Journal or Book Title
Journal of the American Chemical Society
The secondary ent-beyeran-16-yl carbocation (7) is a key branch point intermediate in mechanistic schemes to rationalize the cyclic structures of many tetra- and pentacyclic diterpenes, including ent-beyerene, ent-kaurene, ent-trachylobane, and ent-atiserene, presumed precursors to >1000 known diterpenes. To evaluate these mechanistic hypotheses, we synthesized the heterocyclic analogues 16-aza-ent-beyerane (12) and 16-aza-ent-trachylobane (13) by means of Hg(II)- and Pb(IV)-induced cyclizations onto the ∆12 double bonds of tricyclic intermediates bearing carbamoylmethyl and aminomethyl groups at C-8. The 13,16-seco-16-norcarbamate (20a) was obtained from ent-beyeran-16-one oxime (17) by Beckmann fragmentation, hydrolysis, and Curtius rearrangement. The aza analogues inhibited recombinant ent-kaurene synthase from Arabidopsis thaliana (GST-rAtKS) with inhibition constants (IC50 ) 1 × 10-7 and 1 × 10-6 M) similar in magnitude to the pseudo-binding constant of the bicyclic ent-copalyl diphosphate substrate (Km ) 3 × 10-7 M). Large enhancements of binding affinities (IC50 ) 4 × 10-9 and 2 × 10-8 M) were observed in the presence of 1 mM pyrophosphate, which is consistent with a tightly bound ent-beyeranyl+/ pyrophosphate- ion pair intermediate in the cyclization-rearrangement catalyzed by this diterpene synthase. The weak inhibition (IC50 ) 1 × 10-5 M) exhibited by ent-beyeran-16-exo-yl diphosphate (11) and its failure to undergo bridge rearrangement to kaurene appear to rule out the covalent diphosphate as a free intermediate. 16-Aza-ent-beyerane is proposed as an effective mimic for the ent-beyeran-16-yl carbocation with potential applications as an active site probe for the various ent-diterpene cyclases and as a novel, selective inhibitor of gibberellin biosynthesis in plants.
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American Chemical Society
Roy, Amab; Roberts, Frank G.; Wilderman, P. Ross; Zhou, Ke; Peters, Reuben J.; and Coates, Robert M., "16-Aza-ent-beyerane and 16-Aza-ent-trachylobane: Potent Mechanism-Based Inhibitors of Recombinant ent-Kaurene Synthase from Arabidopsis thaliana" (2007). Biochemistry, Biophysics and Molecular Biology Publications. 11.