Review on Recent Developments in Biocatalysts for Friedel–Crafts Reactions

RSM optimization of Friedel-Crafts C-acylation of para-fluorophenol over the catalysis of phosphomolybdic acid encapsulated in MIL-53 (Fe) metal organic frameworks

In this research, a heterogeneous acid catalyst was synthesized by room temperature encapsulation of phosphomolybdic acid (PMA) in the pores of the MIL-53 (Fe) metal organic framework (MOF) under ultrasonic conditions. Then the catalytic activity of PMA@MIL-53 (Fe) was investigated in Friedel-Crafts C-acylation of para-fluorophenol, and this procedure was optimized using response surface methodology based on central composite design (RSM-CCD). The impact of critical reaction parameters including reaction duration, catalyst dosage, and PMA amount in the catalyst was optimized, leading to the formation of the target product in excellent yield at a short reaction time.

Friedel-Crafts reactions for biomolecular chemistry

Chemical tools and principles have become central to biological and medical research/applications by leveraging a range of classical organic chemistry reactions. Friedel-Crafts alkylation and acylation are arguably some of the most well-known and used synthetic methods for the preparation of small molecules but their use in biological and medical fields is relatively less frequent than the other reactions, possibly owing to the notion of their plausible incompatibility with biological systems. This review demonstrates advances in Friedel-Crafts alkylation and acylation reactions in a variety of biomolecular chemistry fields. With the discoveries and applications of numerous biomolecule-catalyzed or -assisted processes, these reactions have garnered considerable interest in biochemistry, enzymology, and biocatalysis. Despite the challenges of reactivity and selectivity of biomolecular reactions, the alkylation and acylation reactions demonstrated their utility for the construction and functionalization of all the four major biomolecules (i.e., nucleosides, carbohydrates/saccharides, lipids/fatty acids, and amino acids/peptides/proteins), and their diverse applications in biological, medical, and material fields are discussed. As the alkylation and acylation reactions are often fundamental educational components of organic chemistry courses, this review is intended for both experts and nonexperts by discussing their basic reaction patterns (with the depiction of each reaction mechanism in the ESI) and relevant real-world impacts in order to enrich chemical research and education. The significant growth of biomolecular Friedel-Crafts reactions described here is a testament to their broad importance and utility, and further development and investigations of the reactions will surely be the focus in the organic biomolecular chemistry fields.

Using BpyAla to generate copper artificial metalloenzymes: a catalytic and structural study

Artificial metalloenzymes (ArMs) have emerged as a promising avenue in the field of biocatalysis, offering new reactivity. However, their design remains challenging due to the limited understanding of their protein dynamics and how the introduced cofactors alter the protein scaffold structure. Here we present the structures and catalytic activity of novel copper ArMs capable of (R)- or (S)-stereoselective control, utilizing a steroid carrier protein (SCP) scaffold. To incorporate 2,2'-bipyridine (Bpy) into SCP, two distinct strategies were employed: either Bpy was introduced as an unnatural amino acid (2,2'-bipyridin-5-yl)alanine (BpyAla) using amber stop codon expression or via bioconjugation of bromomethyl-Bpy to cysteine residues. The resulting ArMs proved to be effective at catalysing an enantioselective Friedel-Crafts reaction with SCP_Q111BpyAla achieving the best selectivity with an enantioselectivity of 72% ee (S). Interestingly, despite using the same protein scaffold, different attachment strategies for Bpy at the same residue (Q111) led to a switch in the enantiopreference of the ArM. X-ray crystal structures of SCP_Q111CBpy and SCP_Q111BpyAla ArMs with bound Cu(ii) ions unveiled crucial differences in the orientation of the catalytic centre. Combining structural information, alanine scanning studies, and computational analysis shed light on the distinct active sites of the ArMs, clarifying that these active sites stabilise the nucleophilic substrate on different sides of the electrophile leading to the observed switch in enantioselectivity. This work underscores the importance of integrating structural studies with catalytic screening to unravel the intricacies of ArM behaviour and facilitate their development for targeted applications in biocatalysis.

