Fatty aldehyde bisulfite adducts as a purification handle in ionizable lipid synthesis.

Rapid access to ALC-0315, a crucial component of the formulated Pfizer Covid vaccine, was obtained by employing solid adduct formation and filtration after an oxidation step in place of the standard chromatographic separation, allowing for a more scalable synthesis. Impurities were removed by formation of this fatty aldehyde bisulfite adduct at the penultimate step and by performing the final reductive amination directly with the fatty aldehyde bisulfite adduct. This eliminates chromatographic separations for all prepared aldehyde containing intermediates. Along with ALC-0315, FTT5 and SM-102 ionizable lipids were prepared utilizing this strategy. This work paves the way for more sustainable access to these critical ionizable lipids that would de-risk the world supply of important vaccines and medicines in the future.

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

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.

A Bifunctional Nucleoside Probe for the Inhibition of the Human Immunodeficiency Virus-Type 1 Reverse Transcriptase.

Nucleoside analogs have proven effective for the inhibition of viral polymerases and are the foundation of many antiviral therapies. In this work, the antiretroviral potential of 6-azauracil analogs was assessed using activity-based protein profiling techniques and functional assays. Probes based on the 6-azauracil scaffold were examined and found to bind to HCV polymerase and HIV-1 reverse transcriptase through covalent modification of residues near the active site. The modified sites on the HIV-1 RT were examined using a mass spectrometry approach, and it was discovered that the azauracil moieties modified the enzyme in proximity to its active site. However, these scaffolds gave little or no inhibition of enzyme activity. Instead, a bifunctional inhibitor was prepared using click chemistry to link the 6-azauracil moiety to azidothymidine (AzT) and the corresponding triphosphate (AzTTP). These bifunctional inhibitors were found to have potent inhibitory function through a mode of action that includes both alkylation and chain termination. An in vitro assay demonstrated that the bifunctional inhibitor was 23-fold more effective in inhibiting HIV-1 RT activity than the parent AzTTP. The bifunctional inhibitor was also tested in HIV-1 permissive T cells where it decreased Gag expression similarly to the front-line drug Efavirenz with no evidence of cytotoxicity. This new bifunctional scaffold represents an interesting tool for inhibiting HIV-1 by covalently anchoring a chain-terminating nucleoside analog in the active site of the reverse transcriptase, preventing its removal and abolishing enzymatic activity, and represents a novel mode of action for inhibiting polymerases including reverse transcriptases.

Synthesis of Cyclic Azomethine Imines by Cycloaddition Reactions of N-Isocyanates and N-Isothiocyanates.

Various nitrogen-substituted iso(thio)cyanates engage in [3 + 2]-cycloaddition reactions to form azomethine imines containing triazolone, triazole-thione, and pyrazole-thione cores. First, iminoisothiocyanates are shown to undergo aminothiocarbonylation reactions with strained alkenes, and a comparison with recently reported reactions of iminoisocyanates highlights their reduced reactivity. In contrast, amino(thio)carbonylation reactions of imines with iminoisocyanates and iminoisothiocyanates proved more efficient, providing access to triazolone and triazole-thione cores. The dipole products can be converted to valuable heterocyclic cores through simple derivatization reactions.

Intramolecular Alkene Aminocarbonylation Using Concerted Cycloadditions of Amino-Isocyanates.

The ubiquity of nitrogen heterocycles in biologically active molecules challenges synthetic chemists to develop a variety of tools for their construction. While developing metal-free hydroamination reactions of hydrazine derivatives, it was discovered that carbazates and semicarbazides can also lead to alkene aminocarbonylation products if nitrogen-substituted isocyanates (N-isocyanates) are formed in situ as reactive intermediates. At first this reaction required high temperatures (150-200 °C), and issues included competing hydroamination and N-isocyanate dimerization pathways. Herein, improved conditions for concerted intramolecular alkene aminocarbonylation with N-isocyanates are reported. The use of ßN-benzyl carbazate precursors allows the effective minimization of N-isocyanate dimerization. Diminished dimerization leads to higher yields of alkene aminocarbonylation products, to reactivity at lower temperatures, and to an improved scope for a reaction sequence involving alkene aminocarbonylation followed by 1,2-migration of the benzyl group. Furthermore, fine-tuning of the blocking (masking) group on the N-isocyanate precursor, and reaction conditions relying on base catalysis for N-isocyanate formation from simpler precursors resulted in room temperature reactivity, consequently minimizing the competing hydroamination pathway. Collectively, this work highlights that controlled reactivity of aminoisocyanates is possible, and provides a broadly applicable alkene aminocarbonylation approach to heterocycles possessing the ß-aminocarbonyl motif.

Divergent Reactivity of N-Isocyanates with Primary and Secondary Amines: Access to Pyridazinones and Triazinones.

