The thiopyran route to polypropionates

Magnesium catalyzed [3 + 3] heteroannulation of α-enolic dithioesters with MBH acetate: access to functionalized 3,4-dihydro-2H-thiopyrans

Magnesium catalysis proved to be efficient towards [3 + 3] chemo- and diastereoselective heteroannulation employing racemic Morita-Baylis-Hillman (MBH) acetate as the C3 unit and α-enolic dithioester as the C2S1 unit, leading to highly substituted 3,4-dihydro-2H-thiopyrans in excellent yields. The compatibility with a wide range of functional groups makes this domino formation of C-C and C-S bonds interesting. DFT analyses for the regioselective formation of the intermediate was performed.

An amine template strategy to construct successive C-C bonds: synthesis of benzo[h]quinolines by a deaminative ring contraction cascade

We developed a convergent strategy to build, cyclize and excise nitrogen from tertiary amines for the synthesis of polyheterocyclic aromatics. Biaryl-linked azepine intermediates can undergo a deaminative ring contraction cascade reaction, excising nitrogen with the formation of an aromatic core. This strategy and deaminative ring contraction reaction are useful for the synthesis of benzo[h]quinolines.

Total Synthesis of Dolabriferol C by a Highly Stereoselective One-Pot Coupling of a meso-3,7-Diketone with Two Chiral Aldehydes

Total synthesis of the noncontiguous polypropionate dolabriferol C was achieved by retro-Claisen fragmentation of its putative contiguous precursor under mild conditions, thus establishing the former as a plausible isolation artifact. The precursor was prepared by a novel one-pot three-component bisaldol coupling of a meso (Z,Z)-bisenolate (generated in situ from a 3,7-diketone) with two enantioenriched aldehydes to set the absolute configuration of seven stereocenters in one step. The first aldol reaction proceeded with enantioselective desymmetrization of the bisenolate to produce an enantiomerically pure enolate-aldolate. Quenching at this stage enabled a streamlined synthesis of dolabriferol. Addition of a racemic aldehyde to the enolate-aldolate resulted in aldol coupling with kinetic resolution of the "matched" aldehyde; overall, a sequential enantiotopic-group-selective (SEGS) bisaldol reaction. Because the desired adduct results from the "mismatched" aldol reaction, use of enantioenriched aldehyde was required.

Expeditious Asymmetric Synthesis of Polypropionates Relying on Sulfur Dioxide-Induced C–C Bond Forming Reactions

For a long time, the organic chemistry of sulfur dioxide (SO2) consisted of sulfinates that react with carbon electrophiles to generate sulfones. With alkenes and other unsaturated compounds, SO2 generates polymeric materials such as polysulfones. More recently, H-ene, sila-ene and hetero-Diels–Alder reactions of SO2 have been realized under conditions that avoid polymer formation. Sultines resulting from the hetero-Diels–Alder reactions of conjugated dienes and SO2 are formed more rapidly than the corresponding more stable sulfolenes resulting from the cheletropic additions. In the presence of a protic or Lewis acid catalyst, the sultines derived from 1-alkoxydienes are ionized into zwitterionic intermediates bearing 1-alkoxyallylic cation moieties which react with electro-rich alkenes such as enol silyl ethers and allylsilanes with high stereoselectivity. (C–C-bond formation through Umpolung induced by SO2). This produces silyl sulfinates that react with carbon electrophiles to give sulfones (one-pot four component asymmetric synthesis of sulfones), or with Cl2, generating the corresponding sulfonamides that can be reacted in situ with primary and secondary amines (one-pot four component asymmetric synthesis of sulfonamides). Alternatively, Pd-catalyzed desulfinylation generates enantiomerically pure polypropionate stereotriads in one-pot operations. The chirons so obtained are flanked by an ethyl ketone moiety on one side and by a prop-1-en-1-yl carboxylate group on the other. They are ready for two-directional chain elongations, realizing expeditious synthesis of long-chain polypropionates and polyketides. The stereotriads have also been converted into simpler polypropionates such as the cyclohexanone moiety of baconipyrone A and B, Kishi’s stereoheptad unit of rifamycin S, Nicolaou’s C1–C11-fragment and Koert’s C16–CI fragment of apoptolidin A. This has also permitted the first total synthesis of (-)-dolabriferol.

