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Multidecagram Scale Synthesis of an Endoperoxide, Precursor of Anti-malarial and Anti-leishmanial Agents, via Free-Radical [2 + 2 + 2] Annulation with Molecular Oxygen

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journal contribution
posted on 25.11.2021, 18:03 by Davide Lardani, Roger Marti, Arianna Quintavalla, Marco Lombardo, Claudio Trombini
From screening experimental parameters to scaling up, a safe approach is reported for the multidecagram scale preparation of methyl ester of 6,6-dibutyl-3-hydroxy-3-methyl-1,2-dioxane-4-carboxylic acid (7), a flexible common intermediate for the synthesis of a variety of anti-malarial and/or anti-leishmanial compounds. The critical feature of the reaction is the use of a pure oxygen flow under batch conditions using methyl acetoacetate and 2-butyl-1-hexene as reagents, and a mixture of Mn­(II) and Mn­(III) acetates as catalysts, in acetic acid. Once the best solvent, catalyst loading, temperature, and reaction time in the small-scale procedure (10 mL vial) were defined, the process was adapted up to a 3 L flask, eventually resulting in the production of about 70 g of 7. The developed reaction protocol gives a satisfactory result considering: (i) the inexpensive starting materials used, (ii) the 100% atom economy of the [2 + 2 + 2]-annulation reaction, (iii) the stoichiometric ratio of the two reagents (only oxygen is used in excess) coupled with low catalyst loading, and (iv) the mild conditions adopted. The calorimetric studies allow us to classify this reaction as an inherently safe process. On a larger scale, the possible formation of a flammable vapor mixture in the reactor headspace was minimized using two parallel continuous flows of oxygen and argon, thus ensuring a constant headspace vapor phase renewal.