Gold(I)‐Catalyzed Tandem Alkoxylation/Lactonization of γ‐Hydroxy‐α,β‐Acetylenic Esters

The formation of 4‐alkoxy‐2(5H)‐furanones was achieved via tandem alkoxylation/lactonization of γ‐hydroxy‐α,β‐acetylenic esters catalyzed by 2 mol% of [2,6‐bis(diisopropylphenyl)imidazol‐2‐ylidine]gold bis(trifluoromethanesulfonyl)imidate [Au(IPr)(NTf₂)]. The economic and simple procedure was applied to a series of various secondary propargylic alcohols allowing for yields of desired product of up to 95%. In addition, tertiary propargylic alcohols bearing mostly cyclic substituents were converted into the corresponding spiro derivatives. Both primary and secondary alcohols reacted with propargylic alcohols at moderate temperatures (65–80 °C) in either neat reactions or using 1,2‐dichloroethane as a reaction medium allowing for yields of 23–95%. In contrast to [Au(IPr)(NTf₂)], reactions with cationic complexes such as [2,6‐bis(diisopropylphenyl)imidazol‐2‐ylidine](acetonitrile)gold tetrafluoroborate [Au(IPr)(CH₃CN)][BF₄] or (μ‐hydroxy)bis{[2,6‐bis(diisopropylphenyl)imidazol‐2‐ylidine]gold} tetrafluoroborate or bis(trifluoromethanesulfonyl)imidate – [{Au(IPr)}₂(μ‐OH)][X] (X=BF₄, NTf₂) – mostly stop after the alkoxylation. Analysis of the intermediate proved the exclusive formation of the E‐isomer which allows for the subsequent lactonization.     

Efficient Synthesis of Unsaturated δ‐ and ε‐Lactones/Lactams by Catalytic Cycloisomerization: When Pt Outperforms Pd

Platinum pincer complexes featuring an SCS indenediide backbone have been prepared and evaluated in the catalytic cycloisomerization of alkynoic acids and N‐tosyl alkynylamides. One of the platinum complexes significantly outperforms its palladium analogue for the formation of 6‐ and 7‐membered rings. The catalytic system takes advantage of the alkynophilicity of Pt and of the non‐innocent character of the indenediide framework. Like for Pd, the catalytic performance is significantly improved by using an H‐bond donor additive such as pyrogallol. For the first time, a large variety of ω‐unsaturated δ‐ and ε‐lactones/lactams could be prepared with high selectivities and in very good yields.

     

Ring-opening polymerization of ε-caprolactone and δ-valerolactone using new Sm(III) μ-halo-bis(trimethylsilyl)amido complexes

New Sm(III) μ-halo-bis(trimethylsilyl)amido complexes [Sm(μ-X) {N(SiMe₃)₂}₂(THF)]₂ (X = Cl, Br) were synthesized and their crystal structures were analyzed. These complexes were used as catalysts for the homopolymerization of ε-caprolactone and δ-valerolactone and found to initiate ring-opening polymerization (ROP) of these monomers in high conversion over a short period, giving homopolymers with a moderate polydispersity index (∼ 2.0). The effect of temperature and the catalyst amount on the molecular weight distribution was also studied. Mechanistic studies were conducted using 1 : 1 and 1 : 6 molar ratios of μ-halo-bis-(trimethylsilyl)amido complexes and ε-caprolactone by ¹H-NMR and ¹³C-NMR spectroscopic techniques and revealed the presence of more than one initiating species. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1669–1674, 1999     

Copper(I)‐Catalyzed Stereoselective Synthesis of (E)‐α‐Alkynyl‐α,β‐unsaturated Esters from a Terminal Alkyne,Diazoesters and Aldehydes

A new synthetic method for α‐alkynyl‐α,β‐unsaturated esters is presented herein. The method is based on a copper(I)‐catalyzed three‐component reaction of a terminal alkyne, diazoesters and aldehydes. The reaction is featured by mild conditions, high yields and excellent stereoselectivity. Cu(I) carbene migratory insertion is proposed as the key step in the transformation.

     

Gold‐Catalyzed Efficient Formation of α,β‐Unsaturated Ketones from Propargylic Acetates

An efficient gold‐catalyzed method for the preparation of α,β‐unsaturated ketones from propargylic acetates has been developed. Under mild reaction conditions, β‐monosubstituted enones were formed mostly with excellent E‐selectivity. β,β‐Disubstituted enones can be prepared from propargylic acetates derived from ketones. The high efficiency and mild nature of this reaction render it a viable alternative for the synthesis of α,β‐unsaturated ketones.     

Practical Enantioselective Synthesis of β‐Lactones Catalyzed by Aluminum Bissulfonamide Complexes

The development of an efficient and practical aluminum‐bissulfonamide complex catalyzed enantioselective formation of β‐lactones by [2+2] cycloaddition of ketene (generated in situ from acetyl bromide by dehydrobromination) with various α‐unbranched and ‐branched aliphatic aldehydes is presented. The methodology offers the advantage of operational simplicity not only as the ligand synthesis requires just a single sulfonylation step from commercially available enantiomerically pure diamines. The products are formed in high to excellent yields with ee values typically ranging from 78 to 90 % using 10 mol % of the bissulfonamide ligand. The key finding of this work was a remarkable rate acceleration by using an aluminum/ligand ratio of 1.5:1.     

