Biocatalytic Friedel-Crafts Reactions

Abstract

Friedel-Crafts alkylation and acylation reactions are important methodologies in synthetic and industrial chemistry for the construction of aryl-alkyl and aryl-acyl linkages that are ubiquitous in bioactive molecules. Nature also exploits these reactions in many biosynthetic processes. Much work has been done to expand the synthetic application of these enzymes to unnatural substrates through directed evolution. The promise of such biocatalysts is their potential to supersede inefficient and toxic chemical approaches to these reactions, with mild operating conditions - the hallmark of enzymes. Complementary work has created many bio-hybrid Friedel-Crafts catalysts consisting of chemical catalysts anchored into biomolecular scaffolds, which display many of the same desirable characteristics. In this Review, we summarise these efforts, focussing on both mechanistic aspects and synthetic considerations, concluding with an overview of the frontiers of this field and routes towards more efficient and benign Friedel-Crafts reactions for the future of humankind.

Keywords: Acylation; Artificial Enzymes; Biocatalysis; DNA Catalysis; Enzymes; Friedel-Crafts Alkylation.

Scheme 1

Mechanism of action of the PLP dependent enzymes TrpS and TPL. For TPL, tyrosine synthesis, as shown, is the reverse process with respect to natural reactivity. Amino acrylate intermediates are formed from either serine or pyruvate and ammonia by reaction with lysine‐bound PLP, which can undergo nucleophilic attack by indole or phenol. The amino acid products are released after transimination by the same lysine residue. P=−OPO₃ ²⁻.

Scheme 2

Early examples of tryptophan analogue synthesis exploiting the natural substrate promiscuity of tryptophan synthase from E. coli. (A) or S. enterica (B) for non‐native indole derivatives, full reaction conditions can be found in references.

Figure 1

A. TrpA (pink) provides allosteric activation to TrpB (blue), which is responsible for tryptophan synthase activity in the heterodimeric complex from Pyrococcus furiosus, the lysine‐bound PLP cofactor is shown as yellow spheres with hetero‐atoms coloured (PDB: 5E0K). B. Screening libraries of PfTrpB over multiple rounds produced a ‘stand‐alone’ variant bearing six mutations, PfTrpB^2B9 that does not require the TrpA subunit. Positions of the mutations are shown as spheres on the PfTrpB structure and the lysine‐bound PLP cofactor is shown as in (A) (PDB: 5DVZ). C. Some of all of these mutations could be transferred by homology to TrpB homologues from Thermotoga maritima (TmTrpB) and Archaeoglobus fulgidus (AfTrpB) to create further ‘stand‐alone’ TrpB enzymes. TmTrpB already contains alanine at position 321 in the wild‐type sequence and thus this mutation did not need to be transferred. Structures shown were produced with AlphaFold, obtained from the uniprot database.

Scheme 3

A. tryptophan analogues with β‐alkyl substituents synthesised from e. g., threonine and indole using various tryptophan synthase mutants.[ ²⁵ , ²⁶ ] B. Synthesis of tryptophan derivatives with deactivating substituents on the indole moiety using a variety of engineered TrpB mutants. C. Synthesis of a deep‐blue fluorescent amino acid from azulene by an engineered TrpB mutant. See references for reaction conditions and complete description of mutations.

Figure 2

A. active site of CfTPL showing the lysine‐bound PLP cofactor as balls and sticks coloured according to the atom type, the positions of important mutations are shown as spheres (PDB: 2EZ1). B. Synthesis of 3‐substituted tyrosine derivatives in one pot from ortho‐substituted phenols using a rationally designed CfTPL mutant. C. Preparative synthesis of unnatural amino acids with application in biochemical studies employing CfTPL mutants identified by screening site‐saturation libraries with TLC. Full details of reaction conditions can be found in the references listed.

Scheme 4

Cascade reactions involving TPL. A. In situ P450 catalysed synthesis of phenols as substrates for TPL to form 3‐substituted tyrosine derivatives. B. Derivatisation of the tyrosine products of TPL by sequential de‐amination and decarboxylation to form styrenes that can be optionally further hydrated in a regio‐ and enantioselective manner.[ ⁴² , ⁴³ ]

Scheme 5

A. aTPases from a variety of fungal organisms that prenylate tryptophan regioselectively at the 4‐, 5‐, 6‐ or 7‐ positions.[ ^48a , ^48b , ^48d , ⁵³ ] B. aTPases that can accept non‐natural prenylation substrates.[ ⁵¹ , ⁵² ] See references for detailed reaction conditions. OPP=pyrophosphate.

