Barbara Gabor

Indenylidene Complexes of Ruthenium: Optimized Synthesis, Structure Elucidation, and Performance as Catalysts for Olefin Metathesis—Application to the Synthesis of the ADE-Ring System of Nakadomarin A

An optimized and large scale adaptable synthesis of the ruthenium phenylindenylidene complex 3 is described which employs commercially available diphenyl propargyl alcohol 5 as a stable and convenient carbene source. Previous ambiguities as to the actual structure of the complex have been ruled out by a full analysis of its NMR spectra. A series of applications to ring closing metathesis (RCM) reactions shows that complex 3 is as good as or even superior to the classical Grubbs carbene 1 in terms of yield, reaction rate, and tolerance towards polar functional groups. Complex 3 turns out to be the catalyst of choice for the synthesis of the enantiopure core segment 77 of the marine alkaloid nakadomarin A 60 comprising the ADE rings of this target. Together with a series of other examples, this particular application illustrates that catalyst 3 is particularly well suited for the cyclization of medium-sized rings by RCM. Other key steps en route to nakadomarin A are a highly selective intramolecular Michael addition setting the quaternary center at the juncture of the A and D rings and a Takai-Nozaki olefination of aldehyde 73 with CH2I2, Ti(OiPr)4 and activated zinc dust.

Catalysis-Based and Protecting-Group-Free Total Syntheses of the Marine Oxylipins Hybridalactone and the Ecklonialactones A, B, and C

Concise and protecting-group-free total syntheses of the marine oxylipins hybridalactone (1) and three members of the ecklonialactone family (2-4) were developed. They deliver these targets in optically pure form in 14 or 13 steps, respectively, in the longest linear sequence; five of these steps are metal-catalyzed and four others are metal-mediated. The route to either 1 or 2-4 diverges from the common building block 22, which is accessible in 7 steps from 2[5H]furanone by recourse to a rhodium-catalyzed asymmetric 1,4-addition reaction controlled by the carvone-derived diene ligand 35 and a ring-closing alkene metathesis (RCM) catalyzed by the ruthenium indenylidene complex 17 as the key operations. Alternatively, 22 can be made in 10 steps from furfural via a diastereoselective three-component coupling process. The further elaboration of 22 into hybridalactone as the structurally most complex target with seven contiguous chiral centers was based upon a sequence of cyclopropanation followed by a vanadium-catalyzed epoxidation, both of which were directed by the same free hydroxy group at C15. The macrocyclic scaffold was annulated to the headgroup by means of a ring-closing alkyne metathesis reaction (RCAM). In response to the unusually high propensity of the oxirane of the targeted oxylipins for ring opening, this transformation had to be performed with complexes of the type [(Ar(3)SiO)(4)Mo≡CPh][K·OEt(2)] (43), which represent a new generation of exceedingly tolerant yet remarkably efficient catalysts. Their ancillary triarylsilanolate ligands temper the Lewis acidity of the molybdenum center but are not sufficiently nucleophilic to engage in the opening of the fragile epoxide ring. A final semireduction of the cycloalkyne formed in the RCAM step to the required (Z)-alkene completed the total synthesis of (-)-1. The fact that the route from the common fragment 22 to the ecklonialactones could follow a similar logic showcased the flexibility inherent to the chosen approach.

Second-Generation Total Synthesis of Spirastrellolide F Methyl Ester: The Alkyne Route†

The spirastrellolides are a small family of exquisitely potent antimitotic agents of marine origin which are reported to exert their function by selective inhibition of the serine/threonine protein phosphatase PP2A (IC50 ca. 1 nm). This activity predestines them as tools for chemical biology and as possible lead structures in the quest for novel therapeutic agents for the treatment of cancer and other metabolic disorders. Although it took more than four years from the initial isolation to the full elucidation of the stereostructure of these compounds, their intricate topology and very limited supply from the natural source aroused considerable interest in the synthetic community even before the molecular architecture had been fully elucidated. These efforts culminated in the first total synthesis of spirastrellolide A methyl ester (2) by Paterson et al., followed by the conquest of the sister compound spirastrellolide F methyl ester (1) by our research group shortly thereafter (Scheme 1). Since only the relative configuration of the four color-coded stereoclusters embedded into the framework of these macrolides but neither

A Total Synthesis of Spirastrellolide A Methyl Ester

A concise total synthesis of spirastrellolide A methyl ester (1 a, R(1) =Me) as the parent compound of a series of highly cytotoxic marine macrolides is disclosed, which exploits and expands the flexibility of a synthesis plan previously developed by our group en route to the sister compound spirastrellolide F methyl ester (6 a, R(1) =Me). Key to success was the masking of the signature Δ(15,16) -bond of 1 a as a C16-carbonyl group until after the stereogenic center at C24 had been properly set by a highly selective hydrogenation of the C24 exo-methylene precursor 66. Conformational control over the macrocyclic frame allowed the proper stereochemical course to be dialed into this reduction process. The elaboration of the C16 ketone to the C15-C16 double bond was accomplished by a chemoselective alkenyl triflate formation followed by a palladium-catalyzed hydride delivery. The role of the ketone at C16 as a strategic design element is also evident up-stream of the key intermediate 66, the assembly of which hinged upon the addition of the polyfunctionalized dithiane 37 to the similarly elaborate aldehyde fragment 46. Other crucial steps of the total synthesis were an alkyl-Suzuki coupling and a Yamaguchi lactonization that allowed the Northern and the Southern sector of the target to be stitched together and the macrocyclic perimeter to be forged. The lateral chain comprising the remote C46 stereocenter was finally attached to the core region by a modified Julia-Kocienski olefination. The preparation of the individual building blocks led to some methodological spin-offs, amongst which the improved procedure for the N-O-bond cleavage of isoxazolines by zero-valent molybdenum and the ozonolysis of a double bond in the presence of other oxidation-prone functionality are most noteworthy. Preliminary biological data suggest that the entire carbon framework, that is the macrocyclic core plus the lateral chain, might be necessary for high cytotoxicity.

