Vinyl Grignard Reagents Research Articles

Asymmetric conjugate 1,6-addition of organocuprates to N-dienoyl oxazolidinones

A highly regioselective and diastereoselective conjugate 1,6-addition of organocuprates to N-dienoyl oxazolidinones to provide the 1,6-addition products, is described. These products have a β,γ-unsaturated olefin and a new asymmetric center at C(δ). The 1,6-addition products were obtained as a mixture of geometric isomers (E,Z), where its prevalence depends on reaction conditions and the type of Grignard reagent. This methodology can be carried out with small or voluminous alkyl Grignard reagents, aromatic or vinylic Grignard reagents, unlike other methodologies where there are difficulties with bulky Grignard reagents. It is the first paper describing a competition between geometric isomers (E, Z) into the 1,6- addition products depending on the employed reaction conditions. Chiral oxazolidinone moiety removal furnished chiral δ-substituted β,γ-unsaturated carboxylic acids.

Synthesis of Boronic Esters from Organometallic Reagents and Bis(pinacolato)diboron.

Synthesis of alkyl, aryl, and vinyl boronic esters carrying various chiral and achiral diol-protecting groups were synthesized starting from the corresponding alkyl, aryl, and vinyl lithium or Grignard reagents. Good to excellent yields were obtained for a large range of substrates. The reaction can be conducted in a gram scale to obtain the product over 80 % yield. This approach provides direct access to neopentyl, pinene, and other boronic esters that are difficult to achieve. Using trimethoxyborane or 2-isopropoxy pinacolboronic ester. Detailed mechanistic studies have been conducted to understand the mechanism behind the formation of boronic ester starting from organometallic reagents.

Synthesis of tubuvaline (Tuv) fragment of tubulysin via diastereoselective dihydroxylation of homoallylamine

Tubulysins are natural anticancer molecules that directly bind and inhibit tubulin polymerization in actively dividing cells leading to apoptosis and cell death. Structurally, tubulysins are linear tetrapeptides, constituted by a natural amino acid (Ile) and three non-canonical amino acids (Mep, Tuv, and Tup). Herein, we report a convenient strategy for the practical synthesis of tubuvaline fragment of tubulysin natural products. In this approach, we describe the regioselective ring opening of a chiral aziridine 7 with vinyl Grignard reagent to obtain (R)-tert-butyl(2-methylhex-5-en-3-yl)(tosyl)carbamate 8 which was further subjected to Sharpless asymmetric dihydroxylation with AD-mix-β resulting in the formation of (2 R,4R)-4-((tert-butoxycarbonyl)amino)-2-(methoxymethoxy)-5-methylhexanoic acid 3 that was finally transformed to tubuvaline by heterocyclization.

Synthesis of the Deacetoxytubuvaline Fragment of Pretubulysin and its Lipophilic Analogues for Enhanced Permeability in Cancer Cell Lines

In the last two decades, tubulysins have emerged as alternatives to microtubule depolymerizing agents such as colchicine and vinblastine, which are well-established anticancer agents. However, the complex structure of tubulysins has always posed a challenge for synthetic chemists to scale up the production of these compounds. We report a new strategy for the practical gram-scale synthesis of a (4R)-4-[(tert-butoxycarbonyl)amino]-5-methylhexanoic acid through regioselective cleavage of a chiral aziridine ring with a vinyl Grignard reagent to afford tert-butyl [(1R)-1-isopropylbut-3-en-1-yl]carbamate, which was subjected to regioselective hydroboration–oxidation with 9-BBN. The resulting (4R)-4-[(tert-butoxycarbonyl)amino]-5-methylhexanoic acid was successfully transformed into the deacetoxytubuvaline fragment of pretubulysin or its highly lipophilic methyl-substituted thiazole and oxazole analogues for incorporation into pretubulysins. Increasing the lipophilicity of tubulysin or pretubulysin molecules should enhance their cell permeability and cytotoxicity in cancer cell lines.

Enantioselective vinylation of aldehydes with the vinyl Grignard reagent catalyzed by magnesium complex of chiral BINOLs.

