Synthesis of highly polarized [3]dendralenes and their Diels–Alder reactions†
Abstract
The diene-transmissive Diels–Alder (DTDA) reactions of dendralenes are emerging as a powerful synthetic tool. To date, these processes have been studied with non-polarized or mildly polarized species. We now present an expedient synthesis of strongly electron-deficient [3]dendralenes and demonstrate, for the first time, their DTDA reactions with electron-poor dienophiles. While the combination of two electron-poor partners is believed to be generally disfavored, DTDA reactions reported herein proceed at 100 °C with high yields and stereoselectivities. DFT calculations show that this electronically disfavored process is encouraged by a steric effect of the vinylic moiety within the dendralene core, driving the diene segment into the s-cis conformation, thereby lowering the activation energy by 2–3 kcal mol−1. While the free energy barrier is typically lower for the second cycloaddition, the two barriers become similar for dendralenes with a cyclic enone fragment, which allows a controlled stepwise addition of two different dienophiles.
![Graphical abstract: Synthesis of highly polarized [3]dendralenes and their Diels–Alder reactions](/en/Image/Get?imageInfo.ImageType=GA&imageInfo.ImageIdentifier.ManuscriptID=D3QO01221B&imageInfo.ImageIdentifier.Year=2023)
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