Enol-keto tautomerism of alpha-ketophosphonates

Diketo-Ketoenol Tautomers in Curcuminoids: Synthesis, Separation of Tautomers, and Kinetic and Structural Studies

Curcumin and its congeners exist in an equilibrium between diketo and ketoenol tautomers, which have different potencies to bind biomolecules. This work describes procedures for the preparation of 4-alkylated curcumin derivatives and the separation of their two tautomeric forms. Comprehensive NMR studies of the tautomer equilibria in various solvents have been accomplished. Additionally, a pure ketoenol tautomeric form of the active pharmaceutical ingredient (API) ASC-JM17 has been unequivocally determined by X-ray crystallography. Two different polymorphs of this API have been microscopically identified in the X-ray sample and manually separated, and a solid-state NMR study of the two polymorphs has also been performed. This work reports on the slow kinetics of diketo-ketoenol tautomerization in particular solvents that allow the separation and full characterization of both curcuminoids' tautomers.

A multifaceted approach towards understanding the peculiar behavior of (α)-hydroxyiminophosphonates

A unique multifaceted approach involving the interplay of NMR, XRD, LC, DFT and MS/IM-MS techniques toward the understanding of the peculiar isomer-selective reduction of (α)-hydroxyiminophosphonates into (α)-hydroxyaminophosphonate derivatives.

Synthesis, Spectroscopic Studies and Keto-Enol Tautomerism of Novel 1,3,4-Thiadiazole Derivative Containing 3-Mercaptobutan-2-one and Quinazolin-4-one Moieties

In this study, a novel 1,3,4-thiadiazole derivative containing 3-mercaptobutan-2-one and quinazolin-4-one moieties (Compound 3) is synthesized by the coupling of 2-amino-1,3,4-thiadiazole-5-(3-mercaptobutan-2-one) (Compound 1) with 2-Phenyl-4H-3,1-benzoxazin-4-one (Compound 2) in one molecule moiety. Compound 3 is found to exist as two types of intra-molecular hydrogen bonding with keto-enol tautomerism characters, which is further confirmed using FTIR, ¹H-NMR, ¹³C-NMR, mass spectrometer, and UV-Visible spectra. The ¹H-NMR and UV-Visible spectra of Compound 3 are investigated in different solvents such as methanol, chloroform, and DMSO. Compound 3 exhibits keto-enol tautomeric forms in solvents with different percentage ratios depending on the solvent polarity. The ¹H-NMR and UV-Visible spectral results show that Compound 3 favors the keto over the enol form in polar aprotic solvents such as DMSO and the enol over the keto form in non-polar solvents such as chloroform. The ¹³C-NMR spectrum gives two singles at δ 204.5 ppm, due to ketonic carbon, and δ 155.5 ppm, due to enolic carbon, confirming the keto-enol tautomerism of Compound 3. Furthermore, the molecular ion at m/z 43 and m/z 407 in the mass spectrum of Compound 3 and fragmentation mechanisms proposed reveal the existence of the keto and enol forms, respectively.

Phosphonic Acid Analogs of Fluorophenylalanines as Inhibitors of Human and Porcine Aminopeptidases N: Validation of the Importance of the Substitution of the Aromatic Ring

A library of phosphonic acid analogs of phenylalanine substituted with fluorine, chlorine and trifluoromethyl moieties on the aromatic ring was synthesized and evaluated for inhibitory activity against human (hAPN) and porcine (pAPN) aminopeptidases. Fluorogenic screening indicated that these analogs are micromolar or submicromolar inhibitors, both enzymes being more active against hAPN. In order to better understand the mode of the action of the most active compounds, molecular modeling was used. It confirmed that aminophosphonic portion of the enzyme is bound nearly identically in the case of all the studied compounds, whereas the difference in activity results from the placement of aromatic side chain of an inhibitor. Interestingly, both enantiomers of the individual compounds are usually bound quite similarly.

CO2 activation through C–N, C–O and C–C bond formation

CO₂ fixation reactions have been explored using different organic substrates upon activation with a superbase.

Identification of ACE pharmacophore in the phosphonopeptide metabolite K-26

The naturally occurring phosphonotripeptide K-26 is a potent angiotensin converting enzyme (ACE) inhibitor containing an alpha-amino phosphonic acid analogue of tyrosine. Previous studies have demonstrated that canonical peptide analogues of K-26 are micromolar inhibitors of ACE. To ascertain the structure-activity relationships in this class of ACE inhibitory natural products, K-26 and eight analogues were chemically synthesized and evaluated. Phosphonyl substitution was found to be the critical determinant of activity, resulting in a 1500-fold increase in ACE inhibition versus carboxyl analogues. Secondarily, the absolute configuration of the terminal alpha-amino phosphonate and N-acetylation were found to significantly modulate ACE inhibitory activity.