Figure 3

A. crystal structure of AtaPT from Aspergillus terreus (PDB: 5KCL) with (inlay) tyrosine shield thought to be responsible for substrate activation. The substrate analogue dimethylallyl S‐thiolodiphosphate (DMASPP) is shown in in blue with the heteroatoms coloured. B. Application of AtaPT to modulate the anti‐cancer activity of genistein, major products shown, minor products are other regioisomers. See reference for detailed reaction conditions.

Scheme 6

Prenylation of positional isomers of tyrosine as well as para‐amino phenylalanine by FgaPT2 and a single mutant.[ ⁵⁶ , ⁵⁷ ] See reference for detailed reaction conditions.

Scheme 7

Geranylation of hydroxy‐benzoic acid by XimB, where the substrates can be produced in vivo from glucose by an engineered E. coli strain. See reference for detailed reaction conditions.

Figure 4

Overlay of the crystal structures of Coq5 (orange, PDB: 4OBW), NovO (blue, PDB: 5MGZ) and (CouO green, PDB: 5 M58) and their native methylation reactivities. CouO is active for methylation at several points through the synthesis of the dimeric coumermycin A₁ biosynthesis, an intermediate with half of the total dimeric‐coumarin structure is shown.

Scheme 8

A. Chemical synthesis of SAM analogues by Lewis‐acid catalysis. Application of SAM analogues for the alkylation of (B.) coumarin derivatives by CouO and NovO and (C.) tyrosine by SacF and SfmM2.[ ^72a , ⁷⁴ ] D. In situ synthesis of SAM or SAE from CIDA by SalL, and their concurrent application as substrates in the methylation or ethylation of a coumarin substrate by NovO.

Figure 5

A. Heterotrimer structure (part of the larger heterododecamer) of PpATase with PG bound, and inlay showing key residues in the active site (PDB: 5MG5). B. Native reaction of PpATase. Application of PpATase (C.) with resorcinol and native and non‐native acyl‐donors, (D.) on a variety of resorcinol derivatives and, (E.) application of a single‐mutant for the transfer of bulky acyl‐groups to resorcinol.[ ^85b , ⁸⁸ ] See references for full description of reaction conditions.

Figure 6

A. Active site structure of AacSHC showing the catalytic aspartic acid residue, and residues targeted for mutagenesis shown as spheres (PDB: 1UMP). B. Unnatural reactions catalysed by AacSHC mutants where the wild‐type gives less than 1 % conversion. C. Friedel‐Crafts alkylation and hydration side‐product by wild‐type AacSHC and the best performing double mutant identified. See references for detailed reaction conditions.

Figure 7

A. Action of CylK in the final biosynthetic step of Cylindrocyclophane F. Application of CylK with unnatural substrates with different halide groups (B.) and (C.) with simple resorcinols. D. Crystal structure of CylK (PDB:7FH6) reveals fused Ca²⁺ binding (light orange) and propeller (pink) domains, supported by many Ca²⁺ binding sites (Ca shown in blue). Important residues in the active site for binding chloride (green) and resorcinols are shown in the insert.

Figure 8

A. Biomolecular scaffolds to create bio‐hybrid catalysts including DNA and the proteins QacR (blue, PDB: 1JTY) and LmrR (orange, PDB: 3F8B) whose homodimeric structures are shown in different shades. Methods to assemble the biohybrid catalysts, including (B.) supramolecular assembly, and (C.) the use of unnatural amino acids incorporated into proteins biosynthetically.

Scheme 9

A. Inter‐ and (B.) intra‐molecular Friedel‐Crafts reactions catalysed by DNA catalysts consisting of copper(II) complexes bound to DNA. C. Tandem Friedel‐Crafts alkylation‐enantioselective protonation reaction by DNA catalysis. See references for full reaction conditions.

Scheme 10

A. Friedel‐Crafts alkylation catalysed by artificial enzymes based on copper(II) and MDR protein scaffolds.[ ¹¹⁶ , ¹¹⁸ , ¹¹⁹ , ¹²⁰ ] B. Organocatalytic artificial enzyme catalysed Friedel‐Crafts alkylation and (C.) tandem‐enantioselective protonation reactions. See references for full reaction conditions.