The article is about versatile synthetic strategy for preparing enantiomerically pure (N)-methanocarba ring containing monophosphates as a potent agonist for P2 receptors.
I have successfully reproduced the synthesis of compound (2, MRS2339) from intermidiate compound (12).
Compound (12) was treated with sodium borohydride in ethanol at room temperature to chemoselectively reduce the carobonyl group to 2R-Hydroxy Compound (14), in similar yield (55% - 65% yield; reported yield-69%).
Compound (14) was subjected to isomerization (rearrangement) using Trifluoromethanesulfonic acid in dry Dichloromethane instead of trifluroacetic acid in acetone. The change in acid (due to quick availbility in the lab), didnot change the reaction fate or product yield. Repeated fractional recrystallization in cyclohexane gave the required isomer (90% pure by NMR) Compound (15) in similar yield (31% - 35%; reported yield-35%).
Compound (15) was used to build the purine nucleoside and its different analogs. Mitsunobu reaction strategy was used to condense 2,6-dichloro purine to the alcohol using typical reagents - DEAD, triphenylphosphine in dry condition. The reaction was usually stirred for overnight ( 16 - 17 hrs, instead of reported 8 hrs). This may be due to moisture contained in commercially available THF. Also the product and the alcohol has similar Rf values. So, using gradient solvent system and longer TLC plates it was easier to detect the completion of the reaction. Anisaldehyde dye test was used in all the cases to monitor the formation of nucleosides or modified ribose.
Mitsunobu reaction product Compound (16) was then subjected to reduction condition to generate 5' hydroxy group, where further phosphorylations will take place. DIBAL-H was the choice of reducing agent in this case. Two equivalent was used as reported, to reduce ethyl ester to aldehyde and then to required alcohol. The reaction was performed in -78 degree Celcius (dry ice/ acetone bath) and carefully controlled warming up of the reaction mixture in acidic condition gave higher yield ( 70%, reported 56%) of Compound (18). The reaction time ( used 1 hour instead of reported 3 hours) and excess of reducing agent made the difference in yield.
Compound (18) was stirred with 2(N) Ammonia dissolved in Iso-propanol to give N6- substitution of the purine, Compound (19), in higher yield ( 70%, reported 46%).
2 position of Compound (19) was substituted sometime with different groups. Presence of Thio group substitution generates complicacy in following reactions.
Compound (19) was used to develope the phosphate moiety at 5' position. Di-t-butyl N,N'-diethyl-phosphoramidite was used to generate the phosphite. Tetrazole solution in Acetonitrile was used instead of solid Tetrazole in this case. Phosphite was subjected to oxidation using equivalent amount of m-chloroperbenzoic acid to generate phosphate. In the presence of thio group in 2-position of purine ring, mCPBA oxidation was done with half equivalent of oxidant and 1 min stirring only. Phosphate- Compound (20) was obtained in reproducible yield (70% - 80%, reported 77% yield).
Finally Compound (20) was hydrolyzed in mild acidic condition using Dowex-50 resin and stirring at 70 degree Celcius for 5 hrs (instead of 3 hrs reported). Ion exchange chormatography purification gave Compound (2) in 38% yield (reported 25%).
Monophosphate Compound (2) was synthesized and the reaction methodology and synthetic steps were reproducible. Moisture elimination and hence the reaction time was the only major changes to be made. Synthesis of N-methanocarba purine nucleosides and its phosphates are a major breakthrough in understanding the P2 receptors ligands and the authors have sucessfully depicted a reproducible innovative versatile method in this article.