Clifford W. J. Chang University of West Florida Pensacola. Florida 32504
I
Marine Natural Products Revisited
Some of the most recent intriguing reports on the chemistry of marine natural products include work with saxitoxin, the paralytic shellfish ooison. with other toxins, and with chemical signal and defense agents. Only fourteen years ago Kreig (I) published her popular treatise on pharmacognosy, the study of hioactive naturally occurring isolates which may have potential applications in the health-related fields. Although the extracts and compounds discussed were derived chiefly from terrestrial plants, one of her chapters entitled "Drugs from the Sea" appeared to he a forecast of things to come. At that time the unique structure of the puffer fish poison, tetrodotoxin, was resolved independently liy three different groups (2-?I; tarirhatoxin. ohrainerl from thr trrrestrial newr, surprisingly, was shown to be identical to tetrodotoxin (5);and the structural work on the paralytic shellfish toxin, saxitoxin, was already actively under investigation (68). The progress on the chemistry of marine secondary metabolites, particularly the quinonoid pigments (9) and sterols from the echinoderms (10, II), was evidenced by the increasing number of puhlications in the chemical journals. Scheuer's work in collating the diverse reports from the biological and chemical literature resulted in the 1973 puhlication, "Chemistry of Marine Natural Products" (11). Today, this field of research receives the attention of teachers and investigators whose varied backgrounds include, inter alia, marine biology, marine ecology, biochemistry, chemistry, and pharmacology. Since my summary reports were published in this Journal five years ago (12,131, the tempoof puhlications on marine natural oroducts has increased tremendouslv. No doubt, Ruggieri's recent a r n u n t ( 1 4 ) on " l h g s from the Sea" und similar r)~~hlivations ha\.e ~romuleated the intrrrst in this field of r e s e k h . In this paper I shall discuss some of the more recent intriguing reports on the chemistry of marine natural products. Although the scope of this article is necessarily limited, a varietv isolated from marine -~~~ of hioloeicallv active com~ounds organisms are chosen for discussion. For more comprehensive and snecialized reviews. the reader is referred to the Droeress repor& by Baker (15),'~aulkner(16), Fenical (17); ina ale (18), Minale et al. (19), Moore (20), Scheuer (21), and Tursch (22). Saxitoxin, a Revised Structure Saxitoxin, more commonly known as the paralytic shellfish poison, has its origin from dinoflagellates. In 1971 the structure (I), possessing the unique tetrahetero-bonded carhon, was postulated mainly on the basis of chemical degradations and spectral data (23). However, thepKb data for saxitoxin cannot he rationalized adequately with the proposed formulation (I), and coupled with the fact that (I) itself has an unprecedented structure, major efforts were mounted to resolve these ambiguities. Since saxitoxiu was not amenable to crystallization, two derivatives suitahle for X-ray crystallography were finally A~
~~
~- .
Figure 1. Saxitoxin (Iia)from Gonyavlax catenella (above)and gonyautoxin (Ilb) from G. tamerensis. The structure I was revised to I1 in 1975. prepared. Thv revised structure (Ila) was reported independcntl!. hv the cumhined Srhantzand Clardvaruups (2.1) and and coworkers (25) using the reipective p by hromo-benzenesulfonate and ethyl hemiketal dihydrochloride derivatives (see Fig. 1). Shimizu's group in Rhode Island has since discovered other saxitoxin-related compounds, gonyautoxins (IIb),from marine organisms in "red tide" infested areas (26). Their studies employing liquid chromatography equipped with a fluorescent detector (27) have been extended to samples obtained world-wide from other infested areas. The guanidinium moieties in these toxins, shown in the partial structure (III), deserve some comments. The Berkeley
~~
I
II
a-c-c-H
%
I
CI-C-C-H
/
I
The Cover: Photograph provided by University of West Florida Information Services. 684 1 Journal of Chemical Education
C1
V
chloral hydrate
From behavioral and chemical studies of inter- and intro-species associations, the food chain hypothesis lends credence to the existence of structurally related organic compounds found in morphologically dissimilar organisms. Molecularly similar compounds can originate biogenetically from a single precursor and undergo minor dietary modifications within predator organisms.