Electrifying Friedel-Crafts Intramolecular Alkylation toward 1,1-Disubstituted Tetrahydronaphthalenes

In this work, we successfully employed electrochemical conditions to promote a Hofer-Moest, intramolecular Friedel-Crafts alkylation sequence. The reaction proceeds under mild conditions, employing carboxylic acids as starting materials. Notably, the electrochemical process performed in batch was adapted to a continuous flow electrolysis apparatus to provide a significant improvement. This catalyst-free, electrochemical approach produces an array of tetrahydronaphthalenes that could be used for API synthesis.

Biosynthesis of 4-Acyl-5-aminoimidazole Alkaloids Featuring a New Friedel-Crafts Acyltransferase

Friedel-Crafts acylation (FCA) is a highly beneficial approach in organic chemistry for creating the important C-C bonds that are necessary for building intricate frameworks between aromatic substrates and an acyl group. However, there are few reports about enzyme catalyzed FCA reactions. In this study, 4-acyl-5-aminoimidazole alkaloids (AAIAs), streptimidazoles A-C (1-3), and the enantiopure (+)-nocarimidazole C (4) as well as their ribosides, streptimidazolesides A-D (5-8), were identified from the fermentation broth of Streptomyces sp. OUCMDZ-944 or heterologous S. coelicolor M1154 mutant. The biosynthetic gene cluster (smz) was identified, and the biosynthetic pathway of AAIAs was elucidated for the first time. In vivo and in vitro studies proved the catalytic activity of the four essential genes smzB, -C, -E, and -F for AAIAs biosynthesis and clarified the biosynthetic process of the alkaloids. The ligase SmzE activates fatty acyl groups and connects them to the acyl carrier protein (ACP) holo-SmzF. Then, the acyl group is transferred onto the key residue Cys49 of SmzB, a new Friedel-Crafts acyltransferase (FCase). Subsequently, the FCA reaction between the acyl groups and 5-aminoimidazole ribonucleotide (AIR) occurs to generate the key intermediate AAIA-nucleotides catalyzed by SmzB. Finally, the hydrolase SmzC catalyzes the N-glycosidic bond cleavage of the intermediates to form AAIAs. Structural simulation, molecular modeling, and mutational analysis of SmzB showed that Tyr26, Cys49, and Tyr93 are the key catalytic residues in the C-C bond formation of the acyl chain of AAIAs, providing mechanistic insights into the enzymatic FCA reaction.

Template-Directed Incorporation of Functional Molecules into DNA

We report a versatile method for the incorporation of functional molecules into oligonucleotides carrying reactive groups by using a template-directed postsynthetic approach in the solution phase. For this purpose, we prepared oligonucleotides carrying an amino group on the backbone by using an acylic threoninol scaffold. The resulting oligonucleotides could be used to introduce almost any molecule carrying aldehyde, which can be, among other things, a metal-binding ligand or a fluorophore. In our study, we incorporated aldehyde-bearing phenanthroline, a metal-binding ligand, into oligonucleotides by template-directed reversible conjugation. We observed that the use of an abasic sugar site instead of a natural nucleobase in the template strand increased the yield of conjugation and induced selective incorporation of the phenanthroline. This method could lead progress in the development of probes for the recognition of abasic regions in duplex DNA. Moreover, template-directed formation of metal ligand-oligonucleotide conjugates might have potential applications in hybrid biocatalysis for enantioselective transformations.

Mutation of the eunicellane synthase Bnd4 alters its product profile and expands its prenylation ability

Bnd4 catalyzes the first committed step in the biosynthesis of the bacterial diterpenoid benditerpenoic acid and was the first eunicellane synthase identified from nature. We investigated the catalytic roles of the aromatic residues in the active site of Bnd4 through a series of mutation studies. These experiments revealed that large hydrophobic or aromatic side chains are required at F162 and Y197 for eunicellane formation and that selected mutations at W316 converted Bnd4 into a cembrane synthase. In addition, the Bnd4Y197A variant expanded the native prenylation ability of Bnd4 from accepting C5 and C10 prenyl donors to C15. This study supports the mechanism of eunicellane formation by Bnd4 and encourages further engineering of terpene synthases into practical and efficient prenyltransferases.