Stereoselective total synthesis of all the stereoisomers of (+)- and (−)-febrifugine and halofuginone

A Stereocontrolled Synthesis of (+)-Febrifugine via Azide and Azide-Free Pathways

(+)-Febrifugine, a natural antimalarial compound with a promising therapeutic profile, has become a hot target for synthetic chemists seeking to optimize its biological activity and expand its therapeutic applications. In this research, we present a stereocontrolled synthesis of (+)-febrifugine using both azide and azide-free approaches. Starting from the commercially available chiral pool precursor, d-glucose, the synthesis was completed in 20 steps for both approaches. Key reactions included the Zn-mediated Bernet-Vasella reaction, Horner-Wadsworth-Emmons reaction, and cyclization for constructing the chiral substituted piperidine ring. Additionally, α-bromoketone alkylation of quinazolinone was employed to assemble the (+)-febrifugine core structure.

Stereoselective synthesis of analogues of deoxyfebrifugine

The preparation of six new optically active analogues of the natural product febrifugine (1) is reported. These analogues, lacking the hydroxy group from the natural product, were prepared from optically active N-protected S-pelletierine (7) and differ in terms of the specific quinazolinone portion included. The required S-7 (80% enantiomeric excess) was prepared from an asymmetric Mannich reaction between piperideine (8) and acetone in the presence of l-proline. The differently substituted quinazolinone used in this study (10a–10g) was either commercially available or was prepared from the corresponding substituted anthranilic acid and were installed via a bromination–alkylation sequence. N-Deprotection of the subsequent adducts (12a–12g) gave target compounds 13a–13f and completed the synthetic sequence.

Enantiodivergent Synthesis of Halofuginone by Candida antarctica Lipase B (CAL-B)-Catalyzed Kinetic Resolution in Cyclopentyl Methyl Ether (CPME)

The synthesis of both enantiomers of a key intermediate in the synthesis of halofuginone was accomplished by a Candida antarctica lipase B (CAL-B)-catalyzed kinetic resolution of the corresponding racemate. When the resolution was carried out in the versatile solvent cyclopentyl methyl ether (CPME) using p-chlorophenylbutyrate (PCPB) as the acylating reagent, the highest enantiomeric ratio (E) values were measured, and highly enantioenriched (95% ee) compounds could be obtained in a single iteration. As an example, one of the two enantiomers was used as a starting material to prepare (+)-halofuginone in a three-step procedure.