Synthesis and Isolation of 5,6-Dihydro-4H-1,3-Oxazine Hydrobromides by Autocyclization of N-(3-Bromopropyl)amides

Photo- and Metal-Mediated Deconstructive Approaches to Cyclic Aliphatic Amine Diversification

Described herein are studies toward the core modification of cyclic aliphatic amines using either a riboflavin/photo-irradiation approach or Cu(I) and Ag(I) to mediate the process. Structural remodeling of cyclic amines is explored through oxidative C-N and C-C bond cleavage using peroxydisulfate (persulfate) as an oxidant. Ring-opening reactions to access linear aldehydes or carboxylic acids with flavin-derived photocatalysis or Cu salts, respectively, are demonstrated. A complementary ring-opening process mediated by Ag(I) facilitates decarboxylative Csp³-Csp² coupling in Minisci-type reactions through a key alkyl radical intermediate. Heterocycle interconversion is demonstrated through the transformation of N-acyl cyclic amines to oxazines using Cu(II) oxidation of the alkyl radical. These transformations are investigated by computation to inform the proposed mechanistic pathways. Computational studies indicate that persulfate mediates oxidation of cyclic amines with concomitant reduction of riboflavin. Persulfate is subsequently reduced by formal hydride transfer from the reduced riboflavin catalyst. Oxidation of the cyclic aliphatic amines with a Cu(I) salt is proposed to be initiated by homolysis of the peroxy bond of persulfate followed by α-HAT from the cyclic amine and radical recombination to form an α-sulfate adduct, which is hydrolyzed to the hemiaminal. Investigation of the pathway to form oxazines indicates a kinetic preference for cyclization over more typical elimination pathways to form olefins through Cu(II) oxidation of alkyl radicals.

A kinetic method for detecting intramolecular peptide H-bonds

The current method for the detection of peptide hydrogen bond (PHB) formation places charge donors/acceptors (D/A) at the N-/C-termini of the peptides involved in the putative PHB.

Chiral Phosphoric Acid Catalyzed Enantioselective [4 + 2] Cycloaddition Reaction of α-Fluorostyrenes with Imines

An enantioselective [4 + 2] cycloaddition reaction of α-fluorostyrenes with N-benzoyl imines was demonstrated using a chiral phosphoric acid catalyst. Cycloaddition products having fluorine functionality were formed in high yields with excellent diastereo- and enantioselectivities. Further manipulation of the enantioenriched cycloaddition product with silyl enol ether in the presence of BiCl₃ catalyst afforded substitution product with retention of the dihydro-4H-1,3-oxazine framework through selective carbon-fluorine bond cleavage without loss of enantiomeric excess.

Dual targeting of PTP1B and glucosidases with new bifunctional iminosugar inhibitors to address type 2 diabetes

The diffusion of type 2 diabetes (T2D) throughout the world represents one of the most important health problems of this century. Patients suffering from this disease can currently be treated with numerous oral anti-hyperglycaemic drugs, but none is capable of reproducing the physiological action of insulin and, in several cases, they induce severe side effects. Developing new anti-diabetic drugs remains one of the most urgent challenges of the pharmaceutical industry. Multi-target drugs could offer new therapeutic opportunities for the treatment of T2D, and the reported data on type 2 diabetic mice models indicate that these drugs could be more effective and have fewer side effects than mono-target drugs. α-Glucosidases and Protein Tyrosine Phosphatase 1B (PTP1B) are considered important targets for the treatment of T2D: the first digest oligo- and disaccharides in the gut, while the latter regulates the insulin-signaling pathway. With the aim of generating new drugs able to target both enzymes, we synthesized a series of bifunctional compounds bearing both a nitro aromatic group and an iminosugar moiety. The results of tests carried out both in vitro and in a cell-based model, show that these bifunctional compounds maintain activity on both target enzymes and, more importantly, show a good insulin-mimetic activity, increasing phosphorylation levels of Akt in the absence of insulin stimulation. These compounds could be used to develop a new generation of anti-hyperglycemic drugs useful for the treatment of patients affected by T2D.

