Three-Component Aminoalkylations Yielding Dihydronaphthoxazine-Based Sirtuin Inhibitors: Scaffold Modification and Exploration of Space for Polar Side-Chains.

Nonpolar derivatives of heterocyclic aromatic screening hits like the non-selective sirtuin inhibitor splitomicin tend to be poorly soluble in biological fluids. Unlike sp3 -rich natural products, flat aromatic compounds are prone to stacking and often difficult to optimize into leads with activity in cellular systems. The aim of this work was to identify anchor points for the introduction of sp3 -rich fragments with polar functional groups into the newly discovered active (IC50 = 5 µM) but nonpolar scaffold 1,2-dihydro-3H-naphth[1,2-e][1,3]oxazine-3-thione by a molecular modeling approach. Docking studies were conducted with structural data from crystallized human SIRT2 enzyme. Subsequent evaluation of the in silico hypotheses through synthesis and biological evaluation of the designed structures was accomplished with the aim to discover new SIRT2 inhibitors with improved aqueous solubility. Derivatives of 8-bromo-1,2-dihydro-3H-naphth[1,2-e][1,3]oxazine-3-thione N-alkylated with a hydrophilic morpholino-alkyl chain at the thiocarbamate group intended for binding in the acetyl-lysine pocket of the enzyme appeared to be promising. Both the sulfur of the thiocarbamate and the bromo substituent were assumed to result in favorable hydrophobic interactions and the basic morpholino-nitrogen was predicted to build a hydrogen bond with the backbone Ile196. While the brominated scaffold showed moderately improved activity (IC50 = 1.8 µM), none of the new compounds displayed submicromolar activity. Synthesis and characterization of the new compounds are reported and the possible reasons for the outcome are discussed.

Tetrahydroindoles as Multipurpose Screening Compounds and Novel Sirtuin Inhibitors.

Indoles are privileged structures in medicinal and bioorganic chemistry that are particularly well suited to serve as platforms for diversity. Among many other therapeutic areas, the indole scaffold has been used to design aromatic compounds useful to interfere with enzymes engaged in the regulation of substrate acylation status, such as sirtuins. However, the planarity of the indole ring is not necessarily optimal for all target enzymes, especially when functionalization with aromatic side chains is required. Replacement of flat scaffolds by nonplanar molecular cores dominated by sp3 hybridization is a common strategy to avoid the disadvantages associated with poor solubility and high promiscuity, while covering less-well-explored areas of chemical space. Thus, we synthesized fragment-like tetrahydroindoles suitable for fragment-based drug discovery as well as a well-characterized small library intended as multipurpose screening compounds. For proof of principle, these compounds were screened against sirtuins 1-3, enzymes known to be addressable by indoles. We found that 2,6,6-trimethyl-4-oxo-4,5,6,7-tetrahydro-1H-indole-3-carboxamides are potent and selective SIRT2 inhibitors. Compound 16 t displayed an IC50 value of 0.98 µm and could serve as exquisite starting point for hit-to-lead profiling.