Q. Michaudel Pentacyclic Triterpenes Baran Lab GM 2013-04-27 Me Me Me Triterpenes (C30): Me * More than 20,000 triterpenes have been isolated thus far. Among them, tetracyclic and pentacyclic triterpenes are the most abundant. Me Me H CO2H Me Me H R * Triterpenes can be found in their free form (sapogenins), or bound to glycosides HO (saponins). H Me H Me HO HO * Pentacyclic triterpenes are divided into many subgroups: gammaceranes, hopanes, H H lupanes, oleananes, ursanes, etc. based on their carbon skeleton. Me Me Me Me Me corosolic acid β-amyrin, R = Me Me ≥98%, $12,480/g $21,450/g E Isol. Crape-myrtle (Lagerstroemia Tot. synth. (+/–) Johnson, J. Am. Chem. H H H Me H speciosa). Soc. 1993, 115, 515. Me C Isol. Rubber trees. Me Me Me Me Me Me Me H Me Me Me H Me Corey, J. Am. Chem. Soc. 1993, 115, 8873. D erythrodiol, R = CH2OH A H Me H Me H Me $12,550/g B Isol. Olives. H H H Tot. synth. Corey, J. Am. Chem. Soc. 1993, Me Me Me Me Me Me 115, 8873. gammacerane hopane lupane oleanolic acid, R = CO2H ≥97%, $569/g Me Me Me Isol. mistletoe, clove, sugar beet, olive Me Wikimedia Commons leaves, beeches, American Pokeweed (Phytolacca americana)... H H Me Me Tot. synth. Corey, J. Am. Chem. Soc. 1993, 115, 8873. Me Me H Me Me Me H Me H Me Me H Me H Me Me Me Me H H Me Me Me Me Me H Me H H Me oleanane ursane HO H Me Me * Since 1985, Connelly and Hill have been publishing an annual review on the newly Me Me isolated triterpenoids. e.g. Nat. Prod. Rep. 2011, 28, 1087. germanicol O Tot. synth. (+/–) Ireland, Johnson, Me * For an exhaustive review on pentacyclic triterpenes, see Sheng et al., Nat. Prod. J. Am. Chem. Soc. 1970, 92, Me Rep. 2011, 28, 543. 5743. friedelin Van Tamalen, J. Am. Chem. Soc. $297.50/g * Pentacyclic triterpenes are often bioactive (antitumor, antiviral, antidiabetic, 1972, 94, 152. antiinflammatory...) and present a huge therapeutic potential. A few compounds like Isol. Cork oak (Quercus suber, Fagaceae). corosolic acid (dietary supplement against diabetes) are already on the market and Tot. synth. (+/–) Ireland, Tetrahedron Lett. several others are under clinical trials or ready to be launched on the market. 1976, 14, 1071. Prices from Sigma-Aldrich, 2013. 1 Q. Michaudel Pentacyclic Triterpenes Baran Lab GM 2013-04-27 * Betulin was one of the first isolated natural products * The bark of white birch contains both betulin (ca. 25%) and betulinic acid (ca. in 1788: Lowitz, M. (1788) in Chemische Annalen 0.025%). The latter can be tedious to isolate, and therefore semi-syntheses from Me (Crell, L., ed.) Vol. 2. p. 312. In 1876, Hausmann betulin have been developed. H performed detailed investigations including elemental analysis on betulin: Ann. 1876, 182, 368. For a review on betulin, see Phytochemistry, 1989, 28, 2229. Me Me Me Me H R H H * Betulin shows antitumor and antiviral activity. Birch OH 1. CrO3, H2SO4, H Me bark (10-14% betulin) has been used as an antiseptic H acetone, 0 °C H CO H and in folk medicine against a broad variety of Me Me Me Me 2 HO H diseases. Also, native Americans used red alder bark 2. NaBH4, THF Me Me (containing betulin and lupeol) against poison oak, H Me 56 % H Me insects bites and skin diseases. HO HO lupeol, R = Me H H ≥94% $2,570/g * Betulinic acid is the most biologically active molecule Me Me Synth. Commun., 1997, 27, 1607. Me Me Isol. Husk of Lupin seeds of the family. It was shown to be a selective inhibitor of * As an alternative, an electrochemical oxidation of betulin to betulin aldehyde using a (Lupinus luteus). human melanoma that functions by induction of catalytic amount of TEMPO has been disclosed recently. Subsequent oxidation with Tot. synth. (+/–) Stork, J. Am. apoptosis (Nat. Med. 1995, 1, 1046). Betulinic acid NaClO affords betulinic acid. Chem. Soc. 1971, 93, 4945. also presents anti-HIV activity by inhibiting the 2 maturation of the virus. Some derivatives are part of Corey, J. Am. Chem. Soc., 2009, the current leading natural products for anti-HIV drugs 131, 13928. (Med. Res. Rev. 2000, 20, 323). Me Me betulin, R = CH2OH ≥98% $121.50/g H H OH TEMPO (cat.) Isol. Bark of birch trees (Betula). Me H anodic oxidaton H CHO betulinic acid, R = CO2H Me Me Me Me ≥90% $572/g H ≥98% $4700/g H Me H Me Isol. Bark of birch trees (Betula). Me Me H CO2H HO HO Semi. synth. Ruzicka, Helv. H H Me Me O Me Me Me Me Chim. Acta. 1938, 21, 1706. H Me NaClO2 Synth. Commun. 