Читать книгу Catalytic Asymmetric Synthesis - Группа авторов - Страница 52

List reported a Torgov reaction catalyzed by chiral DSI 9b, which yielded enantioenriched tri‐ and tetracyclic dienes. The Torgov reaction is proposed to proceed by the isomerization of alkene followed by the Prins reaction, a subsequent isomerization, and dehydration (Scheme 2.15). The method was successfully applied to a concise synthesis of (+)‐estrone [57].

List subsequently attempted to perform Prins cyclization, but chiral DSI 9 was not sufficiently acidic enough to promote the Prins cyclization. List developed confined imino‐imidodiphosphates (iIDPs) 19a and 19b as a new class of highly acidic Brønsted acids and reported the Prins cyclization. Both aliphatic and aromatic aldehydes participated in the reaction successfully to furnish tetrahydropyran derivatives with 80–96% ee (Scheme 2.16) [58].

List reported an intramolecular carbonyl‐ene reaction of olefinic aldehyde using confined IDP 10a as chiral Brønsted acid to afford diverse trans‐3,4‐disubstituted five‐membered carbo‐ and heterocycles in 77–96% yields and with trans selectivity (4 : 1 to >20 : 1), and with 84–96% ee (Scheme 2.17) [59].

Rueping reported intermolecular carbonyl‐ene reaction of 1,1‐disubstituted alkene with ethyl trifluoromethylpyruvate using N‐triflyl phosphoramide. But the enophile was limited to trifluoromethylpyruvate [60]. In order to achieve carbonyl‐ene reaction with wider substrate scope, Terada designed stronger Brønsted acid by perfluorination of the BINOL moiety. CPA 20, derived from F₁₀‐BINOL, catalyzed the intermolecular carbonyl‐ene reaction between exocyclic alkene and ethyl glyoxylate to furnish homoallylic alcohols with 62–93% ee (Scheme 2.18) [61].

Scheme 2.15. Torgov reaction.

Scheme 2.16. Prins cyclization.

Source: Based on [58].

Scheme 2.17. Intramolecular carbonyl‐ene reaction.

Source: Based on [59].

In 2010, List independently synthesized chiral phosphoric acids derived from (S)‐SPINOL. A highly enantioselective kinetic resolution of homoaldols was achieved using CPA 13b (STRIP), which is derived from (S)‐SPINOL bearing 2,4,6‐(i‐Pr)₃C₆H₂ groups at 6,6′‐positions, to furnish tetrahydrofuran derivatives in a highly enantioselective manner by transacetalization (Scheme 2.19) [20].

Scheme 2.18. Intermolecular carbonyl‐ene reaction.

Source: Based on [61].

Scheme 2.19. Kinetic resolution of homoaldols.

Source: Based on [20].

The enantioselective allylation of aldehyde is an important reaction for the preparation of homoallylic alcohols. Antilla reported an enantioselective allylation of aromatic and aliphatic aldehydes with allylboronate using CPA 6e to furnish homoallylic alcohols with 73–99% ee (Scheme 2.20a) [62]. Allenyl boronate also participated in the reaction to furnish homopropargyl alcohols in 87–96% yields and with 77–96% ee (Scheme 2.20b) [63]. Based on the density‐functional theory (DFT) studies, Goodman [64], and Houk‐Antilla [65] independently proposed a transition state model of the allylboration and the propargylation. Phosphoric acid forms a hydrogen bond with boronate pseudo axial oxygen and at the same time, phosphoryl oxygen forms a hydrogen bond with formyl hydrogen (Figure 2.6).

Scheme 2.20. Allylation of aldehyde by allylboronate (a) (

Source: Based on [62]

), and proparygylation with allenylboronate (b) (

Source: Based on [63]).

Figure 2.6. Transition state model of the allylation with allylboronate.

Scheme 2.21. Allylation of aldehyde with γ‐silyl boronate.

Barrio employed γ‐silyl boronate to form α‐silyl homoallylic alcohols with excellent diastereoselectivity and with 59–96% ee catalyzed by CPA 6h (Scheme 2.21). Subsequent treatment with Selectfluor furnished fluorinated allylic alcohols [66].

Chen reported an enantioselective addition of achiral α‐vinyl allylboronate to aldehyde to furnish dienyl homoallylic alcohols with high Z‐selectivity and with 90–97% ee (Scheme 2.22) [67, 68].

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].

Scheme 2.26. Enantioselective synthesis of isoindolinones.

Source: Based on [75].

Scheme 2.27. Kinetic resolution of 2‐amido benzyl alcohols.

Scheme 2.28. Oxa‐Pictet‐Spengler reaction of 2‐arylethanols.

Source: Based on [77].

Subsequently, Scheidt developed a cooperative catalyst system consisting of achiral hydrogen donor 21 and CPA 6b, and achieved an oxa‐Pictet‐Spengler reaction (Scheme 2.29). The reaction proceeded via oxocarbenium ions bearing chiral counteranion intermediate [78].

Seidel developed a dual catalyst system employing both chiral amine HCl salt and a chiral bisthiourea to generate oxocarbenium ion intermediate from aldehyde and realized an oxa‐Pictet‐Spengler reaction between tryptophol and aldehydes (Scheme 2.30) [79].

Seidel was the first to report an oxa‐Pictet‐Spengler reaction with ketals by developing novel chiral carboxylic acid 24, bearing a urea moiety derived from 1,2‐diaminocyclohexane (Scheme 2.31) [80]. They measured the pK _a value of the catalysts in CH₃CN and found that 24 was one order of magnitude more acidic than CPA 6e: 12.7 for 24 and 13.6 for CPA 6e.

Scheme 2.29. Oxa‐Pictet‐Spengler reaction.

Source: Based on [78].

Scheme 2.30. Oxa‐Pictet‐Spengler reaction between tryptophol and aldehydes.

Source: Based on [79].

Scheme 2.31. Oxa‐Pictet‐Spengler reaction with ketal.