(L)-Monomethyl Tyrosine (Mmt): New Synthetic Strategy via Bulky 'Forced-Traceless' Regioselective Pd-Catalyzed C(sp2)-H Activation.

The enormous influence in terms of bioactivity, affinity, and selectivity represented by the replacement of (L)-2,6-dimethyl tyrosine (Dmt) instead of Phenylalanine (Phe) into Nociceptin/orphanin (N/OFQ) neuropeptide analogues has been well documented in the literature. More recently, the non-natural amino acid (L)-2-methyl tyrosine (Mmt), with steric hindrance included between Tyr and Dmt, has been studied because of the modulation of steric effects in opioid peptide chains. Here, we report a new synthetic strategy to obtain Mmt based on the well-known Pd-catalyzed ortho-C(sp2)-H activation approach, because there is a paucity of other synthetic routes in the literature to achieve it. The aim of this work was to force only the mono-ortho-methylation process over the double ortho-methylation one. In this regard, we are pleased to report that the introduction of the dibenzylamine moiety on a Tyr aromatic nucleus is a convenient and traceless solution to achieve such a goal. Interestingly, our method provided the aimed Mmt either as N-Boc or N-Fmoc derivatives ready to be inserted into peptide chains through solid-phase peptide synthesis (SPPS). Importantly, the introduction of Mmt in place of Phe1 in the sequence of N/OFQ(1-13)-NH2 was very well tolerated in terms of pharmacological profile and bioactivity.

Synthesis and NLRP3-Inflammasome Inhibitory Activity of the Naturally Occurring Velutone F and of Its Non-Natural Regioisomeric Chalconoids.

Plant-derived remedies rich in chalcone-based compounds have been known for centuries in the treatment of specific diseases, and nowadays, the fascinating chalcone framework is considered a useful and, above all, abundant natural chemotype. Velutone F, a new chalconoid from Millettia velutina, exhibits a potent effect as an NLRP3-inflammasome inhibitor; the search for new natural/non-natural lead compounds as NLRP3 inhibitors is a current topical subject in medicinal chemistry. The details of our work toward the synthesis of velutone F and the unknown non-natural regioisomers are herein reported. We used different synthetic strategies both for the construction of the distinctive benzofuran nucleus (BF) and for the key phenylpropenone system (PhP). Importantly, we have disclosed a facile entry to the velutone F via synthetic routes that can also be useful for preparing non-natural analogs, a prerequisite for extensive SAR studies on the new flavonoid class of NLRP3-inhibitors.

The Fascinating Chemistry of α-Haloamides.

The aim of this review is to highlight the rich chemistry of α-haloamides originally mainly used to discover new C-N, C-O and C-S bond forming reactions, and later widely employed in C-C cross-coupling reactions with C(sp3), C(sp2) and C(sp) coupling partners. Radical-mediated transformations of α-haloamides bearing a suitable located unsaturated bond has proven to be a straightforward alternative to access diverse cyclic compounds by means of either radical initiators, transition metal redox catalysis or visible light photoredox catalysis. On the other hand, cycloadditions with α-halohydroxamate-based azaoxyallyl cations have garnered significant attention. Moreover, in view of the important role in life and materials science of difluoroalkylated compounds, a wide range of catalysts has been developed for the efficient incorporation of difluoroacetamido moieties into activated as well as unactivated substrates.

Bile Acid Binding Protein Functionalization Leads to a Fully Synthetic Rhodopsin Mimic.

Rhodopsins are photoreceptive proteins using light to drive a plethora of biological functions such as vision, proton and ion pumping, cation and anion channeling, and gene and enzyme regulation. Here we combine organic synthesis, NMR structural studies, and photochemical characterization to show that it is possible to prepare a fully synthetic mimic of rhodopsin photoreceptors. More specifically, we conjugate a bile acid binding protein with a synthetic mimic of the rhodopsin protonated Schiff base chromophore to achieve a covalent complex featuring an unnatural protein host, photoswitch, and photoswitch-protein linkage with a reverse orientation. We show that, in spite of its molecular-level diversity, light irradiation of the prepared mimic fuels a photochromic cycle driven by sequential photochemical and thermal Z/E isomerizations reminiscent of the photocycles of microbial rhodopsins.

