Calenzanane Sesquiterpenes from the Red Seaweed Laurencia microcladia from the Bay of Calenzana, Elba Island: Acid‐Catalyzed Stereospecific Conversion of Calenzanol into Indene‐ and Guaiazulene‐Type Sesquiterpenes

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Abstract
It is shown here that calenzanane sesquiterpenes (1 and 6) can be isolated from organic extracts from the red seaweed Laurencia microcladia Kützing from the Bay of Calenzana, Elba Island, provided contact with acidic media is minimized. Such contact induces rearrangements of 1 in dry solvents to indene‐type 5 and the blue‐colored guaiazulenium‐type ion 17, via spectrometrically (NMR) characterized indene‐type transient intermediates 10, 14, and 12. Addition of NEt3 to the reaction mixture at appropriate stages allowed the isolation of 12 (and 8 on workup on SiO2), and guaiazulene (18). Prolonged contact with silica gel led to complete degradation of 1, giving calenzanane‐type epimeric enones 20 a/20 b as well as indene‐type epimeric carbinols 22 a/22 b and fulvene 7. The latter was also formed during silica‐gel flash chromatography of the algal extracts. A unifying mechanistic view of these branching and cascade transformations may have both heuristic value, suggesting possible artefact origin of azulenoids, and synthetic applications.

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