Acetylene as a Dicarbene Equivalent for Gold(I) Catalysis: Total Synthesis of Waitziacuminone in One Step

A Gold(I)-Acetylene Complex Synthesised using Single-Crystal Reactivity

Using single-crystal to single-crystal solid/gas reactivity the gold(I) acetylene complex [Au(L1)(η2-HC≡CH)][BArF 4] is cleanly synthesized by addition of acetylene gas to single crystals of [Au(L1)(CO)][BArF 4] [L1=tris-2-(4,4'-di-tert-butylbiphenyl)phosphine, ArF=3,5-(CF3)2C6H3]. This simplest gold-alkyne complex has been characterized by single crystal X-ray diffraction, solution and solid-state NMR spectroscopy and periodic DFT. Bonding of HC≡CH with [Au(L1)]+ comprises both σ-donation and π-backdonation with additional dispersion interactions within the cavity-shaped phosphine.

Ligand Control of Copper-Mediated Cycloadditions of Acetylene to Azides: Chemo- and Regio-Selective Formation of Deutero- and Iodo-Substituted 1,2,3-Triazoles

The participation of σ-monocopper and σ-bis-copper acetylide in mechanistic pathways for copper-catalyzed cycloaddition (CuAAC) reactions of acetylene with azides was probed by analysis of deuterium distributions in the 1,2,3-triazole product formed by deuterolysis of initially formed mono- and bis-copper triazoles. The results show that, when Cu(Phen)(PPh3)2NO3 is used as the catalyst for reactions of acetylene with azides in DMF/D2O, 1-substituted-5-deutero-1,2,3-triazoles are generated selectively. This finding demonstrates that the Cu(Phen)(PPh3)2NO3-catalyzed cycloadditions utilize monocopper acetylide as the substrate and produce 5-copper-1,2,3-triazoles initially. Conversely, when DBU or Et3N is the copper ligand, the process takes place through initial formation and cycloaddition of bis-copper acetylide to produce 4,5-bis-copper-triazole, which reacts with D2O to form the corresponding 4,5-bis-deutero-triazole. Moreover, when C2D2 is used as the substrate, Cu(Phen)(PPh3)2NO3 as the Cu ligand, and H2O/DMF as the solvent, mono-C4-deutreo 1,2,3-triazoles are generated in high yields and excellent levels of regioselectivity. Lastly, CuAAC reactions of acetylene with azides, promoted by CuCl2·2H2O and NaI, yield 4,5-diiodo-1,2,3-triazoles with moderate to high efficiencies.

Gold(I)-Catalyzed Intermolecular Aryloxyvinylation with Acetylene Gas

Acetylene gas is an important feedstock for chemical production, although it is underutilized in organic synthesis. We have developed an intermolecular gold(I)-catalyzed alkyne/alkene reaction of o-allylphenols with acetylene gas that gives rise to chromanes by a stereospecific aryloxycyclization through the nucleophilic regioselective opening of cyclopropyl gold(I)-carbene intermediates. The synthetic application of this method was demonstrated in the late-stage functionalization of the natural product lapachol.

Synthesis of vinyl-substituted alcohols using acetylene as a C2 building block

Vinyl-substituted alcohols represent a highly useful class of molecular skeletons. The current method typically requires either stoichiometric metallic reagents or preformed precursors. Herein, we report a nickel catalysis-enabled synthesis of vinyl-substituted alcohols via a 5-membered oxa-metallacycle. In this protocol, acetylene, the simplest alkyne and abundant feedstock, is employed as an ideal C2 synthon. The reaction features mild conditions, good functional group tolerance and broad substrate scope. Mechanistic exploration implies that the oxa-metallacycle originated from the cyclometallation of aldehyde and acetylene is the key intermediate for this transformation, which is then terminated by a silane-mediated σ-bond metathesis and subsequent reductive elimination.

