Synthesis of Amides and Lactams in Supercritical Carbon Dioxide

Synthesis of Bench-Stable N-Quaternized Ketene N,O-Acetals and Preliminary Evaluation as Reagents in Organic Synthesis

N-Quaternized ketene N,O-acetals are typically an unstable, transient class of compounds most commonly observed as reactive intermediates. In this report, we describe a general synthetic approach to a variety of bench-stable N-quaternized ketene N,O-acetals via treatment of pyridine or aniline bases with acetylenic ethers and an appropriate Brønsted or Lewis acid (triflic acid, triflimide, or scandium(III) triflate). The resulting pyridinium and anilinium salts can be used as reagents or synthetic intermediates in multiple reaction types. For example, N-(1-ethoxyvinyl)pyridinium or anilinium salts can thermally release highly reactive O-ethyl ketenium ions for use in acid catalyst-free electrophilic aromatic substitutions. N-(1-Ethoxyvinyl)-2-halopyridinium salts can be employed in peptide couplings as a derivative of Mukaiyama reagents or react with amines in nucleophilic aromatic substitutions under mild conditions. These preliminary reactions illustrate the broad potential of these currently understudied compounds in organic synthesis.

Indirect reduction of CO2 and recycling of polymers by manganese-catalyzed transfer hydrogenation of amides, carbamates, urea derivatives, and polyurethanes

The reduction of polar bonds, in particular carbonyl groups, is of fundamental importance in organic chemistry and biology. Herein, we report a manganese pincer complex as a versatile catalyst for the transfer hydrogenation of amides, carbamates, urea derivatives, and even polyurethanes leading to the corresponding alcohols, amines, and methanol as products. Since these compound classes can be prepared using CO2 as a C1 building block the reported reaction represents an approach to the indirect reduction of CO2. Notably, these are the first examples on the reduction of carbamates and urea derivatives as well as on the C-N bond cleavage in amides by transfer hydrogenation. The general applicability of this methodology is highlighted by the successful reduction of 12 urea derivatives, 26 carbamates and 11 amides. The corresponding amines, alcohols and methanol were obtained in good to excellent yields up to 97%. Furthermore, polyurethanes were successfully converted which represents a viable strategy towards a circular economy. Based on control experiments and the observed intermediates a feasible mechanism is proposed.

Impregnation of poly(L-lactide-ran-δ-valerolactone) with essential bark oil using supercritical carbon dioxide

This work studied the incorporation of essential bark oil from Thujopsis dolabrata var. hondae, which is known to repel various insects, in poly(L-lactide-ran-δ-valerolactone) [poly(L-LA-ran-VL)] using supercritical carbon dioxide (scCO₂). The poly(L-LA-ran-VL) was synthesized by first purifying the monomers by azeotropic distillation with benzene, followed by polymerization with Sn(oct)₂ using the same equipment, representing an efficient one-pot process. The copolymerization of L-LA with VL using this technique at a feed ratio of 90/10 mol/mol gave poly(L-LA-ran-VL) (91/9) with a molecular weight of 6.48 × 10⁴ g/mol and a high yield of 74.9%. Products with molecular weights over 5.0 × 10⁴ g/mol were obtained at L-LA feed proportions of 70 to 90%. Impregnation trials were conducted between 40 and 120 °C at 14 MPa for 3 h. The oil content of a 73/27 specimen was found to increase significantly during processing at 100 or 120 °C. During enzymatic degradation with proteinase K, the 91/9 specimen showed the fastest degradation rate. Although the 71/29 sample was slowly hydrolyzed in a phosphate buffer at pH 7.0, the release of oil vapor from this material was slightly higher than that from the 91/9 specimen, and the vapor release rate continuously increased throughout the hydrolysis process.

Ce(iii)-catalyzed highly efficient synthesis of pyridyl benzamides from aminopyridines and nitroolefins without external oxidants

An efficient synthesis of a variety of pyridyl benzamides from 2-aminopyridines and nitroolefins is described. This rare-earth-metal-catalyzed reaction provides the corresponding products with broad substrate scope in moderate to excellent yields, in the absence of additives and external oxidants. Water is used as the source of the carbonyl oxygen atom in pyridyl benzamides. Furthermore, 2-substituted oxazolo[4,5-b]pyridines are formed in good yields under the standard conditions when 2-aminopyridin-3-ols are used as the substrates.

Diastereoselective Base-Catalyzed Formal [4 + 2] Cycloadditions of N-Sulfonyl Imines and Cyclic Anhydrides

A diastereoselective base-catalyzed Mannich reaction of cyclic, enolizable anhydrides and N-sulfonyl imines for the synthesis of δ-lactams is reported. This anhydride Mannich reaction tolerates imines derived from aryl and enolizable aldehydes. A base-catalyzed product epimerization pathway ensures high anti diastereoselectivity in aryl and achiral enolizable imines.

