Castilleja rhexifolia

From the seeds of Castilleja rhexifolia deoxyrhexifoline (24) was isolated (35). The molecular ion was observed at m/z 191, 16 amu less than 23, and the difference of one oxygen atom was confirmed by high-resolution measurement. The base peak was observed at m/z 176 due to loss of a methyl radical. No hydroxyl absorption was seen in the IR spectrum, and the NMR spectrum showed no downfield oxymethine proton. Rather, two sets of methylene protons (3.38,3.13 ppm 2.38,1.67 ppm) were seen. Thus, the alkaloid was deduced to be the 5-deoxy derivative and was named deoxyrhexifoline (24) (35). The relationship between methyl boschniaki-nate and deoxyrhexifoline has not been established, but the available data would appear to indicate that they are the same alkaloid. Neither group reported an [a]D for the respective isolates. 

Following the isolation and structure elucidation of rhexifoline (23) from Castilleja rhexifolia (35), further work on the flower heads yielded three iridoid glucosides, one of which was penstemonoside (302). Recognizing the potential relationship between 23 and 302, Roby and Stermitz carried out the synthetic transformation of 302 to 23 (81). Treatment of penstemonoside (302) with /3-glucosidase afforded a 2 1 epimeric mixture of the lactol 303. Reaction with methanolic HC1 followed by ammonia afforded rhexifoline (23) in 31% overall yield (Scheme 30) (81). Euphroside (304), the principal iridoid of Orthocarpus spp., was used for the structure confirmation of euphrosine (25) (40). Acid hydrolysis of 304 in methanol afforded 305, which was treated with aqueous acid followed by basification with ammonia to afford a low yield of euphrosine (25), identical with the natural product (Scheme 31) (40). 

Roby, M. R. and Stermitz, F. R. (1984) Penstemonoside and other iridoids from Castilleja rhexifolia. Conversion of penstemonoside to the pyridine monoterpene alkaloid rhexifoline J Nat Prod 47, 854-857. 

Senecionine (35) has been identified in Castilleja rhexifolia Rydb. This is the first report of any alkaloids occurring in the genus Castilleja and also the first report of pyrrolizidine alkaloids in the Scrophulariaceae. 

The stems and leaves of Castilleja rhexifolia (Scrophula-riaceae) contain senecionine (5) and its A -oxide (obtained from Senecio triangularis) as their main alkaloids. The major alkaloid of the blossoms and seeds is rhexifoline, an iridoid-monoterpene-derived alkaloid, which is found in many Castilleja species. These plants serve as a host for the plume moth, Platyptilia pica. The larvae primarily eat the green seeds. Both the larvae and the adults of this insect contain rhexifoline (Roby and Stermitz, 1984). Plants of C. hispida contain quinolizidine alkaloids, and occasional plants of C. sulphurea contain quinolizidine alkaloids, whereas those of C. occidentalis do not contain alkaloids other than rhexifoline. The iridoid monoterpene content of all these alkaloids is similar. The larvae were found to excrete the monoterpene iridoids (Stermitz et al., 1986). 

Suess TR, Stermitz FR 1981 The constituents of Castilleja rhexifolia, Mahonia repens and Oncidium cebolleta. Diss Abstr Int B 42 1025-1026... 

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