Marine Terpenic Endoperoxides

Organic extracts of marine invertebrates, mainly sponges, from seas all over the world are well known for their high in vitro anticancer and antibiotic activities which make them promising sources of compounds with potential use as pharmaceutical leads. Most of the structures discovered so far have a peculiar structural feature in common: a 1,2-dioxane ring. This is a highly reactive heterocycle that can be considered as an endoperoxide function. Together with other structural features, this group could be responsible for the strong biological activities of the substances present in the extracts. Numerous research programs have focused on their structural elucidation and total synthesis since the seventies. As a consequence, the number of established chiral centres and the similarity between different naturally occurring substances is increasingly higher. Most of these compounds have a terpenoid nature, mainly diterpene and sesterterpene, with several peculiar structural features, such as the loss of one carbon atom. Although there are many reviews dealing with the occurrence of marine peroxides, their activities, or potential pharmaceutical uses, no one has focused on those having a terpene origin and the endoperoxide function. We present here a comprehensive review of these compounds paying special attention to their structural features and their biological activity.

Antiprotozoal and Antitumor Activity of Natural Polycyclic Endoperoxides: Origin, Structures and Biological Activity

Polycyclic endoperoxides are rare natural metabolites found and isolated in plants, fungi, and marine invertebrates. The purpose of this review is a comparative analysis of the pharmacological potential of these natural products. According to PASS (Prediction of Activity Spectra for Substances) estimates, they are more likely to exhibit antiprotozoal and antitumor properties. Some of them are now widely used in clinical medicine. All polycyclic endoperoxides presented in this article demonstrate antiprotozoal activity and can be divided into three groups. The third group includes endoperoxides, which show weak antiprotozoal activity with a reliability of up to 70%, and this group includes only 1.1% of metabolites. The second group includes the largest number of endoperoxides, which are 65% and show average antiprotozoal activity with a confidence level of 70 to 90%. Lastly, the third group includes endoperoxides, which are 33.9% and show strong antiprotozoal activity with a confidence level of 90 to 99.6%. Interestingly, artemisinin and its analogs show strong antiprotozoal activity with 79 to 99.6% confidence against obligate intracellular parasites which belong to the genera Plasmodium, Toxoplasma, Leishmania, and Coccidia. In addition to antiprotozoal activities, polycyclic endoperoxides show antitumor activity in the proportion: 4.6% show weak activity with a reliability of up to 70%, 65.6% show an average activity with a reliability of 70 to 90%, and 29.8% show strong activity with a reliability of 90 to 98.3%. It should also be noted that some polycyclic endoperoxides, in addition to antiprotozoal and antitumor properties, show other strong activities with a confidence level of 90 to 97%. These include antifungal activity against the genera Aspergillus, Candida, and Cryptococcus, as well as anti-inflammatory activity. This review provides insights on further utilization of polycyclic endoperoxides by medicinal chemists, pharmacologists, and the pharmaceutical industry.

Anti-Inflammatory Dembranoids from the Soft Coral Lobophytum crassum

Cembrane-type diterpenoids are among the most frequently encountered natural products from the soft corals of the genus Lobophytum. In the course of our investigation to identify anti-inflammatory constituents from a wild-type soft coral Lobophytum crassum, two new cembranoids, lobophyolide A (1) and B (2), along with five known compounds (3–7), were isolated. The structures of these natural products were identified using NMR and MS spectroscopic analyses. Compound 1 was found to possess the first identified α-epoxylactone group among all cembrane-type diterpenoids. The in vitro anti-inflammatory effect of compounds 1–5 was evaluated. The results showed that compounds 1–5 not only reduced IL-12 release, but also attenuated NO production in LPS-activated dendritic cells. Our data indicated that the isolated series of cembrane-type diterpenoids demonstrated interesting structural features and anti-inflammatory activity which could be further developed into therapeutic entities.

Crassocolides G-M, cembranoids from the Formosan soft coral Sarcophyton crassocaule

Seven new polyoxygenated cembranoids possessing an alpha-methylene-gamma-lactone group, crassocolides G-M (1-7, resp.), have been isolated from the AcOEt extract of the Formosan soft coral Sarcophyton crassocaule. The structures of compounds 1-7 were established by detailed spectroscopic analyses, including 2D-NMR spectroscopy ((1)H,(1)H-COSY, HMQC, HMBC, and NOESY), while the absolute configuration of 1 was determined using a modified reaction of Mosher's method. The cytotoxicity of compounds 1-7 against a limited panel of cancer cell lines was also determined.

Crassolides A-F, cembranoids with a trans-fused lactone from the soft coral Sarcophyton crassocaule

Six new polyoxygenated cembrane-based diterpenoids possessing a trans-fused alpha-methylene-gamma-lactone, crassocolides A-F (1-6), have been isolated along with the known compound 7 from the ethyl acetate extract of a Taiwanese soft coral, Sarcophyton crassocaule. The structures of 1-6 have been established by detailed spectroscopic analysis, including 2D NMR (1H-1H COSY, HMQC, HMBC, and NOESY) spectroscopy, while their absolute configurations were determined using a modified Mosher's method for 1. The structure of 5 was further proven by X-ray diffraction analysis. The full assignment of NMR data of 7 is reported herein for the first time. The cytotoxicity of crassocolides 1-4, 6, and 7 against a limited panel of cancer cells was also determined.

Alcyonacean metabolites VII--chemical constituents of Lobophytum denticulatum and Lobophytum strictum of the Indian Ocean

A rare cembranoid diterpene, (7E,11E.1 R,2S,3R,4R,l4S)-14-acetoxy-3,4-epoxycembra-7,11,15-triene-17,2-olide (1), was isolated from Lobophytum denticulatum and a new polyhydroxysterol, 7-hydroxyandamansterol (9) has been identified as peracetyl derivative from Lobophytum strictum. Several known polyhydroxysterols have also been isolated from these organisms. 1 exhibited moderate antibacterial activity.