<div>The present work seeks to use high throughput experimentation (HTE) to guide chemical synthesis. We demonstrate the use of an HTE system utilizing a robotic liquid handler to prepare arrays of reactions and print them onto a surface to be analyzed by desorption electrospray ionization mass spectrometry (DESI-MS) as a tool to guide reaction optimization, synthetic route selection, and reaction discovery. DESI-MS was employed as a high throughput experimentation tool to provide qualitative predictions of the outcome of a reaction, so that vast regions of chemical reactivity space may be more rapidly explored and areas of optimal efficiency identified. This work is part of a larger effort to accelerate reaction optimization to enable the rapid development of continuous-flow syntheses of small molecules in high yield. In the present iteration of this system, reactions are scaled up from these nanogram surface printed reactions to milligram scale microfluidic reactions, where more detailed analysis and further optimization may be performed. In the earliest iterations of this screening system prior to the use of DESI, the initial screening reactions were performed in electrospray (ESI) droplets and leidenfrost droplets before scaling up to microfluidic reactions which were analyzed by ESI-MS. The insights from this combined droplet and microfluidic screening/rapid ESI-MS analysis approach, helped guide the synthesis of diazepam. The system was further refined to by the use of liquid handling robots and DESI-MS analysis, greatly accelerating the overall pace of screening. In order to build confidence in this approach, however, it is necessary to establish a robust predictive connection between reactions performed under analogous DESI-MS, batch, and microfluidic reaction conditions. To achieve this goal, we first explored the potential of high throughput DESI-MS experiments to identify trends in reactivity based on chemical structure, solvent, temperature, and stoichiometry that are consistent across these platforms. While DESI-MS narrowed the scope of possibilities for reaction</div><div>13</div><div>selection with some parameters such as solvent, others like stoichiometry and temperature still required further optimization under continuous synthesis conditions. With our increased confidence in DESI-MS HTE, we proceeded to explore it’s application to rapidly evaluate large sets of aldol reactions of triacetic acid lactone (TAL), a compound well studied for use as a bio-based platform molecule that may be converted to a range of useful commodity chemicals, agrochemicals, and advanced pharmaceutical intermediates. Our DESI-MS HTE screening technique was used to rapidly evaluate known reactions of triacetic acid lactone, in an effort to accelerate reaction discovery with platform chemicals. Our rapid experimentation system, when applied to reaction discovery in this manner, may help to shorten the time scale of platform chemical development.</div>