Catalytic Asymmetric Synthesis. Группа авторов
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Название: Catalytic Asymmetric Synthesis
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119736417
isbn:
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].
Scheme 2.48. 1,3‐Dipolar cycloaddition reaction between azomethine ylide and isatin‐derived ketimines.
Source: СКАЧАТЬ