Sequential condensation and double desulfonylative cyclopropanation of 1,2-bis(sulfonylmethyl)arenes with 3-arylacroleins: access to biscyclopropane-fused tetralins

Efficient tBuOK-mediated sequential condensation and double desulfonylative cyclopropanation of readily accessible 1,2-bis(sulfonylmethyl)arenes with 3-arylacroleins is described. This high-yielding, single-step strategy provides a variety of polysubstituted biscyclopropane-fused tetralins with six contiguous stereogenic centers via the construction of five carbon-carbon single bonds. A plausible mechanism is proposed and discussed. In the overall reaction process, water and sulfinic acid salts were generated as the byproducts.

[13 C+D] Double Labeling with Calcium Carbide: Incorporation of Two Labels in One Step

D-labeling is a valuable tool in advanced synthetic chemistry and pharmacy. However, D-incorporation significantly complicates the identification of products. In fact, D labels are invisible in ¹ H-NMR spectra and cause undesirable splitting in ¹³ C-NMR spectra which decreases the detectable limits. At the same time, ² H-NMR spectra are not effective for precise identification due to low sensitivity and the absence of correlations with ¹ H atoms. Here, ¹³ C-label was considered as an accompanying label for D-label in [¹³ C+D] unit for identification of D-containing sites and to track D-labels. [¹³ C+D]-doubly labeled vinyl derivatives and triazoles were synthesized using ¹³ C-labeled calcium carbide as a source of ¹³ C-label and deuterium oxide as a source of D-label. The reaction occurred in one-step manner accompanied with in situ doubly labeled acetylene formation. Non-labeled, mono-labeled and doubly labeled substrates were isolated in 25-80% yields.

Cu-catalyzed carboboration of acetylene with Michael acceptors

A copper-catalyzed three-component carboboration of acetylene with B2Pin2 and Michael acceptors is reported. In this reaction, a cheap and abundant C2 chemical feedstock, acetylene, was used as a starting material to afford cis-alkenyl boronates bearing a homoallylic carbonyl group. The reaction was robust and could be reliably performed on the molar scale. Furthermore, the resulting cis-alkenyl boronates could be converted to diverse functionalized molecules with ease.

Diverse synthesis of C2-linked functionalized molecules via molecular glue strategy with acetylene

As the simplest alkyne and an abundant chemical feedstock, acetylene is an ideal two-carbon building block. However, in contrast to substituted alkynes, catalytic methods to incorporate acetylene into fine chemicals are quite limited. Herein, we developed a photoredox-catalyzed synthetic protocol for diverse C2-linked molecules via a molecular glue strategy using gaseous acetylene under mild conditions. Initiated by addition of an acyl radical to acetylene, two cascade transformations follow. One involves a double addition for the formation of 1,4-diketones and the other where the intermediate vinyl ketone is intercepted by a radical formed from a heterocycle. In addition to making two new C-C bonds, two C-H bonds are also created in two mechanistically distinct ways: one via a C-H abstraction and the other via protonation. This system offers a reliable and safe way to incorporate gaseous acetylene into fine chemicals and expands the utility of acetylene in organic synthesis.

Although acetylene is an ideal two-carbon building block, very few catalytic methods can be applied to incorporate acetylene into fine chemicals. Here, the authors show photoredox-catalyzed syntheses of C2- linked molecules with gaseous acetylene under mild conditions.