Enantioselective vinylation of aldehydes via direct catalytic asymmetric Grignard reaction of aldehdyes and the vinyl Grinard reagent is a long-standing challenge. This work demonstrated that the magnesium (S)-3,3'-dimethyl BINOLate enantioselectively catalyze the direct vinylation of aldehydes with the deactivated vinylmagnesium bromide by bis(2-[N,N'-dimethylamino]ethyl) ether (BDMAEE) in the addition of n-butylmagnesium chloride. The highest ee of 63% was achieved up to date.

Total Synthesis of (±)-Prostratin

Summary Prostratin, a highly complex diterpene natural product, is a pre-clinical candidate in developing a cure for HIV-1 infection and also a potent and selective inhibitor of several tumors. Despite nearly 40 years of progress, prostratin and related natural products still present formidable challenges for synthetic chemists. Here, we report a total synthesis of (±)-prostratin that proceeds in 23 steps from cyclopentadiene. The synthetic strategy includes rapid assembly of the tricyclic core and highly controlled establishment of the densely functionalized C ring, featuring an array of stereoselective transformations including alkoxide-guided 1,4-additions and cyclopropane formation. This work should enable further explorations of chemical and biological spaces based on prostratin and related natural products.

Applications of γ,δ-Unsaturated Ketones Synthesized by Copper-Catalyzed Cascade Addition of Vinyl Grignard Reagents to Esters.

γ,δ-Unsaturated ketones, so-called homoallylic ketones, have served as versatile building blocks for the synthesis of a variety of heterocycles, carbocycles, natural products, and reactive intermediates. Procured by a variety of processes, including conjugate addition of vinyl organometallic reagents to unsaturated ketones, allylation of silyl enol ethers, and rearrangements, homoallylic ketones are often synthesized by step-intensive methods. The cascade addition of 2 equiv of vinyl Grignard reagent to a carboxylate was reported by the Lubell laboratory in 2003 to give effective access to homoallylic ketones from a variety of aromatic, aliphatic, and α-amino methyl esters. Employing readily accessible vinyl magnesium halides in the presence of a catalytic amount of copper salt, this cascade reaction provides high yields of homoallylic ketones with minimal side product by a process featuring the assembly and collapse of a tetrahedral intermediate with expulsion of alkoxide ion, followed by conjugate addition to the resulting enone. Application of the cascade reaction to the synthesis of various homoallylic ketones has provided versatile building blocks for the synthesis of targets for different applications. For example, by employing (hetero)aryl di- and tricarboxylates as precursors, copper-catalyzed cascade additions have provided donor-acceptor and star-shaped monomers for optical-electronic materials. Amino ester starting materials have given homoallylic ketones for the synthesis of various peptidomimetics, including heteroarylalanines, hydroxyethylene isoesters, and diazepinone turn mimics. Moreover, anthranilate has served as building block to prepare various pyrrole, quinoline, benzodiazepine, and benzotriazepine heterocyles. In addition, cascade additions on hydroxyprolinates have given access to bipyrrole precursors of the prodigiosin family of natural products. In the interest to highlight the utility of the copper-catalyzed cascade addition of vinyl Grignard reagents to carboxylates, this Account provides details on the broad scope of substrates that deliver homoallylic ketone products as well as an overview of the wide range of applications in which this method may impact including materials and peptide science, heterocycle and natural product synthesis, and medicinal chemistry.

Selective Boryl-Anion Migration in a Vinyl sp2 -sp3 Diborane Induced by Soft Borane Lewis Acids.

An intramolecular 1,2‐boryl‐anion migration from boron to carbon has been achieved by selective activation of the π system in [(vinyl)B2Pin2)]− using “soft” BR3 electrophiles (BR3=BPh3 or 9‐aryl‐BBN). The soft character is key to ensure 1,2‐migration proceeds instead of oxygen coordination/B−O activation. The BR3‐induced 1,2‐boryl‐anion migration represents a triple borylation of a vinyl Grignard reagent using only B2Pin2 and BR3 and forms differentially protected 1,1,2‐triborylated alkanes. Notably, by increasing the steric bulk at the β position of the vinyl Grignard reagent used to activate B2Pin2, 1,2‐boryl‐anion migration can be suppressed in favor of intermolecular {BPin}− transfer to BPh3, thus enabling simple access to unsymmetrical sp2−sp3 diboranes.

