By optimizing a reagent's residence time within a packed-bed reactor, it is possible to overcome selectivity issues frequently encountered in stirred reaction vessels. This important feature is demonstrated for the chemoselective protection of 4- acetylbenzaldehyde whereby 1-[4-1, 3-dithian-2-yl-phenyl] ethanone is obtained in excellent yield and purity. In addition, the generality of the technique is highlighted via the protection ...