o-DPPB-Directed Copper-Mediated and -Catalyzed Allylic Substitution with Grignard Reagents

Migratory allylic arylation of 1,n-enols enabled by nickel catalysis

Transition-metal-catalyzed allylic substitution reactions (Tsuji-Trost reactions) proceeding via a π-allyl metal intermediate have been demonstrated as a powerful tool in synthetic chemistry. Herein, we disclose an unprecedented π-allyl metal species migration, walking on the carbon chain involving 1,4-hydride shift as confirmed by deuterium labeling experiments. This migratory allylic arylation can be realized under dual catalysis of nickel and lanthanide triflate, a Lewis acid. Olefin migration has been observed to preferentially occur with the substrate of 1,n-enols (n ≥ 3). The robust nature of the allylic substitution strategy is reflected by a broad scope of substrates with the control of regio- and stereoselectivity. DFT studies suggest that π-allyl metal species migration consists of the sequential β-H elimination and migratory insertion, with diene not being allowed to release from the metal center before producing a new π-allyl nickel species.

Asymmetric conjugate addition of alkylzirconium reagents to α,β-unsaturated thioesters: access to fragrances and acyclic stereochemical arrays

An asymmetric iterative route for the synthesis of acyclic stereochemical arrays is obtained by addition to α,β-unsaturated thioesters.

Palladium-catalysed formation of vicinal all-carbon quaternary centres via propargylation

Construction of two vicinal all-carbon quaternary carbon centres is of great importance due to the common presence of such units in natural and unnatural molecules with attractive functions. However, it remains a significant challenge. Here, we have developed a palladium-catalysed general coupling for the efficient connection of two tertiary carbon atoms: Specifically, propargylic carbonate has been treated with a fully loaded soft functionalized nucleophile to connect such two fully loaded carbon atoms with a simple palladium catalyst. It is observed that the central chirality in the optically active tertiary propargylic carbonates has been remembered and transferred into the products with very high efficiency. The triple bond and the functional groups such as ester, cyano and unsaturated C–C bonds make this method a relatively general solution for such a purpose due to their high synthetic versatility.

Formation of vicinal, quaternary stereocentres is challenging, but such products are commonly encountered in nature. Here, the authors report a palladium catalysed process to form vicinal quaternary carbons via propargylation, including in enantiopure form when optically active starting materials are employed.

Copper-free arylation of 3,3-disubstituted allylic halides with triazene-softened aryl Grignard reagents

A copper-free allylic arylation reaction between 3,3-disubstituted allylic halides and triazene-softened aryl Grignard reagents has been developed. This protocol presents a direct and efficient way to construct both α- or γ-isomers with high regioselectivity under environmentally benign conditions. Various functional groups can be tolerated in the reaction and the products are of high value for multiple synthetic applications. The α- and γ-isomers can be converted to the corresponding 3H-indole and indole derivatives in multigram scale respectively.

A general, asymmetric, and noniterative synthesis of trideoxypropionates. Straightforward syntheses of the pheromones (+)-vittatalactone and (+)-norvittatalactone

A novel, highly stereocontrolled, and very flexible synthetic access to biologically relevant trideoxypropionate building blocks in optically pure form has been developed. On the basis of a three-step sequence comprising a thermal oxy-Cope rearrangement, an iridium-catalyzed hydrogenation, and an auxiliary-controlled enolate methylation, trideoxypropionates with easily adjustable relative configuration were synthesized in excellent yields. In addition, the functionalized end groups allow for chemoselective manipulations and further elongation of the chain. The underlying strategy constitutes the first noniterative process for the assembly of polydeoxypropionates and has further been applied in total syntheses of the pheromones (+)-vittatalactone and (+)-norvittatalactone, which had been isolated from the striped cucumber beetle Acalymma vittatum.

Removable directing groups in organic synthesis and catalysis

Directing groups have been widely used in recent years to achieve control over all aspects of reaction selectivity in a wide range of transformations involving transition-metal catalysis and organometallic reagents. In cases when the existing functional group within a substrate is unsuited to achieve efficient intramolecular delivery of a reagent or catalyst, the specific introduction of an appropriately designed removable reagent-directing group can be a solution to this problem. In this Review we give an overview of the state of the art in this area, including the stoichiometric and catalytic use of directing groups.

Iterative strategies for the synthesis of deoxypropionates

This feature article gives an overview of iterative synthetic methods for the construction of deoxypropionates, an important class of polyketides. The catalytic and non-catalytic methodologies discussed are based on highly stereoselective reactions which can be carried out in an iterative fashion. Non-catalytic methods are described first, followed by state of the art catalytic iterative protocols. The application of the iterative methods in the synthesis of natural products is discussed as well.

Total synthesis of (+)-bourgeanic acid utilizing o-DPPB-directed allylic substitution

The lichen metabolite (+)-bourgeanic acid has been synthesized utilizing a new strategy for the construction of propionate motifs relying on the o-DPPB-directed copper-mediated allylic substitution. This synthesis features the o-DPPB-directed allylic substitution employing a chiral Grignard reagent, Sharpless asymmetric epoxidation, and reductive epoxide ring opening with a higher order dimethylcuprate to set the four stereogenic centers of the aliphatic depside.

A unified strategy for the stereospecific construction of propionates and acetate-propionates relying on a directed allylic substitution

Flexible friends: A new strategy that relies on o-DPPB-directed allylic substitution has been implemented for the flexible and stereospecific construction of major polyketide and isoprenoid structural elements (see scheme; o-DPPB=ortho-diphenylphosphanyl benzoate; PG=protecting group).

A New Method for the Synthesis of Chiral β-Branched α-Amino Acids

A new method for the synthesis of chiral beta-branched alpha-amino acids based on a copper-mediated directed allylic substitution reaction with Grignard reagents is reported. This is the first case in which a delta-stereogenic center is controlling the diastereoselectivity of an o-DPPB-directed allylic substitution. Depending on the alkene geometry of the starting material either diastereomer, anti or syn, is accessible with good levels of acyclic stereocontrol.

Stereospecific and stereodivergent construction of tertiary and quaternary carbon centers through switchable directed/nondirected allylic substitution

This study introduces the ortho-diphenylphosphanylbenzoate (o-DPPB)/o-DPPB oxide system as a switchable directing/nondirecting leaving group in a copper-mediated allylic substitution with Grignard and organozinc reagents. With this system, the regioselective, stereospecific, and stereodivergent construction of quaternary as well as tertiary carbon centers is possible in a reliable and predictable fashion. Starting from one substrate enantiomer, both optical antipodes of the substitution products are readily available. Hence, this methodology features reversed polarity in comparison to established enolate alkylation chemistry and may be an interesting alternative, particularly for the construction of quaternary stereogenic carbon centers.

Highly Diastereoselective Preparation of (E)-Alkenylsilanes Bearing an α-Chiral Center

The copper(I)-mediated anti-S(N)2' allylic substitution allows a highly stereoselective preparation of alkenylsilanes bearing a chiral center in the alpha-position with high transfer of chirality. These alkenylsilanes were converted into alpha,beta-unsaturated ketones or into the corresponding boronic esters without loss of the chiral information. [reaction: see text]