Selective Boryl‐Anion Migration in a Vinyl sp2−sp3 Diborane Induced by Soft Borane Lewis Acids

AbstractAn intramolecular 1,2‐boryl‐anion migration from boron to carbon has been achieved by selective activation of the π system in [(vinyl)B2Pin2)]− using “soft” BR3 electrophiles (BR3=BPh3 or 9‐aryl‐BBN). The soft character is key to ensure 1,2‐migration proceeds instead of oxygen coordination/B−O activation. The BR3‐induced 1,2‐boryl‐anion migration represents a triple borylation of a vinyl Grignard reagent using only B2Pin2 and BR3 and forms differentially protected 1,1,2‐triborylated alkanes. Notably, by increasing the steric bulk at the β position of the vinyl Grignard reagent used to activate B2Pin2, 1,2‐boryl‐anion migration can be suppressed in favor of intermolecular {BPin}− transfer to BPh3, thus enabling simple access to unsymmetrical sp2−sp3 diboranes.

Regioselectivity in Reactions between Bis(2-benzothiazolyl)ketone and Vinyl Grignard Reagents: C- versus O-alkylation-Part III.

The reaction between bis(2-benzothiazolyl)ketone and vinyl Grignard reagents bearing different substituents on the vinyl moiety gave the product derived from attack on the carbonylic carbon- and/or oxygen-atom. The regioselectivity of the attack depends on the kind of substituents bound to the vinylic carbon atoms and on their relative position. The reaction between vinylmagnesium bromide and 2-methyl-1-propenylmagnesium bromide was carried out under different experimental conditions and in the presence of radical scavengers. The results indicate a plausible mechanistic pathway involving radical intermediates in the case of O-alkylation, but a polar ones in the case of classic C-alkylation. This agrees with our previous reports indicating a key role played by the delocalization ability of the substituents bound to the carbonyl group in driving the regioselectivity of the vinylmagnesium bromide attack towards O-alkylation. Further support of this was obtained by diffractometric analysis of four distinct bis(heteroaryl)ketones.

Sequential Addition Reaction of Sulfanylmethyllithiums and Grignard Reagents to Thioformamides Leading to the Formation of 2-Phenyl-2-sulfanylethyl Tertiary Amines.

The reaction of sulfanylmethyllithiums generated from benzylsulfanes and n-BuLi with N,N-dimethylthioformamide followed by the addition of Grignard reagents gave 2-phenyl-2-sulfanyl tertiary amines in moderate to good yields. A range of Grignard reagents involving primary alkyl, aryl, vinyl, and alkynyl Grignard reagents were used. Two carbon-carbon bond-forming reactions were achieved through a one-pot reaction. The reaction showed good to high diastereoselectivity, particularly with alkynyl Grignard reagents.

Further studies on sultones derived from carbene cyclization cycloaddition cascades

A sulfonyl-stabilized diazoketone underwent an efficient rhodium-catalyzed cascade reaction with an internal vinylsulfonate unit to give a tricyclic sultone as a single diastereomer. Nucleophilic addition of a vinyl Grignard reagent to the ketone of the resultant arylsulfonyl-substituted cycloadduct was stereocomplementary with respect to vinyl Grignard addition to corresponding ester-substituted substrates. The latter tricyclic compounds were chemoselectively transformed into vinylsultones via β-elimination in good yield.

Iron catalysed cross-couplings of azetidines - application to the formal synthesis of a pharmacologically active molecule.

A protocol for the coupling of 3-iodoazetidines with Grignard reagents in the presence of an iron catalyst has been developed. A variety of aryl, heteroaryl, vinyl and alkyl Grignards were shown to participate in the coupling process to give the products in good to excellent yields. Furthermore, a short formal synthesis towards a pharmacologically active molecule was shown.

ChemInform Abstract: A Novel One‐Pot Synthesis of Carbazole‐1,4‐quinones Through Pd‐Catalyzed Cyclocarbonylation, Desilylation and Oxidation Processes from 3‐Iodo‐2‐propenylindoles.

