Natural bioactive compounds from marine fungi (2017-2020).

Secondary metabolites generated by marine fungi have relatively small molecular weights and excellent activities and have become an important source for developing drug lead compounds. The review summarizes the structures of novel small-molecule compounds derived from marine fungi in recent years; introduces representative monomers in antimicrobial, antitumor, anti-viral, and anti-neuritis aspects; and discusses their biological activities and molecular mechanisms. This review will act as a guide for further discovering marine-derived drugs with novel chemical structures and specific targeting mechanisms.

Facile synthesis of the naturally cytotoxic triterpenoid saponin Patrinia-glycoside B-II and its conformer.

The first chemical synthesis of the natural triterpenoid saponin Patrinia-glycoside B-II, namely oleanolic acid 3-O-α-L-rhamnopyranosyl-(1→2)-[ß-D-gluco-pyranosyl-(1→3)]-α-L-arabinopyranoside, has been accomplished in a linear 11-step sequence 11 with 9.4% overall yield. The abnormal 1C4 conformation of the arabinose residue was found to occur via conformational fluctuation during preparation of the intermediates. Molecular mechanism and quantum chemistry calculations showed that Patrinia-glycoside B-II and its conformer 1 cannot interconvert under normal conditions. Preliminary structure-activity relationships studies indicated that the 4C1 chair conformation of the arabinose residue in the unique α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranosyl disaccharide moiety is one of the chief positive factors responsible for its cytotoxic activity against tumors.

Synthesis and tumor cytotoxicity of novel amide derivatives of ß-hederin.

Thirteen novel triterpenoid saponins, designed as amide derivatives of the natural cytotoxic saponin ß-hederin, were synthesized by a stepwise glycosylation strategy. The in vitro cytotoxic activity of these compounds was evaluated against five different tumor cell lines. Most of the evaluated compounds showed effective inhibitory activity against at least one tumor cell line at micromolar concentrations. The preliminary structure-activity relationships (SAR) indicate that mide derivatization at C-28 resulted in highly cytotoxic derivatives on specific tumor cell lines, and also resulted in an increase in the antitumor selectivity of ß-hederin. 

Molecular dynamics simulations of HIV-1 protease monomer: Assembly of N-terminus and C-terminus into beta-sheet in water solution.

HIV-1 protease (HIV-PR) consists of two identical subunits that are united together through a four-stranded antiparallel beta-sheet formed of the peptide termini of each monomer. Since the active site exists only in the dimer, a strategy that is attracting more and more attention in inhibitor design and which may overcome the serious drug resistance caused by competitive inhibitors is to block the peptide termini of the monomer, thereby interfering with formation of the active dimer. In the present work, we performed several extensive molecular dynamics (MD) simulations of the HIV-PR monomer in water to illustrate its solvated conformation and dynamics behavior. We found that the peptide termini usually assembled into beta-sheet after several nanoseconds' simulation, and became much less flexible. This beta-sheet is stabilized by intramolecular interactions and is not easily disaggregated under the present MD simulation conditions. This transformation may be an important transition during the relaxing and equilibrating of the HIV-PR monomer in aqueous solution, and the terminal beta-sheet may be one of the major conformations of the solvated HIV-PR monomer termini in water. This work may provide new insights into the dynamics behavior and dimerization mechanism of HIV-PR, and more significantly, offer a more rational receptor model for the design and discovery of novel dimerization inhibitors than crystalline structures.

Facile synthesis and cytotoxicity of triterpenoid saponins bearing a unique disaccharide moiety: hederacolchiside A1 and its analogues.

An improved synthetic approach toward hederacolchiside A1, an antitumor triterpenoid saponin bearing a unique disaccharide moiety, was established. This approach began from a partially protected intermediate and avoided tedious protection-deprotection manipulation. An abnormal ring conformation (1C4) of the center arabinose residue was found in the intermediate, which may account for the unusual regioselectivity between 3-OH and 4-OH of arabinose. Two analogues of hederacolchiside A1 were then facilely prepared by this approach and exhibited significant cytotoxicity in preliminary in vitro assay.

Facile synthesis of oleanolic acid monoglycosides and diglycosides.

Oleanolic acid and its glycosides are important natural products, possessing various attractive biological activities such as antitumor, antivirus and anti-inflammatory properties. In the present work, fifteen oleanolic acid saponins bearing various saccharide moieties, including 3-monoglycoside, 28-monoglycoside and 3,28-diglycoside, were easily synthesized in high yields. Benzyl was chosen as the protective group for the COOH(28) group, instead of commonly used methyl and allyl, to avoid difficulties in the final deprotection. Alkali-promoted condensation of the carboxylic acid with bromo-glycosides was found to be more efficient in the synthesis of 28-glycosides. Two approaches were investigated and proved practicable in the preparation of 3,28- diglycosides. This method is suitable for preparing oleanolic acid glycosides with structural diversity for extensive biological evaluation and structure-activity relationship study, and it also apply new idea for the corresponding synthetic methods to the glycoside derivatives of other triterpenoid.

Synthesis of beta-hederin and Hederacolchiside A1: triterpenoid saponins bearing a unique cytotoxicity-inducing disaccharide moiety.

A facile synthetic approach toward oleanolic acid glycoside bearing alpha-L-rhamnopyranosyl-(1-->2)-alpha-L-arabinopyranosyl moiety, a unique oligosaccharide that strongly induces antitumor activity of oleanane-type triterpenoid saponins, was developed. Based on this approach beta-hederin (oleanolic acid 3-O-alpha-L-rhamnopyranosyl-(1-->2)-alpha-L-arabinopyranoside) was efficiently prepared from oleanolic acid through stepwise glycosylation in linear eight steps with 52% overall yield, while Hederacolchiside A1 (oleanolic acid 3-O-alpha-L-rhamnopyranosyl-(1-->2)-[beta-D-glucopyranosyl-(1-->4)]-alpha-L-arabinopyranoside) in linear 13 steps with 20% overall yield.

Synthesis and antitumor activity of two natural N-acetylglucosamine-bearing triterpenoid saponins: lotoidoside D and E.

Two natural triterpenoid saponins bearing N-acetylglucosamine, lotoidoside D and lotoidoside E, which had been available only from Glinus lotoides growing in Egyptian desert, were facilely synthesized from readily available oleanolic acid. Preliminary pharmacological research showed their antitumor activity against HeLa cell.