The lindenane dimeric sesquiterpenoids with versatile biological activities are accessible via biometric synthesis, in which the endo -selective Diels-Alder reaction plays an important role. To explore the endo -selectivity of the Diels-Alder reaction between lindenane sesquiterpenes promoted by pyridines, density functional theory (DFT) calculations were performed to explore the reaction mechanism between pyridines and D-A monomers. The calculations performed on the reaction pathways explain why pyridines can promote endo -selectivity via hydrogen bonding, and the hydrogen bond strength is a key factor driving the Diels-Alder reaction in major biochemical systems. These DFT-level insights will pave the way for designing better promoters for Diels-Alder reactions in biometric synthesis applications.