Abstract3‐Iodo‐2‐propenylindoles (I) are easily prepared from corresponding indole‐2‐carbaldehydes and vinyl Grignard reagents.

Allylic Functionalization of Unactivated Olefins with Grignard Reagents

New advances in the functionalization of unactivated olefins with carbon nucleophiles have provided more efficient and practical approaches to convert inexpensive starting materials into valuable products. Recent examples have been reported with stabilized carbon nucleophiles, tethered carbon nucleophiles, diazoesters, and trifluoromethane donors. A general method for functionalizing olefins with aromatic, aliphatic, and vinyl Grignard reagents was developed. In a one-pot process, olefins are oxidized by a commercially available reagent to allylic electrophiles, which undergo selective copper-catalyzed allylic alkylation with Grignard reagents. Products are formed in high yield and with high regioselectivity. This was utilized to synthesize a series of skipped dienes, a class of compounds that are prevalent in natural products and are difficult to synthesize by known allylic alkylation methods.

Allylic Functionalization of Unactivated Olefins with Grignard Reagents

AbstractNew advances in the functionalization of unactivated olefins with carbon nucleophiles have provided more efficient and practical approaches to convert inexpensive starting materials into valuable products. Recent examples have been reported with stabilized carbon nucleophiles, tethered carbon nucleophiles, diazoesters, and trifluoromethane donors. A general method for functionalizing olefins with aromatic, aliphatic, and vinyl Grignard reagents was developed. In a one‐pot process, olefins are oxidized by a commercially available reagent to allylic electrophiles, which undergo selective copper‐catalyzed allylic alkylation with Grignard reagents. Products are formed in high yield and with high regioselectivity. This was utilized to synthesize a series of skipped dienes, a class of compounds that are prevalent in natural products and are difficult to synthesize by known allylic alkylation methods.

Iron-catalyzed cross-coupling between C-bromo mannopyranoside derivatives and a vinyl Grignard reagent: toward the synthesis of the C31–C52 fragment of amphidinol 3

A chemo- and diastereoselective iron-catalyzed cross-coupling between C-bromo mannopyranoside derivatives and 2-methyl-1-propenylmagnesium bromide was developed. This method was used as the key step for the synthesis of the mirror image of the C31–C40 and C43–C52 fragments of amphidinol 3 (AM3). These syntheses were achieved from a common trans-tetrahydropyran derived from d-mannose.

Improved methods for the stereoselective synthesis of mannoheptosyl donors and their glycosides: toward the synthesis of the trisaccharide repeating unit of the Campylobacter jejuni RM1221 capsular polysaccharide

In view of the importance of 6-deoxymannoheptosides as structural units in the Campylobacter jejuni RM1221 capsular polysaccharide, the development of effective synthetic protocols for 4-O-6-S-α-cyanobenzylidene thio-d-mannoheptapyranoside donors carrying either 3-O-naphthylmethyl or 3-O-acetyl groups is described starting from d-mannose. In particular, tris(phenylthio)methyllithium is found to undergo highly stereoselective addition to a mannose-6-aldehyde in sharp contrast to the vinyl Grignard reagent whose reactions were essentially devoid of selectivity. A brief survey of coupling reactions with the two donors indicted the 3-O-acetyl system to be highly α-selective whereas the 3-O-naphthylmethyl congener was highly β-selective indicating that the presence of the seventh carbon atom in these donors is not detrimental to coupling selectivity in either instance.

ChemInform Abstract: Diastereoselective Metal‐Catalyzed Synthesis of C‐Aryl and C‐Vinyl Glycosides.

AbstractCobalt‐catalyzed cross‐coupling of aryl and vinyl Grignard reagents with mannose (I) and galactose (X) is highly diastereoselective to give only the α‐products.

Chemo and diastereoselective conjugate addition of Grignard reagents on parthenin, a bioactive natural sesquiterpene lactone

The conjugate addition on a natural bioactive pseudoguaianolide sesquiterpene lactone parthenin with various Grignard reagents leads to 13-C alkylated products chemo and diastereoselectively in fair overall yields. Compared with other Grignard reagents vinyl and smaller straight chain alkyl Grignard reagents gave better yields.