Α· Sesquiterpenoids
3
1. PREVIOUSLY KNOWN COMPOUNDS
One of the earliest reports of marine-derived sesquiterpenoids was published by Takaoka and Ando (1951). In this rarely quoted study the Japanese
authors steam-distilled the brown alga Dictyopteris divaricata and obtained
an oil with a "beach odor." The oil was said to consist of sesquiterpene
hydrocarbons and alcohols as evidenced by the azulene which was obtained
on dehydrogenation. One of the alcohols was designated as " 1-cadinol,"
mp 137.5°-138.5°. This work was performed before sesquiterpene research
had become attractive through efficient separation methods and non-destructive tools for structural determination. Irie and co-workers (1964) reexamined
this research and confirmed, inter alia, that the sesquiterpene alcohol of
Takaoka and Ando was ( —)-8-cadinol (1), known previously from the
terrestrial genus Pinus.
By far the bulk of the sesquiterpenoids up to now have been isolated from
the phylum Coelenterata, first reported on by Ciereszko et al. (1960). The
phylum Coelenterata (Cnidaria) comprises some 9000 species in three
classes: the Hydrozoa (e.g., the Portuguese man-of-war), the Scyphozoa
(the jellyfish), and the Anthozoa (the sea anemones; soft and stony corals).
The Anthozoa constitute the largest class (some 6000 species) and are divided
into two subclasses: the Zoantharia (sea anemones and corals) and the
Alcyonaria (sea fans, sea pansies, etc.). One of four orders of the alcyonarians
are the Gorgonacea or gorgonians. All sesquiterpenes that have so far been
reported from the marine coelenterates have been isolated from gorgonians,
and all of them by one research group at the University of Oklahoma. Collection of these animals is an esthetic experience since gorgonians contribute
some spectacularly beautiful species to tropical coral reefs.
In their first publication on terpenoid constituents of the gorgonians
Ciereszko et al. (1960) reported the isolation of several partially characterized
compounds in addition to a dextrorotatory cadinene from Plexaura crassa,
which had been collected at Bermuda. The identity of this widely occurring
sesquiterpene diene was secured by its conversion to a (+)-cadinene dihydrochloride and by selenium dehydrogenation to cadalene (l,6-dimethyl-4isopropylnaphthalene). The infrared spectrum of the dihydrochloride was
identical with that of a levorotatory sample prepared from oil of cade. The
authors did not state which of the cadinene isomers was originally isolated. As
Gallagher and co-workers (1964) pointed out, that might have been a difficult
task without rigorous proof of homogeneity. In fact, the Australian workers
further caution that cadinene dihydrohalides can arise not only from the
nine cadinene isomers, but from other structures, e.g., copaene (vide infra, 14)
as well.
Regardless of the correct structure of the sesquiterpene, its isolation
3
1. PREVIOUSLY KNOWN COMPOUNDS
One of the earliest reports of marine-derived sesquiterpenoids was published by Takaoka and Ando (1951). In this rarely quoted study the Japanese
authors steam-distilled the brown alga Dictyopteris divaricata and obtained
an oil with a "beach odor." The oil was said to consist of sesquiterpene
hydrocarbons and alcohols as evidenced by the azulene which was obtained
on dehydrogenation. One of the alcohols was designated as " 1-cadinol,"
mp 137.5°-138.5°. This work was performed before sesquiterpene research
had become attractive through efficient separation methods and non-destructive tools for structural determination. Irie and co-workers (1964) reexamined
this research and confirmed, inter alia, that the sesquiterpene alcohol of
Takaoka and Ando was ( —)-8-cadinol (1), known previously from the
terrestrial genus Pinus.
By far the bulk of the sesquiterpenoids up to now have been isolated from
the phylum Coelenterata, first reported on by Ciereszko et al. (1960). The
phylum Coelenterata (Cnidaria) comprises some 9000 species in three
classes: the Hydrozoa (e.g., the Portuguese man-of-war), the Scyphozoa
(the jellyfish), and the Anthozoa (the sea anemones; soft and stony corals).
The Anthozoa constitute the largest class (some 6000 species) and are divided
into two subclasses: the Zoantharia (sea anemones and corals) and the
Alcyonaria (sea fans, sea pansies, etc.). One of four orders of the alcyonarians
are the Gorgonacea or gorgonians. All sesquiterpenes that have so far been
reported from the marine coelenterates have been isolated from gorgonians,
and all of them by one research group at the University of Oklahoma. Collection of these animals is an esthetic experience since gorgonians contribute
some spectacularly beautiful species to tropical coral reefs.
In their first publication on terpenoid constituents of the gorgonians
Ciereszko et al. (1960) reported the isolation of several partially characterized
compounds in addition to a dextrorotatory cadinene from Plexaura crassa,
which had been collected at Bermuda. The identity of this widely occurring
sesquiterpene diene was secured by its conversion to a (+)-cadinene dihydrochloride and by selenium dehydrogenation to cadalene (l,6-dimethyl-4isopropylnaphthalene). The infrared spectrum of the dihydrochloride was
identical with that of a levorotatory sample prepared from oil of cade. The
authors did not state which of the cadinene isomers was originally isolated. As
Gallagher and co-workers (1964) pointed out, that might have been a difficult
task without rigorous proof of homogeneity. In fact, the Australian workers
further caution that cadinene dihydrohalides can arise not only from the
nine cadinene isomers, but from other structures, e.g., copaene (vide infra, 14)
as well.
Regardless of the correct structure of the sesquiterpene, its isolation