A novel approach is reported for the enantioselective flow synthesis of rolipram comprising a telescoped asymmetric conjugate addition–oxidative aldehyde esterification sequence followed by trichlorosilane-mediated nitro group reduction and concomitant lactamization. The telescoped process takes advantage of a polystyrene-supported chiral organocatalyst along with in situ -generated persulfuric acid as a robust and scalable oxidant for direct aldehyde esterification. This approach demonstrates significantly improved productivity compared with earlier methodologies while ensuring environmentally benign metal-free conditions.

The efficacy of synthetic methodologies is based not only on potent chemistries but also on processing strategies that amplify reaction performance and overall safety. The N-hydroxyphtalimide-catalyzed aerobic oxidation of aldehydes is an environmentally reliable and inexpensive approach for the synthesis of valuable carboxylic acids. However, the reaction may lead to the accumulation of significant amounts of peracid in crude product mixtures thereby generating a considerable safety hazard. Also, in the case of deactivated substrates, long reaction times, high catalyst loading, and stoichiometric additives are required to promote oxidation. Therefore, we developed a flow chemistry-based process for N-hydroxyphtalimidecatalyzed aldehyde oxidations and exploited its benefits to minimize peracid buildup, thereby enhancing the process safety. By performing the aerobic oxidation of aldehydes under continuous flow conditions, chemically intensified and scalable carboxylic acid formation was obtained even in the reactions of deactivated substrates. Our findings are presented herein.

Persulfuric acid is a well‐known oxidant in various industrial‐scale purification procedures. However, due to its tendency toward explosive decomposition, its usefulness in organic synthesis remained largely underexplored. Herein, a continuous in situ persulfuric acid generator was developed and applied for oxidative esterification of aldehydes under flow conditions. Sulfuric acid served as a readily available and benign precursor to form persulfuric acid in situ. By taking advantage of the continuous‐flow generator concept, safety hazards were significantly reduced, whilst a robust and effective approach was ensured for direct transformations of aldehydes to valuable esters. The process proved useful for the transformation of diverse aliphatic as well as aromatic aldehydes, while its preparative capability was verified by the multigram‐scale synthesis of a pharmaceutically relevant key intermediate. The present flow protocol demonstrates the safe, sustainable, and scalable application of persulfuric acid in a manner that would not be amenable to conventional batch processing.