Progress toward the Total Synthesis of Lymphostins: Preparation of a Functionalized Tetrahydropyrrolo[4,3,2-de]quinoline and Unusual Oxidative Dimerization.

The lymphostins are a family of closely related pyrrolo[4,3,2-de]quinoline natural products produced by Streptomyces and Salinispora actinobacteria. Neolymphostin A was recently shown to strongly inhibit phosphoinositide 3-kinase (PI3K) and the mammalian target of rapamycin (mTOR) in a covalent manner via conjugation to a catalytic lysine residue in the ATP-binding pocket of the enzymes, making this metabolite the first reported covalent kinase inhibitor from a bacterium. A flexible and efficient synthetic route toward these alkaloids would allow for improvements in their solubility, stability, and selectivity and help to deliver a viable drug candidate. We have since established a short synthesis to methyl 8-bromo-1,3,4,5-tetrahydropyrrolo[4,3,2-de]quinoline-4-carboxylate via a conjugate addition/intramolecular Ullman reaction sequence. However, attempts to oxidize this intermediate to the pyrrolo[4,3,2-de]quinoline characteristic of the lymphostins resulted in formation of either a 2-oxo-1,2-dihydropyrrolo[4,3,2-de]quinoline or an unusual N,C-linked tetrahydropyrroloquinoline-pyrroloquinoline heterodimer. We expect that key modifications to the tetrahydropyrroloquinoline intermediate prior to oxidation should prevent these side reactions and pave the way for the completion of the synthesis.

Nature's Combinatorial Biosynthesis Produces Vatiamides A-F.

Hybrid type I PKS/NRPS biosynthetic pathways typically proceed in a collinear manner wherein one molecular building block is enzymatically incorporated in a sequence that corresponds to gene arrangement. In this work, genome mining combined with the use of a fluorogenic azide-based click probe led to the discovery and characterization of vatiamides A-F, three structurally diverse alkynylated lipopeptides, and their brominated analogues, from the cyanobacterium Moorea producens ASI16Jul14-2. These derive from a unique combinatorial non-collinear PKS/NRPS system encoded by a 90 kb gene cluster in which an upstream PKS cassette interacts with three separate cognate NRPS partners. This is facilitated by a series of promiscuous intermodule PKS-NRPS docking motifs possessing identical amino acid sequences. This interaction confers a new type of combinatorial capacity for creating molecular diversity in microbial systems.

Neolymphostin A Is a Covalent Phosphoinositide 3-Kinase (PI3K)/Mammalian Target of Rapamycin (mTOR) Dual Inhibitor That Employs an Unusual Electrophilic Vinylogous Ester.

Using a novel chemistry-based assay for identifying electrophilic natural products in unprocessed extracts, we identified the PI3-kinase/mTOR dual inhibitor neolymphostin A from Salinispora arenicola CNY-486. The method further showed that the vinylogous ester substituent on the neolymphostin core was the exact site for enzyme conjugation. Tandem MS/MS experiments on PI3Kα treated with the inhibitor revealed that neolymphostin covalently modified Lys802 with a shift in mass of +306 amu, corresponding to addition of the inhibitor and elimination of methanol. The binding pose of the inhibitor bound to PI3Kα was modeled, and hydrogen-deuterium exchange mass spectrometry experiments supported this model. Against a panel of kinases, neolymphostin showed good selectivity for PI3-kinase and mTOR. In addition, the natural product blocked AKT phosphorylation in live cells with an IC50 of ∼3 nM. Taken together, neolymphostin is the first reported example of a covalent kinase inhibitor from the bacterial domain of life.