Total synthesis of (+) Artemisinin

(+) Artemisinin is a sesquiterpene endoperoxide lactone with an unprecedented structure is a natural medicine for the treatment of malaria in particular drug against drug resistant malaria and cerebral malaria. The total synthesis of this novel sesquiterpene is described using an inter-molecular radical reaction on important intermediate iodolactone starting from terpene (+) isolimonene


Introduction
Malaria is probably as old as mankind and continues to affect millions of people throughout the world. 1 Today some 500 million people in Africa, India, South East Asia and South America are exposed to endemic malaria and it is estimated to cause two and half million deaths annually, one million of which are children. 2Certainly malaria is a serious problem all over the globe.As a consequence, effective therapeutic agents against malaria are continuously being sought, especially against those strains of Plasmodium falciparum, which are resistant to conventional quinine and acridine based drugs.Artemisinin, which has been isolated 3,4 from Artemisia Annua L. Compositae (Qinghao), is an active constituent of traditional Chinese herbal medicine which is used for the treatment of malaria in China for more than 1000 years.(+) Artemisinin 1, a sesquiterpene endoperoxide lactone with an unprecedented structure is a natural medicine for the treatment of malaria, in particular drug against drug resistant and cerebral malaria.The exceptional pharmacological potential and extreme scarcity of the natural material together with its complex structure prompted us to study the total synthesis of (+) Artemisinin.The architectural complexity is attributed to the presence of 7 chiral centers with tetracyclic framework with an endoperoxide unit.Though many valuable contributions [5][6][7][8][9] have been made towards the total synthesis of this unique structurally complex molecule, the need for a simple strategic route still remains, encouraging us to take up the total synthesis of this potent antimalarial drug.