Design and Synthesis of 99mTcN-Labeled Dextran-Mannose Derivatives for Sentinel Lymph Node Detection.

BACKGROUND: New approaches based on the receptor-targeted molecular interaction have been recently developed with the aim to investigate specific probes for sentinel lymph nodes. In particular, the mannose receptors expressed by lymph node macrophages became an attractive target and different multifunctional mannose derivate ligands for the labeling with 99mTc have been developed. In this study, we report the synthesis of a specific class of dextran-based, macromolecular, multifunctional ligands specially designed for labeling with the highly stable [99mTc≡N]2+ core. METHODS: The ligands have been obtained by appending to a macromolecular dextran scaffold pendant arms bearing a chelating moiety for the metallic group and a mannosyl residue for allowing the interaction of the resulting macromolecular 99mTc conjugate with specific receptors on the external membrane of macrophages. Two different chelating systems have been selected, S-methyl dithiocarbazate [H2N‒NH‒C(=S)SCH3=HDTCZ] and a sequence of two cysteine residues, that in combination with a monophosphine coligand, are able to bind the [99mTc≡N]2+ core. CONCLUSIONS: High-specific-activity labeling has been obtained by simple mixing and heating of the [99mTc≡N]2+ group with the new mannose-dextran derivatives.

Sometimes, the Least Demanding Solution Is the Most Suitable: A Careful Survey of NMR Spectra of a Phosphonium Salt Accounted for Its "Unusual Wittig Reaction".

Careful product analysis in combination with appropriate supporting experiments unambiguously proves that in contrast to what was previously reported, generation of the phosphonium salt from a ß-cyclocitral-derived allylic alcohol and PPh3HBr takes place via a proton-driven elimination/addition path.

Recent Developments in General Methodologies for the Synthesis of α-Ketoamides.

The α-ketoamide motif is widely found in many natural products and drug candidates with relevant biological activities. Furthermore, α-ketoamides are attractive candidates to synthetic chemists due to the ability of the motif to access a wide range of functional group transformations, including multiple bond-forming processes. For these reasons, a vast array of synthetic procedures for the preparation of α-ketoamides have been developed over the past decades, and the search for expeditious and efficient protocols continues unabated. The aim of this review is to give an overview of the diverse methodologies that have emerged since the 1990s up to the present. The different synthetic routes have been grouped according to the way the α-ketoamide moiety has been created. Thus, syntheses of α-ketoamides proceeding via C(2)-oxidation of amide starting compounds are detailed, as are amidation approaches installing the α-ketoamide residue through C(1)-N bond formation. Also discussed are the methodologies centered on C(1)-C(2) σ-bond construction and C(2)-R/Ar bond-forming processes. Finally, the literature regarding the synthesis of α-ketoamide compounds by palladium-catalyzed double-carbonylative amination reactions is discussed.

Toward a stable α-cycloalkyl amino acid with a photoswitchable cationic side chain.

The N-alkylated indanylidenepyrroline (NAIP) Schiff base 3 is an unnatural α-amino acid precursor potentially useful for the preparation of semisynthetic peptides and proteins incorporating charged side chains whose structure can be modulated via Z/E photoisomerization. Here we report that the heteroallylic protons of 3 led to partial loss of ethanol accompanied by formation of the novel heterocyclic system 4 during attempted deprotection. We also show that the same protons catalyze the thermal isomerization of 3, making the light-driven conformational control concept ineffective for times longer than a few hours. These problems are not present in the previously unreported compound 5 where the acidic methyl group is replaced by an H atom. Therefore, 5, rather than 3, constitutes a promising prototype for the design of building blocks capable to modulate the electrostatic potential of a protein in specific locations via light irradiation.

