y -Alkyliden-tetronsaure-derivate kommen als Inhaltsstoffe einiger Flechtenarten vor. Ein Beispiel ist die sog. Vulpinsaure (11)2) 3), die in der Usneacee Evernia vulpina (Achatius) u. a. Flechten gefunden wird. Nahe Verwandte davon sind das Calycin4) und die P i n a~t r i n s i i u r e~) aus einigen Cetraria-und Lepraria-Arten.
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