Deconstructive diversification of cyclic amines

Deconstructive functionalization involves C–C bond cleavage followed by bond construction on one or more of the constituent carbons. For example, ozonolysis¹ and olefin metathesis^{2, 3} have allowed each carbon in C–C double bonds to be viewed as a functional group. Despite the significant advances in deconstructive functionalizations involving scission of C–C double bonds, there are very few methods that achieve C(sp³)–C(sp³) single bond cleavage/functionalization, especially in relatively unstrained cyclic systems. Here, we report a deconstructive strategy to transform saturated nitrogen heterocycles such as piperidines and pyrrolidines, important moities in bioactive molecules, into halogen-containing acyclic amine derivatives through sequential C(sp³)–N/C(sp³)–C(sp³) single bond cleavage followed by C(sp³)–halogen bond formation. The resulting acyclic haloamines serve as versatile intermediates that are transformed into a variety of structural motifs through substitution reactions. In this way, skeletal remodeling of cyclic amines, which constitutes a scaffold hop, can be achieved. The value of this deconstructive strategy has been demonstrated through the late-stage diversification of proline-containing peptides.

Diastereospecific Gold(I)-Catalyzed Cyclization Cascade for the Controlled Preparation of N- and N,O-Heterocycles

The reaction of oxime-tethered 1,6-enynes with a cationic gold(I) catalyst demonstrates a great potential for the synthesis of a range of heterocycles in a diastereospecific fashion. The control of the configuration of the oxime and the alkene of the enyne moiety is the key to selectively obtain dihydro-1,2-oxazines, isoxazolines or dihydropyrrole-N-oxides as single diastereoisomers. As supported by DFT calculations, these cascade reactions proceed stepwise, by the intramolecular addition of the O or N atom of the oxime onto cyclopropyl gold(I) carbene intermediates. In this study, a rare [3,3]-sigmatropic rearrangement of nitrones is also observed.

Exploring the consequences of a representative "disallowed" conformation of Aib on a 3₁₀-helical fold

The structural effects of a representative "disallowed" conformation of Aib on the 3(10)-helical fold of an octapeptidomimetic are explored. The 1D ((1)H, (13)C) & 2D NMR, FT-IR and CD data reveal that the octapeptide 1, adopts a 3(10)-helical conformation in solution, as it does in its crystal structure. The C-terminal methyl carboxylate (CO2Me) of 1 was modified into an 1,3-oxazine (Oxa) functional group in the peptidomimetic 2. This modification results in the stabilization of the backbone of the C-terminal Aib (Aib*-Oxa) of 2, in a conformation (ϕ, ψ = 180, 0) that is natively disallowed to Aib. Consequent to the presence of this natively disallowed conformation, the 3(10)-helical fold is not disrupted in the body of the peptidomimetic 2. But the structural distortions that do occur in 2 are primarily in residues in the immediate vicinity of the natively disallowed conformation, rather than in the whole peptide body. Non-native electronic effects resulting from modifications in backbone functional groups can be at the origin of stabilizing residues in natively disallowed conformations.

Stereoselective bromocyclization of allylated aldoxime ethers

The intramolecular bromoamination of allylated aldoxime ethers leads first to isoxazolidinium salts which then undergo a skeletal rearrangement to form bromo-5,6-dihydro-4H-1,3-oxazines. Aliphatic aldoxime ethers with α-protons undergo multiple brominations before rearrangement.

Accessing the disallowed conformations of peptides employing amide-to-imidate modification

Selective modification of the C-terminal amide in peptides to dihydrooxazine (a novel stable imidate isostere) by intramolecular nucleophilic cyclo-O-alkylation of the corresponding N-(3-bromopropyl)amides results in constraining of the C-terminal residue in natively disallowed conformations both in crystals and in solution.