Gold(I)‐Catalyzed Carboaminations of Allenes by N‐Allyltetrahydro‐β‐carbolines: An Experimental and Theoretical Study

N‐Allyltetrahydro‐β‐carbolines bearing a pendant allene undergo unprecedented gold(I)‐catalyzed cyclizations proceeding with an allyl migration from the nitrogen to the allene moiety. The initial product of the gold‐catalyzed cyclization evolves either via isomerization or Cope rearrangement, leading to different scaffolds. Density function theory (DFT) calculations established that the allyl migration occurs in an asynchronous suprafacial manner.

     

An Efficient Cobalt(I)‐Catalysed Reformatsky Reaction using α‐Chloro Esters

An efficient cobalt(I )‐catalysed Reformatsky reaction using α‐chloro esters has been developed. The catalyst is prepared by reducing the cobalt(II ) chloride (5 %)/1,2‐bis(diphenylphosphino)ethane (dppe)(5 %)/zinc iodide (10 %) system with zinc metal in acetonitrile in the presence of both the α‐chloro ester and the carbonyl compound; good to excellent conversions to β‐hydroxy esters are obtained at room temperature in 2.5 h.     

Microwave‐Assisted Synthesis of Pyrazino[2,1‐b]quinazolines and 3‐Indolyl‐2(1H)‐pyrazinones Employing a Chemoselective Silver(I)‐ and Gold(I)‐Catalyzed Reaction

Novel microwave‐assisted syntheses of pyrazino[2,1‐b]quinazolines and 3‐indolyl‐2(1H)‐pyrazinones employing silver(I)‐ or gold(I)‐catalyzed protocols have been elaborated. The scope and limitations of these processes have been investigated.     

Organocatalytic Asymmetric Synthesis of Protected α,β‐Diamino Acids

In the presence of the readily available quinine‐derived catalyst 4d , highly diastereo‐ and enantioselective Mannich reactions of tosyl‐protected imines and α‐isothiocyanato imides proceeded to afford the protected α,β‐diamino acids, useful building blocks for natural products and biologically active compounds, in good to excellent yields.     

Gold Catalysis: β‐Ketonaphthalenes via Molecular Gymnastics of 1,6‐Diyne‐4‐en‐3‐ols

1,6‐Diyne‐4‐en‐3‐ols with one terminal alkyne were applied as test substrates for a possible dual catalyzed cyclization. Instead of a dual catalysis cycle, naphthyl ketone derivatives were obtained as single products. The regioselectivity of the obtained products is unprecedented. Instead of the expected naphthyl ketones bearing the keto group in the α‐position, the keto group is positioned in the ß‐position of the naphthyl skeleton by a complex rearrangement of the starting materials.     

Gold(I)‐Catalyzed Intramolecular Carbon‐Oxygen Bond Cleavage Reaction via Gold Carbenes Derived from Vinylidenecyclopropanes

Under gold(I) catalysis, the in situ generated gold carbene from vinylidenecyclopropanes bearing a methoxymethyl group can undergo an intramolecular nucleophilic attack to give the carbon‐oxygen bond cleavage product in good yield within 15 min. and different phosphine ligands could control both Z‐ and E‐configurations, respectively. The reaction mechanism has been clarified on the basis of deuterium‐ and ¹⁸O‐labeling experiments. This method provides a new and efficient approach to stereoselectively synthesize alkylidenecyclobutanones under mild conditions.

     

Ligand‐Free Nickel‐Catalysed 1,4‐Addition of Arylboronic Acids to α,β‐Unsaturated Carbonyl Compounds

A simple and efficient ligand‐free nickel‐based catalytic system has been developed for the 1,4‐addition of arylboronic acids to α,β‐unsaturated carbonyl compounds. With catalyst loadings of 1–2 mol%, a series of 1,4‐adducts from chalcones and cinnamates was obtained in moderate to excellent yields within 5–30 min under a nitrogen atmosphere and microwave irradiation. The 1,4‐addition of arylboronic acids to acrylates is less efficient.