One-Pot and Two-Chamber Methodologies for Using Acetylene Surrogates in the Synthesis of Pyridazines and Their D-Labeled Derivatives

Acetylene surrogates are efficient tools in modern organic chemistry with largely unexplored potential in the construction of heterocyclic cores. Two novel synthetic paths to 3,6-disubstituted pyridazines were proposed using readily available acetylene surrogates through flexible C₂ unit installation procedures in a common reaction space mode (one-pot) and distributed reaction space mode (two-chamber): (1) an interaction of 1,2,4,5-tetrazine and its acceptor-functionalized derivatives with a CaC₂ -H₂ O mixture performed in a two-chamber reactor led to the corresponding pyridazines in quantitative yields; (2) [4+2] cycloaddition of 1,2,4,5-tetrazines to benzyl vinyl ether can be considered a universal synthetic path to a wide range of pyridazines. Replacing water with D₂ O and vinyl ether with its trideuterated analog in the developed procedures, a range of 4,5-dideuteropyridazines of 95-99% deuteration degree was synthesized for the first time. Quantum chemical modeling allowed to quantify the substituent effect in both synthetic pathways.

Advances in polycyclization cascades in natural product synthesis

Cascade reactions are among the most powerful means to achieve the construction of multiple ring systems in a single step. This tutorial review describes recent advances in the use of polycyclization cascades in natural product synthesis.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Recent Advances in Construction of Polycyclic Natural Product Scaffolds via One-Pot Reactions Involving Alkyne Annulation

Polycyclic scaffolds are omnipresent in natural products and drugs, and the synthetic strategies and methods toward construction of these scaffolds are of particular importance. Compared to simple cyclic ring systems, polycyclic scaffolds have higher structure complexity and diversity, making them suitable for charting broader chemical space, yet bringing challenges for the syntheses. In this review, we surveyed progress in the past decade on synthetic methods for polycyclic natural product scaffolds, in which the key steps are one-pot reactions involving intermolecular or intramolecular alkyne annulation. Synthetic strategies of selected polycyclic carbocycles and heterocycles with at least three fused, bridged, or spiro rings are discussed with emphasis on the synthetic efficiency and product diversity. Recent examples containing newly developed synthetic concepts or toolkits such as collective and divergent total synthesis, gold catalysis, C–H functionalization, and dearomative cyclization are highlighted. Finally, several “privileged synthetic strategies” for “privileged polycyclic scaffolds” are summarized, with discussion of remained challenges and future perspectives.

Calcium Carbide Looping System for Acetaldehyde Manufacturing from Virtually any Carbon Source

A vinylation/devinylation looping system for acetaldehyde manufacturing was evaluated. Vinylation of iso-butanol with calcium carbide under solvent-free conditions was combined with hydrolysis of the resulting iso-butyl vinyl ether under slightly acidic conditions. Acetaldehyde produced by hydrolysis was collected from the reaction mixture by simple distillation, and the remaining alcohol was redirected to the vinylation step. All the inorganic co-reagents can be looped as well, and the full sequence is totally sustainable. A complete acetaldehyde manufacturing cycle was proposed on the basis of the developed procedure. The cycle was fed with calcium carbide and produced the aldehyde as a single product in a total preparative yield of 97 %. No solvents, hydrocarbons, or metal catalysts were needed to maintain the cycle. As calcium carbide in principle can be synthesized from virtually any source of carbon, the developed technology represents an excellent example of biomass and waste conversion into a valuable industrial product.

Acetylene as a Dicarbene Equivalent for Gold(I) Catalysis: Total Synthesis of Waitziacuminone in One Step

The gold(I)-catalyzed reaction of acetylene gas with alkenes leads to (Z,Z)-1,4-disubstituted 1,3-butadienes and biscyclopropanes depending on the donor ligand on gold(I). Acetylene was generated in situ from calcium carbide and water in a user-friendly procedure. Reaction of acetylene with 1,5-dienes gives rise stereoselectively to tricyclo[5.1.0.02,4 ]octanes. This novel double cyclopropanation has been applied to the one step total synthesis of the natural product waitziacuminone from acetylene and geranyl acetone.

Examining the vinyl moiety as a protecting group for hydroxyl (–OH) functionality under basic conditions

A method for the protection and deprotection of alcohols via vinylation and devinylation reactions is proposed. Stability of the vinyl protecting group under various conditions is studied and synthetic applicability is demonstrated.