Recent Advancements in the Utilization of s-Block Organometallic Reagents in Organic Synthesis with Sustainable Solvents.

This mini-review offers a comprehensive overview of the advancements made over the last three years in utilizing highly polar s-block organometallic reagents (specifically, RLi, RNa and RMgX compounds) in organic synthesis run under bench-type reaction conditions. These conditions involve exposure to air/moisture and are carried out at room temperature, with the use of sustainable solvents as reaction media. In the examples provided, the adoption of Deep Eutectic Solvents (DESs) or even water as non-conventional and protic reaction media has not only replicated the traditional chemistry of these organometallic reagents in conventional and toxic volatile organic compounds under Schlenk-type reaction conditions (typically involving low temperatures of -78 °C to 0 °C and a protective atmosphere of N2 or Ar), but has also resulted in higher conversions and selectivities within remarkably short reaction times (measured in s/min). Furthermore, the application of the aforementioned polar organometallics under bench-type reaction conditions (at room temperature/under air) has been extended to other environmentally responsible reaction media, such as more sustainable ethereal solvents (e.g., CPME or 2-MeTHF). Notably, this innovative approach contributes to enhancing the overall sustainability of s-block-metal-mediated organic processes, thereby aligning with several key principles of Green Chemistry.

FeIII -Based Eutectic Mixtures as Multi-task and Reusable Reaction Media for Efficient and Selective Conversion of Alkynes into Carbonyl Compounds.

Invited for the cover of this issue is the group of Vicente del Amo, Alejandro Presa Soto and Joaquín García-Álvarez (QuimSinSos Group) at the University of Oviedo. The image depicts the use of the FeIII -based deep eutectic mixture [FeCl3 ⋅6 H2 O/Gly (3:1)] (Gly = glycerol) as both promoter and solvent for the straightforward and selective hydration of alkynes, working under mild (45 °C), bench-type reaction conditions (air) and in the absence of ligands, co-catalysts or co-solvents. Read the full text of the article at 10.1002/chem.202301736.

FeIII -Based Eutectic Mixtures as Multi-task and Reusable Reaction Media for Efficient and Selective Conversion of Alkynes into Carbonyl Compounds.

An efficient, simple and general protocol for the selective hydration of terminal alkynes into the corresponding methyl ketones has been developed by using a cheap, easy-to-synthesise and sustainable FeIII -based eutectic mixture [FeCl3 ⋅ 6H2 O/Gly (3 : 1)] as both promoter and solvent for the hydration reaction, working: i) under mild (45 °C) and bench-type reaction conditions (air); and ii) in the absence of ligands, co-catalysts, co-solvents or toxic, non-abundant and expensive noble transition metals (Au, Ru, Pd). When the final methyl ketones are solid/insoluble in the eutectic mixture, the hydration reaction takes place in 30 min, and the obtained methyl ketones can be isolated by simply decanting the liquid FeIII -DES, allowing the direct isolation of the desired ketones without VOC solvents. By using this straightforward and simple isolation protocol, we have been able to recycle the FeIII -based eutectic mixture system up to eight consecutive times. Furthermore, the FeIII -eutectic mixture is able to promote the selective and efficient formal oxidation of internal alkynes into 1,2-diketones, with the possibility of recycling this system up to three consecutive times. Preliminary investigations into a possible mechanism for the oxidation of the internal alkynes seem to indicate that it proceeds through the formation of the corresponding methyl ketones and α-chloroketones.

Nicotinamide N-methyltransferase (NNMT) regulates the glucocorticoid signaling pathway during the early phase of adipogenesis.

