The cyclization of 4-chloro-3,3-dimethylbutyronitrile (I) with benzylmagnesium chloride (II) in ethyl ether containing some iodine gives 2-benzyl-4,4-dimethyl-1-pyrroline (III), which is cyclized with 4'-chlorophenacyl bromide (IV) by means of NaHCO3 in methanol, yielding 6-(4-chlorophenyl)-2,2-dimethyl-7-phenyl-2,3-dihydro.1H-pyrrolizine (V). The condensation of (V) with oxalyl chloride (VI) in THF affords 2-[6-(4-chlorophenyl)-2,2-dimethyl-7-phenyl-2,3-dihydro-1H-pyrrolizin-5-yl]-2-oxoacetyl chloride (VII), which is finally reduced and hydrolyzed with hydrazine in hot diethyleneglycol to provide the target licofelone.
The cyclization of 5-benzyl-3,3-dimethyl-3,4-dihydro-2H-pyrrole (I) with 4-chlorophenacyl bromide (II) ethyl ether/ethanol gives 6-(4-chlorophenyl)-2,2-dimethyl-7-phenyl-2,3-dihydro-1H-pyrrolizine (III), which is condensed with ethyl diazoacetate (IV) by means of copper powder in refluxing toluene to afford 2-[6-(4-chlorophenyl)-2,2-dimethyl-7-phenyl-2,3-dihydro-1H-pyrrolizin-5-yl]acetic acid ethyl ester (V). Finally, this ester is hydrolyzed with NaOH in ethanol/water.
A new synthesis of ML-3000 has been published: The reaction of 3-phenyl-2-propynyl chloride (I) with isobutyraldehyde (II) by means of tetrabutylammonium iodide/NaI/NaOH in toluene/water gives 2,2-dimethyl-5-phenyl-4-pentynal (III), which is condensed with glycine methyl ester by means of NaBH(OAc)3 and triethylamine in dichloromethane yielding the N-alkyl-glycine (V). The cyclization of (V) by means of pivalic acid at 150 C affords the bicyclic ketone (VI), which is condensed with diethyl oxalate (VII) by means of sodium ethoxide in ethanol giving the ethoxalyl derivative (VIII). The esterification of (VIII) with the triflic amide (IX) yields the triflate (X), which is condensed with 4-chlorophenylboronic acid (XI) by means of palladium tetrakis(triphenylphosphine) as catalyst in refluxing THF affording the compound (XII). The reduction of the oxoacetic group with tosyl hydrazide (XIII) in refluxing ethanol gives the expected acetate derivative (XIV), which is finally hydrolyzed with NaOH in hot ethanol/water.
The synthesis of the acyl glucuronide of ML-3000 has been described: The cleavage of the lactone ring of D-glucuronic acid gamma-lactone (I) gives the bromide derivative (II), which is treated with silver oxide and benzyl alcohol to yield compound (III). The methanolysis of (III) with sodium methoxide in methanol, followed by treatment with tert-butyldimethylsilyl chloride (TBDMS-Cl) and imidazole affords silylated compound (IV). The hydrolysis of the ester group of (IV) with NaOH in THF/water, followed by reesterification with diethyl chlorophosphate, 2,2,2-trichloroehanol and DMAP gives the silylated trichloroethyl ester (V), which is debenzylated by hydrogenation with H2 over Pd/C in ethyl acetate yielding D-glucuronate (VI). The esterification of the acid group of ML-3000 (VII) with (VI) by means of triphenylphosphine and DIAD in THF affords the corresponding ester (VIII), which is finally deprotected first with Zn and KH2PO4 to eliminate the trichloroethyl group, and then with THF in acetonitrile to eliminate the silyl groups. The product wasisolated as a 1:2 mixture of the alpha- and beta-anomers.
An alternative route to improve the yield of a previous reported synthesis of ML-3000 [Cossy, J. and Belotti, D. J Org Chem 1997, 62(23): 7900] has been reported: The reduction of the oxoacetic ester intermediate (I) can also be performed stepwise by treatment of (I) with tosylhydrazine (II), isolation of the corresponding tosylhydrazone (III), and reduction with sodium cyanoborohydride in ethanol to yield the expected acetate (IV). Finally, this compound is hydrolyzed with NaOH as already described.