The Reaction Mechanism of Chromanol‐Ring Formation Catalyzed by tocopherol cyclase from Anabaena variabilis K ÜTZING (Cyanobacteria)

Incubation of the synthetic 18O‐labelled phytyl‐hydroquinone (O4‐18O)‐2 with the tocopherol cyclase from Anabaena variabilis KÜTZING (Cyanobacteria) in D2O furnished the doubly labelled γ‐tocopherol, (2R,3S,4′R,8′R)‐(1‐18O,3‐2H)‐1. The chirality at C(3) was determined by two independent routes providing interlocking evidence that the enzyme‐catalyzed ring closure proceeds by si‐protonation of the double bond of 2 and concomitant re‐attack of the phenolic O‐atom.