Current Organic Chemistry - Volume 16, Issue 21, 2012

Ionic Liquids (ILs) have emerged in the last decade as a novel technology to be applied in biocatalysis (among other areas of chemistry). Most of the work has focused on 2nd generation of ILs, a broad palette of tuneable solvents yet with still high prices for commercial applications. It is thus expected that such know-how will be transferred to 3rd generation of ILs and to deep-eutectic-solvents (DES), as the greener and more economic neoteric solvents for many future applications. In general, the use of a given IL must be triggered by firstly identifying an “added-value” that such derivative may bring to a certain application. In the field of oxidative biocatalysis, IL have been used as replacement of solvents for enzymatic reactions, as well as in more subtle applications as performance additives, protein stabilizers, in immobilization of enzymes, biosensors, etc. This article discusses the most relevant cases, providing emphasis, when possible, on the added-value that ILs may bring to the different areas, and that therefore justify their use.

Laccases represent versatile catalysts being able to oxidize a wide range of aromatic substrates and are susceptible of several industrial applications based on both oxidative degradation reactions and synthetic chemistry. The range of laccase based synthetic reactions is extremely wide. Laccases are able to catalyze transformation of antibiotics based on both β-lactams functionalization and phtalides functionalization. These enzymes can also catalyze derivatization of amino acids to obtain metabolically stable amino acid analogues, maximizing biological response while minimizing toxicity, thus representing an useful system for drug development. Biomolecules having antioxidative and anticancer activity can also be produced by laccase-mediated reactions of flavonoids oxidative coupling and phenoxazinones synthesis. Application of laccases to production of new derivatives of the hormones resveratrol, 17ß-estradiol, totarol and isoeugenol and oligomerization products of substituted imidazoles was also reported, with applications for pharmacological purposes due to hormonal activity of the products. The enzymatic preparation of aromatic polymeric materials by the action of laccases represents a viable and non-toxic alternative to the usual formaldehyde-based chemical production of these compounds and it has been reported for several substrates such as 2,6-dimethylphenol, 4-hydroxybenzoic acid derivatives, 3,5-dimethoxy-4-hydroxybenzoic acid and 3,5-dimethyl-4-hydroxybenzoic acid, aniline and acrylamide. Moreover, laccase-mediated biografting of phenols or certain other types of low-molecular weight compounds provides a method for tailoring the surface of lignocellulosics or for adhesion enhancement in binderless wood boards under mild conditions and usually without harmful solvents. Laccase-mediated modification of lignocellulosic materials is accomplished through two main routes: coupling of low-molecular weight compounds onto lignocellulosic materials and laccase mediated cross-linking of lignin molecules in situ. Depending on the choice of laccase substrate, properties such as improved strength properties, increased antimicrobial resistance, or hydrophilicity/ hydrophobicity can be imparted to lignocellulosic materials.

Nowadays, the design of sustainable processes applicable at industrial scale is highly desirable due to environmental reasons. The use of biocatalytic reactions to carry out oxidative transformations is one of the possible strategies framed in here due to the mildness and usually high selectivities achieved with these methods. Anyway, while implementing this type of system at industrial scale several drawbacks must be overcome in order to obtain efficient setups. Historically, one of the main issues has been the cofactor-dependency of these biocatalysts to be active. Herein, we will show the state-of-the-art concerning the recent efforts developed in the design of (potentially) efficient methods to regenerate the cofactor in oxidative transformations. Thus, the more studied enzymatic methods will be discussed to highlight some recent examples dealing with the co-expression of both oxidative and recycling enzymes in one host or the development of self-sufficient biocatalysts. Furthermore, novel applications of these systems to couple two productive synthetic reactions will also be reviewed. Subsequently, we will focus on some recent examples related with the employment of electrochemical, photochemical, and chemical strategies to carry out the nicotinamide coenzyme or the flavin/heme prosthetic group regeneration. In the case of biocatalysts that use NAD(P) as electron shuttle, these systems have also been employed to replace it, allowing the design of nicotinamide- free recycling setups. In all cases, the (dis)advantages that these methodologies present will be briefly discussed.