VI ninhydrin group (23) was able to demonstrate that saxitoxin existed as the hydrate (IIa) by drying the compound (high vacuum a t llO°C) to yield (111). That the hydrated ketone (IIa) is stable can be explained by the resonance contributingstructure (N), a system which is analogous to chloral (V) and triketohydrindene (VI), which readily form their respective familiar hydrates.
Other Toxins Some rather unusual toxins (VII) derived from the midgut eland of several Hawaiian sea hares (molluscs) were iuvestigated by Kato and Scheuer (28-30):~s unique creatures of the family Aplysiidae, the sea hares are known to possess a large digestive gland which comprises about a third of their body cavity. The digestive gland, also referred to as the midgut gland or hepatopancreas, contains residues of algae which constitute a major portion of the sea hare's diet. Using -12 g of toxin extracted from 50 kg of the Hawaiian Stylocheilus longicauda (about 5,000 animals), Kato and Scheuer were able to deduce the structures of the aplysiatoxins by degradative and spectra means (see Fig. 2).
Chemlcal Signal and Defense Agents One of the more challenging areas of marine research involves the nature of intra- and inter-species communication between organisms (35, 36). Considering the plethora of marine organisms, our current knowledge of chemical communication among the various species is sparse, although chemists, usually in collaboration with their marine biological counterparts, have begun to probe into the mechanisms of species' luring and avoidance behavior. Scientists have recognized within the immediate surroundings of our natural hahitats that small organic molecules exuded by larger dominant trees can he a factor in governing and affecting the growth of other smaller plants (37). In addition, pheromones, substances which elicit behavioral and physiological responses intra-specifically, are well known modes of chemical communication in arthropods (38,39). In contrast to the many documented cases within the realm of terrestrial ecology our present knowledge of chemical communication among marine organisms is sparse (35, 36). Sleeper and Fenical (40), for example, have identified the trail-breaking alarm pheromone from the sea slug, Nauanax inermis. As the authors observed: "When this (mixture of secreted substances) is encountered by a trail-following Nauanax, an immediate alarm and avoidance response is induced. which terminates trail-followine behavior and eeneratrs a deviution in direction of grcatrr than W0C"' (40)"l'hrse pheromones are the navenones A-C \Vllla,b,c), which 3rr 0
Figure 2.Debromaapiysiataxin (Vila)and apiysiatoxin (Vllb) from a sea hare. In independent work relating to antileukemia activity of alea " constituents. Moore and his associates have investinated several algae species from Enewetak. Having served as a member of the thesis advisory committee of Kato, Moore was able to associate the similarities of the nmr spectra of their nurified com~oundsto those obtained previously by Kato. (;u~rI~!dIly biuasjay, a crysrallinc. rn;~rt!rinlfrum the fractiunntiun oithe I.>nb>uprucili.~ extract was shown to he idmtical ~to dehromoaplysiatoxin 131). These observations obtained independently by the Hawaiian workers were particularly interesting. That the sea hare Stylocheilus longicauda's preferential food source is the alga Lynbya (31) provides yet another example of the algal-herbivore relationships which have been implicitly demonstrated by the previous work of Stallard and Faulkner (32,33) and Minale, and coworkers (34). Furthermore, debromoaplysiatoxin (VIIa) is active against P-388 lymphocytic mouse leukemia (31). Moore and coworkers in the same work have also implicated this compound, and possibly similar compounds, as the probable active principle(s) causing "swimmer's itch" in Hawaiian waters. As in the case of red tide blooms. these deleterious infestations are infrcquent nnd iaolnted phenomena uhlch presently are not well u n d e r s ~ w d11) thv scirnrific community. ~
~
~