Obesity is associated with adipose tissue dysfunction through the differentiation and expansion of pre-adipocytes to adipocytes (hyperplasia) and/or increases in size of pre-existing adipocytes (hypertrophy). A cascade of transcriptional events coordinates the differentiation of pre-adipocytes into fully differentiated adipocytes; the process of adipogenesis. Although nicotinamide N-methyltransferase (NNMT) has been associated with obesity, how NNMT is regulated during adipogenesis, and the underlying regulatory mechanisms, remain undefined. In present study we used genetic and pharmacological approaches to elucidate the molecular signals driving NNMT activation and its role during adipogenesis. Firstly, we demonstrated that during the early phase of adipocyte differentiation NNMT is transactivated by CCAAT/Enhancer Binding Protein beta (CEBPB) in response to glucocorticoid (GC) induction. We found that Nnmt knockout, using CRISPR/Cas9 approach, impaired terminal adipogenesis by influencing the timing of cellular commitment and cell cycle exit during mitotic clonal expansion, as demonstrated by cell cycle analysis and RNA sequencing experiments. Biochemical and computational methods showed that a novel small molecule, called CC-410, stably binds to and highly specifically inhibits NNMT. CC-410 was, therefore, used to modulate protein activity during pre-adipocyte differentiation stages, demonstrating that, in line with the genetic approach, chemical inhibition of NNMT at the early stages of adipogenesis impairs terminal differentiation by deregulating the GC network. These congruent results conclusively demonstrate that NNMT is a key component of the GC-CEBP axis during the early stages of adipogenesis and could be a potential therapeutic target for both early-onset obesity and glucocorticoid-induced obesity.

Broadening the Scope of Steroidal Scaffolds: The Umpolung of a Bis-Primary Amine Precatalyst for the Insertion of CO2 into Epoxides.

A cholic acid-based bis-primary amine is capable of promoting the insertion of CO2 into epoxides with the cooperative aid of an iodide anion. This framework is transformed in situ into a bis-carbamic acid. The latter is the active catalytic species, operating through H-bonding interactions. Our system works with complete atom economy, under solvent-free, metal-free, and mild conditions. Also, it can be recycled.

Mimicking Enzymes: Asymmetric Induction inside a Carbamate-Based Steroidal Cleft.

Cholic acid has been elaborated into a carbamate-based tripodal architecture, which is able to promote an asymmetric organic transformation inside its chiral cavity. The nature of this steroidal catalyst has been disclosed by quantum-chemical calculations. It comprises the preorganization and confinement of the reagents within the cavity of the steroid to form a supramolecular complex held together by means of cooperative H-bond contacts. This operational mode resembles that of some enzymes.

l-Isoleucine in a Choline Chloride/Ethylene Glycol Deep Eutectic Solvent: A Reusable Reaction Kit for the Asymmetric Cross-Aldol Carboligation.

l-Isoleucine is able to catalyze the cross-aldol reaction between cyclohexanone and aromatic aldehydes in a deep eutectic solvent consisting in choline chloride and ethylene glycol, rendering products with high diatereo- and enantioselectivity. This protocol is straightforward and green: the organocatalyst and the reaction medium can be recycled up to five times, allowing the preparation of different substrates with a single load of solvent and catalyst.

Highly enantioselective synthesis of α-azido-ß-hydroxy methyl ketones catalyzed by a cooperative proline-guanidinium salt system.

The combined activity of (S)-proline and an achiral tetraphenylborate TBD-derived guanidinium salt permits the aldol reaction between azidoacetone and aromatic, or heteroaromatic aldehydes. The α-azido-ß-hydroxy methyl ketones obtained as products can be isolated in good yield, with high diastereo- and enantioselectivity.

General metal-free Baeyer-Villiger-type synthesis of vinyl acetates.

Oxone, a cheap, stable, and nonhazardous oxidizing reagent, transforms α,ß-unsaturated ketones of defined stereochemistry into their corresponding vinyl acetates through a Baeyer-Villiger reaction. This process is general and straightforward, tolerating a wide range of functional groups.

Switching diastereoselectivity in proline-catalyzed aldol reactions.

The choice of the anion of an achiral TBD-derived guanidinium salt, used as cocatalyst for proline, allows reacting cycloketones with aromatic aldehydes and preparing either anti- or syn-aldol adducts with very high enantioselectivity. As a proof of principle, we show how the judicious choice of an additive allows individual access to all possible products, thus controlling the stereochemical outcome of the asymmetric aldol reaction. The origin of the syn diastereoselectivity unfolds from an unusual equilibrium process coupled to the enamine-based catalytic cycle standard for proline.