Tandem Bond-Forming Reactions of 1-Alkynyl Ethers

Electron-rich alkynes, such as ynamines, ynamides, and ynol ethers, are functional groups that possess significant potential in organic chemistry for the formation of carbon-carbon bonds. While the synthetic utility of ynamides has recently been expanded considerably, 1-alkynyl ethers, which possess many of the reactivity features of ynamides, have traditionally been far less investigated because of concerns about their stability. Like ynamides, ynol ethers are relatively unhindered to approach by functional groups present in the same or different molecules because of their linear geometry, and they can potentially form up to four new bonds in a single transformation. Ynol ethers also possess unique reactivity features that make them complementary to ynamides. Research over the past decade has shown that ynol ethers formed in situ from stable precursors engage in a variety of useful carbon-carbon bond-forming processes. Upon formation at -78 °C, allyl alkynyl ethers undergo a rapid [3,3]-sigmatropic rearrangement to form allyl ketene intermediates, which may be trapped with alcohol or amine nucleophiles to form γ,δ-unsaturated carboxylic acid derivatives. The process is stereospecific, takes place in minutes at cryogenic temperatures, and affords products containing (quaternary) stereogenic carbon atoms. Trapping of the intermediate allyl ketene with carbonyl compounds, epoxides, or oxetanes instead leads to complex α-functionalized β-, γ-, or δ-lactones, respectively. [3,3]-Sigmatropic rearrangement of benzyl alkynyl ethers also takes place at temperatures ranging from -78 to 60 °C to afford substituted 2-indanones via intramolecular carbocyclization of the ketene intermediate. tert-Butyl alkynyl ethers containing pendant di- and trisubstituted alkenes and enol ethers are stable to chromatographic isolation and undergo a retro-ene/[2 + 2] cycloaddition reaction upon mild thermolysis (90 °C) to afford cis-fused cyclobutanones and donor-acceptor cyclobutanones in good to excellent yields and diastereoselectivities. This process, which takes place under neutral conditions and proceeds through an aldoketene intermediate, obviates the need to employ moisture-sensitive and/or unstable acid chlorides under basic conditions for intramolecular [2 + 2] cycloaddition reactions. Furthermore, Lewis acid-catalyzed intramolecular condensations of both ethyl and tert-butyl ynol ethers with tethered acetals efficiently provide protected five-, six-, and seven-membered cyclic Baylis-Hilman adducts. Metalated ethoxyacetylene can also participate in multiple bond-forming reactions that avoid isolation of the alkynyl ether intermediate. Lewis acid-promoted tandem additions employing epoxides/oxetanes and carbonyl compounds give rise to (Z)-α-alkylidene and α-benzylidene lactones stereoselectively in high overall yields. Three new carbon-carbon bonds and a ring are formed in this atom-economical single-flask transformation, resulting in a significant increase in molecular complexity. This Account provides a detailed overview of these useful transformations with the intention of stimulating further interest in and research on ynol ethers and their application in organic synthesis.

The chemistry of ynol and thioynol ethers

Alkynyl ethers and thioethers are versatile building blocks in organic synthesis allowing various transformations including a number of C–C bond forming processes.

Catalytic Ring Expansion of Cyclic Hemiaminals for the Synthesis of Medium-Ring Lactams

A mild and efficient intermolecular ring-expansion approach was developed for the synthesis of medium-ring lactams by using siloxy alkynes. Key to success is the suitable combination of a superior catalyst and an exceptional nitrogen-protecting group. Control experiments indicated that the reaction is remarkably selective toward the desired lactam formation, even with many possible non-productive pathways.

Catalytic chemical amide synthesis at room temperature: one more step toward peptide synthesis

An efficient method has been developed for direct amide bond synthesis between carboxylic acids and amines via (2-(thiophen-2-ylmethyl)phenyl)boronic acid as a highly active bench-stable catalyst. This catalyst was found to be very effective at room temperature for a large range of substrates with slightly higher temperatures required for challenging ones. This methodology can be applied to aliphatic, α-hydroxyl, aromatic, and heteroaromatic acids as well as primary, secondary, heterocyclic, and even functionalized amines. Notably, N-Boc-protected amino acids were successfully coupled in good yields with very little racemization. An example of catalytic dipeptide synthesis is reported.

