The Diels-Alder reaction is a cornerstone of modern organic synthesis. Despite this, it remains essentially inaccessible to biosynthetic approaches. Only a few natural enzymes catalyze,even a formal [4 + 2] cycloaddition, and it remains uncertain if any of them proceed via the Diels-Alder mechanism. In this study, we focus on the [4 + 2] cycloaddition step in the biosynthesis of spinosyn A, a reaction catalyzed by SpnF enzyme, one of the most promising "true Diels-Alderdse" candidates. The four currently proposed mechanisms (including the Diels-Alder one) for this reaction in water (as a first-order approximation of the enzymatic reaction) are evaluated by an exhaustive quantum mechanical search for possible transition states (728 were found in total). We find that the line between the recently proposed biS-pericyclic [J. Am. Chem. Soc. 2016, 138 (11), 3631] and Diels Alder routes is blurred; and favorable transition states of both types may coexist. Application of the Curtin-Hammett principle, however, reveals that the bis-pericyclic mechanism accounts, for similar to 83% of the reaction flow in water, while the classical Diels Alder mechanism contributes only similar to 17%. The current findings provide a route for modeling this reaction inside the SpnF active site and inferring the catalytic architecture of possible Diels-Aldetases.