Structure‐Proving Syntheses of the Polyenoyltetramic Acids Pyranonigrin J and I

Abstract

AbstractThe polyenoyltetramic acids pyranonigrin J (3) and pyranonigrin I (4) had been isolated from Aspergillus niger. Their origins from and roles in biosynthesis as well as the S‐configurations of their stereocenter had been deduced from expression experiments with modifications of the corresponding gene cluster. We corroborated this stereochemical assignment after executing the first total syntheses of both compounds because they had essentially the same specific rotations as their natural counterparts. Our syntheses used the β‐ketothioester 18 as a conjunctive reagent. It was combined with the l‐serine derivatives (S)‐19 or (S)‐21 (“Western building blocks”), respectively, through aminolyses. Stille couplings with the stannane 13 (“Eastern building block”) followed. The resulting β‐ketoamides (S)‐11 and (S)‐12 underwent desilylative Lacey‐Dieckmann cyclizations when exposed to 8 equiv. of Bu4NF. They rendered the polyenoyltetramic acids (S)‐26 [acidolysis: → (S)‐3] and (S)‐4, respectively.