Diversity-oriented synthesis of glycomimetics

Glycomimetics are structural mimics of naturally occurring carbohydrates and represent important therapeutic leads in several disease treatments. However, the structural and stereochemical complexity inherent to glycomimetics often challenges medicinal chemistry efforts and is incompatible with diversity-oriented synthesis approaches. Here, we describe a one-pot proline-catalyzed aldehyde α-functionalization/aldol reaction that produces an array of stereochemically well-defined glycomimetic building blocks containing fluoro, chloro, bromo, trifluoromethylthio and azodicarboxylate functional groups. Using density functional theory calculations, we demonstrate both steric and electrostatic interactions play key diastereodiscriminating roles in the dynamic kinetic resolution. The utility of this simple process for generating large and diverse libraries of glycomimetics is demonstrated in the rapid production of iminosugars, nucleoside analogues, carbasugars and carbohydrates from common intermediates.

Enantioselective Pd-Catalyzed Decarboxylative Allylic Alkylation of Thiopyranones. Access to Acyclic, Stereogenic α-Quaternary Ketones

A catalytic, enantioselective decarboxylative allylic alkylation of 4-thiopyranones is reported. The α-quaternary 4-thiopyranones produced are challenging to access by standard enolate alkylation owing to facile ring-opening β-sulfur elimination. In addition, reduction of the carbon-sulfur bonds provides access to elusive acyclic α-quaternary ketones. The alkylated products are obtained in up to 92% yield and 94% enantiomeric excess.

Synthesis of polysubstituted 3,4-dihydro-2H-thiopyrans by regioselective annulation of 3,3-disubstituted allylic alcohols with a β-oxodithioester

Polyfunctionalized 3,4-dihydro-2H-thiopyrans were obtained in high yields via the regioselective annulation of 3,3-disubstituted allylic alcohols with a β-oxodithioester under BF₃·Et₂O catalysis.

A convenient electro-catalyzed multicomponent synthesis of 4H-thiopyran derivatives

A convenient one pot electro-synthesis of 4H-thiopyrans by condensation of aromatic aldehydes, malononitrile, carbon disulfide and primary amines.

Bioinspired iterative synthesis of polyketides

Diverse array of biopolymers and second metabolites (particularly polyketide natural products) has been manufactured in nature through an enzymatic iterative assembly of simple building blocks. Inspired by this strategy, molecules with inherent modularity can be efficiently synthesized by repeated succession of similar reaction sequences. This privileged strategy has been widely adopted in synthetic supramolecular chemistry. Its value also has been reorganized in natural product synthesis. A brief overview of this approach is given with a particular emphasis on the total synthesis of polyol-embedded polyketides, a class of vastly diverse structures and biologically significant natural products. This viewpoint also illustrates the limits of known individual modules in terms of diastereoselectivity and enantioselectivity. More efficient and practical iterative strategies are anticipated to emerge in the future development.

Stereospecific cross-coupling reactions of aryl-substituted tetrahydrofurans, tetrahydropyrans, and lactones

The stereospecific ring-opening of O-heterocycles to provide acyclic alcohols and carboxylic acids with controlled formation of a new C-C bond is reported. These reactions provide new methods for synthesis of acyclic polyketide analogs with complex stereochemical arrays. Stereoselective synthesis of the cyclic template is utilized to control relative configuration; subsequent stereospecific nickel-catalyzed ring-opening affords the acyclic product. Aryl-substituted tetrahydrofurans and tetrahydropyrans undergo nickel-catalyzed Kumada-type coupling with a range of Grignard reagents to furnish acyclic alcohols with high diastereoselectivity. Enantioenriched lactones undergo Negishi-type cross-coupling with dimethylzinc to afford enantioenriched carboxylic acids. Application in a two-step enantioselective synthesis of an anti-dyslipidemia agent is demonstrated.

Programmed synthesis of a contiguous stereotriad motif by triple stereospecific reagent-controlled homologation

All distinct diastereoisomers of a contiguous stereotriad motif were separately targeted by a triple chain extension of B-phenethyl boronic esters using four unique presentation sequences of enantiomorphs of 1-[(2)H]-1-chloro-2-(1,3-dioxolan-2-yl)ethyllithium. The (R)- or (S)-configured chloroalkyllithium reagents were generated by sulfoxide-lithium exchange from the appropriate scalemic p-tolyl chloroalkyl sulfoxides using phenyllithium (THF, -78 °C). Stereotriad synthesis was accomplished in a single reaction vessel [7-19% yield, typical dr ≥ 74 (target):26 (∑ all other isomers)] and implemented by a simple algorithm consisting of reagent charging and temperature cycling events.

Total synthesis of 6-deoxyerythronolide B via C-C bond-forming transfer hydrogenation

The 14-membered macrolide 6-deoxyerythronolide B is prepared in 14 steps (longest linear sequence) and 20 total steps. Two different methods for alcohol CH-crotylation via transfer hydrogenation are deployed for the first time in target-oriented synthesis. Enyne metathesis is used to form the 14-membered ring. The present approach represents the most concise construction of any erythronolide reported, to date.