Cascade reactions for the synthesis of 1,2,4-triazinones and 5-aminopyridazinones are reported using α-ketocarbazones as N-isocyanate precursors and exploiting the divergent reactivity observed with primary and secondary amines. Triazinones were formed with primary amines, likely through addition of the amine on the N-isocyanate, followed by cyclization (condensation) on the ketone. In contrast, such cyclization is impossible for secondary amines; this allows in situ formation of enamines, which, upon cyclization, generate 5-amino pyridazinones. This sequence further illustrates the versatility of N-isocyanates in heterocyclic synthesis and provides a rare example of carbon nucleophiles reacting with N-isocyanates.

Cascade reactions of nitrogen-substituted isocyanates: a new tool in heterocyclic chemistry.

In contrast to normal C-substituted isocyanates, nitrogen-substituted isocyanates (N-isocyanates) are rare. Their high reactivity and amphoteric/ambident nature has prevented the scientific community from exploiting their synthetic potential. Recently, we have developed an in situ formation approach using a reversible equilibrium, which allows controlled generation and reactivity of N-isocyanates and prevents the dimerization that is typically observed with these intermediates. This blocked (masked) N-isocyanate approach enables the use of various N-isocyanate precursors to assemble heterocycles possessing the N-N-C[double bond, length as m-dash]O motif, which is often found in agrochemicals and pharmaceuticals. Cascade reactions for the rapid assembly of several valuable 5- and 6-membered heterocycles are reported, including amino-hydantoins, acyl-pyrazoles, acyl-phthalazinones and azauracils. Over 100 different compounds were synthesized using amino-, imino- and amido-substituted N-isocyanates, demonstrating their potential as powerful intermediates in heterocyclic synthesis. Their reactivity also enables access to unprecedented bicyclic derivatives and to substitution patterns of azauracils that are difficult to access using known methods, illustrating that controlled reactivity of N-isocyanates provides new disconnections, and a new tool to assemble complex N-N-C[double bond, length as m-dash]O containing motifs.

One-Pot Synthesis of Aza-Diketopiperazines Enabled by Controlled Reactivity of N-Isocyanate Precursors.

A one-pot sequence for the synthesis of aza-diketopiperazines is reported, involving carbazate acylation with chloroacetyl chloride, SN2 with a primary amine, N-isocyanate formation, and cyclization. Nitrogen-substituted isocyanates (N-isocyanates) are a rare class of amphoteric isocyanate with high, but severely underdeveloped synthetic potential. This approach highlights that ßN-acyl carbazates can act as blocked (masked) N-isocyanates, thus allowing a challenging intermolecular SN2 reaction of a primary amine to proceed while the N-isocyanate is "protected", and then cyclization once it is unmasked. Control experiments show that the alternate pathway--N-isocyanate substitution and then cyclization by an intramolecular SN2 reaction--is not operating.

Diversity-oriented heterocyclic synthesis using divergent reactivity of N-substituted iso(thio)cyanates.

Carbon-substituted isocyanates and isothiocyanates are common building blocks in organic synthesis. In contrast, synthetic uses of N-substituted isocyanates and isothiocyanates are severely underdeveloped: few have been reported and their reactivity had not been compared. Herein, we compare the reactivity of blocked (masked) N-isocyanate and N-isothiocyanate precursors in cascade reactions. Divergent reactivity is observed with secondary propargylic and allylic systems, leading to new syntheses of imidazolones, thiazolidines, and a tool to form complex azomethine imines.

A cascade synthesis of aminohydantoins using in situ-generated N-substituted isocyanates.

Nitrogen-substituted isocyanates are rarely utilized but powerful building blocks for the development of cascade reactions in heterocyclic synthesis. These reactive amphoteric intermediates can be accessed in situ via an equilibrium that allows controlled reactivity in the presence of bifunctional partners such as α-amino esters. A cascade reaction has been carried out that forms 3-aminohydantoin derivatives using simple phenoxycarbonyl derivatives of hydrazides and hydrazones as precursors of N-substituted-isocyanates. This method allows rapid assembly of complex aminohydantoin derivatives, including analogues of medicinally-relevant compounds, using simple reactants.

Studies on difficult intramolecular hydroaminations in the context of four syntheses of alkaloid natural products.

Examples of intramolecular alkene hydroaminations forming six-membered ring systems are rare, especially for systems in which the double bond is disubstituted. Such cyclizations have important synthetic relevance. Herein we report a systematic study of these cyclizations using recently developed Cope-type hydroamination methodologies. Difficult intramolecular alkene hydroaminations were used as key steps in syntheses of 2-epi-pumiliotoxin C, coniine, N-norreticuline and desbromoarborescidine A. This effort required the development of optimized hydroamination conditions to improve the efficiency of the cyclizations. Collectively, our results show that Cope-type cyclizations can be achieved on a variety of challenging substrates and proceed under similar conditions for both N-H and N-substituted hydroxylamines.