Andrés R. Alcántara

2-Methyltetrahydrofuran (2-MeTHF): A Biomass-Derived Solvent with Broad Application in Organic Chemistry

2-Methyl-tetrahydrofuran (2-MeTHF) can be derived from renewable resources (e.g., furfural or levulinic acid) and is a promising alternative solvent in the search for environmentally benign synthesis strategies. Its physical and chemical properties, such as its low miscibility with water, boiling point, remarkable stability compared to other cyclic-based solvents such as THF, and others make it appealing for applications in syntheses involving organometallics, organocatalysis, and biotransformations or for processing lignocellulosic materials. Interestingly, a significant number of industries have also started to assess 2-MeTHF in several synthetic procedures, often with excellent results and prospects. Likewise, preliminary toxicology assessments suggest that the use of 2-MeTHF might even be extended to more processes in pharmaceutical chemistry. This Minireview describes the properties of 2-MeTHF, the state-of-the-art of its use in synthesis, and covers several outstanding examples of its application from both industry and academia.

Biocatalytic Strategies for the Asymmetric Synthesis of α-Hydroxy Ketones

The development of efficient syntheses for enantiomerically enriched alpha-hydroxy ketones is an important research focus in the pharmaceutical industry. For example, alpha-hydroxy ketones are found in antidepressants, in selective inhibitors of amyloid-beta protein production (used in the treatment of Alzheimers), in farnesyl transferase inhibitors (Kurasoin A and B), and in antitumor antibiotics (Olivomycin A and Chromomycin A3). Moreover, alpha-hydroxy ketones are of particular value as fine chemicals because of their utility as building blocks for the production of larger molecules. They can also be used in preparing many other important structures, such as amino alcohols, diols, and so forth. Several purely chemical synthetic approaches have been proposed to afford these compounds, together with some organocatalytic strategies (thiazolium-based carboligations, proline alpha-hydroxylations, and so forth). However, many of these chemical approaches are not straightforward, lack selectivity, or are economically unattractive because of the large number of chemical steps required (usually combined with low enantioselectivities). In this Account, we describe three different biocatalytic approaches that have been developed to efficiently produce alpha-hydroxy ketones: (i) The use of thiamine diphosphate-dependent lyases (ThDP-lyases) to catalyze the umpolung carboligation of aldehydes. Enantiopure alpha-hydroxy ketones are formed from inexpensive aldehydes with this method. Some lyases with a broad substrate spectrum have been successfully characterized. Furthermore, the use of biphasic media with recombinant whole cells overexpressing lyases leads to productivities of approximately 80-100 g/L with high enantiomeric excesses (up to >99%). (ii) The use of hydrolases to produce alpha-hydroxy ketones by means of (in situ) dynamic kinetic resolutions (DKRs). Lipases are able to successfully resolve racemates, and many outstanding examples have been reported. However, this approach leads to a maximum theoretical yield of 50%. As a means of overcoming this problem, these traditional lipase-catalyzed kinetic resolutions are combined with racemization of remnant substrate, which can be done in situ or in separate compartments. Examples showing high conversions (>90%) and enantiomeric excesses (>99%) are described. (iii) Whole-cell redox processes, catalyzed by several microorganisms, either by means of free enzymes (applying a cofactor regeneration system) or by whole cells. Through the use of redox machineries, different strategies can lead to high yields and enantiomeric excesses. Some enantiopure alpha-hydroxy ketones can be formed by reductions of diketones and by selective oxidations of vicinal diols. Likewise, some redox processes involving sugar chemistry (involving alpha-hydroxy ketones) have been developed on the industrial scale. Finally, the redox whole-cell concept allows racemizations (and deracemizations) as well. These three strategies provide a useful and environmentally friendly synthetic toolbox. Likewise, the field represents an illustrative example of how biocatalysis can assist practical synthetic processes, and how problems derived from the integration of natural tools in synthetic pathways can be efficiently tackled to afford high yields and enantioselectivities.

Applied Biotransformations in Green Solvents

The definite interest in implementing sustainable industrial technologies has impelled the use of biocatalysts (enzymes or cells), leading to high chemo-, regio- and stereoselectivities under mild conditions. As usual substrates are not soluble in water, the employ of organic solvents is mandatory. We will focus on different attempts to combine the valuable properties of green solvents with the advantages of using biocatalysts for developing cleaner synthetic processes.

