Catalytic Asymmetric Synthesis. Группа авторов
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and in situ generated aza‐o‐quinone‐methide, giving rise to tetrahydroacridines using CPA 6n [94]. Subsequent transformation gave free hexahydroacridines with a total of three new stereogenic centers (Scheme 2.42).
Ishihara developed a chiral magnesium potassium binaphthyl disulfonate cluster as a chiral Brønsted acid catalyst for the cycloaddition reaction between styrene derivatives and aldimines to afford cyclic carbamates with 79–98% ee (Scheme 2.43) [95]. They obtained a 3 : 1 : 4 aqua complex of (R)‐8d/Mg/K as a crystal and determined the structure of the cluster by X‐ray structural analysis and electrospray ionization mass spectrometry (ESI‐MS) analysis (Figure 2.11). Although the cluster itself did not exhibit the catalytic activity, the addition of TfOH restored the catalytic activity through H+ exchange. It was found that the strong acidity of the catalyst dissolved MS3Å and took up leached Mg2+ and K+.
Scheme 2.42. Addition of enamides to ortho‐quinone methide.
Scheme 2.43. Cycloaddition reaction between styrenes and aldimines.
Source: Based on [95].
Figure 2.11. 3 : 1 : 4 Aqua complex of (R)‐8d/Mg/K.
Terada reported an enantioselective synthesis of β‐amino secondary amides by [4+2] cycloaddition reaction between vinyl azides and N‐acyl imines using chiral phosphoramide 14c. The [4+2] cycloaddition was followed by subsequent ring‐opening of iminodiazonium ion intermediate, Schmidt‐type 1,2‐aryl migration, recyclization of the resulting nitrilium ion, and acid hydrolysis (Scheme 2.44) [96].
Scheme 2.44. [4+2] cycloaddition reaction between vinyl azides and N‐acyl imines.
Source: Based on [96].
2.4.3. Oxa‐Diels‐Alder Reactions
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].
2.4.4. Other Cycloaddition Reactions
Shi reported a 1,3‐dipolar cycloaddition reaction between azomethine ylide and isatin‐derived ketimines to afford a spiro‐imidazolidine‐2,3′‐oxindole framework with a single diastereomer and with 73–94% ee using CPA 6h (Scheme 2.48) [100].
Shi reported a formal [3+2] cycloaddition reaction of isatin‐derived 3‐indolylmethanols with 3‐methyl‐2‐vinylindole using CPA 12c (Scheme 2.49) [101]. Indolylmethanols are useful precursors for the generation of the indolenium ion intermediate and have been extensively investigated for numerous transformations (Scheme 2.50) [102, 103].
Masson reported a [4+3] cycloaddition reaction between indolyl alcohol and 1,3‐diene‐1‐carbamates catalyzed by CPA 6e (Scheme 2.51) [104].
In addition to 3‐indolylmethanol, 2‐indolylmethanol was employed successfully. For example, Schneider reported a [3+2] cycloaddition reaction between 2‐vinylindoles and in situ generated 2‐methide‐2H‐indoles to afford pyrrolo[1,2‐a]indoles using CPA 6q (Scheme 2.52) [105]. Subsequently, Shi reported a similar [3+2] cycloaddition reaction of 3‐indolylmethanol [106].