Oxidases represent a distinct and interesting class of oxidative biocatalysts. A major portion of the known oxidases contain a flavin as cofactor, with glucose oxidase as best known example. While a number of oxidases are well known in the field of biocatalysis, the total number of available oxidases is rather limited. However, by analysis of literature data and genome sequences a clear picture emerges: nature harbors a large number of unexplored flavoprotein oxidases that can catalyze a breadth of oxidative reactions. In this review a summary is provided of the unique and intriguing catalytic potential of newly discovered flavoprotein oxidases. The reactions that have been shown to be catalyzed range from enantio- and/or regioselective alcohol and amine oxidations to oxidative C-O or C-C bond formations. This illustrates that nature harbors a large collection of oxidases that can be of use for catalyzing complex oxidative reactions.

Research on steroid biooxidation is being pursued for development of processes exploitable by the pharmaceutical industry, and also for the purpose of preparation of potentially useful steroid analogues which are otherwise inaccessible. The use of enzymes provides high regio- and stereoselectivity of the reaction to be performed. In this work a review of recent and important findings related to microbial and enzymatic biotransformations of steroidal compounds including hydroxylation, Baeyer-Villiger oxidation, dehydrogenation and sterol side-chain cleavage is presented, with emphasis on processes of practical biotechnological importance. Although some of these bioconversions are well-established, efforts are ongoing in order to increase their efficiency as well as to discover novel steroidal compounds and acquire recombinant microorganisms capable of performing desired steroid biotransformations.

Haloperoxidases have been the only halogenating enzymes know for more than 35 years. They produce in general free hypohalous acid which acts as the halogenating agent and leads thus to a product formation almost identical to chemical halogenation reactions using electrophilic halogen species. With the detection of FADH2-dependent halogenases a type of enzymes was found that shows high substrate specificity and catalyzes regioselective halogenation reactions. FADH2-dependent halogenases are involved in many biosynthetic pathways and catalyze the halogenation of aromatic and aliphatic substrates activated for electrophilic attack. These enzymes also produce hypohalous acids. However, in contrast to haloperoxidases, the hypohalous acid generated cannot leave the active site but can only react with substrates at the active site resulting in selective halogenation reactions. FADH2-dependent halogenases are a twocomponent system with a flavin reductase as the second component. They show similarity to flavin-dependent monooxygenases in forming an enzyme-bound flavin hydroperoxide intermediate. This flavin hydroperoxide reacts with a halide to form hypohalous acid which may then react in a selective reaction with the organic substrate. For halogenation of unactivated carbon atoms, non-heme iron, α- ketoglutarate- and O2-dependent halogenases use a radical mechanism. A substrate radical intermediate which is formed by abstraction of a H. abstracts a halide radical from the non-heme iron coordination complex resulting in the formation of a halogenated methyl group. While in vitro application of both types of halogenases is problematic, due to issues with cofactor recycling, in vivo use which overcomes this problem seems to have a very high potential.

Oxidative enzymes constitute privileged catalysts in organic synthesis. Environmentally benign reaction conditions along with high selectivity are the most appealing characteristic shown by these biocatalysts in contrast to classical metal-based reagents. The present review surveys new perspectives and concepts derived from oxidative enzymatic processes, comprising oxidative C-C bond forming reactions, atroposelective oxidations, oxidative dynamic processes, interconnected reactions, cyclic deracemizations, oxidative desymmetrizations and artificial oxidative enzymes. Selected examples taken from the recent literature are discussed, highlighting relevant aspects from a synthetic point of view. Thus, application of these biocatalyzed reactions in the preparation of chiral high-added value compounds is also outlined. Finally, future perspectives for the development of novel oxidative enzymatic processes and further applications of well settled ones are presented.

Cashew nut shell liquid (CNSL) is a largely available by-product of the cashew agro-industry. In this paper, the main constituents (cardanol, cardol and methylcardol) obtained after separation of the CNSL mixture have been used as starting material to prepare a variety of novel mono- and bis-benzoxazines via cyclization reactions with formaldehyde and aromatic amine derivatives under mild conditions. These model reactions were successfully extended to the cardanol and cardols mixture affording the one pot synthesis monoand bis-benzoxazines.