Recycling Preparative Liquid Chromatography, the Overlooked Methodology for the Purification of Natural Products

Frequently, preparative high-performance liquid chromatography separations of complex natural product mixtures by adsorption chromatography are erratic to achieve full baseline separation. Purification of metabolites with similar or identical polarity, such as epimers, diastereoisomers, homologs in a series, and geometric or positional isomers, by a single chromatographic run, is not properly achieved. Consequently, recycling preparative high-performance liquid chromatography has been proposed with a closed-loop recycling valve designed to increase the capacity of separation of mixtures of low-resolution peaks by a series of consecutive passes through the same column. Thus, the sample zone is basically recycled back into the column to continue the separation process in a closed-loop system. A consequence of this recycling mode is the increment in the number of theoretical plates with each cycle and maintaining a minium peak dispersion in the resulting chromatogram with no additional solvent needed for recycling. Pure samples are collected when the baseline resolution is achieved. Therefore, compounds with comparable physicochemical characteristics are fully separated to provide pure single chemical entities suitable for structure elucidation and further biological assessments. This review article examines the essential fundaments of this efficient method and its advances in isolation of natural products that have made the laborious purification processes less demanding and less time-consuming. Several applications that show the purification of natural products from small to large bioactive natural molecules by recycling preparative high-performance liquid chromatography are revised.