Root Inhibitors in Eucalyptus grandis. 2. Synthesis of the Inhibitors and Origin of the Peroxide Linkage

Abstract

Syntheses are described for the three inhibitors G₃ (1-hydroxy- 4,4,8,8,10,10-hexamethyl-2,3-dioxabicyclo[4,4,0]dec-5-ene-7,9-dione), G₁ and G₂ (the diastereoisomers 4-ethyl-1-hydroxy-4,8,8,10,10-pentamethyl- 2,3-dioxabicyclo[4,4,0]dec-5-ene-7,9-dione). Knoevenagel condensation of syncarpic acid with 2-methylpropanal or 2-methylbutanal respectively is followed by slow spontaneous uptake of atmospheric oxygen and cyclization to give the products named.     Isolation of the naturally occurring inhibitors from Eucalyptus grandis in an atmosphere of ¹⁸O₂ results in no incorporation of isotopic oxygen into the peroxide linkage; this indicates that this structural feature was present in the inhibitors in vivo.