Metathesis reactions in total synthesis

Recovery of enlarged olefin metathesis catalysts by nanofiltration in an eco-friendly solvent

This study was aimed at integrating a green separation process without phase change, namely nanofiltration, with olefin metathesis to recover the homogeneous catalyst. As the commercially available Hoveyda II catalyst was not sufficiently retained by the membrane, a set of homogeneous ruthenium-based catalysts were prepared to enhance the recovery of the catalyst by solvent-resistant commercial membranes made of polyimide (Starmem 228). The molecular weights of the catalysts were gradually increased from 627 to 2195 g mol(-1), and recovery was found to increase from around 70 % to 90 % both in toluene and dimethyl carbonate. The most retained catalyst was then engaged in a series of model ring-closing metathesis reactions associated to a final nanofiltration step to recover and recycle the catalyst. Up to five cycles could be performed before a deterioration in the performance of the process was observed.

Total synthesis of complex heterocyclic natural products

Total synthesis campaigns toward complex heterocyclic natural products are a prime source of inspiration for the design and execution of complex cascade sequences, powerful reactions, and efficient synthetic strategies. We highlight selected examples of such innovations in the course of our total syntheses of diazonamide A, azaspiracid-1, thiostrepton, 2,2'-epi-cytoskyrin A and rugulosin, abyssomycin C, platensimycin, and uncialamycin.

Synthesis and structural reassignment of (+)-epicalyxin F

We have established the structure of (+)-epicalyxin F through chemical synthesis. An acid-promoted rearrangement of synthetic benzopyran 6 led to the identification of the natural product as (3S,5S,7R)-epicalyxin F (22). Comparison with NMR spectra and optical rotation of the natural product confirms our assignment, and the reassigned structure is compatible with the proposed biosynthetic pathway.

Total Synthesis of Myxovirescin A1

A convergent total synthesis of the antibiotic macrolide myxovirescin A1 (1) is described that is largely based on reagent- and catalyst-controlled transformations. This includes a highly regioselective Negishi reaction of dibromo-alkene 48 with an alkynylzinc reagent, and a palladium catalyzed alkyl-Suzuki coupling of the resulting enyne derivative 12 with the 9-BBN-adduct derived from alkene 61. The latter was obtained via an asymmetric hydrogenation of the chlorinated beta-ketoester 49 and an anti-selective oxyallylation of the functionalized aldehyde 53 as the key steps. The preparation of the bis-borylated allyl-donor 57 used in the oxyallylation step, however, required careful optimization and led to important insights into the nature of the classical hydroborating agent "di(isopinocampheyl)borane (Ipc2BH)". It was unambiguously shown by X-ray crystallography that in the solid state this compound is dimeric, but it is prone to undergo an essentially quantitative mono-deborylation when dissolved in CH2Cl2 or benzene; its composition in ethereal solvents is even more complex as evident from 11B NMR data. Product 71 derived from 12 and 61 was elaborated into the enyne-yne derivative 75, which served as the substrate for an exquisitely selective ring closing alkyne metathesis reaction (RCAM) catalyzed by the molybdenum tris-amido complex 20 activated in situ with CH2Cl2. The resulting cyclic enyne 76 was subjected to a ruthenium catalyzed trans-hydrosilylation/proto-desilylation tandem. Although [Cp*Ru(MeCN)3]PF6 had previously been recommended as catalyst of choice for trans-hydrosilylation reactions of internal alkynes, this complex failed to afford the desired product, whereas its sterically less hindered congener [CpRu(MeCN)3]PF6 permitted the reaction to be performed in appreciable yield, but at the expense of a lower stereoselectivity. AgF-mediated proto-desilylation of the isomeric silanes 79 and 80 followed by cleavage of the remaining acetal protecting groups afforded myxovirescin A1 and its hitherto unknown 14Z-isomer 81, respectively.

Olefin metathesis in homogeneous aqueous media catalyzed by conventional ruthenium catalysts

Olefin metathesis in aqueous solvents is sought for applications in green chemistry and with the hydrophilic substrates of chemical biology, such as proteins and polysaccharides. Most demonstrations of metathesis in water, however, utilize exotic complexes. We have examined the performance of conventional catalysts in homogeneous water/organic mixtures, finding that the second-generation Hoveyda-Grubbs catalyst has extraordinary efficiency in aqueous dimethoxyethane and aqueous acetone. High (71-95%) conversions are achieved for ring-closing and cross metathesis of a variety of substrates in these solvent systems.

