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Mit Stickstoffwasserstoffsäure in den Mikroreaktor! Ein Hochtemperatur‐Hochdruckprozess‐Prozessintensivierungsansatz lieferte Tetrazole auf sehr effiziente Art. Trotz des toxischen und explosiven Charakters von Stickstoffwasserstoffsäure gelang die sichere Synthese im kontinuierlichen Fluss mit Verweildauern von weniger als 2.5 Minuten bei 260 °C (siehe Bild).

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Hydrazoic acid (HN 3 ) was used in a safe and reliable way for the synthesis of 5-substitued-1H-tetrazoles and for the preparation of N-(2-azidoethyl)acylamides in a continuous flow format. Hydrazoic acid was generated in situ either from an aqueous feed of sodium azide upon mixing with acetic acid, or from neat trimethylsilyl azide upon mixing with methanol. For both processes, subsequent reaction of the in situ generated hydrazoic acid with either organic nitriles (tetrazole formation) or 2-oxazolines (ring opening to b-azido-carboxamides) was performed in a coil reactor in an elevated temperature/pressure regime. Despite the explosive properties of HN 3 , the reactions could be performed safely at very high temperatures to yield the desired products in short reaction times and in excellent product yields. The scalability of both protocols was demonstrated for selected examples. Employing a commercially available benchtop flow reactor, productivities of 18.9 g/h of 5-phenyltetrazole and 23.0 g/h of N-(1-azido-2-methylpropan-2-yl)acetamide were achieved.

We isolated, characterised, and cloned an enantio-specific amidase from Klebsiella oxytoca and used it to resolve (R,S)-3,3,3-trifluoro-2-hydroxy-2-methylpropionamide, giving (R)-3,3,3-trifluoro-2-hydroxy-2-methylpropionic acid and (S)-3,3,3trifluoro-2-hydroxy-2-methylpropionamide. The (S)-amide could then be hydrolysed chemically to (S)-3,3,3-trifluoro-2-hydroxy-2-methylpropionic acid. The process can therefore be adapted to produce both (R)-and (S)-enantiomers of 3,3,3-trifluoro-2hydroxy-2-methylpropionic acid, or (S)-3,3,3-trifluoro-2-hydroxy-2-methylpropionamide. The biocatalytic step is part of a combined chemical and biocatalytic route that starts from ethyl trifluoroacetoacetate. The products typically have a purity of greater than 98% and ee values of essentially 100% after isolation. The process has been used to produce 100-g amounts of the (S)-acid, and successfully scaled up to produce 100-kg amounts of the (R)-acid, with the biotransformation carried out at the 1500-L scale.