Synthetic Route to 4,4a‐ and 3,4‐Dihydroxanthones through [4+2] Cycloaddition and Base‐Assisted Sigmatropic Rearrangement

摘要

> The [4+2] cycloaddition of chromone‐fused dienes with enamines was found to be an efficient method for the synthesis of 4,4a‐ and 3,4‐dihydroxanthone derivatives. Either product type could be obtained by choosing the proper conditions [reaction time and addition of La(NO ₃ ) ₃ as a Lewis acid]. Calculations using density functional theory and M?ller–Plesset perturbation theory showed that the isomerization of 4,4a‐dihydroxanthones to 3,4‐dihydroxanthones occurred through a base‐assisted sigmatropic rearrangement. The described reaction provides a convenient route to a natural‐product‐inspired group of 4,4a‐dihydroxanthones with cytotoxic activity.