N-Heterocyclic Carbene and Phosphine Ruthenium Indenylidene Precatalysts: A Comparative Study in Olefin Metathesis

Kinetic studies on ring-closing metathesis of unhindered and hindered substrates using phosphine and N-heterocyclic carbene (NHC)-containing ruthenium-indenylidene complexes (first and second generation precatalysts, respectively) have been carried out. These studies reveal an appealing difference, between the phosphine and NHC-containing catalysts, associated with a distinctive rate-determining step in the reaction mechanism. These catalysts have been compared with the benzylidene generation catalysts and their respective representative substrates. Finally, the reaction scope of the two most interesting precatalysts, complexes that contain tricyclohexylphosphine and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (SIMes), has been investigated for the ring-closing and enyne metathesis for a large range of olefins. Owing to their high thermal stability, the SIMes-based indenylidene complexes were more efficient than their benzylidene analogues in the ring-closing metathesis of tetrasubstituted dienes. Importantly, none of the indenylidene precatalysts were found to be the most efficient for all of the substrates, indeed, a complementary complex-to-substrate activity relationship was observed.

Ring-Rearrangement Metathesis

Ring-rearrangement metathesis (RRM) refers to the combination of several metathesis transformations into a domino process, in which an endocyclic double bond of a cycloolefin reacts with an exocyclic alkene. RRM has proven to be a powerful method for the rapid construction of complex structures. The extension of the basic ring-opening-ring-closing metathesis process by further metathesis steps as well as an examination of the driving forces, limits, scope, recent advantages, and future perspectives of these domino sequences is presented with various examples, thus reflecting the high efficiency and utility of RRM in organic synthesis.

Efficient preparation of chiral non-racemic sulfur compounds

p-Menthane-3-carboxaldehyde serves as an efficient chiral auxiliary in the preparation of chiral non-racemic S-alkylthiocarbamates or S-dithiocarbonates via the 3,3-sigmatropic rearrangement of the corresponding alkylthionocarbamates or xanthates. The transfer of chirality during rearrangement is complete, and the final products possess a chiral tertiary or quaternary carbon bearing sulfur. The rearranged products are then transformed into enantiopure cyclic or acyclic sulfur-containing products, depending on the means of clivage of the auxiliary. The synthesis of a potent MMP-13 inhibitor is presented.Key words: 3,3-sigmatropic rearrangement, p-menthane-3-carboxaldehyde, S-alkylthiocarbamate, S-dithiocarbonate, alkylthionocarbamate, xanthate, ring-closing metathesis.

Pestalotiopsin A. Enantioselective Construction of Potential Building Blocks Derived from Antipodal Cyclobutanol Intermediates

D-Glyceraldehyde acetonide has been used as the starting point for accessing the enantiomeric cyclobutanols 11 in optically pure condition. The dextrorotatory enantiomer has been transformed in five steps into the [3.2.0] bicyclic lactone 22. While the deoxygenation of 22 proved to be problematical, the uncyclized variant 25 underwent the Barton process smoothly. These findings guided the related conversion of (-)-11 into 34. Use was also made of ring-closing metathesis to bring about the conversion of (+)-11 into [4.2.0] bicyclic lactone building blocks. In general, all three pathways are efficient and offer the prospect of practical side-chain appendage for the purpose of installing the nine-membered ring of pestalotiopsin A (1).

S-adenosylhomocysteine analogues with the carbon-5' and sulfur atoms replaced by a vinyl unit

Cross-metathesis of suitably protected 5'-deoxy-5'-methyleneadenosines with racemic and chiral N-Boc-protected six-carbon amino acids bearing a terminal double bond in the presence of the Hoveyda-Grubbs catalyst gave adenosylhomocysteine analogues with the C5'-C6' double bond. Bromination with pyridinium tribromide and dehydrobromination with DBU followed by standard deprotections yielded the 5'-(bromo)vinyl analogue. [structure: see text]

Practical total syntheses of epiquinamide enantiomers

Short and practical syntheses of epiquinamide and its enantiomer were accomplished with high overall yields and high stereoselectivity from readily available starting materials.

Highly recoverable pyridinium-tagged Hoveyda-Grubbs pre-catalyst for olefin metathesis. Design of the boomerang ligand toward the optimal compromise between activity and reusability

Whereas the boomerang ligand of Hoveyda-Grubbs pre-catalysts can be modified by attachment of a pyridinium tag to its benzylidene moiety, a precise adjustment of the length of the spacer allows the optimum balance to be reached between the activity of the catalyst and its recoverability, exceeding 98% after 6 catalytic runs in the best case.

