As an extension of structure-activity relationship studies of pancratistatin (1), various techniques were first evaluated for separating the mixtures of 7-deoxynarciclasine (2b) and 7-deoxy-trans-dihydronarciclasine (3a) isolated from Hymenocallis littoralis. An efficient solution for that otherwise difficult separation then allowed the lactam carbonyl group of protected (4c and 5c) alcohols 2b and 3a to be reduced employing lithium aluminum hydride. Cleavage (TBAF followed by H2SO4) of the silyl ester/acetonide protected 6a gave amine 8. X-ray crystal structure determinations were employed to confirm the structures of 3,4-acetonide-5-aza-6-deoxynarciclasine (6b), 5-aza-6-deoxynarciclasine (8a), and 5-aza-6-deoxy-trans-dihydronarciclasine (9a, 9b). Against the murine P388 lymphocytic leukemia and a panel of human cancer cell lines, the parent natural products, 7-deoxynarciclasine (2b) and 7-deoxy-trans-dihydronarciclasine (3a), were found to generally be more cancer cell growth inhibitory (GI50 0.1 to <0.01 microg/mL) than the compounds with structural modifications such as amine 8 by a factor of 10 or more. The trans ring juncture of isocarbostyril 3a proved to be an important modification of narciclasine (2a) for improving cancer cell growth inhibition in this series.

To further pursue the antineoplastic leads offered by our isolation of trans-dihydronarciclasine (1a) and 7-deoxy-trans-dihydronarciclasine (1c) from two medicinal plant species of the Amaryllidaceae family, a practical palladium-catalyzed hydrogenation procedure was developed for synthesis of these isocarbostyrils from narciclasine (2a) and 7-deoxynarciclasine (2c).From a 1982 collection (bulbs) of the Chinese medicinal plant Zephyranthes candida (Amaryllidaceae) 1b we isolated the strong (ED 50 0.0032 μg/mL) P388 lymphocytic leukemia cell growth inhibitor trans-dihydronarciclasine (1a). The structure was established by detailed spectroscopic analyses of its peracetate derivative (1b) 1b and confirmed by comparison with the minor product from catalytic hydrogenation of narciclasine (2a). 2 Hydrogenation afforded as the major product the expected cis-dihydronarciclasine (3a) accompanied by the trans isomer (1a) and iso-narciclasine (4a). Subsequently, trans-dihydronarciclasine (1a) was found to exhibit strong cancer cell growth inhibition (mean panel GI 50 12.6 nM) against the U.S. National Cancer Institute (NCI) panel of cancer cell lines3a,b whereas its cis isomer (3a) 3c was only very weakly active (mean panel GI 50 3800 nM). Importantly, the trans isomer (1a) gave an active Compare correlation coefficient of 0.92 in respect to (+)-pancratistatin (5). 3a The trans isomer (1a) also showed strong activity against a range of RNA viruses while the synthetic cis isomer (3a) was completely inactive. 4 Because of the close structural and biological relationship of trans-dihydronarciclasine (1a) to (+)-pancratistatin (5), already in preclinical development, it became necessary to increase the availability of trans isomer 1a and the closely related 7-deoxy-trans-dihydronarciclasine (1c), by synthesis. The latter was isolated from Hymenocallis sp. (P388 ED 50 0.02 μg/mL) and gave an active Compare correlation coefficient of 0.89 in respect to (+)-pancratistatin (5). 3 Those structural and biological relationships of 1a and 1c to (+)-pancratistatin (5) are of special importance owing to its well known 5 in vitro and in vivo anticancer activity augmented by the rapidly increasing knowledge of the very promising activation of the mitochondrial route to cancer cell apoptosis.6a Importantly, pancratistatin has been shown to induce apoptosis routinely in various cancer cell lines at sub-micromolar concentrations while being nontoxic to normal human fibroblasts and endothelial cells at the same drug concentrations.6b,c As anticipated, narciclasine (2a) has recently been found to induce the mitochondrial and/or caspase-8/caspase-10 death receptor pathway in human MCF-7 breast as well as PC-3 prostate cancer cell lines. 5 Presumably, the strong cancer cell growth inhibitors 1a and 1c will also display related mechanisms of action selectively targeting cancer cells. The need to begin more detailed preclinical development of 1a and 1c led to our syntheses of the potentially useful phosphate prodrugs trans-dihydronarcis...