Results and Discussion
In the retrosynthetic analysis (scheme 1), among all the valuable contributions for the total synthesis of Artemisinin, we believe that the key intermediate would be α-hydroperoxy aldehyde which can be easily photo oxygenated from methylvinylether 2 because, in a ketalization-like process, simple cyclodehydration of α-hydroperoxy aldehyde should readily furnish the tetracyclic natural product 1.Thus, the next intermediate in our analysis was the iodolactone 3 which has required stereochemistry and further it can be easily prepared from the starting (+) isolimonene 4 which has two asymmetric carbon atoms having the same absolute configuration as the target molecule.
(+) Isolimonene 4 with exocyclic and endocyclic double bonds, was subjected to regioselective hydroboration.The regioselective exocyclic hydroboration was achieved using dicyclohexyl borane 10 to get the required alcohol 5 in 82 % yield.The resulting alcohol 5 was oxidized with Jones reagent 11 in acetone at 0 o C to the corresponding acid 6 in 80 % yield.The γ,δ-unsaturated acid 6 was subjected to iodolactonization 12 using KI, I 2 in aq.NaHCO 3 to afford iodolactone 3, 3a as separable diastereomers, isomeric at C3 in 68:32 (β:α) ratio in 70 % yield.While this work was under progress the iodolactones were reported by Chavan et.al. 13 for the synthesis of Wine lactone.The next step in the synthesis called for the introduction of the side-chain appendage at carbon bearing iodine on intermediate 3 with the four requisite stereo centers embedded within its cyclohexane ring.Michael addition reaction was attempted with methylvinyl ketone using Bu 3 SnH 14 and a catalytic amount of AIBN in refluxing toluene.The reduced product 8 was the major product from the reaction and the desired product 7 was isolated in 10 % yield (scheme 4).Use of Ph 3 SnH in the place of Bu 3 SnH also gave the same results.Since homologation was essential for proceeding further in the synthesis of Artemisinin, we envisioned cyclofuntionalisation of γ,δ-unsaturated acid with bezeneselenyl bromide for construction of selenolactone in place of iodolactone.Phenyl selenyl bromide 15 was prepared from diphenyl diselenide 16 using bromine in CH 2 Cl 2 at 0 o C, and treated with unsaturated acid 6 to afford selenolactone 9, 9a as 1:1 diasteriomeric mixture in good yield.But attempted reaction of the selenolactone 9 with methylvinyl ketone using tri-n-butyltin hydride and catalytic amount of AIBN gave the same product distribution as observed with 3 (scheme 5).Since intermolecular C-C bond formation reactions have been increasingly achieved by radical addition to alkenes.But in efforts, alkylation reaction on iodolactone 3 and selenolactone 9 with methylvinyl ketone met with failure using Bu 3 SnH, Ph 3 SnH.We therefore, selected the silane functionality as mediator in the formation of inter molecular C-C bond.Using Chatgilialoglu's reagent 19 tris(trimethylsilyl)silane ((CH 3 ) 3 Si) 3 SiH, the reaction of iodolactone 3 with methylvinyl ketone, with AIBN in refluxing toluene gave product 7 in very good yield by slow addition of tris(trimethylsilyl)silane using syringe pump (Scheme 7).The product 7 was obtained in 72 % yield as an inseparable epimeric mixture at C 7 (8:2 β/α) along with 12 ( To proceed further towards the synthesis of Artemisinin 1, the ketogroup of lactone 7 has to be protected.Ethane diol and propane diol were used for protecting the ketone using PTSA in refluxing benzene but decomposition of starting material was observed.The keto group of lactone 7 was treated (scheme 8) with ethane dithiol and BF 3 .Et 2 O in CH 2 Cl 2 at 0 o C to afford thioketal lactone 13, 14 in quantitative yield. 6Thioketalization reaction facilitated the separation of the major isomer 14 in pure form, which was expected as all three side chains are in equitorial position.Which was further subjected to hydrolysis and esterification with diazomethane provided hydroxyl ester 15 in 50% yield (The thioketal lactone 14 was also isolated due to competent lactonization of the hydroxy acid.).The hydroxy methylester 15 was transformed to the keto ester 16 using PCC as the oxidizing agent in CH 2 Cl 2 at room temperature (Scheme 8).
In a two dimensional 1 H NMR study of compound 16 revealed the strong nOe between the two di axial protons adjacent to the carbonyl group.These observations confirmed its structure.The second key intermediate methylvinyl ether 17 was achieved by Wittig reaction using methoxymethyl triphenylphosphonium chloride and KHMDS in THF, HMPA at 50 o C. 20 The reaction was also carried out using n-butyl lithium in THF, n-butyl lithium in ether, NaH in DMSO, NaH in DMSO at benzene reflux conditions as reported in the literature.But due to hindrance at the two sides of the carbonyl group yields were moderate.These reactions were successful with high yields when attempted on the unhindered ketone such as menthone 18 (Scheme 9).The deprotection of thioketal 17 using HgCl 2 , CaCO 3 resulting the key intermediate 2 in 80% yield.Compound 2 has been transformed to the target molecule 1 by photooxidation using an ordinary tungsten light source (250 V, 500W), followed by acid hydrolysis reaction with 70 % HClO 4 (scheme 10).Photooxidation of methyl vinyl ether 2 with O 2 with Rose Bengal forms 1,2 dioxetane and which on acid hydrolysis with HCl gives the hydroperoxy-aldehyde, thereafter, cyclization with HClO 4 forms the final product 1.The synthetic material was found to be identical with natural Artemisinin in every respect (

Conclusions
In conclusion, a stereoselective total synthesis of the highly potent antimalarial drug (+) Artemisinin has been achieved in the shortest possible route.Synthetic intermediate iodolactone and its inter molecular C-C bond formation using silane functionality as the mediator have been achieved as key steps in the total synthesis with very good yields.Further, the synthetic route is very much useful for the large-scale synthesis of structurally diverse natural sesquiterpene endoperoxide lactone (+) Artemisinin 1 (Qinghaosu).

