Expedient synthesis and structure-activity relationships of phenanthroindolizidine and phenanthroquinolizidine alkaloids

Total synthesis of phenanthroindolizidine alkaloids (+/-)-antofine, (+/-)-deoxypergularinine, and their dehydro congeners and evaluation of their cytotoxic activity

Due to their limited natural abundance and significant biochemical effects, we synthesized the alkaloids (+/-)-antofine (1a), (+/-)-deoxypergularinine (1b), and their dehydro congeners (2 and 3) starting from the corresponding phenanthrene-9-carboxaldehydes. We also evaluated their in vitro cytotoxic activity. Compounds 1a and 1b showed significant potency against various human tumor cell lines, including a drug-resistant variant, with EC(50) values ranging from 0.16 to 16ng/mL. Structure-activity correlations of these alkaloids and some of their synthetic intermediates were also ascertained. The non-planar structure between the two major moieties, phenanthrene and indolizidine, plays a crucial role in the cytotoxic activity of phenanthroindolizidines. Increasing the planarity and rigidity of the indolizidine moiety significantly reduced potency. A methoxy group at the 2-position (1a) was more favorable for cytotoxic activity than a hydrogen atom (1b).

Synthesis of naphthalenes through three-component coupling of alkynes, Fischer carbene complexes, and benzaldehyde hydrazones via isoindole intermediates

The synthesis of naphthalene derivatives through three-component coupling of 2-alkynylbenzaldehyde hydrazones with carbene complexes and electron-deficient alkynes has been examined. The reaction involves formation of an isoindole derivative, followed by intramolecular Diels-Alder reaction, followed by nitrene extrusion. The reaction was highly regioselective using unsymmetrical alkynes.

Structural analogs of tylophora alkaloids may not be functional analogs

Phenanthroindolizidine-based tylophora alkaloids have been reported to have potential antitumor, anti-immuno and, anti-inflammatory activity. The structure-activity relationships of a series of tylophora alkaloids were studied to guide future drug design. Our results indicate that although these compounds are structural analogs, their potency of cytotoxicity, selectivity against NF-kappaB signaling pathway, and their inhibitory effects against protein and nucleic acid synthesis are different. Because they do not have an identical spectrum of targets, the studied compounds are structural, but may not be functional analogs.

Efficient Synthesis of a New Structural Phenanthro[9,10,3‘,4‘]indolizidine Starting from Pyrrole

A new structural phenanthroindolizidine, 2,3,6,7-tetramethoxyphenanthro[9,10,3',4']indolizidine, has been synthesized efficiently from pyrrole. An important feature of this synthesis is that intramolecular oxidative coupling and rearrangement of 6,7-bis(3,4-dimethoxyphenyl)-8-methoxy-1,2,3-trihydroindolizin-5-one by using VOF3 and TFA have been achieved in one pot.

Structure-activity studies of phenanthroindolizidine alkaloids as potential antitumor agents

Five phenanthroindolizidine alkaloids (PA) were chemically synthesized and seven were isolated from Tylophora atrofolliculata. To facilitate future drug design of phenanthroindolizidine alkaloids as potential antitumor agents, we have explored the structure-activity relationships (SAR) of this class of compounds. We demonstrated that DCB-3503 and tylophorinidine (PA-7) were among the most active compounds against tumor growth both in vitro and in vivo. In the hepatocellular carcinoma cell line HepG2, the GI(50)s of DCB-3503 and PA-7 were 35+/-5 nM and 11+/-5 nM, respectively. DCB-3503 and PA-7 significantly inhibited HepG2 tumor growth in nude mice at a dose of 9 mg/kg given by intraperitoneal (ip) injections twice a day every third day for a total of four cycles (P<0.05 for DCB-3503 and P<0.01 for PA-7). Their potent antitumor activities correlated with their potent NF-kappaB-inhibitory effects and their cyclin D1 down-regulatory effects.

Anti-inflammatory effects of 7-methoxycryptopleurine and structure–activity relations of phenanthroindolizidines and phenanthroquinolizidines

A cryptopleurine analogue, 7-methoxycryptopleurine, a phenanthroquinolizidine, was first found to exert potent anti-inflammatory activity in vitro and in vivo as well as have remarkable cytotoxic activity against cancer cells. The non-planar structure between the two major moieties, phenanthrene and indolizidine/quinolizidine, played a crucial role in the activity of phenanthroindolizidines or phenanthroquinolizidines in terms of cytotoxic effects on cancer cells and anti-inflammatory activity. We also showed that increase in planarity and rigidity of the indolizidine/quinolizidine moiety and change of the amine group into an amide by introducing a keto group to phenanthroindolizidines or phenanthroquinolizidines at the equivalent position 9 of tylophorine significantly reduced their activities. Moreover, in general, phenanthroquinolizidines are more potent than their respective phenanthroindolizines.

Cytotoxic 5-alkylresorcinol metabolites from the leaves of Grevillea robusta

Bioassay-guided fractionation of the MeOH extract of the leaves of Grevillea robusta led to the isolation of six new 5-alkylresorcinols, gravicycle (1), dehydrogravicycle (2), bisgravillol (3), dehydrobisgravillol (4), dehydrograviphane (5), and methyldehydrograviphane (6), as well as eight known compounds. The structures of these compounds were determined by spectroscopic and chemical methods. Graviphane (7) and methylgraviphane (8) were isolated in the pure form for the first time from a natural source. The compounds all showed marginal cytotoxicity against MCF-7, NCI-H460, and SF-268 cell lines.

Synthesis and structure–activity studies of antofine analogues as potential anticancer agents

Due to the profound cytotoxicities and interesting biochemical aspects, phenanthroindolizidine alkaloids have received an attention as potential therapeutic leads. To define the features of the molecule that are essential for cytotoxicity, we have synthesized and evaluated a series of phenanthroindolizidine alkaloid, antofine, analogues with different substituents on the phenanthrene ring. The systematic structure activity relationship studies elucidate the essential functional group requirement of phenanthrene ring, providing the basis for further development of phenanthroindolizidine alkaloids.

Indole alkaloids from Cephalanceropsis gracilis

Purification of CHCl3 and EtOAc solubles of the MeOH extract of Cephalanceropsis gracilis afforded seven new indole alkaloids, cephalinones A (1), B (2), C (3), and D (4) and cephalandoles A (5), B (6), and C (7), besides eight known compounds. The structures of the new compounds were determined by spectroscopic analysis. All 15 indole alkaloids were evaluated for their cytotoxic effects on MCF-7, NCI-H460, and SF-268 cell lines by the MTT method. Only cephalinone-F (6) showed significant cytotoxicity.