An optimized route to an iodo-imidazole intermediate in the synthesis of 4-ethynyl-2,5-dimethyl-1-aryl-1H-imidazoles (6) was devised. Important data for the optimization work was obtained by carrying out a DOE study to gain understanding of the parameters that affect the key intramolecular cyclization to build the imidazole ring. Additional information on the reaction mechanism of this step was obtained by carrying out a flow NMR experiment. In order to complete the proof of concept, the iodo-imidazole intermediate was converted to two ethynyl imidazoles (6a, b) using metal-catalyzed reactions.
An exothermic decomposition was observed during a metalation/acylation of 3,4-difluoroanisole (5), resulting in a significant thermal hazard. The lithiated anion 6 was found to decompose exothermically at temperatures above −47 °C showing an adiabatic temperature rise at a peak rate of 120 °C/min. A literature search revealed similar observations for metalation/acylation in analogous aromatic difluoro compounds. This sequence of reactions was evaluated thermochemically. Control experiments at −55 °C over 2 h indicated anion 6 was stable at temperatures below −55 °C under dilute reaction concentrations. This runaway hazard could be addressed using MgCl2 to stabilize the reactive species and thereby decrease its decomposition rate. Thermochemical experiments suggested MgCl2 forms a weak interaction with lithiated species 6, rather than via complete lithium−magnesium exchange. The process was successfully piloted on a multikilo scale by use of MgCl2 as an additive.
Many oxidation reactions can be hazardous when run on large scale. The manufacturing process for the production of R411, a developmental compound indicated for the treatment of asthma, includes the oxidation of 2-chloro-6-methylbenzaldehyde to the corresponding carboxylic acid. The use of sodium chlorite in this transformation was efficient and economical, but there were safety concerns regarding the use of hydrogen peroxide to scavenge unwanted hypochlorite, which was generated as a byproduct of the reaction. During the development of the R411 manufacturing process, an inherently safer oxidation system was discovered using a stoichiometric quantity of dimethyl sulfoxide (DMSO) as scavenger. The new process provided equivalent yields and purities to the hydrogen peroxide procedure, thus maintaining the economic viability of the process. The developed process was demonstrated in fixed equipment on a 300 gal scale.
A novel nitration process was developed for the production of 4-(4methoxy-3-nitrophenyl)morpholine. Crude 4-(4-methoxyphenyl-)morpholine produced in step 1 was converted to its nitric acid salt. The nitration reaction was carried out by adding a dichloromethane solution of the isolated salt to concentrated sulfuric acid. This protocol provided an easy and reliable way to obtain a 1:1 molar ratio of the substrate and nitric acid in the reaction mixture and was proven to be the most effective method to prevent under-/ over-nitration. The incorporation of the protocol into the process resulted in substantial improvement of the robustness and safety profile of the whole process. In addition, 59% overall yield improvement, 30% capacity increase, 40% waste reduction, and simplified operations were achieved. A detailed thermal hazard analysis of the process was also performed.
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