Bidentate phosphines of heteroarenes: 1,9-bis(diphenylphosphino)-dibenzothiophene and 4,6-bis(diphenylphosphino)dibenzothiophene

Abstract Twofold lithiation of dibenzothiophene with n-butyllithium and N,N,N′,N′-tetramethylethylenediamine (TMEDA) in boiling n-heptane followed by reaction with chlorodiphenylphosphine (Ph 2 PCl) yielded the title compounds 2 (colourless) and 3 (yellow). Both phosphine were characterised by single crystal X-ray structure analysis and 13 C, 1 H shift correlated 2D NMR spectroscopy, based on which our previous assignment of the yellow compound to structure 2 has to be revised.

Concise Total Synthesis of Enigmazole A.

An efficient entry into the phosphorylated marine macrolide enigmazole A is described. Enigmazole A interferes with c-Kit signaling by an as yet unknown mode of action and is therefore a potential lead in the quest for novel anticancer agents. Key to success is a gold-catalyzed cascade comprising a [3,3]-sigmatropic rearrangement of a propargyl acetate along the periphery of a macrocyclic scaffold, followed by a transannular hydroalkoxylation of the resulting transient allenyl acetate. This transformation mandated the use of a chiral gold catalyst to ensure a matching double-asymmetric setting. Other noteworthy steps are the preparation of the oxazole building block by a palladium-catalyzed C-H activation, as well as the smooth ring-closing alkyne metathesis of a diyne substrate bearing a propargylic leaving group, which has only little precedent.

Total Synthesis of Nominal Gobienine A

The lichen-derived glycoconjugate gobienine A is structurally more complex than most glycolipids isolated from higher plants by virtue of the all-cis substituted γ-lactone substructure embedded into its macrocyclic frame. A concise entry into this very epimerization-prone and hence challenging structural motif is presented, which relies on an enantioselective cyanohydrin formation, an intramolecular Blaise reaction, a palladium-catalyzed alkoxycarbonylation, and a diastereoselective hydrogenation of the tetrasubstituted alkene in the resulting butenolide. This strategy, in combination with ring-closing olefin metathesis for the formation of the macrocyclic perimeter, allowed the proposed structure of gobienine A (1) to be formed in high overall yield. The recorded spectral data show that the structure originally attributed to gobienine A is incorrect and that it is not the epimerization-prone ester site on the butanolide ring that is the locus of misassignment; rather, the discrepancy must be more profound.

Unprecedented McMurry Reactions with Acylsilanes: Enedisilane Formation versus Brook Rearrangement

Abstract The first inter- and intramolecular McMurry reactions of aroyltrimethylsilanes to substituted 1,2-bis(trimethylsilyl)ethene derivatives 2a-c and 7 are described. A low-valent titanium reagent prepared by the reduction of TiCl3 with Na on inorganic supports (Al2O3, NaCl, TiO2) turned out to be best suited. Depending on the reaction conditions and on the particular substitution patterns of the substrates, Brook rearrangements of the intermediate 1,2-disilylated titanium-1,2-diolates leading to the formation of C,O-disilyl-enol ethers may compete with the McMurry deoxygenation pathway.

A New Method for the Preparation of Non-Terminal Alkynes: Application to the Total Syntheses of Tulearin A and C

Lactones are known to react with the reagent generated in situ from CCl4 and PPh3 in a Wittig-type fashion to give gem-dichloro-olefin derivatives. Such compounds are now shown to undergo reductive alkylation on treatment with organolithium reagents RLi to furnish acetylene derivatives bearing the substituent R at their termini (R=Me, n-, sec-, tert-alkyl, silyl); the reaction can be catalyzed with either Cu(acac)2 or Fe(acac)3 /1,2-diaminobenzene. Two alkynol derivatives prepared in this way from readily accessible lactone precursors served as the key building blocks for the total syntheses of the cytotoxic marine macrolides tulearin A (1) and C (2). The assembly of these fragile targets hinged upon ring closing alkyne metathesis (RCAM) followed by a formal trans-reduction of the resulting cycloalkynes via trans-hydrosilylation/protodesilylation.

2,4,6-Tri-tert-butyl-1,3,5-triphosphabenzene as an η6-Ligand in Transition Metal Complexes

The η6-1,3,5-triphosphabenzene transition metal complexes 12a,b, 14, and 16 have been synthesized for the first time by the displacement of η6-arene ligands (toluene, benzene and naphthalene) with 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene (9). The new complexes have been fully characterized by a combination of elemental analysis, mass spectrometry, and 1H-, 13C- and 31 P-NMR spectroscopy.