The most abundant member of this enzyme family, cyclophilin A, is the cellular receptor of the immunosuppressive drug cyclosporine A (CsA). As a consequence of the pathophysiological role of cyclophilins, particularly in viral infections, there is a broad interest in cyclophilin inhibition devoid of immunosuppressive activity. This Review first gives an introduction into the physiological and pathophysiological roles of cyclophilins. The presentation of non-immunosuppressive cyclophilin inhibitors will commence with drugs based on chemical modifications of CsA. The naturally occurring macrocyclic sanglifehrins have become other lead structures for cyclophilin-inhibiting drugs. Finally, de novo designed compounds, whose structures are not derived from or inspired by natural products, will be presented. Relevant synthetic concepts will be discussed, but the focus will also be on biochemical studies, structure-activity relationships, and clinical studies.

Enantioselective Synthesis and Biological Evaluation of Sanglifehrin A and B and Analogs

Sanglifehrin A and B are immunosuppressive macrocyclic natural products endowed with and differentiated by a unique spirocyclic lactam. Herein, we report an enantioselective total synthesis and biological evaluation of sanglifehrin A and B and analogs. Access to the spirocyclic lactam was achieved through convergent assembly of a key pyranone intermediate followed by a stereo-controlled spirocyclization. The 22-membered macrocyclic core was synthesized by ring-closing metathesis in the presence of 2,6-bis(trifluoromethyl) benzeneboronic acid (BFBB). The spirocyclic lactam and macrocycle fragments were united by a Stille coupling to furnish sanglifehrin A and B. Additional sanglifehrin B analogs with variation at the C40 position were additionally prepared. Biological evaluation revealed that the 2-CF₃ analog of sanglifehrin B exhibited higher anti-proliferative activity than the natural products sanglifehrin A and B in Jurkat cells. Both natural products induced higher-order homodimerization of cyclophilin A (CypA), but only sanglifehrin A promoted CypA complexation with inosine-5'-monophosphate dehydrogenase 2 (IMPDH2). The synthesis reported herein will enable further evaluation of the spirolactam and its contribution to sanglifehrin-dependent immunosuppressive activity.

Alkene chemoselectivity in ruthenium-catalyzed Z-selective olefin metathesis

Chelated ruthenium catalysts have achieved highly chemoselective olefin metathesis reactions. Terminal and internal Z olefins were selectively reacted in the presence of internal E olefins. Products were produced in good yield and high stereoselectivity for formation of a new Z olefin. No products of metathesis with the internal E olefin were observed. Chemoselectivity for terminal olefins was also observed over both sterically hindered and electronically deactivated alkenes.

Chemoenzymatic Approaches toward Dechloroansamitocin P-3

[reaction: see text] The enantioselective total synthesis of proansamitocin, a key biosynthetic intermediate of the highly potent antitumor agent ansamitocin P-3, is described which bears a diene-ene RCM as the key macrocyclization step. Feeding of proansamitocin to an AHBA block mutant Actinosynnema pretiosum (HGF073) yielded ansamitocin P-3 as well as dechloroansamitocin P-3, the latter also being formed upon fermentation in the presence of 3-amino-5-methoxybenzoic acid.

Development of a Concise and Diversity-Oriented Approach for the Synthesis of Plecomacrolides via the Diene−Ene RCM

[reaction: see text] A concise synthesis of the core structures of plecomacrolide with ring sizes varying from 16 to 19 atoms was achieved for the first time by the diene-ene ring-closing olefin metathesis reaction. This approach should allow access to the structurally diverse analogues of plecomacrolide.

Chemoselective Construction of Substituted Conjugated Dienes Using an Olefin Cross-Metathesis Protocol

[Reaction: see text] Various substituted conjugated dienes have been made by olefin cross-metathesis. Using either electronic or steric "protection," one of the olefins of the conjugated diene was deactivated relative to the other for cross-metathesis. The reactions proceed with very high chemoselectivity and, when steric deactivation is used, very high diastereoselectivity.

Ring-Closing Olefin Metathesis and Radical Cyclization as Competing Pathways

First and second generation Grubbs' catalyst mediate under otherwise identical conditions two different cyclization modes with high selectivity: a ring-closing metathesis and an atom-transfer radical addition (ATRA) pathway.