Total Synthesis of Iejimalide A−D and Assessment of the Remarkable Actin-Depolymerizing Capacity of These Polyene Macrolides

A concise and convergent total synthesis of the highly cytotoxic marine natural products iejimalide A-D (1-4) is reported, which relies on an effective ring-closing metathesis (RCM) reaction of a cyclization precursor containing no less than 10 double bonds. Because of the exceptional sensitivity of this polyunsaturated intermediate and its immediate precursors toward acid, base, and even gentle warming, the assembly process hinged upon the judicious choice of protecting groups and the careful optimization of all individual transformations. As a consequence, particularly mild protocols for Stille as well as Suzuki reactions of elaborate coupling partners have been developed that hold considerable promise for applications in other complex settings. Moreover, a series of non-natural "iejimalide-like" compounds has been prepared, differing from the natural lead in the polar head groups linked to the macrolide's N-terminus. With the aid of these compounds it was possible to uncover the hitherto unknown effect of iejimalide and analogues on the actin cytoskeleton. Their capacity to depolymerize this microfilament network rivals that of the latrunculins which constitute the standard in the field. Structural modifications of the peptidic terminus in 2 are thereby well accommodated, without compromising the biological effects. The iejimalides hence constitute an important new class of probe molecules for chemical biology in addition to their role as promising lead structures for the development of novel anticancer agents.

Modern synthetic efforts toward biologically active terpenes

Terpenes represent one of the largest and most diverse classes of secondary metabolites, with over 55,000 members isolated to date. The terpene cyclase enzymes used in nature convert simple, linear hydrocarbon phosphates into an exotic array of chiral, carbocyclic skeletons. Further oxidation and rearrangement results in an almost endless number of conceivable structures. The enormous structural diversity presented by this class of natural products ensures a broad range of biological properties-ranging from anti-cancer and anti-malarial activities to tumor promotion and ion-channel binding. The marked structural differences of terpenes also largely thwart the development of any truly general strategies for their synthetic construction. This review focuses on synthetic strategies directed toward some of the most complex, biologically relevant terpenes prepared by total synthesis within the past decade. Of crucial importance are both the obstacles that modern synthetic chemists must confront when trying to construct such natural products and the key chemical transformations and strategies that have been developed to meet these challenges.

Dienyne Ring-Closing Metathesis Approach for the Construction of Taxosteroids

A cascade dienyne ring-closing metathesis approach has been applied to the synthesis of the tetracyclic carbon framework of a new class of hybrid compounds-the taxosteroids-possessing carbon frameworks incorporating moieties characteristic of both taxanes (such as AB rings) and steroids (i.e., CD system and side chain). This tandem cyclization is highly stereoselective, allowing the one-step formation of the bicyclo[5.3.1]undecene system characteristic of taxol. In this work we describe the scope and limitations of such cyclizations.

An Efficient Synthesis of Nitroalkenes by Alkene Cross Metathesis: Facile Access to Small Ring Systems

A synthesis of highly functionalized nitroalkenes is reported that utilizes a cross metathesis (CM) reaction between simple aliphatic nitro compounds and a range of substituted alkenes. This chemistry offers a simple and attractive route to nitroalkenes that would otherwise be difficult to prepare, and that have a very useful application as precursors to a variety of heterocyclic entities.

Total Synthesis of Amphidinolide E and Amphidinolide E Stereoisomers

Four amphidinolide E stereoisomers, amphidinolide E (1), 2-epi-amphidinolide E (2), 19-epi-amphidinolide E (3), and 2-epi-19-epi-amphidinolide E (4), have been synthesized via the judicious union of aldehyde 5, allylsilanes 7 or 8, acids 9 or 10, and vinylstannane 6. The C19 stereocenters of the C19 epimeric allylsilanes 7 and 8 were introduced via crotylboration reactions early in the synthesis. [3+2]-Annulation reactions of aldehyde 5 with allylsilanes 7 and 8 were employed to set the core tetrahydrofuran units of 1-4. Finally, the C2 stereocenter was installed by esterification using acid 9, without incident, or with acid 10, in which case an unexpected and completely stereoselective inversion of C2 occurs.

Domino Metathesis Involving ROM-RCM of Substituted Norbornenes. Rapid Access to Densely Functionalized Tricyclic Bridged and Condensed Ring Systems

Domino metathesis involving ROM-RCM of appropriately constructed norbornene derivatives having multiple alkene chains leads to direct access of highly functionalized bridged tricyclic compounds while that of a compound having two norbornene units tethered through one carbon produces a linearly arrayed condensed tricyclic system.