Expanding the reactivity of metal- and boron-mediated asymmetric mutli-component condensation reactions
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β-amino esters are useful building blocks for the synthesis of biologically active β-amino carbonyl compounds. We developed a three-component condensation reaction for directly accessing β-amino esters using diazo esters, arylboranes, and carbamoyl imines. Catalyzed by Cu(II) salts, the reaction affords the corresponding isocyanate Mannich product: one that can be easily trapped in situ by other nucleophiles. The overall Mannich condensation is an α,α-disubstituted enolate addition to imines. The reaction was rendered asymmetric using (-)-phenylmenthol ester in good yields and selectivities. The tetrahydrochromeno polycyclic core is a characteristically unique structure among the large number of biologically active phenolic compounds isolated from nature. We have developed an iron-catalyzed tandem rearrangementlhetero-Diels--Alder approach to access tetrahydrochromeno heterocycles. The process can occur as a homodimerization and cycloaddition process using electron-rich dienophiles. Deuterium labeling and mechanistic studies revealed a hydride shift and ortho-quinone methide cycloaddition reaction pathway. Short route synthesis of (±)-dependensin using the homodimerization approach has also been demonstrated. The enantioselective addition of vinyl- and aryl boronates to ortho-quinone methides would result in compounds with a benzylic chiral carbon center. This methodology could readily be utilized in the synthesis of a variety of natural products, such as (+)-cochinchinenene B- D, myristinin A, (-)-(S)-4-methoxy-dalbergione and (+)-mimosifoliol. We have developed a novel enantioselective addition ofboronates to orthoquinone methides catalyzed by chiral biphenols. Substituted 2-styryl phenols were obtained in good yields (up to 95%) and high enantiomeric ratios (up to 98:2) in presence of 10 mol % of 3,3'-Br2-BINOL. A two-step synthesis of (S)-4-Methoxy-dalbergione is achieved in good yield and selectivity. Dihydronaphthalene derivatives are useful starting materials for the synthesis of biologically active cyclic molecules. We developed a new enantioselective [4 + 2] cycloaddition reaction, which uses alkenyl boronate as the dienophile. The reaction is dual catalyzed by a chiral Bmnsted acid and an achiral Lewis acid. The 1,2-dihydronaphthalene products are obtained in high yields, diastereoselectivities and enantioselectivities.
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