Experimental Section
General Procedures.Infrared spectra were recorded on a GC-FTIR spectrometer. 1 H NMR and 13 C NMR spectra were recorded using a Varian-Gemini 200 or Varian Unity 400 MHz spectrometer.Mass spectra were recorded on Finningan Mat 1020B mass spectrometer.Column chromatography refers to silica gel chromatography using Acme 60-120 mesh.Analytical thin layer chromatography was performed on pre-coated E-Merck silica glass plates.Acetonitrile was used as it is purchased from E-Merck.

2-(4-Methyl-2-cyclohexenyl)-1-propanol (5).
A 250 mL flask equipped with a septum inlet, a magnetic stirring bar was charged with 5.05 mL of BH 3 SMe 2 (50 mmol) and 18 mL of freshly distilled THF.It was cooled to 0 o C and 18.3 mL (115 mmol) of cyclohexene (neat) was added drop wise.After the mixture was stirred at 0 o C for 1 hour (C 6 H 5 ) 2 BH separated as white solid during this time), the flask was stored at 0 o C in a refrigerator for 7 days.
To the (C 6 H 5 ) 2 BH (solid, 50 mmol) was added (18.3 g, 75 mmol) of neat olefin 4. The reaction mixture was stirred at -25 o C for 1 hour and kept in the refrigerator for a day.The trialkyl borane was treated with 50 mL of 3N sodium hydroxide, 7.5 mL of 30% hydrogen peroxide and stirred at 25 o C for 5 hours.This was extracted with ether, dried (Na 2 SO 4 ) and the ether was evaporated.The residue was filtered through silica gel (pet.ether-ethylacetate 9:1 used as eluent) to remove the olefin and cyclohexyl alcohol and then eluted with pet.ether-ethylacetate (1:1) mixture to give the pure alcohol 5 in (17.0 g) 82 % yield. 1

2-(4-Methyl-2-cyclohexenyl)proanoic acid (6).
Jones reagent was prepared by drop wise addition of sulfuric acid (17 mL) to a cooled solution of CrO 3 (20 g, 200 mmol) in water (30 mL) and the resulting solution was diluted with water until the total volume of this solution 60 mL.

3,6-Dimethyl-7-(3-oxobutyl)perhydrobenzo[b]furan-2-one (7).
A 500 mL round-bottemed flask equipped with a magnetic stirring bar, dry nitrogen inlet, reflux condenser, and septum was flushed with nitrogen and charged with 5 g (17.0 mmol) of iodolactone formula IV and 1.06 mL (12.75 mmol) of methylvinyl ketone in 150 mL of toluene.The mixture was brought to reflux; 3.96 g (12.75 mmol) of TTMSS and 279 mg (1.7 mmol) of AIBN dissolved in 20 mL of toluene were added over 4 hours through a long needle using a syringe pump.Similarly, second portion of 1.06 mL (12.75 mmol) of methylvinyl ketone was added and 3.96 g (12.75 mmol) of TTMSS and 279 mg (1.7 mmol) of AIBN dissolved in 6 mL of toluene were added over 4 hours through a long needle using a syringe pump.The reaction mixture was brought to room temperature, concentrated in vacuum.The residue was subjected to column chromatography on silica gel using eluent (80:20 hexane/ethyl acetate) to provide the pure ketolactone 7 (2.954 g, 73 % yield) as brown colour semisolid.9).Benzeneselenyl bromide (PhSeBr) 1.545 g, 6.54 mmol was prepared by dissolving Ph 2 Se 2 (1.021g, 3.27 mmol) in 3 mL THF and 0.16 mL (0.523g, 3.27 mmol) of bromine were added drop wise while stirring under N 2 atmosphere.The reaction is essentially instantaneous and solution can be used directly for the next reaction.
To a solution of above crude product in ether (5 mL) was added a solution of 70% HClO 4 (1 mL) and water (5 mL).The resulting reaction mixture was stirred at 25 o C for 28 hours.The ethereal layer was separated and the aqueous layer was further extracted with ether.The combined ethereal solution was washed, dried and concentrated to obtain the target compound.The crude product was purified on preparative TLC (eluent petroleum ether/ethyl acetate, 90/10) to give 1 (6 mg) in 10% yield. 1