Linear dichroism amplification: adapting a long-known technique for ultrasensitive femtosecond IR spectroscopy.

We demonstrate strong amplification of polarization-sensitive transient IR signals using a pseudo-null crossed polarizer technique first proposed by Keston and Lospalluto [Fed. Proc. 10, 207 (1951)] and applied for nanosecond flash photolysis in the visible by Che et al. [Chem. Phys. Lett. 224, 145 (1994)]. We adapted the technique to ultrafast pulsed laser spectroscopy in the infrared using photoelastic modulators, which allow us to measure amplified linear dichroism at kilohertz repetition rates. The method was applied to a photoswitch of the N-alkylated Schiff base family in order to demonstrate its potential of strongly enhancing sensitivity and signal to noise in ultrafast transient IR experiments, to simplify spectra and to determine intramolecular transition dipole orientations.

Modeling, preparation, and characterization of a dipole moment switch driven by Z/E photoisomerization.

We report the results of a multidisciplinary research effort where the methods of computational photochemistry and retrosynthetic analysis/synthesis have contributed to the preparation of a novel N-alkylated indanylidene-pyrroline Schiff base featuring an exocyclic double bond and a permanent zwitterionic head. We show that, due to its large dipole moment and efficient photoisomerization, such a system may constitute the prototype of a novel generation of electrostatic switches achieving a reversible light-induced dipole moment change on the order of 30 D. The modeling of a peptide fragment incorporating the zwitterionic head into a conformationally rigid side chain shows that the switch can effectively modulate the fluorescence of a tryptophan probe.

Fluorenylidene-pyrroline biomimetic light-driven molecular switches.

A new family of biomimetic photoactivated molecular switches based in the retinal chromophore is described. Expedient synthesis allows a library of compounds with a different substitution pattern, including chiral substituents, to be obtained. The effect of substitution, solvent, and light source on the photoisomerization step has been assessed. The absorption maximum has been red-shifted ca. 50 nm with respect to related systems and rotation is now easily achieved by using visible light.

An artificial molecular switch that mimics the visual pigment and completes its photocycle in picoseconds.

Single molecules that act as light-energy transducers (e.g., converting the energy of a photon into atomic-level mechanical motion) are examples of minimal molecular devices. Here, we focus on a molecular switch designed by merging a conformationally locked diarylidene skeleton with a retinal-like Schiff base and capable of mimicking, in solution, different aspects of the transduction of the visual pigment Rhodopsin. Complementary ab initio multiconfigurational quantum chemistry-based computations and time-resolved spectroscopy are used to follow the light-induced isomerization of the switch in methanol. The results show that, similar to rhodopsin, the isomerization occurs on a 0.3-ps time scale and is followed by <10-ps cooling and solvation. The entire (2-photon-powered) switch cycle was traced by following the evolution of its infrared spectrum. These measurements indicate that a full cycle can be completed within 20 ps.

Design and photochemical characterization of a biomimetic light-driven Z/E switcher.

Protonated Schiff bases (PSBs) of polyenals constitute a class of light-driven switchers selected by biological evolution that provide model compounds for the development of artificial light-driven molecular devices or motors. In the present paper, our primary target is to show, through combined computational and experimental studies, that it is possible to approach the design of artificial PSBs suitable for such applications. Below, we use the methods of computational photochemistry to design and characterize the prototype biomimetic molecular switchers 4-cyclopenten-2'-enylidene-3,4-dihydro-2H-pyrrolinium and its 5,5'-dimethyl derivative both containing the penta-2,4-dieniminium chromophore. To find support for the predicted behavior, we also report the photochemical reaction path of the synthetically accessible compound 4-benzylidene-3,4-dihydro-2H-pyrrolinium. We show that the preparation and photochemical characterization of this compound (together with three different N-methyl derivatives) provide both support for the predicted photoisomerization mechanism and information on its sensitivity to the molecular environment.