Department of Chemistry and Centre for Biotechnology, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario L2S 3A1, Canada
Chemoenzymatic synthesis of complex natural and unnatural products: morphine, pancratistatin, and their analogs
ARKIVOC 2006 (vii) 276-291
pp. 276 – 291
Tomas Hudlicky: Canada Research Chair in Biocatalysis; Professor, Chemistry
Organic synthesis, biocatalysis, electrochemistry, asymmeric catalysis
Our group is engaged in a variety of projects ranging from total synthesis to investigations of new reactions and the design of enzyme inhibitors. In total synthesis, we work on implementing reliable and efficient routes to target molecules. Our ventures are exact and logical pursuits, yet serendipity, intuition, and art all form an integral part of designing a total synthesis.
We have exploited the biooxidation of aromatic compounds in an exhaustive approach to the synthetic design of carbohydrates and their derivatives. Our guiding principles are symmetry, simplicity, and precise order of operations so that any derivative or stereoisomer with a sugar backbone can be constructed. These products are tested for glycosidase inhibition, a process important in viral expression. In addition, carbocyclic sugars can act as cell messengers, and their availability through synthesis allows greater understanding of cellular communication. Oligomers of inositols can also be exploited in a rational design of templates for asymmetric synthesis and in the design of chiral polymers.
Morphine, pancratistatin, and taxol are other important molecules in which our group has invested much synthetic effort. Their total synthesis permits the investigation of new reactions and mechanistic pathways, which can then be applied in subsequent syntheses. Current effort is focused on designing a practical synthesis of morphine and analogs and in probing the active pharmacophore of pancratistatin in hopes of designing a more bio-available anti-tumor agent.
To address environmentally benign manufacturing, or Green Chemistry, we are exploiting organic electrochemistry as replacement technology for metal-based oxidizing and reducing agents.
Finally we are devoting some effort to studies in the mechanism of prokaryotic oxygenase enzymes. Our ultimate goal is the design of a synthetic enzyme mimic that can be used as a chiral reagent for aromatic cis-hydroxylation.
Research: organic synthesis, green chemistry, chemoenzymatic synthesis, biomanufacturing, biocatalysis
When people are trying to find Brock University they are often told to use the Schmon Tower, which can be seen throughout Niagara, as their guide. In the world of organic chemistry, Tomas Hudlicky, a Canada Research Chair in Biocatalysis, has earned the same sort of status.
The goal of Hudlicky’s research is the practical and efficient synthesis of new medicinal agents by asymmetric synthesis and total synthesis of natural products. His work related to the total synthesis of morphine and the anticancer drug pancratistatin is concerned with refinements and production of the alkaloids in a more efficient and environmentally benign manner. Analogs of both compounds are also being synthesized and evaluated for biological activities.
Hudlicky also conducts research in the area of organic electrochemistry, which provides “green” alternatives to oxidation and reduction methodology. His current research has led to several patent applications and licensing agreements with the Johnson & Johnson subsidiary Noramco. He has also developed a new, simpler route to Tamiflu, one of the few compounds effective against the illness known as H5N1 virus or bird flu.
Recognized as a “green” scientist, Hudlicky converts pharmaceutical waste into a variety of desirable pharmaceutical compounds. His research is responsible for giving the harmful waste of the past a new life as analgesic and anti-tumour products, specifically compounds used in the treatment of cancer, bio-infection and diabetes.
Hudlicky receives daily requests from across the globe to join his research team. The Cairns Family Health and Bioscience Research Complex will greatly improve the size and capacity of Hudlicky’s research facilities, allowing him to accept more graduate students to study with his group.
|Systematic (IUPAC) name|
Pancratistatin occurs naturally in Hawaiian Spider Lily, a flowering plant within the Amaryllidaceae family. Pancratistatin is mostly found in the bulb tissues of Spider Lilies. It has been shown that the enrichment of atmospheric CO2 can enhance the production ofantiviral secondary metabolites, including Pancratistatin, in these plants. Pancratistatin can be isolated from the tropical bulbs ofHymenocallis littoralis in the order of 100 to 150 mg/kg when bulbs are obtained from the wild type in Hawaii. However, the compound has to be commercially extracted from field- and greenhouse-grown bulbs or from tissue cultures cultivated, for example, in Arizona, which generate lower levels of Pancratistatin (a maximum of 22 mg/kg) even in the peak month of October. After October, when the bulb becomes dormant, levels of Pancratistatin drop, down to only 4 mg/kg by May. Field-grown bulbs, which show monthly changes in Pancratistatin content, generate somewhat smaller amounts (2–5 mg/kg) compared to those grown in greenhouses cultivated over the same period. There are about 40 different Spider Lily species worldwide and they are mainly native to theAndes of South America.