TBD/Al2O3: a novel catalytic system for dynamic intermolecular aldol reactions that exhibit complex system behaviour.

The heterogeneous system TBD/Al(2)O(3) is an efficient catalyst for the intermolecular aldol reaction between ketones and aromatic aldehydes. This system operates with low catalysts loading (10%), in water or organic solvents, and with short reaction times. The desired aldol products are rendered cleanly. Experiments confirmed that this aldol protocol is reversible, and allowed for the preparation of dynamic combinatorial libraries (DCLs) of interconverting aldols. Analysis of these DCLs showed up how properties such as diastereoselectivity can emerge unpredictably from the library when it is considered as a whole.

Chromium-mediated stereoselective synthesis of carbohydrate-derived (E)-α,ß-unsaturated esters or amides.

A chromium-mediated novel synthesis of carbohydrate-derived di- and trisubstituted (E)-α,ß-unsaturated esters or amides from a range of dichloroesters or amides and a variety of sugar aldehydes is reported. The process took place with total stereoselectivity and in high yields. A mechanism based on a sequential chromium-promoted aldol-type reaction and a completely stereoselective ß-elimination reaction is proposed to explain these results.

Direct aldol reactions catalyzed by a heterogeneous guanidinium salt/proline system under solvent-free conditions.

The combined activity of (S)-proline and an achiral cocatalyst (a TBD-derived guanidinium salt) allow direct aldol reactions to be carried out with high diastereoselectivity and enantioselectivity under solvent-free conditions with a rather simple reaction setup where stirring is not required.

Structure-enantioselectivity effects in 3,4-dihydropyrimido[2,1-b]benzothiazole-based isothioureas as enantioselective acylation catalysts.

The catalytic activity and enantioselectivity in the kinetic resolution of (±)-1-naphthylethanol with a range of structurally related 3,4-dihydropyrimido[2,1-b]benzothiazole-based catalysts is examined. Of the isothiourea catalysts screened, (2S,3R)-2-phenyl-3-isopropyl substitution proved optimal, giving good levels of selectivity in the kinetic resolution of a number of secondary alcohols (S values up to >100 at ~50% conversion). Low catalyst loadings (0.10-0.25 mol%) of the optimal isothiourea can be used to generate enantiopure alcohols (>99% ee) in good yields.

Stereospecific and highly stereoselective cyclopropanation reactions promoted by samarium.

Samarium metal and samarium diiodide have become important tools as selective cyclopropanating agents in organic synthesis due to their high chemo- and stereoselectivity. Therefore, Sm and SmI(2) are the ideal reagents to prepare cyclopropane derivatives. This tutorial review highlights C-C multiple bond cyclopropanation processes promoted by samarium or samarium diiodide.

Sequential synthesis of (E)-alpha,beta-unsaturated primary amides with complete stereoselectivity.

A novel, efficient, and totally stereoselective synthesis of (E)-alpha,beta-unsaturated primary amides is reported. This process is consistent with a SmI(2)-mediated sequential reaction of an unmasked samarium chloroacetamide enolate with an aldehyde, followed by a beta-elimination to produce (E)-alpha,beta-unsaturated primary amides in good yields.

Amidine catalysed O- to C-carboxyl transfer of heterocyclic carbonate derivatives.

The structural requirements of amidines necessary to act as efficient O- to C-carboxyl transfer agents are delineated and the scope of this process outlined through its application to a range of oxazolyl, benzofuranyl and indolyl carbonates.

Totally selective synthesis of enantiopure (3S,5S)- and (3R,5R)-4-amino-3,5-dihydroxypiperidines from aminodiepoxides derived from serine.

The transformation of enantiopure (2R,4R)- and (2S,4S)-N,N-dibenzyl-1,2:4,5-diepoxypentan-3-amine, 1 and 2, into the corresponding enantiopure (3S,5S)- and (3R,5R)-3,5-dihydroxy-3-aminopiperidines, 3 and 4 respectively, is described. The opening of the two epoxide rings with a range of amines takes place with total regioselectivity and high yields, in the presence of LiClO4. A mechanism to explain this transformation is proposed.