Synthesis of a 3-Amino-2,3-dihydropyrid-4-one and Related Heterocyclic Analogues as Mechanism-Based Inhibitors of BioA, a Pyridoxal Phosphate-Dependent Enzyme

Abstract

Amiclenomycin (ACM) is a chemically unstable antibiotic with selective activity against Mycobacterium tuberculosis (Mtb) due to mechanism-based inhibition of BioA, a pyridoxal 5′-phosphate (PLP) dependent aminotransferase. The first-generation ACM analogue dihydro-2-pyridone 1 is semi-stable and maintains a similar bioactivation mechanism concluding with covalent labeling of the PLP cofactor. To improve on 1, we report the synthesis of dihydro-4-pyranone 2, dihydro-4-pyridone 3 and dihydro-4-thiopyranone 13, which were rationally designed to boost the rate of enzyme inactivation by lowering the pKa of their α-protons. We employed a unified synthetic strategy for construction of the desired heterocycles featuring α-amino ynone generation followed by 6-endo-dig cyclization. However, competitive 5-exo-dig cyclization, β-elimination of the ynone, and dimerization of the resultant α-amino carbonyls all complicated the syntheses of the dihydro-4-pyranone and dihydro-4-pyridone scaffolds. These obstacles were overcome by Teoc protection of the β-amino group in the assembly of 3 and Boc-MOM protection of the α-amino group in the synthesis of 2, enabling the efficient construction of 2 and 3 in seven steps from commercially available starting materials. Dihydro-4-pyridone 3 possessed improved enzyme inhibition as measured by its kinact value against BioA, validating our pKa–based design strategy.

Recent advances for identification of new scaffolds and drug targets for Mycobacterium tuberculosis

“…These enzymes are also essential for replication and persistence of M. tuberculosis in vivo . Among these, BioA, a key enzyme in biotin biosynthesis is a target for various scaffolds and these have been demonstrated to inhibit M. tuberculosis growth in vitro .…”

Gold-Catalyzed Reactions of Specially Activated Alkynes, Allenes, and Alkenes

“…323 This strategy has also been used to synthesize analogs of a BioA inhibitor. 324 A 6-exo-dig hydroamidation-initiated double cyclization of an alkynyl ester has also been used by Tokuyama and coworkers in a short total synthesis of lupinine. 325 Intramolecular hydroamination has also been used for the formation of large heterocycles.…”

“…Gouault and coworkers applied the method they developed in the syntheses of pipecolic acid derivatives, dendrobate alkaloid (+)-241D, isosolenopsin and isosolenopsin A, and (−)-epimyrtine . This strategy has also been used to synthesize analogs of a BioA inhibitor . A 6- exo - dig hydroamidation-initiated double cyclization of an alkynyl ester has also been used by Tokuyama and coworkers in a short total synthesis of lupinine …”

The endo-aza-Michael addition in the synthesis of piperidines and pyrrolidines

“…The reaction has also been used in the synthesis of enzyme inhibitors. 72 Scheme 43. Alkaloid synthesis using gold catalysis Silver has also been used to catalyse the cyclisation, although only one example of an endo-aza-Michael reaction, the conversion of ynone 177 to heterocycle 178, was given in a paper otherwise devoted to the endo-oxa-Michael addition (Scheme 44).…”

“…There are a number of examples in which the simple nucleophilicity of an amine is sufficient (Scheme 45). 68,72,74,75 Scheme 44. Silver catalysed cyclisation Scheme 45.…”