Helenalin systems

Allergenic effects. A number of secondary metabolites influence the immune system of animals, such as coumarins, furanocoumarins, hypericin, and helenalin. Common to these compounds is a strong allergenic effect on those parts of the skin or mucosa that have come into contact with the compounds (4,17,312). Activation or repression of the immune response is certainly a target that was selected during evolution as an antiherbivore strategy. The function of alkaloids in this context is hardly known. 

Grieco and coworkers have utilized intramolecular aldolization of keto aldehydes to form seven-mem-bered rings. In a synthesis of ( )-helenalin, keto aldehyde (54) was cyclized to aldol (55 equation 1In a later modification of the basic approach, keto aldehyde (56) was cyclized to cycloheptenone (57 equation 113).The success of these cyclizations is related to the fact that the normally preferred five-membered ring closure would yield a strained rnzns-fused bicyclo[3.3.0]octene system. 

New approaches to the hydroazulene system found in the sesquiterpene pseudoquaianolides continue to appear. Schlessinger et al. have used the route shown in Scheme 15 to synthesize (212)/ a key intermediate which was then used in syntheses of damsin (213), confertin (214), and helenalin (215). 

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