Biocatalytic Friedel-Crafts alkylation using non-natural cofactors.

The formation of C C bonds is a central aspect of synthetic organic chemistry. However, in biocatalysis only few enzymes capable to perform this reaction are known, among which aldolases, transketolases, and hydroxynitril lyases have been investigated thoroughly. Some have even found their way into industrial applications. Friedel–Crafts alkylation is a classic organic reaction of great importance. However, in particular for large scale application, this transformation is ecologically very critical and regiospecific monoalkylation is difficult to achieve. Therefore, an environmentally friendly and selective alternative would be highly desirable. In nature methyl groups are selectively introduced into reactive aromatic rings by methyltransferases (Mtases), in particular with S-adenosyl-lmethionine (SAM) as the cofactor. Furthermore, enzyme-catalyzed reactions are important for access to isoprenoids. Also, prenylation of aromatic rings has been performed. For phenylalanine ammonia lyases a Friedel–Crafts-type mechanism has been proposed. Recently, it has been shown that besides the methyl group other alkyl, alkenyl, and alkinyl groups can be introduced into S-adenosyl-l-homocystein. These modified cofactors of transferases were used for a sequence-specific alkylation of DNA. Having cofactor and modified cofactors in hand, we investigated the possibility of alkylation of aromatic substrates, thus transferring the biosynthesis into the laboratory (Scheme 1). Aminocoumarins are antibiotics produced by some Streptomyces species and are targets for the methyl transfer from the natural cofactor SAM. TheMtase A and B are involved in the biosynthesis of the antibiotics coumermycin A1 [7] (produced by Streptomyces rishiriensis) and novobiocin (produced by Streptomyces spheroides ; Scheme 1). SAM analogues were synthesized by modifying the strategy published by Klimašauskas, Weinhold, and co-workers. S-Adenosyl-l-homocysteine (SAH) was alkylated by seven different alkyl bromides using formic acid as the solvent and AgOTf as a Lewis acid activator and catalyst. We observed quantitative conversion in less than 2 days (average reaction time 24 h; Table 1). The chemical synthesis of SAM analogues results in approximately 1:1 diastereomeric mixtures at the sulfonium center. In the first screenings the diastereomers were separated by preparative HPLC and used as cofactors for the alkylation of coumarin compound 3a. Both epimers were accepted by the enzymes NovO and CouO Scheme 1. C-Mtases involved in the biosynthesis of the antibiotics coumermycin A1 in Streptomyces rishiriensis and Novobiocin in Streptomyces spheroides.