Generation and Trapping of Ketenes in Flow

Ketenes were generated by the thermolysis of alkoxyalkynes under flow conditions, and then trapped with amines and alcohols to cleanly give amides and esters. For a 10 min reaction time, temperatures of 180, 160, and 140 °C were required for >95 % conversion of EtO, iPrO, and tBuO alkoxyalkynes, respectively. Variation of the temperature and flow rate with inline monitoring of the output by IR spectroscopy allowed the kinetic parameters for the conversion of 1-ethoxy-1-octyne to be easily estimated (Ea = 105.4 kJ/mol). Trapping of the in-situ-generated ketenes by alcohols to give esters required the addition of a tertiary amine catalyst to prevent competitive [2+2] addition of the ketene to the alkoxyalkyne precursor.

The ketene-surrogate coupling: catalytic conversion of aryl iodides into aryl ketenes through ynol ethers

tert-Butoxyacetylene is shown to undergo Sonogashira coupling with aryl iodides to yield aryl-substituted tert-butyl ynol ethers. These intermediates participate in a [1,5]-hydride shift, which results in the extrusion of isobutylene and the generation of aryl ketenes. The ketenes are trapped in situ with multiple nucleophiles or undergo electrocyclic ring closure to yield hydroxynaphthalenes and quinolines.

SYNTHESIS OF DE NOVO CHIRAL γ-AMINO-YNAMIDES USING LITHIATED YNAMIDES. OBSERVATION OF A UNIQUE 5-ENDO-DIG CYCLIZATION WITH AN INVERSION OF S-CENTER

We describe herein details of our efforts in developing a highly stereoselective synthesis of de novo chiral γ-amino-ynamides through additions of lithiated ynamides to Ellman-Davis chiral N-tert-butanesulfinyl imines. While additions of ynamides could be highly stereoselective even without Lewis acids, the use of BF3-OEt2 completely reversed the stereoselectivity. On the other hand, additions of oxazolidinone-substituted, oxazinanone-substituted and tetrahydropyrimidinone-substituted ynamides behaved quite differently and functioned better with BF3-OEt2. The chirality of the oxazolidinone ring exerts no impact on the selectivity. This work also features a unique 5-endo-dig cyclization of oxazolidinone-substituted γ-amino-ynamides that could be promoted with acid, leading to isothiazoles and 2,3-dihydro-isothiazole S-oxides.

A Convenient Synthesis of γ-Amino-Ynamides via Additions of Lithiated Ynamides to Aryl Imines. Observation of an Aza-Meyer-Schuster Rearrangement

Efforts in developing an expeditious and convenient method for synthesizing γ-amino-ynamides via nucleophilic addition of lithiated ynamides to aryl imines are described. This work also features an aza-variant of a Meyer-Schuster rearrangement of γ-amino-ynamides and the synthetic utility of γ-amino-ynamides in an intramolecular ketenimine-[2 + 2] cycloaddition.

A highly stereoselective addition of lithiated ynamides to Ellman-Davis chiral N-tert-butanesulfinyl imines

A highly diastereoselective addition of lithiated ynamides to Ellman-Davis chiral imines is described. While additions of N-sulfonyl ynamides are highly stereoselective even without Lewis acids, the use of BF3-OEt2 completely reversed the stereoselectivity. In addition, oxazolidinone-substituted ynamides behaved differently and functioned better with BF3-OEt2, and the chirality of the oxazolidinone ring exerts no impact on the selectivity.

Lewis acid catalyzed intramolecular condensation of ynol ether-acetals. Synthesis of alkoxycycloalkene carboxylates

Treatment of ynol ether-tethered dialkyl acetals with catalytic quantities of scandium triflate in CH(3)CN gives rise to five-, six-, and seven-membered alkoxycycloalkene carboxylates in good to excellent yields. Tri- and tetrasubstituted carbocyclic and heterocyclic alkenes may be formed by this method, and the products obtained may serve as useful intermediates for natural product synthesis.

Intramolecular [2 + 2] cycloaddition reactions of alkynyl ether derived ketenes. A convenient synthesis of donor-acceptor cyclobutanes

Mild thermolysis of tert-butyl alkynyl ethers furnishes aldoketenes, which undergo facile [2 + 2] cycloaddition reactions with pendant di- and trisubstituted alkenes. A wide variety of cis-fused cyclobutanones are produced in moderate to high diastereoselectivity and good to excellent yields by this method, and free hydroxyl groups are tolerated in the ene-ynol ether starting materials. Enol-ynol ethers also undergo efficient reaction to produce donor-acceptor cyclobutanes in high yields.

On the role of CO2 in NHC-catalyzed oxidation of aldehydes

NHC-catalyzed oxidations using carbon dioxide as the stoichiometric oxidant have been carefully investigated. These studies support a secondary role of CO(2) in suppressing side reactions and exogenous oxygen as the actual oxidant.