Heck Coupling Using a Vinyliodo-MIDA Boronate: An Efficient and Modular Access to Polyene Frameworks

Ru(0)-catalysed synthesis of borylated polyene building blocks by cross-dimerisation toward cross-coupling

Conjugated and non-conjugated polyenes are important substructures and are often found in biologically active compounds and natural products. Their preparation often needs multiple steps or iterative reactions and as a result, they have poor step economies. In this feature article, we show a new methodology to prepare these substructures by combinations of cross-dimerisation giving borylated polyenes and subsequent cross-coupling reactions. This divergent reaction strategy allows for the opportunity to access many bioactive compounds and natural products as well as some electronic materials.

Direct synthesis of conjugated tetraenes from 1,3-enynes with 1,3-dienes

New direct access to conjugated tetraenes has been achieved. A Ru(0)-catalysed reaction of 1,3-enynes with 1,3-dienes gives 1,3,5,7-octatetraene derivatives by formal regioselective insertion of the alkynyl group of 1,3-enynes into the terminal C-H bond in 1,3-dienes. With a silyl substituent on the alkynyl side in 1,3-enynes, the reaction regioselectively proceeds to give the linear cross-dimerisation product having the silyl group at the internal position. Stoichiometric and DFT calculations support the oxidative coupling mechanism for the linear cross-dimerisation. Methyl (2E,4E,6E,8E)-10-hydroxy-2,4,6,8-decatetraenoate, a versatile polyene intermediate, is accessed by this method as a formal synthesis of biologically active compounds.

Pd Nanoparticle-Catalyzed Stereospecific Mizoroki-Heck Arylation of cis-1,2-Disilylarylethylenes

In the presence of catalytic amounts of Pd nanoparticles, generated from Pd2dba3/Ag(I), cis-1,2-ditrimethylsilylarylethylenes undergo with aryl iodides a stereospecific Mizoroki-Heck arylation leading to trans-ditrimethylsilyldiarylethylenes. This chemoselectivity is in contrast to that of their trimethylgermyl analogues, which are arylated at the position of the C-Ge bonds. trans-1,2-Ditrimethylsilylarylethylenes are completely unreactive under the standard reaction conditions. The reaction tolerates the presence of boryl, silyl, or bromine substituents on the aryl iodides. From a mechanistic point of view, the process involves syn-arylpalladation followed by syn-dehydropalladation.

anti-Selective Borylstannylation of Alkynes with (o-Phenylenediaminato)borylstannanes by a Radical Mechanism

We have achieved the first anti-borylstannylation of alkynes by using (o-phenylenediaminato)borylstannanes. This reaction afforded 1-boryl-2-stannylalkenes with excellent regio- and stereoselectivity by a radical mechanism. This anti-addition manner is in sharp contrast to the syn-selectivity obtained during transition metal-catalyzed borylstannylation. The mild radical conditions enabled a broad substrate scope, and various types of aromatic and aliphatic alkynes were applicable. The origin of regio- and stereoselectivity was elucidated by DFT calculation of the reaction mechanism. The application of the borylstannylation products to cross- or homocoupling reactions provided ready access to either triarylethenes or bisborylbutadienes.

On the Structure of Thailandene A: Synthetic Examination of the Cryptic Natural Product Aided by a Theoretical Approach

Phenotype-guided transposon mutagenesis has emerged as a valuable tool to access cryptic metabolites encoded in bacterial genomes. Recently, the method was demonstrated by inducing silent biosynthetic gene clusters in Burkholderia thailandensis. Amongst the isolated metabolic products, thailandene A exhibited promising antibiotic activity. By assignment, the linear polyenic aldehyde contained a labile motif, where an ostensible chiral secondary alcohol was interlaced in an allylic and a homoallylic constellation. Our attention was drawn to the pseudo-symmetric relationship between the heterofunctionalities, indicating the transformation of a dodecapentaenedial scaffold. Centering on an iterative cross-coupling protocol, the assigned all-E-(12R)-hydroxydodecapentaenal moiety was assembled by combining Zincke chemistry with the MIDA-attenuated Suzuki reaction developed in the Burke laboratory. Thus, according to the devised strategy, the mixed 1,2-bisborylated vinyl linchpin was consecutively functionalized with 5-bromodienal derivatives in a doubly orthogonal fashion. However, when the synthetic material was matched against the bacterial isolate, inconsistencies were observed. A re-examination of the cryptic natural product was conducted by juxtaposing analytical data from experiment and density functional theory calculations, in which hydroperoxide was evaluated as a candidate metabolite present in the bacterial isolate.

Halohydroxylation of alkenyl MIDA boronates: switchable stereoselectivity induced by B(MIDA) substituent

A switchable stereoselectivity in the halohydroxylation of alkenyl MIDA boronates was found. C–B(MIDA) bond hyperconjugation plays the key role.