Different phyllosilicates as supports for lipase immobilisation

The aim of this work was to determine the enzymatic activities resulting from the adsorption of Rhizomucor miehei lipase (RML) and Candida cylindracea lipase (CCL) onto three different phyllosilicates (sepiolite, palygorskite and montmorillonite), comparing the resultant activities with those obtained following similar immobilisation technique on a widely used resin (Duolite A-568). Due to the different adsorption mechanisms produced, different derivatives with higher hydrolytic activities can be obtained. Comparing the clays tested, the results showed that, in comparison with the laminar silicate (montmorillonite sample) and Duolite A-568 (spherical particles), fibrous materials (palygorskite and sepiolite) resulted in derivatives with higher hydrolytic activities in the hydrolysis of different ethyl esters. Moreover, according to the data obtained with the electrophoresis, the selectivity of immobilisation for RML in the case of fibrous silicates was optimal. As a conclusion, and according to the activities and selectivities measured, at least two out of the four studied materials (sepiolite and palygorskite) would be useful as supports for immobilisation for proteins of relatively low molecular weight (such as RML) for further use in biotransformations, while for C. cylindracea the immobilisation onto duolite rendered a derivative specially active in the hydrolysis of ethyl formiate (esterasic activity).

2-Methyltetrahydrofuran as a suitable green solvent for phthalimide functionalization promoted by supported KF

An efficient chemoselective nitrogen functionalization of phthalimides by using KF-Alumina in 2-methyltetrahydrofuran, a solvent obtained from renewal sources, is described.

Regioselective enzymatic acylation of pharmacologically interesting nucleosides in 2-methyltetrahydrofuran, a greener substitute for THF

1-β-Arabinofuranosyl uracil, 9-β-arabinofuranosyl adenosine, 2′-O-(2-methoxyethyl)-5-methyl uridine, adenosine and uridine were enzymatically acylated with hexanoic anhydride and vinyl esters by CALB lipase (lipase B from Candida antarctica) with excellent regioselectivity in many cases and analytical reaction yields above 90%. The influence of the stereochemistry of the hydroxyl group on C-2′ was studied. Some of these esterifications were carried out in 2-methyltetrahydrofuran (MeTHF), which is described as an excellent substitute for THF in biocatalysed processes in organic media. This application for this green solvent is a proof-of-concept opening the use of MeTHF in biotransformations.

Highly efficient chemoselective N-TBS protection of anilines under exceptional mild conditions in the eco-friendly solvent 2-methyltetrahydrofuran

A straightforward chemoselective protection of anilines as N-TBS derivatives is described by using a suitable deprotonation of the amine with methyllithium in the environmentally friendly and safer substitute of THF, 2-methyltetrahydrofuran, under exceptional mild reaction conditions (0 °C, 30 min). Interestingly, the protecting group maybe cleaved efficiently by simple treatment of N-TBS-anilines with silica gel in ethanol–water.

Heptyl oleate synthesis as useful tool to discriminate between lipases, proteases and other hydrolases in crude preparations

Heptyl oleate synthesis is a good test to compare crude lipases. Being a reaction selective for lipases, esterases and proteases are not active in this esterification. The heptyl oleate synthesis has been carried out with crude lipases and other hydrolases (such as proteases and esterases) from different suppliers and origins, in order to explore the methodology scope. While the initial rate of esterification seems to be mainly dependent on the concentration of lipases in the sample tested, the final synthetic yield depends on the total amount of water in the reactor. The standard protocol to evaluate crude lipases is described.

RHIZOMUCOR MIEHEI LIPASE AS THE CATALYST IN THE RESOLUTION OF CHIRAL COMPOUNDS : AN OVERVIEW

Abstract Rhizomucor miehei lipase is probably the most used lipase obtained from fungi, even being used as a model for the determination of the structure of some other lipases due to the deep knowledge of its three dimensional structure. In this paper we present an overview of the use of this lipase for the obtention of homochiral compounds via hydrolytic and acyl transfer methodologies. The application for the resolution of both chiral carboxylic acids and alcohol is also presented.

Robust eco-friendly protocol for the preparation of γ-hydroxy-α,β-acetylenic esters by sequential one-pot elimination–addition of 2-bromoacrylates to aldehydes promoted by LTMP in 2-MeTHF

An efficient and widely applicable preparation of γ-hydroxy-α,β-acetylenic esters is described by means of a one-pot dehydrobromination of a 2-bromoacrylate ester with LTMP followed by the electrophilic addition of the transient propiolate to different aldehydes in the eco-friendly solvent 2-MeTHF.