Catalytic Asymmetric Synthesis. Группа авторов

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Scheme 2.22. Allylation of aldehydes with α‐vinyl allylboronate.

      Source: Based on [67, 68].

As an extension of the allylation reaction, Murakami combined allylboration and a Pd‐catalyzed transposition reaction of homoallylic bisboronate. Treatment of 1,1‐di(boryl)alk‐2‐ene with Pd catalyst generated allylboronate, in situ, which underwent an allylboration reaction with an aldehyde using CPA 6e to afford anti‐homoallylic alcohols with high diastereo‐ and enantioselectivities (Scheme 2.23a) [69]. The same group subsequently reported an enantioselective synthesis of anti‐1,2‐oxaborinan‐3‐enes from aldehydes and 1,1‐di(boryl)alk‐3‐enes using Ru(II) complex and CPA 6e (Scheme 2.23b) [70].

Scheme 2.23. Reaction between di(boryl)butane and aldehyde, leading to the formatinof anti‐homoallylic alcohols (a) (

      Source: Based on [69]

      ), and anti‐1,2‐oxaborinan‐3‐enes (b) (

      Source: Based on [70]).

With regard to the allylation reaction using allylsilane, List reported the Hosomi‐Sakurai allylation reaction between allylsilane and aromatic aldehydes catalyzed by chiral DSI 9c (Scheme 2.24a) [71]. DSI 9c acted as a precatalyst, N‐trimethylsilyl sulfonimide was generated as a chiral Lewis acid catalyst, and sulfonimide anion controlled the enantioselectivity by acting as a chiral counteranion. Subsequently, List developed highly acidic IDPs 11b and 11c [14], and reported a catalytic addition reaction between allyltrimethylsilane and aldehydes, which is based on the silylium‐based Lewis acid organocatalysis (Scheme 2.24b) [72]. Both aromatic and aliphatic aldehydes were suitable substrates. It is noted that as low as 0.5 mol% of the catalyst promoted the allylation reaction with aromatic aldehyde, and as low as 0.05 mol% of the catalyst could be employed with aliphatic aldehydes.

Scheme 2.24. Allylation reaction using 9c (a) and 11b,c (b) (

      Source: Based on [72]).

List reported a catalytic Mukaiyama aldol reaction that uses chiral IDPi 11d as the catalyst to furnish aldol products with 80–98% ee. The silylium ion is considered to be the actual catalyst. It is noted that parts per million (ppm) levels of catalyst were sufficient for the aldol reaction [73]. In general, aldehyde is more reactive than ketone. They subsequently developed a ketone‐selective Mukaiyama aldol reaction catalyzed by 11e to afford tetrahydrofuran derivatives with high chemoselectivity and with 84–97% ee (Scheme 2.25). The in situ generated silylium ion pair coordinated to sterically less hindered aldehydes and subsequent intramolecular cyclization gave a highly active cyclic oxocarbenium ion intermediate bearing a chiral counteranion, which appears to be responsible for the chemoselectivity (Figure 2.7) [74].

Scheme 2.25. Mukaiyama aldol reaction with ketones (a) and ketone selective Mukaiyama aldol reaction (b).

Figure 2.7. Reaction intermediate in the ketone‐selective reaction.

      Source: Based on [74].

Seidel developed an enantioselective synthesis of isoindolinones through the condensation of 2‐acylbenzaldehydes with anilines in the presence of CPA 6e (Scheme 2.26) [75]. They proposed that the tautomerization of the hydroxy‐isoindoline intermediate to isoindoline is proposed to be the enantiodetermining step.

Yang developed an efficient method for the synthesis of 4H‐3,1‐benzoxazines by kinetic resolution of 2‐amido benzyl alcohols catalyzed by CPA 6i [76]. A broad range of benzyl alcohols (both tertiary and secondary alcohols) was kinetically resolved, wherein the amide group reacted as the electrophile (Scheme 2.27).

The oxa‐Pictet‐Spengler reaction was reported by several groups. List employed nitrated confined IDP 10b as the chiral strong Brønsted acid catalyst for the oxa‐Pictet‐Spengler reaction between 2‐arylethanols and aldehydes to furnish 1‐substituted isochromans with 90–99% ee (Scheme 2.28) [77].

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