Diels–Alderase-free, bis-pericyclic, [4+2] dimerization in the biosynthesis of (±)-paracaseolide A

Abstract

The natural product paracaseolide A is a tetracyclic dilactone containing six adjacent stereocenters. It has an unprecedented skeleton and occupies unique structural space among the >200,000 characterized secondary metabolites. Six different research groups have reported a chemical synthesis of this compound, five of which used a thermal, net Diels–Alder [4+2] cycloaddition and dehydration at 110 °C to access the target by dimerization of a simple butenolide precursor. Here we report that this dimerization proceeds under much milder conditions and with a different stereochemical outcome than previously recognized. This can be rationalized by invoking a bis-pericyclic transition state. Furthermore, we find that spontaneous epimerization, necessary to correct the configuration at one key stereocenter, is viable and that natural paracaseolide A is racemic. Together these facts point to the absence of enzymatic catalysis (i.e., Diels–Alderase activity) in the cycloaddition and strongly suggest that a non-enzyme-mediated dimerization is the actual event by which paracaseolide A is produced in Nature.

Natural [4 + 2]-Cyclases

“…This is consistent with known pathways where nonenzymatic [4 + 2]-cycloadditions are believed to be the operating biosynthetic transformation such as in the biosynthesis of paracaseolide A. 225 Preorganization in the active sites of the priming, auxiliary or down-pathway enzymes may impose stereochemical constraints on the cyclization following the activation step and even accelerate the cyclization itself. Eventually, the priming activity of the enzyme may no longer be necessary and lost leaving the enzyme as a stand-alone cyclase like VstJ, PyrE3 and PyrI4.…”

Biomimetic syntheses of racemic natural products

“…It exhibits a variety of biological activities including inhibition of relevant phosphatases, kinase inhibition related to cell cycle regulation, and signal transduction activity in the insulin biosynthetic pathway. Hoye recognized that the optical rotation of +0.9 reported for paracaseolide A was within experimental error of zero and that it could be a racemate . This would be consistent with Hoye's proposed biosynthesis of paracaseolide A via a nonenzymatic Diels‐Alder dimerization (Scheme ).…”

“…Dehydration and epimerization of 14 at 7c gave (±)‐paracaseolide. The dimerization was surprisingly exo selective (diene approaches the dienophile exo to the carbonyl group of the dienophile) (Scheme ) . Epimerization studies at room temperature in deuterated methanol showed that the proton at 7c was exchanged spontaneously in a unimolecular process with a half‐life of 4 hours.…”

Non-enzymatic pyridine ring formation in the biosynthesis of the rubrolone tropolone alkaloids

“…Non-enzymatic Diels–Alder reactions have been widely observed en route to natural products such as paracaseolide A (ref. 43 ). Additional pericyclic reactions have been observed to generate impressive diversity of fungal polyketides by exploiting the intrinsic reactivity of an isochromene intermediate 44 .…”