c. R = HO released by the marine opisthohranch as a yellow hydrophobic material from a small gland found near the anus of the animal. The pheromonal compounds are 3-pyridyl-, phenyl-, and 4'hydroxyphenyl-derivatives of deca-3E, 5E, 7E, 9E-tetraene-2-one. which as a mixture exist in a ratio of 4 2 1 . are C-20, monocarbocyclic diterpenoids whose skeletal arraneements follow the i s o ~ r e n erule (41). Amonn the marine organisms only the coelenterates elaborate these compounds. Kashman and coworkers have observed that loholide (IX), like sarcophine (42), is toxic to fishes (43). They have speculated that these compounds obtained from Red Sea soft corals may serve as defense mechanisms against predators. OAc ( -
emb bra no id^
Volume 55, Number 11. November 1978 1 685
summary of Data on Aemplydnim-I (XVa,b) and AemplyslnlnaZ (XVla,b). Compound Aeroplysinin-1
Aeroplysinin-2
Anticancer Agents
The multitude of marine species provides a vast framework on which chemists have endeavored to investigate potentially useful drugs from the sea. From data obtained up to 1974, Weinheimer and Karns (44) have reported that approximately 9% of the 1535 species examined by various investigators have yielded extracts which exhibit confirmed activity in the KB and P S cell lines. The KB and P S cell lines represent two among a number of bioassays which are used to monitor autitumor activity. Among the animal phyla, the Chordata, MoUusca, Porifera, and Coelenterata have been particularly interesting with reference to studies on anticancer agents. A number of marine cembranoids, of which asperdiol (X) and sinularin (XI) are examples, has been reported to he cytotoxic in the in uitro KB and P S cell lines (45,461. Asperdiol has the distinction that it is the sole non-lactonic cembranoid isolated to date which displays antitumor activity. Perhaps it may be surprising that cemhrene-A (XII), though not known as an antitumor agent, occurs naturally in a number of plants, an insect, and a soft coral (coelenterate). Although i t was implicated as a trail-marking pheromone of the termite Nasutitermes exitisosis (47, 48), its biological significance in the soft coral Sinularia flexibilis is not known (49). Several other compounds of varying skeletal structures have been bioassayed for antitumor activity. Juncusol (XIII) from an estuarine marsh plant (50),aplysistatin (XIV) from a sea hare (51), and aeroplysinin-1 (XV), from sponges (52,53) are only a few of the marine isolates which are undergoing further evaluation for anticancer activity.
oca
XVII
.
lanthella adis -198' Vemngia aemphoba +I66 Psammoposiliapwpur~~ + I 9 3 lanthelk a r m 0 Verongia aeropb5.a +22 lanthella sp. 0 PsammoposiNapwpurea 0 oca
xm
~~
~
~
~
~
A
686 1 Journal of Chemical Education
Melting Palm 112-116°C 120 112-113 106-108 127-128 127-128 OCH.
XIX
Interestingly, from the pioneering work of the late Professor W. Bergman on unusual nucleosides from marine sponges (55), a number of nucleosides including cytidine arabinoside (XVII) and adenine arahinoside (XVIII) were synthesized. The synthetic nucleoside (XVII) was used for some time in anticancer studies. More recently, adenine arabinoside, commonlv known as "ara-A", has been evaluated in the treatmen; of h v r p e ~encephalitis. Cancer patients, whuse immunulc&al defenses cnn be weakened by anticanrer drugs. are - ~ - treated with ara-A when thev become affected bv chicken pox or shingles (56).Ara-A also has an inhibitory Hction on DNA polymerase, ribonucleotide reductase, and adenyl cyclase. Although Bergman was able to isolate some uncommon nucleosides from only one Caribbean sponge, Cryptotethya crlpta, the natural and un-natural nuclcosides from other sponges cun he obtained by hydrolysis ( 5 5 ) .The author in his research on another Carihhean sponge, l)a.,)rhalina c ~ a l h i n a , has isolated a numher of tree nucleosides including adenosine ( S I N ) ,u,hichcontsins a rihosr instead of an arahinust,sugHr ~
The aeroplysinin compounds, whose precursor is probably 3,5-dihromotyrosine, are of special interest to the author. In work performed a t the University of Oklahoma (54) and elsewhere, both enantiomers (XVa) and (XVh) and their racemic modification have been isolated. The related aeroulvsinin-2 (XVI) has been r e ~ o r t e das the dextrorotatorv isomer and as a racemic mixture. The levorotatory form has not been located to date. The table summarizes the presently known data. Although the aeroplysinin-2 compounds iXV1a.h) have not demonstrated cvtotoxic activitv in the KB A d PS cell lines, the levorotatory, dextrorotatory, A d racemic aeroplysinin-1 compounds are cytotoxic to some cancer cells.
Optical Rotation
Species
Figure 3. Arsenobetaine (XX) from a lobster
