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(Il)- and Pb(IV)-induced cyclizations onto the A12 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 X 10(-7) and 1 X 10(-6) M) similar in magnitude to the pseudo-binding constant of the bicyclic ent-copalyl cliphosphate substrate (K-m = 3 x 10-7 M). Large enhancements of binding affinities (ICK = 4 x 10(-9) and 2 x 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 x 10(-5) M) exhibited by ent-beyeran-16-exo-yl diphosphate (111) and its failure to undergo bridge rearrangement to kaurene appear to rule out the covalent cliphosphate as a free intermediate. 16-Aza-ent-beyerane is proposed as an effective mimic for the ent-beyeran-1 6-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.