Constituents of Calamintha ashei: Effects on Florida sandhill species

Abstract

Chemical constituents of the perennial shrub Calamintha ashei have been characterized as part of our investigation of the allelopathic properties of this plant. Besides the known monoterpenes, (+)-evodone, (-)-calaminthone and (+)-desacetylcalaminthone, fresh aerial parts of C. ashei provided six new menthofurans, two new germacrane sesquiterpenes, and the six, known flavonoids: 5-desmethoxynobiletin, 5-hydroxy-6,7,8,4′-tetramethoxyflavone, 5,4′-dihydroxy-6,7,8,3′-tetramethoxyflavone, thymonin, 5,4′-dihydroxy-6,7-dimethoxyflavone and 6-hydroxy-7,3′- dimethoxyluteolin. The structures of the new compounds were elucidated by spectroscopic methods and comparison of their 1H NMR spectra with those of structurally related compounds. The molecular structures of (+)-evodone, 5-desmethoxynobiletin (5-hydroxy-6,7,8,3′,4′- pentamethoxyflavone) and the triacetate of thymonin (7,8,3′-trimethoxy-5, 6,4′-triacetoxyflavone), were determined by single crystal X-ray diffraction. Saturated aqueous solutions of menthofuran, (+)-evodone, (-)-calaminthone, (+)-desacetylcalaminthone, 4α,5β- diacetoxymenthofuran, as well as mixtures of (+)-evodone and (+)-desacetylcalaminthone inhibited the germination and root growth of Schizachyrium scoparium and Leptochloa dubia, two native Florida sandhill grasses, as well as Lactuca sativa. (+)-Evodone and (+)-desacetylcalaminthone were the most active. 5-Hydroxy-6,7,8,3′,4′-pentamethoxyflavone and 5,4′-dihydroxy-6,7,8,3′-tetramethoxyflavone and mixtures of the two flavonoids in aqueous as well as in saturated aqueous solutions of ursolic acid were tested on the same species, but showed no significant activity.