     

Microbial Synthesis of Medium‐Chain α,ω‐Dicarboxylic Acids and ω‐Aminocarboxylic Acids from Renewable Long‐Chain Fatty Acids

Biotransformation of long‐chain fatty acids into medium‐chain α,ω‐dicarboxylic acids or ω‐aminocarboxylic acids could be achieved with biocatalysts. This study presents the production of α,ω‐dicarboxylic acids (e.g., C₉, C₁₁, C₁₂, C₁₃) and ω‐aminocarboxylic acids (e.g., C₁₁, C₁₂, C₁₃) directly from fatty acids (e.g., oleic acid, ricinoleic acid, lesquerolic acid) using recombinant Escherichia coli‐based biocatalysts. ω‐Hydroxycarboxylic acids, which were produced from oxidative cleavage of fatty acids via enzymatic reactions involving a fatty acid double bond hydratase, an alcohol dehydrogenase, a Baeyer–Villiger monooxygenase and an esterase, were then oxidized to α,ω‐dicarboxylic acids by alcohol dehydrogenase (ADH, AlkJ) from Pseudomonas putida GPo1 or converted into ω‐aminocarboxylic acids by a serial combination of ADH from P. putida GPo1 and an ω‐transaminase of Silicibacter pomeroyi. The double bonds present in the fatty acids such as ricinoleic acid and lesquerolic acid were reduced by E. coli‐native enzymes during the biotransformations. This study demonstrates that the industrially relevant building blocks (C₉ to C₁₃ saturated α,ω‐dicarboxylic acids and ω‐aminocarboxylic acids) can be produced from renewable fatty acids using biocatalysis.

     

Synthesis of Functionalized Hydrazines: Facile Homogeneous (N‐Heterocyclic Carbene)‐Palladium(0)‐Catalyzed Diboration and Silaboration of Azobenzenes

The bis(N‐heterocyclic carbene)(diphenylacetylene)palladium complex [Pd(ITMe)₂(PhC≡CPh)] (ITMe=1,3,4,5‐tetramethylimidazol‐2‐ylidene) acts as a highly active pre‐catalyst in the diboration and silaboration of azobenzenes to synthesize a series of novel functionalized hydrazines. The reactions proceed using commercially available diboranes and silaboranes under mild reaction conditions.

     

Enantioselective N‐Heterocyclic Carbene‐Catalyzed Annulations of 2‐Bromoenals with 1,3‐Dicarbonyl Compounds and Enamines via Chiral α,β‐Unsaturated Acylazoliums

The N‐heterocyclic carbene (NHC)‐catalyzed generation of chiral α,β‐unsaturated acylazoliums from 2‐bromoenals followed by their interception with 1,3‐dicarbonyl compounds or enamines, the formal [3+3] annulation reaction, is reported. The reaction results in the enantioselective synthesis of synthetically and medicinally important dihydropyranones and dihydropyridinones, and tolerates a wide range of functional groups. It is noteworthy that the reaction takes place under mild reaction conditions utilizing relatively low catalyst loadings. In addition, based on DFT calculations, a mechanistic scenario involving the attack of the nucleophile from below the plane of the α,β‐unsaturated acylazoliums, and the mode of enantioinduction is presented.     

Substrate Control in the Gold(I)‐Catalyzed Cyclization of β‐Propargylamino Acrylic Esters and Further Transformations of the Resultant Dihydropyridines

N‐Protected β‐propargylamino acrylic esters with a push‐pull olefinic bond afforded good to high yields of dihydropyridines upon treatment with 5% tris(2‐furyl)phosphine‐gold(I) chloride/silver(I) tetrafluoroborate [(TFP)AuCl/AgBF₄] in anhydrous benzene. Carbamate and sulfonyl groups were employed for nitrogen protection. On a model enyne, the p‐methoxybenzenesulfonyl (MBS) group was found to be a better protective group than tosyl in terms of cyclization yield, and also the yield of elimination to the corresponding 2,3,4‐trisubstituted pyridines. Boc‐protected dihydropyridines underwent partial deprotection/oxidation under the cyclization conditions, which enabled a more straightforward, one‐pot preparation of the corresponding pyridines. In another application, an appropriately substituted derivative protected as a stable methoxycarbamate was subjected to catalytic hydrogenation affording the known precursor of paroxetine. The chemoselectivity of enyne cyclization (dihydropyridine vs. pyrrole) is governed, among other factors, by C‐3 substitution. Dihydropyridines were obtained as sole products regardless of the catalyst/conditions when C‐3 was unsubstituted.

     

Highly Efficient Asymmetric Epoxidation of Electron‐Deficient α,β‐Enones and Related Applications to Organic Synthesis

The asymmetric epoxidation of electron‐deficient olefins has been achieved using inexpensive and readily available prolinols as catalysts with good to excellent yields and enantioselectivities. The utility of the resulting chiral epoxides was illustrated by elaboration to several synthetically useful compounds featuring a concise synthesis of (−)‐(5R,6S)‐balasubramide.     

Synthesis of Spiro[piperidine‐3,3′‐oxindoles] via Gold(I)‐Catalyzed Dearomatization of N‐Propargyl‐ and N‐Homoallenyl‐2‐bromotryptamines

N‐Propargyl‐ and N‐homoallenyl‐2‐bromo‐β‐tryptamines undergo gold(I)‐catalyzed dearomatizing cyclizations to afford 2‐bromospiroindolenines that are in situ hydrolyzed to furnish spirooxindoles in a one‐pot process. Tryptophane derivatives (R²=CO₂Et) led upon cyclization to chiral spirooxindoles in excellent diastereoselectivities.