Iron- and indium-catalyzed reactions toward nitrogen- and oxygen-containing saturated heterocycles.

A myriad of natural and/or biologically active products include nitrogen- and oxygen-containing saturated heterocycles, which are thus considered as attractive scaffolds in the drug discovery process. As a consequence, a wide range of reactions has been developed for the construction of these frameworks, much effort being specially devoted to the formation of substituted tetrahydropyrans and piperidines. Among the existing methods to form these heterocycles, the metal-catalyzed heterocyclization of amino- or hydroxy-allylic alcohol derivatives has emerged as a powerful and stereoselective strategy that is particularly interesting in terms of both atom-economy and ecocompatibility. For a long time, palladium catalysts have widely dominated this area either in Tsuji-Trost reactions [Pd(0)] or in an electrophilic activation process [Pd(II)]. More recently, gold-catalyzed formation of saturated N- and O-heterocycles has received growing attention because it generally exhibits high efficiency and diastereoselectivity. Despite their demonstrated utility, Pd- and Au-complexes suffer from high costs, toxicity, and limited natural abundance, which can be barriers to their widespread use in industrial processes. Thus, the replacement of precious metals with less expensive and more environmentally benign catalysts has become a challenging issue for organic chemists. In 2010, our group took advantage of the ability of the low-toxicity and inexpensive FeCl3 in activating allylic or benzylic alcohols to develop iron-catalyzed N- and O-heterocylizations. We first focused on N-heterocycles, and a variety of 2,6-disubstituted piperidines as well as pyrrolidines were synthesized in a highly diastereoselective fashion in favor of the cis-compounds. The reaction was further extended to the construction of substituted tetrahydropyrans. Besides triggering the formation of heterocycles, the iron salts were shown to induce a thermodynamic epimerization, which is the key to reach the high diastereoselectivities observed in favor of the most stable cis-isomers. It is worth noting that spiroketals could be prepared by using this method, which was successfully applied to a synthetic approach toward natural products belonging to the bistramide family. We then turned our attention to heterocycles incorporating two heteroatoms such as isoxazolidines. These frameworks can be found in biologically active natural products, and in addition, they can be transformed into 1,3-amino alcohols, which are of importance in organic chemistry. The use of FeCl3·6H2O allowed the access to a large variety of 3,5-disubstituted isoxazolidines from δ-hydroxylamino allylic alcohol derivatives with good yields and diastereoselectivities in favor of the cis-isomer. Recently, a Lewis acid-catalyzed synthesis of six- and five-membered ring carbonates starting from linear tert-butyl carbonates was reported. In some cases, the mild and chemoselective InCl3 was preferred over FeCl3·6H2O to avoid side-product formation. The resulting cyclic carbonates were easily transformed into 1,3- or 1,2-diols, and a total synthesis of (3S,5S)-alpinikatin was achieved.

Isolation, structural determination and synthetic approaches toward amphidinol 3.

This review highlights the isolation and the structural determination of amphidinol 3 (AM3), as well as the synthetic efforts to its preparation. The mechanism of action of AM3 will not be developed herein.

FeCl3·6H2O, a catalyst for the diastereoselective synthesis of cis-isoxazolidines from N-protected δ-hydroxylamino allylic acetates.

An ecofriendly and diastereoselective synthesis of cis-3,5-disubstituted isoxazolidines through the FeCl3·6H2O-catalyzed cyclization of δ-hydroxylamino allylic acetates is described. The synthetic potential of these products is highlighted by the preparation of several functionalized 1,3-amino alcohol precursors.

Cobalt-catalyzed diastereoselective synthesis of C-furanosides. Total synthesis of (-)-isoaltholactone.

An array of C-aryl and C-vinyl furanosides were prepared in good yields and diastereoselectivities from C-halogeno furanosides either with aryl Grignard or with vinyl Grignard using the convenient Co(acac)3/TMEDA catalytic system. This method is illustrated by the total synthesis of the (-)-isoaltholactone.

Diastereoselective synthesis of the C14­C29 fragment of amphidinol.

An efficient stereoselective synthesis of the C14­C29 fragment highlighting a coupling reaction between a 1,3-dithiane derivative and an α-branched aldehyde was realized. This highly convergent synthesis involved two chiral pools, L-malic acid and (+)-camphorsulfonic acid, which are the starting compounds to control the six stereogenic centers present in the C14­C29 fragment of amphidinol 3.

Formal synthesis of dictyostatin and synthesis of two dictyostatin analogues.

A formal convergent synthesis of dictyostatin from (R)-Roche ester is described. Synthetic highlights include a Ni-catalyzed Nozaki-Hiyama-Kishi coupling between an aldehyde and a Z vinyl iodide to assemble the two main fragments, a diastereoselective Myers alkylation, a stereoselective Evans aldolization, two asymmetric Duthaler crotyltitanations, and a stereoselective Pd-catalyzed Marshall allenylindium addition to install the stereogenic centers of dictyostatin. The synthesis of (9R)-epi-dictyostatin and a new ring-contracted dictyostatin isomer were also achieved.

