Endo Rules! Multigram Synthesis of a Molecular Box

Thermal and photochemical cycloadditions are key reactions in second-year undergraduate organic chemistry curricula, valued for their ability to construct complex molecules from simple precursors. The experiments described herein demonstrate how a sequence of thoughtfully designed cycloadditions enables the synthesis of a complex small molecular box in a concise time frame, suitable for undergraduate teaching laboratories. Beginning with a Diels–Alder reaction, students observe the exclusive formation of an endo-product due to secondary orbital stabilization in the transition state. The resulting product undergoes a light-mediated [2 + 2] cycloaddition, reproducible in standard undergraduate laboratory settings without specialized equipment, to yield an open molecular box precursor. The final step─hydration by adding one water molecule─completes the molecular box’s closure. Alongside the synthesis, students analyze intriguing solvent effects in the NMR spectra, enhancing their skills in structural characterization. The methodology is scalable and robust, as demonstrated by a multigram synthesis using leftover student material. These experiments combine fundamental concepts in reaction mechanisms, stereochemistry, and structural analysis with hands-on engagement, offering students a unique opportunity to appreciate the beauty of bottom-up molecular design by constructing a complex, aesthetically intriguing molecular box. Detailed instructions for assessing student learning, a step-by-step protocol, NMR spectral analysis, and crystal structure data are provided, ensuring educational value and facile integration into undergraduate teaching.