Although there may not be a precise elucidation of Pancratistatin biological synthesis, there have been speculations on biosynthesis ofNarciclasine and Lycoricidine that are very similar to Pancratistatin in terms of structure. The biosynthesis is accomplished via synthesis from O-methylnorbelladine 4 by para-para phenol coupling to obtain vittatine 5 as an intermediate. Subsequent elimination of two carbon atoms and hydroxylations of compound 5 (vittatine) then leads to narciclasine.
The first total synthesis of racemic (+/-) Pancratistatin was proposed by Samuel Danishefsky and Joung Yon Lee, which involved a very complex and long (40 steps) total synthesis. According to both Danishefsky and Joung, there were several weak steps in this synthesis that gave rise to a disappointing low synthetic yield. Amongst the most challenging issues, the Moffatt transposition and theorthoamide problem, which required a blocking maneuver to regiospecifically distinguish the C, hydroxyl group for rearrangement were considered to be the severe cases. However, both Danishevsky and Yon Lee stated that their approach towards the PST total synthesis was not out of merit and believed that their work would interest other medicinal scientists to construct a much more practical and efficient way for PST total synthesis.
The work of Danishevsky and Joung provided the foundation for another total synthesis of PST, which was propounded by Li,M. in 2006. This method employed a more sophisticated approach, starting out with the pinitol 30 that its stereocenters are exactly the same as the ones in the C-ring of Pancratistatin. Protection of the diol functions of compound 30 gave compound 31. The free hydroxyl of this was subsequently substituted by an azide to give 32. After removal of the silyl function, a cyclic sulfate was installed to obtain product 33. The Staudinger reaction gave the free amine 34 from azide 33. The coupling reaction between 34 and 35 gave compound 36 with a moderate yield. Methocymethyl protection of both the amide and the free phenol gave compound 37. Treatment of this latter product with t-BuLi followed by addition of cerium chloride gave compound 38. Full deprotection of 38 by BBr3 and methanol afforded pancratistatin 3 in 12 steps from commercially available pinitol with an overall yield of 2.3% 20.
a: TIPDSCl2, imidazole, DMAP, DMF, 24%. b: DMP, p-TsOH, acetone, 81%. c: PPh3, DEAD, CH3SO3H, CH2Cl2, 0 °C to r.t. then NaN3, DMF, 60 °C, 72%. d: TBAF, THF, 0 °C to r.t., 100%. e: SOCl2, Et3N, CH2Cl2, 0 °C. f: NaIO4, RuCl3, aq CH3CN, 87% (more than two steps). g: PPh3, aq THF, 0 °C to r.t., 94%. h: Et2O, 35, 0 °C, 64%. i: K2CO3, MOMCl,DMF, 84%. j: t-BuLi, CeCl3, ultrasound, THF, −78 °C to r.t., 72%. k: BBr3, CH2Cl2, −78 °C to 0 °C, 1 hour then MeOH, −78 °C to 0 °C, 2 hours, 52%.
A very recent approach to a stereocontrolled Pancratistatin synthesis was accomplished by Sanghee Kim from the National University of Seoul, in which claisen rearrangement of dihydropyranethlyene and a cyclic sulfate elimination reaction were employed 21. This reaction has proven to be very highly efficient as it produced an 83% overall synthetis yield. (Proved by H and 13C NMR).
The B ring of the phenanthridone (three membered nitrogen hetrocyclic ring) is formed using the Bischler-Napieralski reaction. The n precursor 3 with its stereocenters in the C ring is stereoselectively synthesized from the cis-disubstituted cyclohexene 4. The presence of unsaturated carbonyl in compound 4 suggested the use of a Claisen rearrangement of 3,4-dihydro-2H-pyranylethylene.
The synthesis starts with the treatment of 6 with excess trimethyl phosphate. This reaction provides phosphate 7 in 97% yield. Using Honer-Wadsworth-Emmons reaction between 7 ands acrolein dimmer 8 in the presence of LHMDS in THF forms (E)-olefin 5 with very high stereoselectivity in 60% yield. Only less than 1% of (Z)-olefin was detected in the final product. The Claisen rearrangement of dihydropyranethylene forms the cis-distributed cyclohexene as a single isomer in 78% yield.
