Abstract
Further chemical investigation of the EtOAc extract of the soft coral Lobophytum varium resulted in the discovery of eleven new diterpenoids lobovarols F–P (1–11) of lobane– and prenyleudesmane–types, along with two known metabolites (12 and 13). The structures of the new metabolites were established by spectroscopic analyses, including 2D NMR experiments. The absolute configuration of 1 was determined using Mosher’s method. The complete assignment of 1H and 13C NMR spectroscopic data of 12 and 13 and the identification of pyran-derived moieties in the prenyleudesmanes were reported for the first time. Anti-inflammatory activities of the isolated compounds in suppressing elastase release and superoxide anion generation in human neutrophils were disclosed for 1, 2, 4, 12, and 13. A stereospecific biosynthesis for lobanes and prenyleudesmanes from the related prenylgermacranes could explain the coexistence of lobanes and prenylgermacranes in L. varium.
Highlights
Soft corals belonging to genus Lobophytum (Alcyoniidae) were shown to be the main biological sources of lobane [1,2,3,4,5,6,7], prenyleudesmane [1,8,9,10,11], and spatane–type diterpenoids [8]
The terpenoids containing fractions of the EtOAc extract of L. varium were repeatedly purified using a series of chromatographic techniques, including HPLC, to afford compounds 1–13 (Figure 1)
A prenylgermacrene has been regarded as the same biosynthetic precursor [8,9] and we further propose a stereospecific biosynthesis for lobane and prenyleudesmane diterpenoids through Cope propose a stereospecific biosynthesis for lobane and prenyleudesmane diterpenoids through Cope rearrangement and acid-catalyzed cyclization, respectively [9,29], as illustrated in Scheme 1
Summary
Soft corals belonging to genus Lobophytum (Alcyoniidae) were shown to be the main biological sources of lobane [1,2,3,4,5,6,7], prenyleudesmane [1,8,9,10,11], and spatane–type diterpenoids [8]. Mar. Drugs 2020, 18, 223 shown to exhibit significant cytotoxic [12,13,14], antiviral [15], and anti-inflammatory activities [14,16,17,18]. Our previous chemical study on the EtOAc extract of a Formosan soft coral, Lobophytum varium has led to the discovery of five new metabolites of lobane- and prenyleudesmane-types, of which some were found to possess anti-inflammatory activity through the suppression of elastase release and/or superoxide anion generation in challenged neutrophils [1]. This study led to the isolation of new metabolites and gave strong evidence for the biosynthetic mechanism of lobane and prenyleudesmane from the corresponding prenylgermacrane biosynthetic intermediate was proposed, which could explain the common coexistence of lobane and prenylgermacranes in L. varium
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