DIASTEREOSELECTION AND INVIVO INHIBITION OF 3-DEHYDROQUINATE SYNTHASE

Epimeric carbaphosphonate [1R-(1-alpha,3-alpha,4-beta,5-alpha)]-1,3,4-trihydroxy-5-(phosphonomethyl)cyclohexane-1-carboxylic acid and carbaphosphonate (1S-(1-alpha,3-beta,4-alpha,5-beta)]-1,3,4-trihydroxy-5-(phosphonomethyl)cyclohexane-1-carboxylic acid are potent inhibitors of purified 3-dehydroquinate (DHQ) synthase. Can either of these carbocyclic diastereomers inhibit DHQ synthase in intact plants? Obtaining sufficient amounts of both diastereomers for in vivo inhibition studies required the development of efficient synthetic routes to both molecules. Carbaphosphonate and epimeric carbaphosphonate were synthesized from a single epoxy alcohol derived from quinic acid. Payne rearrangement of the epoxy alcohol provided an equilibrium mixture of two diastereomeric epoxides which could then be converted into carbaphosphonate and epimeric carbaphosphonate. An essential step in these conversions entailed introduction of a phosphonomethyl functionality via regioselective, nucleophilic attack on the diastereomeric epoxides with lithium diisopropyl methanephosphonate. Enzyme inhibition in whole plants was measured by the accumulation of 3-deoxy-D-arabino-heptulosonic acid (DAH), the dephosphorylated substrate of DHQ synthase, in plant tissue subsequent to foliar application of the carbocyclic inhibitors. Accumulations of DAH of up to 40-fold increases over control DAH levels were observed when carbaphosphonate was applied to Pisum sativum, Echinochloa crusgalli, Setaria viridis, Sorghum halepense, and Avena fatua. Application of epimeric carbaphosphonate to the same range of plant species resulted in a maximum increase of only 2-fold in DAH concentration. This diastereoselection contrasts sharply with the comparable levels of in vitro inhibition of purified DHQ synthase observed for both carbaphosphonate and epimeric carbaphosphonate.