1997, 27, 1607. 85% (2 steps) HO C O 2 H Me Me Bevirimat (Panacos, then Myriad Genetics), anti-HIV drug, not currently FDA approved. Platinum anode, copper cathode in 40 L apparatus. Pilot plant can produce ca. 1kg/day of betulinic acid Me US patent US 20080308426 A1 H N. American Herb & Spice Bet-u-Power Birch Bark Extract: Me Me H CO2H "Is the power of raw, wild, remote-source white birch bark. Birch bark is a top source of potent sterols known as betulin H Me and betulinic acid. These sterols are the subject of extensive research and are heart-healthy. Sterols have high electrical HO charge and are needed by cell membranes. Take wild Bet-u- H Power daily for better health." Me Me * As a side note, buchu leaf oil (Agathosma betulina) that you can buy through Sigma- Image and text found on the website www.soap.com Aldrich does not contain any lupane terpenes, but only smaller terpenes (menthone, $16.31 limonene...). 2 Q. Michaudel Pentacyclic Triterpenes Baran Lab GM 2013-04-27 Biosynthesis of pentacyclic triterpenes from squalene or oxidosqualene: * Squalene and oxidosqualene (6 isoprene units) are formed by condensation of two farnesyl pyrophosphate molecules. See Maimone group meeting 2005. * Many cyclase phases are possible, consequently the structural diversity of the pentacyclic triterpenes is immense. For interesting reviews, see Corey, Liu, Angew. Chem. Int. Ed. 2000, 39, 2812 Matsuda, Phytochemistry, 2004, 65, 261. * Often for pentacyclic triterpenes, cyclization takes place with an all-chair conformation of the (oxido)squalene. O * Mutation of only one amino acid can derail the cyclization pathway to another product. + squalene- Enz-AH Enz-AH+ lupeol For instance, Kushiro et al. have engineered lupeol synthase into β-amyrin synthase (J. hopene synthase cyclase Am. Chem. Soc. 2000, 122, 6816). * Finally, ring cleavage can take place after cyclization leading to even more complexity. For a review on unusually cyclized triterpenes, see Domingo et al.: Nat. Prod. Rep. 2009, 26, 115. Me Me Me Me H H Me H Me Me H Me H HO H Me H H Me H Me H HO H HO H H H H Me Me Me Me deoxydammarenyl dammarenyl α-seco-amyrin cation cation HO H H Me Me Me Me O H O O H H Me OH H baccharenyl Me Me Me Me Me Me cation O Me H Me O H H Me Me H H H H H Me camelliol B yardenone HO H hopyl Main synthetic strategies for pentacyclic triterpenes: H cation H lupyl cation AB + DE ––> ABCDE HO CDE ––> BCDE ––> ABCDE H H D(E) ––> ABCD(E) via polyene cyclization (––> ABCDE) hopene lupeol 3 Q. Michaudel Pentacyclic Triterpenes Baran Lab GM 2013-04-27 Early work, syntheses of degradation products: Me Me H Synthesis of 1,8-dimethylpicens and 1,8 dimethyl-2-methoxypicens: Ruzicka, Helv. Chim. Acta, 1937, 20, 1155. O Me Me Ruzicka and coworkers dehydrogenated various pentacyclic triterpenes using Me 1. KOH, MeOH HClO4, O HO Ac O, Me palladium at 305 °C. Starting with oleanolic acid, they obtained 1,8-dimethylpicens and then NH3, Et2O Me 2 AcOH Me starting with β-amyrin, they obtained 1,8-dimethyl-2-methoxypicens. Then, they Me Me Me H synthesized both picens. 2. electrolysis Me OH PhMe name? rt,10% H Me Me Me Me Me 11-16% 1. Br CO2Et (2 steps) R H γ-onocerene Zn, PhMe Prepared in 1 or 2 H Me Me + steps (isomerization) Me Me from sclareol Me 2. Na, EtOH Me 3. HBr O 38% (3 steps) Br O Synthesis of the oleanane skeleton: Tetrahedron Lett. 1962, 10, 429. Me Me Me Me R R Mg, Et2O, 58% Me Me 1. Pd/C, Br 320 °C Me 1. NaOtAm 68% Me Me 2. AlCl CS 2. Li, NH3, 97% HO 3, 2 + 5% (2 steps) Me Me 3. MeLi, Et2O H Me Me Me Me O H Me Me R = H or OMe, yields are given for R = H. Prepared in 4 steps from sclareol Synthesis of pentacyclosqualene: Corey, J. Am. Chem. Soc. 1959, 81, 1739. Me Me Me Me Me Me AlCl3 H CH NO :Et O OH 3 2 2 rt, 35% H Me Me Me 1. diol oxidation Me Me Me + Me Me Me 2. acetic ring closure Me Me H Me Me Me Me 3. Wolff-Kishner Me H H Barton, J. Chem. Soc., 1955, 2639. Me Me Me Me HO H H Me Me γ-onocerene Me Me onocerin These two products have been previously obtained by isomerization of (–)-olean-12-ene For an enantioselective synthesis of onocerin, see Corey, J. Am. Chem. Soc. 2002, 124, 11290. 4 Q. Michaudel Pentacyclic Triterpenes Baran Lab GM 2013-04-27 Relay synthesis of pentacyclic triterpenes: O O 1.