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Shi reported an inverse electron‐demand oxa‐Diels‐Alder reaction of ortho‐quinone methide, which was generated in situ from ortho‐hydroxybenzyl alcohols catalyzed by CPA 6o (Scheme 2.45) [97]. This is the first example of the oxa‐version of the inverse electron‐demand Diels‐Alder reaction.

Scheme 2.45. Inverse electron‐demand oxa‐Diels‐Alder reaction of ortho‐quinone methide.

Source: Based on [97].

ortho‐Quinone methide intermediate has been employed extensively in numerous transformations. Schneider reported an enantioselective synthesis of 4H‐chromenes, which involves a conjugate addition of β‐diketones to in situ generated ortho‐quinone methides using 6p, followed by a cyclodehydration reaction (Scheme 2.46) [98].

Scheme 2.46. Enantioselective synthesis of 4‐aryl‐4H‐chromenes.

Source: Based on [98].

Schneider reported a domino‐type reaction between diazoesters and ortho‐quinone methides generated in situ to furnish densely functionalized chromans with three contiguous stereogenic centers. A transition metal and a Brønsted acid catalyst 6b acted synergistically to produce a transient oxonium ylide and ortho‐quinone methide, which underwent subsequent coupling in a conjugate‐addition‐hemiacetalization event to afford chromans (Scheme 2.47) [99].

Scheme 2.47. Synergistic rhodium/phosphoric acid catalysis.

Source: Based on [99].