Synthesis of Diverse Macrocyclic Peptidomimetics Utilizing Ring-Closing Metathesis and Solid-Phase Synthesis

The synthesis of a range of highly functionalized peptidomimetic macrocycles has been accomplished using ring-closing metathesis and enyne tandem cross-metathesis-ring-closing metathesis reactions. This approach gives access to rigidified macrocycles modeled on the structures of cyclic peptides and designed to be biologically stable. The potential for peripheral functionalization of these templates has been demonstrated using Diels-Alder reactions, palladium(0) coupling reactions, and amide formation both in the solution phase and using polymer-supported syntheses.

A stereoselective enyne cross metathesis

[reaction: see text] Intermolecular enyne metathesis reaction of alkynes with olefins catalyzed by second-generation Grubbs catalyst (1) proceeded stereoselectively under ethylene atmosphere to produce 1,3-disubstituted butadienes with E stereochemistry.

Total synthesis of the salicylate enamide macrolide oximidine II

The asymmetric synthesis of the salicylate enamide macrolide oximidine II is reported. The synthesis involves a highly regio- and stereoselective ring-closing metathesis of a bis-diene substrate to construct the macrocyclic triene core. Copper(I)-mediated amidation of a (Z)-vinyl iodide was employed to attach the enamide side chain.

A convergent three-component total synthesis of the powerful immunosuppressant (-)-sanglifehrin a

The potent immunosuppressive agent (-)-sanglifehrin A (5), initially discovered in a soil sample from Malawi, has been synthesized in a highly convergent and stereocontrolled manner. The enantioselective approach relies on initial construction of the iodovinyl carboxylic acid 14, which is coupled to tripeptide 59 in advance of a key macrolactonization step that generates 61a. An alternative protocol that involves the linkage of 14 to 46 for possible construction of the large ring failed due to an inability to bring about a corresponding macrolactamization maneuver. An efficient means for elaborating the C26-N42 spirolactam western sector of 5 is also detailed. This requisite fragment was assembled through the proper adaptation of consecutive aldol tactics for construction of the nine stereogenic centers, six of which are contiguous. The first aldol process consisted of the tin triflate-mediated reaction of the aldehyde derived from 72 with enantiopure ketone 73 to generate the syn C36-C37 relationship resident in 75. Once the conversion of 75 to 78 had been completed, the attachment to ketone 66 was effected with (+)-DIPCl, thereby setting the C33-C34 relationship as anti. Once functional group modifications had given rise to 62, spirolactamization was achieved to deliver predominantly 94, thereby setting the stage for the acquisition of vinyl stannane 13 and its subsequent palladium-catalyzed Stille coupling to 61b. Controlled acidic hydrolysis completed the synthesis of 5. Other important features of the present route are addressed where relevant.

Intramolecular silicon-assisted cross-coupling reactions: general synthesis of medium-sized rings containing a 1,3-cis-cis diene unit

The combination of ring-closing metathesis and Pd-catalyzed, silicon-assisted intramolecular cross-coupling has been developed to provide an effective and powerful method for construction of medium-sized rings with an internal 1,3-cis-cis diene unit. Allylic alcohols bearing a Z-iodoalkenyl tether can be silylated with chlorodimethylvinylsilane and subjected to Mo-catalyzed ring-closing metathesis to form unsaturated siloxanes. Activation of the siloxane with tetrabutylammonium fluoride in the presence of [allylPdCl](2) leads to high yielding ring-closing reactions to form 9-, 10-, 11- and 12-membered rings. Extension to the synthesis of 9-membered ring unsaturated ethers has also been accomplished. Noteworthy features of this process include: (1) a highly stereospecific intramolecular coupling process, (2) flexible positioning of the hydroxy group, and (3) potential extension to other medium-sized carbocycles and heterocycles.

Conformationally stable and constrained macrocarbocyclic pseudopeptide mimics of beta-hairpin structures

Subjecting a D-Pro-L-Pro template harboring N- and C-terminal omega-alkenyl amino acids to a ring-closure metathesis reaction afforded the corresponding macrocyclic alkenes. A cis-alkene analogue crystallized with one molecule each of water and chloroform, which were retained even after heating at 100 degrees C. By using the reduced macrocyclic product as a template, the metathesis could be repeated twice on newly installed omega-alkenyl amino acids to give three-tiered macrocarbocyclic pseudopeptides as mixtures of conformers. NMR studies revealed the high conformational stability of these motifs.