Catalytic Asymmetric [4+2] Cycloaddition of o-Hydroxybenzyl Alcohols with o-Hydroxyl Styrenes: Diastereo- and Enantioselective Construction of Chiral Chroman Scaffold

A chiral phosphoric acid-catalyzed asymmetric [4+2] cycloaddition of o-hydroxyl styrenes with o-quinone methides (o-QMs) generated in situ from o-hydroxybenzyl alcohols has been established. O-Hydroxybenzyl alcohols could transform into o-QM intermediates under the catalysis of chiral phosphoric acid (CPA), which are easily activated by CPA via hydrogen-bonding interaction. On the other hand, o-hydroxyl styrenes could also be activated by CPA via forming a hydrogen bond between the hydroxyl group of styrenes and the phosphoryl oxygen of CPA. So, by selecting o-hydroxybenzyl alcohols as precursors of dienes and o-hydroxyl styrenes as dienophiles under the catalysis of CPA, this catalytic asymmetric [4+2] cycloaddition provided an efficient strategy for constructing enantioenriched chroman framework with two stereogenic centers. A variety of substituted o-hydroxybenzyl alcohols and o-hydroxyl styrenes bearing either electron-donating or electron-withdrawing groups could be applicable to the reaction, delivering chiral chroman derivatives in high yields, considerable enantioselectivities and excellent diastereoselectivities (up to 78% yield, 72% ee, most of examples>95 : 5 dr). The electronic nature of the substituents has some effect on the reaction. Namely, the electron-donating groups were beneficial to both the reactivity and the enantioselectivity. Based on the control experiments, it is suggested that the o-hydroxyl styrenes and the o-QM intermediates generated from o-hydroxybenzyl alcohols were simultaneously activated by CPA via forming double hydrogen bonds, thus facilitating the reaction in an enantioselective way. A representative procedure for the enantioselective [4 + 2] cycloaddition reaction is as following: 1,2-dichloroethane (1 mL) was added to the mixture of o-hydroxybenzyl alcohols (0.1 mmol), o-hydroxyl styrenes (0.12 mmol), the chiral phosphoric acid (0.005 mmol), and 3 angstrom molecular sieves (100 mg). After being stirred at 50 degrees C for 12 h, the reaction mixture was filtered to remove the molecular sieves, and the solid powder was washed with ethyl acetate. The resultant solution was concentrated under the reduced pressure to give the residue, which was purified through flash column chromatography on silica gel to afford the pure chiral chroman derivatives.