Dynamically Complex [6+4] and [4+2] Cycloadditions in the Biosynthesis of Spinosyn A

Abstract

SpnF, an enzyme involved in the biosynthesis of spinosyn A, catalyzes a transannular Diels–Alder reaction. Quantum mechanical computations and dynamic simulations now show that this cycloaddition is not well described as either a concerted or stepwise process, and dynamical effects influence the identity and timing of bond formation. The transition state for the reaction is ambimodal and leads directly to both the observed Diels–Alder and an unobserved [6+4] cycloadduct. The potential energy surface bifurcates and the cycloadditions occur by dynamically stepwise modes featuring an “entropic intermediate”. A rapid Cope rearrangement converts the [6+4] adduct into the observed [4+2] adduct. Control of nonstatistical dynamical effects may serve as another way by which enzymes control reactions.

Oxidative Cyclization in Natural Product Biosynthesis

“…This modest but real acceleration indicates that many of the diene/dienophile substructures are poised to react nonenzymatically and the enzymes may be lowering energy barriers but not changing the intrinsic reactivity profiles of their substrates. While a transannular [4+2] mechanism can be written to produce the 6,5 fused bicyclic 585 in the SpnF reaction, calculations of the energy barriers 423 have also suggested that a [6+4] electrocyclization could proceed to first form 586 , followed by a rapid Cope rearrangement to get to the observed product 586 . Glycosylation of 585 by the glycosyltransferase SpnG affords 587.…”

Natural [4 + 2]-Cyclases

“…30 This saddle point was therefore designated as an ambimodal transition state (represented by 69 in Scheme 7), because it can lead directly to two different products without an intermediary local minimum in potential energy. 147–152 Furthermore, the steepest path of decent from the transition state led to 72 , which calculations indicated 30 would readily undergo a Cope rearrangement to generate 63 thereby explaining why the former compound is not observed experimentally. 135,140 At face value, these computations suggested that the nonenzymatic reaction proceeds by both concerted and stepwise pathways.…”

“…In other words, the region where trajectories were briefly trapped in the MD simulations ( 70 ) corresponds to a formal intermediate in what the authors described as a 10- π -electron aromatic state. 30 …”

“…10,21,23,26 It has been concluded only relatively recently that most Diels-Alder reactions, including the reaction between ethylene and butadiene, is concerted and proceeds through a single, pericyclic transition state; 10,23,27–29 however, examples abound where intermediates (e.g., biradicals) have been implicated. 21,26,30 Nevertheless, as the case of macrophomate synthase discussed below shows, some intermediates evidently disqualify a stereospecific [4 + 2]-cycloaddition from being a Diels-Alder reaction.…”

Recent advances of Diels–Alderases involved in natural product biosynthesis

“…Most recently, quantum mechanical computations and dynamic simulations suggested that the observed [4+2] cycloadduct is constructed by a [6+4] cycloaddition followed by a Cope rearrangement. 43 In the recent studies on thiazoyl peptides synthesized ribosomally, 44 an interest has been focusing on the formation of a central pyridine ring at the junction of the macrocyclic ring system, which may be biosynthesized by an intramolecular hetero-Diels-Alder reaction of two dehydroalanines and a neighboring carboxyl group (Figure 2). Gene knockout experiments and biochemical analysis of leader peptide modification enzymes facilitated the identification of crucial linear precursor peptides and putative DAases.…”