Multigram-scale HPLC enantioseparation as a rescue pathway for circumventing racemization problem during enantioselective synthesis of ethyl 3,4-dihydro-2H-1,4-benzoxazine-2-carboxylate

Racemic ethyl 3,4-dihydro-2H-1,4-benzoxazine-2-carboxylate is a key synthon for the design of promising therapeutic drugs. It is mainly synthesized from racemic ethyl 2,3-dibromopropionate and 2-aminophenol in presence of K₂ CO₃ in refluxed acetone. Unfortunately, synthesis of (R)- and (S)-enantiomers using the enantioselective version of this reaction, which should normally be performed with a double S_N 2 mechanism, is unsuitable due to a racemization process involving the dehydrobromination of enantiopure ethyl 2,3-dibromopropionate into ethyl 2-bromoacrylate. For the first time, the enantioselective version is studied (ee ≈ 55-66%), and the percentage of racemization process has estimated to around 34-46% after determination of the optimal experimental conditions for analytical HPLC enantioseparation of racemate. The influence of the experimental and purification conditions on the racemization rate is also studied. The results indicate that racemization occurs faster at the beginning of the reaction but the initiation of the double S_N 2 process takes place more faster to limit the racemization rate. The study of the influence of experimental conditions (reaction times, temperature, solvent or type of base, etc.) on the degree of racemization of the (R)- enantiomer is performed and shows that despite changes in the experimental conditions, the synthesis of the (R)- enantiomer is always accompanied by a racemization rate which is difficult in reducing. In parallel, (R)- and (S)-enantiomers are obtained in high enantiopurity (ee > 99.5%) by preparative HPLC enantioseparation of racemate on multigram scale and characterized by optical rotation measurements, ECD and UV spectra.

Purine derivatives with heterocyclic moieties and related analogs as new antitumor agents

AIM

Identification of new antiproliferative compounds.

METHODOLOGY

Four series of compounds were synthesized by the Mitsunobu reaction. Their antiproliferative activity was studied against several cancer cells and a noncancerous fibroblast cell line. Their apoptotic activity was analyzed using a caspase 3/7 fluorescence assay.

RESULTS & CONCLUSION

9-alkylated-6-halogenated and 2,6-dihalogenated purines show remarkable inhibition of tumor cell proliferation, with the dichloro derivatives being the most potent of all the series. The most promising compound, tetrahydroquinoline 4c, exhibits significant antiproliferative activity against the cancer cells tested, while displaying a 19-fold lower potency against noncancerous fibroblasts, a key feature that indicates potential selectivity against cancer cells. This compound produces a high percentage of apoptosis (58%) after 24 h treatment in human breast cancer MCF-7 cells.