A combined approach toward syntheses of epoxyguaiane sesquiterpenes is presented. By use of a fungus sesquiterpene cyclase, guaian-6,10(14)-diene was produced through metabolic engineering of the isoprenoid pathway in E. coli. (−)-Englerin A, (−)-oxyphyllol, (+)-orientatol E, and (+)-orientalol F have been synthesized in two to six steps. This strategy provided rapid access to the epoxyguaiane core structure and would facilitate syntheses of (−)-englerin A and its analogues for evaluation of their therapeutic potentials in drug discovery.Letter pubs.acs.org/OrgLett

Stereoselective syntheses of terpenoids in a more efficient manner have been a long-term pursuit for synthetic chemists. Herein we describe the two-step, enantiospecific and protecting-group-free synthesis of (+)-schisanwilsonene A from a carotane compound, which was produced in E. coli. We also completed the first enantiomeric synthesis of (+)-tormesol in five steps. The two-stage strategy offers a step- and redox-economical approach to prepare terpene natural products and their analogues.

The Lewis acid-catalyzed Friedel–Crafts alkylation of an aromatic ring with an alkyl halide is extensively used in organic synthesis. However, its biological counterpart was not reported until the elucidation of the cylindrocyclophane biosynthetic pathway in Cylindrospermum licheniforme ATCC 29412 by Balskus and co-workers. CylK is the key enzyme that catalyzes the formation of the cylindrocyclophane scaffold through the Friedel–Crafts alkylation reactions with regioselectivity and stereospecificity. Further research demonstrates that CylK can accept other resorcinol rings and secondary alkyl halides as substrates. To date, the three-dimensional structure of CylK has not been disclosed and the catalytic mechanism remains obscure. Herein we report the crystal structures of CylK alone and complexed with substrate and product analogues. The structures reveal an unprecedented fusion of an RTX-like domain and a seven-bladed β-propeller domain, and the active site architecture that defines the substrate binding mode. Combining the crystal structures, free energy simulations, and the site-directed mutagenesis experiments, we proposed a concerted double-activation mechanism, which could explain the regioselectivity and stereospecificity of this unprecedented enzymatic alkylation consistently. This work provides a foundation for engineering CylK as a biocatalyst to expand its substrate scope and applications in organic synthesis.

The Lewis acid-catalyzed Friedel-Crafts alkylation of an aromatic ring with an alkyl halide is extensively used in organic synthesis. However, its biological counterpart was not reported until the elucidation of the cylindrocyclophane biosynthetic pathway in Cylindrospermum licheniforme ATCC 29412 by Balskus and co-workers. CylK is the key enzyme to catalyze the formation of the cylindrocyclophane scaffold through the Friedel-Crafts alkylation reactions with regioselectivity and stereospecificity. Further research demonstrates that CylK can accept other resorcinol rings and secondary alkyl halides as substrates. To date, the crystal structure of CylK has not been disclosed and the catalytic mechanism remains obscure. Herein we report the crystal structures of CylK in its apo form and its complexes with the analogues of its substrate and reaction intermediate. Combining the crystal structures, free energy simulations and the mutagenesis experiments, we proposed a concerted double-activation mechanism, which could explain the regioselectivity and stereospecificity. This work provides a foundation for engineering CylK as a biocatalyst to expand its substrate scope and applications in organic synthesis.

The Lewis acid-catalyzed Friedel-Crafts alkylation of an aromatic ring with an alkyl halide is extensively used in organic synthesis. However, its biological counterpart was not reported until the elucidation of the cylindrocyclophane biosynthetic pathway in Cylindrospermum licheniforme ATCC 29412 by Balskus and co-workers. CylK is the key enzyme to catalyze the formation of the cylindrocyclophane scaffold through the Friedel-Crafts alkylation reactions with regioselectivity and stereospecificity. Further research demonstrates that CylK can accept other resorcinol rings and secondary alkyl halides as substrates. To date, the crystal structure of CylK has not been disclosed and the catalytic mechanism remains obscure. Herein we report the crystal structures of CylK in its apo form and its complexes with the analogues of its substrate and reaction intermediate. Combining the crystal structures, free energy simulations and the mutagenesis experiments, we proposed a concerted double-activation mechanism, which could explain the regioselectivity and stereospecificity. This work provides a foundation for engineering CylK as a biocatalyst to expand its substrate scope and applications in organic synthesis.