Synthesis of fluorinated amphoteric organoborons via iodofluorination of alkynyl and alkenyl MIDA boronates

The iodofluorination of alkynyl and alkenyl MIDA boronates led to the synthesis of fluorinated organoborons bearing a valuable C–I bond.

Pd-Catalyzed Heck-Type Reaction: Synthesizing Highly Diastereoselective and Multiple Aryl-Substituted P-Ligands

In this work, an efficient palladium-catalyzed Heck-type reaction was successfully used to synthesize a wide range of monoaryl- or diaryl-substituted P-ligands with excellent yields and diastereoselectivity (up to 98% yield, dr >20:1). The preliminary mechanistic studies demonstrated that it possibly underwent a cationic Heck reaction that was assisted by silver salt, and it revealed the significant role of P(O)Ar₂ for excellent yields and diastereoselectivity.

Peridinin Is an Exceptionally Potent and Membrane-Embedded Inhibitor of Bilayer Lipid Peroxidation

Antilipoperoxidant protein dysfunction is associated with many human diseases, suggesting that bilayer lipid peroxidation may contribute broadly to pathogenesis. Small molecule inhibitors of this membrane-localized chemistry could in theory enable better understanding and/or treatment of such diseases, but currently available compounds have important limitations. Many biological questions thus remain unanswered, and clinical trials have largely been disappointing. Enabled by efficient, building block-based syntheses of three atypical carotenoid natural products produced by microorganisms that thrive in environments of extreme oxidative stress, we found that peridinin is a potent inhibitor of nonenzymatic bilayer lipid peroxidation in liposomes and in primary human endothelial cells. We also found that peridinin blocks monocyte-endothelial cell adhesion, a key step in atherogenesis. A series of frontier solid-state NMR experiments with a site-specifically 13C-labeled isotopolog synthesized using the same MIDA boronate building block-based total synthesis approach revealed that peridinin is completely embedded within and physically spans the hydrophobic core of POPC membranes, maximizing its effective molarity at the site of the targeted lipid peroxidation reactions. Alternatively, the widely used carotenoid astaxanthin is significantly less potent and was found to primarily localize extramembranously. Peridinin thus represents a promising and biophysically well-characterized starting point for the development of small molecule antilipoperoxidants that serve as more effective biological probes and/or therapeutics.

The Molecular Industrial Revolution: Automated Synthesis of Small Molecules

Today we are poised for a transition from the highly customized crafting of specific molecular targets by hand to the increasingly general and automated assembly of different types of molecules with the push of a button. Creating machines that are capable of making many different types of small molecules on demand, akin to that which has been achieved on the macroscale with 3D printers, is challenging. Yet important progress is being made toward this objective with two complementary approaches: 1) Automation of customized synthesis routes to different targets by machines that enable the use of many reactions and starting materials, and 2) automation of generalized platforms that make many different targets using common coupling chemistry and building blocks. Continued progress in these directions has the potential to shift the bottleneck in molecular innovation from synthesis to imagination, and thereby help drive a new industrial revolution on the molecular scale.

Toward Generalization of Iterative Small Molecule Synthesis

Small molecules have extensive untapped potential to benefit society, but access to this potential is too often restricted by limitations inherent to the customized approach currently used to synthesize this class of chemical matter. In contrast, the "building block approach", i.e., generalized iterative assembly of interchangeable parts, has now proven to be a highly efficient and flexible way to construct things ranging all the way from skyscrapers to macromolecules to artificial intelligence algorithms. The structural redundancy found in many small molecules suggests that they possess a similar capacity for generalized building block-based construction. It is also encouraging that many customized iterative synthesis methods have been developed that improve access to specific classes of small molecules. There has also been substantial recent progress toward the iterative assembly of many different types of small molecules, including complex natural products, pharmaceuticals, biological probes, and materials, using common building blocks and coupling chemistry. Collectively, these advances suggest that a generalized building block approach for small molecule synthesis may be within reach.

Studies toward the synthesis of strevertenes A and G: stereoselective construction of C1–C19segments of the molecules

Efficient route for the stereoselective synthesis of common C₁–C₁₉segment of strevertenes A and G has been developed.

Synthesis of 3-fluoroalkenyl-3-trifluoromethyl-2-oxindoles by the reaction of indoline-2,3-diones with difluoromethylene phosphabetaine

An unprecedent reaction of indoline-2,3-diones and (triphenylphosphonio)difluoroacetate (PDFA) afforded novel 3-fluoroalkenyl-3-trifluoromethyl-2-oxindoles in moderate to excellent yields. These products could be transformed into other trifluoromethylated oxindole derivatives.

Regio- and stereoselective synthesis of conjugated trienes from silaborated 1,3-enynes

Palladium-catalyzed silaboration of 1,3-enynes followed by Suzuki–Miyaura coupling with alkenyl iodides and Hiyama coupling with aryl iodides provides access to 1,3,5-trienes with defined regio- and stereochemistry.