We have developed an electrochemical method for the synthesis of 1,3,4-oxadiazole-2(3 H )-ones. The electrochemical oxidative carbonylation of hydrazides features mild and green conditions, external oxidant-free, broad substrate scope, short reaction time and ease of scale-up to gram scale. 

Electrochemical oxidative carbonylation of hydrazides for the synthesis of 1,3,4-oxadiazole-2(3H)-ones

4,4'-Methylene diphenyl diisocyanate (MDI) is an industrially crucial compound, being one of the most utilized linkers in the polyurethane industry. However, its long-term stability is limited due to dimerization to form insoluble uretdione. Herein we demonstrate an organometallic "catch-store-release" concept aiming at improving long-term chemical stability of MDI. Treatment of MDI with two equivalents of selected N-heterocyclic carbenes (NHC) forms stable MDI-NHC adducts. Treatment of the adducts with CuCl forms metastable di-Cu(I) complexes, which undergo decomposition to reform MDI (up to 85%), along with Cu-NHC complexes. The yield of reformed MDI can be improved (up to 95%) by the release of the NHC ligands in the form of thiourea, which prevents subsequent MDI dimerization/polymerization by the carbenes. Furthermore, the need to separate MDI from the reaction mixture can be eliminated by the direct reaction of MDI-NHC complexes with alcohols (as model for diols), which form dicarbamate (as model for polyurethane) quantitatively.

"Catch-Store-Release" Strategy for the Stabilization of 4,4'-Methylene Diphenyl Diisocyanate (4,4'-MDI).

Blocked polyisocyanates are used in single component (1 K) water-based and powder coatings to simplify coating synthesis and reduce waste. However, the relationship between the melting point and deblocking behavior of blocked polyisocyanates is not clear. We aim to determine the influence of the melting temperature on the deblocking of methyl ethyl ketoxime (MEKO)-blocked aromatic polyisocyanates. A toluene-2,4-diisocyanate trimer grafted with different mass ratios of methoxypolyethylene glycol (MPEG) is used to achieve compounds with different melting points and is blocked with MEKO. The deblocking behaviors of the polymers are investigated using Fourier transform infrared spectroscopy, differential scanning calorimetry, scanning electron microscopy, and thermogravimetric analysis. The results show that the deblocking temperature and average activation energy of the blocked polyisocyanates decrease with increase in melting point. Further, MEKO is more likely to be lost from blocked polyisocyanates having lower melting points. Furthermore, polyacrylic resins cured with blocked polyisocyanates having low melting points exhibit better properties than those having high melting points. The results reveal that the reduction in melting point induced by grafting with MPEG can significantly reduce the deblocking temperature of blocked polyisocyanates. The improvements in these properties may help achieve the low-temperature crosslinking of blocked polyisocyanates for coating technologies. 

Influence of the melting point on the deblocking reaction of methyl ethyl ketoxime‐blocked aromatic polyisocyanates

For over 50 years of azapeptide synthetic techniques developments have renewed the field of peptidomimetic therapeutics. Azapeptides are close surrogates of natural peptides: they contain a substitution of the amino acid α-carbon by a nitrogen atom. Goserelin (1989) and Atazanavir (2003) are two well-known, FDA-approved azapeptide-based drugs for treatment of cancers and HIV infection, providing evidence for the successful clinical implementation of this class of therapeutic. This review highlights the azapeptides in recent medicinal chemistry applications and synthetic milestones. We describe the current techniques for azapeptide bond formation by introducing azapeptide coupling reagents and chain elongation methods both in solution and solid phase strategies.

Azapeptides -A History of Synthetic Milestones and Key Examples.

We reported the base-promoted cyclization of N-Boc arylimidrazone toward 1,2-dihydro 3H-1,2,4-triazol-3-one. This procedure tolerated methyl, methoxy, chloro, bromo, fluoro, cyano, methylcarbonyl groups well. Thus, it represents a facile and high efficient pathway leading to 1,2-dihydro 3H-1,2,4-triazol-3-one. Mechanism study revealed this procedure proceeded with the intramolecular nucleophilic cyclization rather than N-isocyanate intermediate. 

The base-promoted cyclization of N-Boc arylimidrazone toward 1,2-dihydro 3H-1,2,4-triazol-3-ones

In contrast to carbon-substituted isocyanates that are common building blocks, N-substituted isocyanates remain underdeveloped and reports on their N-acyl derivatives (i. e. amido-isocyanates) are exceedingly rare. Herein, amido-isocyanates were investigated in the context of syntheses of aza-tripeptide and hydantoins subunits starting from simple bench-stable precursors. A key finding is that the amido-isocyanate formed in situ cyclized to yield an oxadiazolone, and that under suitable reaction conditions this heterocycle is a traceless blocked (masked) N-isocyanate. Using organic bases as catalysts and upon heating, oxadiazolone formation is observed, and various nucleophiles to provide the desired aza-dipeptides or hydantoins in moderate to high yields. Further support for an amido-isocyanate intermediate was obtained using carboxylic acids as nucleophiles, affording N-acylhydrazide products.

Investigation of Masked N-Acyl-N-isocyanates: Support for Oxadiazolones as Blocked N-Isocyanate Precursors.

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Investigation of Masked N-Acyl-N-isocyanates: Support for Oxadiazolones as Blocked N-Isocyanate Precursors.