2-Methylbenzo[d]thiazole-6-carbonitrile
was internally
identified as an important building block and therefore, kg amounts
of it needed to be urgently supplied. As the desired benzothiazole
was only available in small quantities for a high price (∼1000
USD/g), a robust and scalable route needed to be rapidly developed.
The key to success was the use of a 2-iodophenyl N-acetamide precursor to construct the 2-methylbenzo[d]thiazole core via a Pd-catalyzed Migita coupling followed by subsequent
intramolecular condensation of the intermediate thiophenol onto the N-acetyl group. To avoid the use of malodorous thiols on
scale, 2-ethylhexyl 3-mercaptopropionate was used as a nonsmelly,
inexpensive thiol surrogate. The Migita coupling was extensively optimized
and allowed the reaction to be performed in a dose-controlled manner
with regard to the addition of the thiol surrogate at 40 °C,
by using only 0.1 mol % Pd2dba3. The subsequent
intramolecular cyclization step was promoted by the one-pot addition
of DBU. After aqueous workup and crystallization, 2-methylbenzo[d]thiazole-6-carbonitrile was obtained in 81% yield and
excellent purity (99.7% a/a) on a 2.0 kg scale.
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