Diastereoselective synthesis of the C17-C30 fragment of amphidinol 3.

The diastereoselective synthesis of the C17-C30 fragment of amphidinol 3 (AM3) 1 was achieved from the enantio-enriched aldehyde 20, Weinreb amide 14 and 2-bromo-3-(trimethylsilyl)propene, which was used as a bifunctional conjunctive reagent. The absolute configuration of the stereogenic centers, in both aldehyde 20 and Weinreb amide 14, were efficiently controlled by using (+)-(R)-methyl-p-tolylsulfoxide as the unique source of chirality.

Diastereoselective metal-catalyzed synthesis of C-aryl and C-vinyl glycosides.

Cobalt, the catalyst of choice: The diastereoselective cobalt-catalyzed cross-coupling of 1-bromo glycosides and aryl or vinyl Grignard reagents is described. A convenient and inexpensive catalyst, [Co(acac)(3)]/tmeda (acac = acetylacetonate, tmeda = N,N'-tetramethylethylenediamine), gives full α selectivity in the mannose and galactose series, and an α selectivity in the glucose series with α/ß ratios of 1.3:1-3:1.

Synthetic efforts toward the spiroketal core of spirangien A.

Synthetic studies toward the spiroketal core of spirangien A are described. Two synthetic approaches were developed. Both of them use a diastereoselective aldol addition of a lithium enolate derived from a methyl ketone on an aldehyde. In the first approach, the introduction of the (E)-trisubstituted terminal olefin was achieved by using an iron-catalyzed cross-coupling between an alkyl iodide and a vinyl Grignard reagent and a randomly protected spiroketal was obtained. In the second approach, a highly functionalized spiroketal carbamate, which includes 13 stereogenic centers, was successfully isolated.

Formal chemoselective synthesis of leucascandrolide A.

A chemoselective synthesis of the macrocyclic core of leucascandrolide A has been achieved, utilizing highly enantioselective allylmetalations, an enantioselective Noyori reduction of a propargylic ketone and olefin metatheses as the key steps.

Total synthesis of (-)-spongidepsin.

[structure: see text] A convergent and rapid stereoselective synthesis of (-)-spongidepsin has been achieved from the Roche ester in 14 steps with an overall yield of 13%.

Synthetic Studies toward the C14-C29 Fragment of Mirabalin.

A convergent synthesis of one isomer of the C14-C29 fragment of mirabalin is disclosed. The key steps include a Marshall allenylation, a Mukaiyama aldol reaction and a Crimmins aldolization, which allow the control of 10 out of 25 stereogenic centers present in the molecule.

Lewis basicity modulation of N-heterocycles: a key for successful cross-metathesis.

Cross-metathesis involving N-heteroaromatic olefinic derivatives is disclosed. The introduction of an appropriate substituent on the heteroaromatic ring decreases the Lewis basicity of the nitrogen atom, thus preventing the deactivation of the ruthenium-centered catalyst. The reaction is quite general in terms of both N-heterocycles and olefinic partners.

Iron- and cobalt-catalyzed arylation of azetidines, pyrrolidines, and piperidines with Grignard reagents.

Iron- and cobalt-catalyzed cross-couplings between iodo-azetidines, -pyrrolidines, -piperidines, and Grignard reagents are disclosed. The reaction is efficient, cheap, chemoselective and tolerates a large variety of (hetero)aryl Grignard reagents.

Synthetic strategy toward the C44-C65 fragment of mirabalin.

A convergent and flexible stereoselective synthesis of one isomer of the C44-C65 fragment of mirabalin is described. The key steps include organocatalytic aldolization, ruthenium-catalyzed asymmetric hydrogenation, amide formation, Marshall stereoselective allenylation, and the Nozaki-Hiyama-Kishi reaction.

Silica gel-mediated rearrangement of allylic acetates. Application to the synthesis of 1,3-enynes.

Silica gel was found to efficiently promote the rearrangement of allylic acetates into their most stable regioisomers under microwave irradiation. The reaction is easy to perform and eco-friendly. This method was applied to the metal-free synthesis of 1,3-enynes.

FeCl(3)-Catalyzed highly diastereoselective synthesis of substituted piperidines and tetrahydropyrans.

The eco-friendly and highly diastereoselective synthesis of substituted cis-2,6-piperidines and cis-2,6-tetrahydropyrans is described. The key step of this method is the iron-catalyzed thermodynamic equilibration of 2-alkenyl 6-substituted piperidines and 2-alkenyl 6-substituted tetrahydropyrans allowing the isolation of enriched mixtures of the most stable cis-isomers.

Synthetic efforts toward the macrolactone core of Leucascandrolide A.

A chemoselective synthesis of 1, the macrocyclic core of leucascandrolide A, has been achieved by utilizing highly enantioselective allylmetalations, an enantioselective Noyori reduction of a propargylic ketone, and olefin metatheses as the key steps.