Desacetyl Apremilast (Impurity B)

Desacetyl Apremilast (Impurity B) is a structurally related degradation byproduct of the phosphodiesterase-4 inhibitor Apremilast, formed via acetyl group removal from the indole nitrogen. This impurity retains the 2,6-dichlorophenyl amide moiety and cyclopentyl substituent but lacks the N-acetyl functionality, exposing a free amine. Its core structure comprises a fused indole-pyridine ring system with a carboxamide linkage and a chlorinated aromatic substituent. The molecule exhibits ester-like hydrolysis susceptibility due to the adjacent amide and cyclopentyl ring. This compound serves as a critical HPLC reference standard for quantifying process-related impurities in Apremilast formulations.