Читать книгу Catalytic Asymmetric Synthesis - Группа авторов - Страница 94

Since the discovery of the cation‐binding properties of cyclic polyethers, there has been a desire to utilize this class of compounds to impart enantioselectivity onto a reaction by utilizing a chiral crown ether. The first example of this concept was demonstrated by Cram in 1981, with the asymmetric Michael addition of a cyclic β‐keto ester into methyl vinyl ketone (MVK) with a BINOL‐derived 22‐crown‐6 catalyst (Scheme 4.27) [87]. In this example, a chiral crown ether acts as a phase‐transfer catalyst for KOtBu. After deprotonation of the substrate, a potassium‐bound crown ether cation is ion‐paired with an enolate, allowing for enantioselective addition to methyl vinyl ketone (MVK).

Since the initial report by Cram, focus in the field of cation‐binding catalysis shifted to utilizing crown ether catalysts derived from carbohydrates. In 1989, a highly symmetric polyether catalyst was reported that catalyzed an asymmetric Michael addition of methyl phenyl acetate to methyl acrylate in high yield and moderate enantioselectivity (Scheme 4.28) [88]. In 1997, the Bakó group reported that an aza‐crown ether catalyst derived from D‐glucose catalyzed a nitro‐Michael reaction in high enantioselectivity [89]. This catalyst architecture proved to be applicable in various asymmetric phase‐transfer settings, such as in glycine imine[90] and aminomethylene phosphonate alkylation[91], as well as asymmetric chalcone epoxidation [92].

In 2009, the Song group demonstrated a novel polyether catalyst based on the BINOL scaffold that competently bound and phase‐transferred KF (Scheme 4.29) [93]. This catalyst was effective for the kinetic resolution of silyl‐protected alcohols via desilylation [94]. A 2015 follow‐up publication revealed that this catalyst architecture was competent for the reverse‐reaction, where an alcohol was kinetically resolved by silylation with the catalyst [95]. In early 2016, the Yan group applied this system to kinetic resolution via an E1cB‐elimination of β‐sulfonyl ketones [96]. Over the next several years, numbers of other kinetic resolutions featuring various leaving groups were shown to be compatible with this strategy, such as poly halogenated ketones[97], and aldols [98].

In addition to initiation with KF, in 2012, an organocatalytic asymmetric Strecker reaction was developed utilizing BINOL‐derived crown ether with KCN (Scheme 4.30) [99]. The functionalization of α‐amidosulfones proved to be a valuable paradigm, as many compatible anions were demonstrated to perform Mannich reactivity in high yields and selectivities. Notable nucleophiles include fluoro‐oxindoles [100], indoles [101], fluoroketones [102], thiocyanato ketones [103], and phthalimides [104]. In addition to kinetic resolutions, this system was also found to be useful for the synthesis of cyclic compounds from unsaturated ketones, such as with nitrones [105], mercaptoacetaldehyde [106], or intramolecular exo‐trig cyclizations [107].

Scheme 4.27. Asymmetric Michael addition via cation binding BINOL‐derived ether catalysts.

Source: Based on [87].

Scheme 4.28. Enantioselective Michael addition via cation binding sugar‐derived ether catalysts.

Source: Based on [88].

Scheme 4.29. Kinetic resolution of silyl‐protected alcohols.

Source: Based on [93].