The next step of the synthesis involves the oxidation of aldehyde of compound 4 using NaClO2 to the corresponding carboxylic acid 9 in 90% yield. Iodolactonization of 9 and subsequent treatment with DBU in refluxing benzene gives rise to the bicyclic lacytone in 78% yield. Mthanolysis of lactone 10 with NaOMe forms a mixture of hydroxyl ester 11 and its C-4a epimer (Pancratistatin numbering). Saponification of the methyl ester 11 with LiOH was followed by a Curtius rearrangement of the resulting acid 12 with diphenylphosphoryl azide in refluxing toluene to afford isocyanate intermediate, which its treatment with NaOMe/MeOH forms the corresponding carbamate 13 in 82% yield.
The next steps of the synthesis involve the regioselevtive elimination of C-3 hydroxyl group and subsequent unsaturation achieved by cyclic sulfate elimination. Diol 16 needs to be treated with thionyl chloride and further oxidation with RuCl3 provides the cyclic sulfate 17 in 83% yield. Treatment of cyclic sulfate with DBU yields the desired allylic alcohol 18 (67% yield).
Reaction with OsO4 forms the single isomerlization 19 in 88% yield. Peracetylation of 19 (77% yield) accompanied by Banwell’s modified Bischler-Napieralski forms the compound 20 with a little amount of isomer 21 ( 7:1 regioselectivity). The removal of protecting groups with NaOMe/MeOH forms Pancratistatin in 83%.
Cheon-Gyu Cho of Hanyang University added (Org. Lett. 2013, 15, 5806. DOI: 10.1021/ol4028623) the activated dienophile 4 to the dienyl lactone to give, after oxidation, the dibromide 5. Debromination followed by oxidation led to the antineoplastic lactam Pancratistatin (6).
- Siedlakowski, P.; McLachlan-Burgess, A.; Griffin, C.; Tirumalai, S. S.; McNulty, J.; Pandey, S. Synergy of pancratistatin and tamoxifen on breast cancer cells in inducing apoptosis by targeting mitochondria. Cancer Biol. Ther. 2008, 7, 376-384.
- Shnyder, S. D.; Cooper, P. A.; Millington, N. J.; Gill, J. H.; Bibby, M. C. Sodium Pancratistatin 3,4-O-Cyclic Phosphate, a Water-Soluble Synthetic Derivative of Pancratistatin, Is Highly Effective in a Human Colon Tumor Model. J. Nat. Prod. 2008, 71, 321-324.
- Ziska, L.; Emche, S.; Johnson, E. Alterations in the production and concentration of selected alkaloids as a function of rising atmospheric carbon dioxide and air temperature: implications for ethno-pharmacology. Global Change Biology 2005, 11, 1798-1807
- Ingrassia, L.; Lefranc, F.; Mathieu, V.; Darro, F.; Kiss, R. Amaryllidaceae isocarbostyril alkaloids and their derivatives as promising antitumor agents. Transl Oncol 2008, 1, 1-13.
- Nair JJ, Bastida J, Codina C, Viladomat F, van Staden J (September 2013). “Alkaloids of the South African Amaryllidaceae: a review”. Nat Prod Commun (Review) 8 (9): 1335–50.PMID 24273880.
- Nair JJ, Bastida J, Viladomat F, van Staden J (December 2012). “Cytotoxic agents of the crinane series of amaryllidaceae alkaloids”. Nat Prod Commun (Review) 7 (12): 1677–88.PMID 23413581.
- Fuganti, C; Staunton, J; Battersby, AR. The biosynthesis of narciclasine. J Chem Soc D: Chem Commun. 1971, 19, 1154–1155.
- anishefsky, S.; Lee, J. Y. Total synthesis of (B1)-pancratistatin. J. Am. Chem. Soc. 1989, 111, 4829-37.
- Jump up^ Li, M; Wu, A; Zhou, P. A concise synthesis of (+)-pancratistatin using pinitol as a chiral building block. Tetrahedron Lett. 2006, 47, 3707–3710.
- Kim, S.; Ko, H.; Kim, E.; Kim, D. Stereocontrolled total synthesis of pancratistatin. Org Lett. 2002, 4, 1343-5.
- Shin, K. J.; Moon, H. R.; George, C.; Marquez, V. E.J. Org.Chem. 2000, 65, 2172.
- Winkler, J. D.; Kim, S.; Harrison, S.; Lewin, N. E.; Blumberg, P. M. J.Am. Chem. Soc. 1999, 121, 296.