Spirotryprostatin A

Biosynthesis of fungal nonribosomal peptides frequently involves redox enzymes such as flavin-containing monooxygenase (FMO) to introduce complexity into the core chemical structure. One such example is the formation of spiro-carbons catalyzed by various oxidases. Because many chemically complex spiro-carbon-bearing natural products exhibit useful biological activities, understanding the mechanism of spiro-carbon biosynthesis is of great interest. We previously identified FqzB, an FMO from the fumiquinazoline biosynthetic pathway responsible for epoxidation of fumiquinazoline F that crosstalks with the fumitremorgin biosynthetic pathway to form spirotryprostatin A via epoxidation of the precursor fumitremorgin C. What makes FqzB more interesting is its relaxed substrate specificity, where it can accept a range of other substrates, including tryprostatins A and B along with its original substrate fumiquinazoline F. Here, we characterized FqzB crystallographically and examined FqzB and its site-specific mutants kinetically to understand how this promiscuous epoxidase works. Furthermore, the mutagenesis studies as well as computational docking experiments between the FqzB crystal structure and its known substrates spirotryprostatin A and B, as well as fumitremorgin C and fumiquinazoline F, provided insight into potential modes of substrate recognition and the source of broad substrate tolerance exhibited by this epoxidase. This study serves as a foundation for further characterization and engineering of this redox enzyme, which has potential utility as a valuable catalyst with broad substrate tolerance and an ability to introduce chemical complexity into carbon frameworks for chemoenzymatic and biosynthetic applications.

In this paper, we disclosed a novel enantioselective total synthesis of spirotryprostatin A (1) in 15 steps with a 7.4% total yield from commercially available 2-iodo-5-methoxyaniline and γ-butyrolactone. The key step features of this synthesis include the copper-catalyzed cascade reaction of o-iodoaniline derivatives with alkynone to introduce the quaternary carbon stereocenter and an aza-Michael tandem reaction to construct the spiro[pyrrolidine-3,3'-oxindole] moiety.

Cyclodipeptide 2,5-diketopiperazines (DKP) are privileged structural units present in drugs and natural alkaloids. This work reports a new method for the synthesis of biologically important DKP scaffolds based on an intramolecular nucleophilic α-addition of general amides towards an alkynamide system. The utility of this umpolung cyclization mediated by trimethyl phosphine and l-glutamic acid is highlighted by its application to the concise total syntheses of 6-methoxyspirotryprostatin B (the first total synthesis), spirotryprostatin A, and spirotryprostatin B.