Читать книгу Catalytic Asymmetric Synthesis - Группа авторов - Страница 97

Following the successful demonstration of asymmetric iminium reactivity with chiral‐anions, there was an increased interest in expanding this reactivity to other cationic intermediates. In 2007, the List group expanded the scope of ion‐pairing catalysis to oxocarbenium intermediates in an asymmetric Mukaiyama aldol reaction by using a highly acidic disulfonimide catalyst (Scheme 4.33) [113]. In a closely related follow‐up publication, the List group demonstrated vinylogous and doubly vinylogous silyl enol ethers as nucleophiles [114].

Scheme 4.33. Asymmetric Mukaiyama aldol reaction catalyzed by a highly acidic BINOL‐derived disulfonimide.

Source: Based on [113].

A variety of stabilized oxocarbenium intermediates has been shown to be compatible with this reaction paradigm. In 2013, the Terada group demonstrated an enantioselective reduction of hydroxy‐pyrans via a pyrylium/TRIP chiral ion‐pair (Scheme 4.34) [115]. A similar pyrylium intermediate was shown to undergo an enantioselective Diels‐Alder via ion‐pairing with a chiral phosphate, followed by rearrangement to an exo‐cyclic ketone [116]. Nonaromatic oxocarbeniums have also been accessed and utilized in chiral‐anion catalysis, as demonstrated by the Nagorny group. In this work, an allyl acetal underwent a Diels‐Alder, via the intermediacy of an allylic oxocarbenium ion‐paired with a chiral phosphate anion [117].

In order to access oxocarbenium intermediates without stabilizing moieties, more acidic chiral acids were required to favor ionization of the substrate. In 2016, the Thiel, De, and List groups developed an oxa‐Pictet‐Spengler reaction, catalyzed by a highly acidic imidodiphosphoric acid (Scheme 4.35) [118]. In 2018, Scheidt demonstrated that a similar oxa‐Pictet‐Spengler reaction could be realized with a less acidic chiral catalyst [119], by introducing exogenous achiral urea to increase the acidity of the catalyst via hydrogen bonding of the phosphate anion. Similarly, a bifunctional catalyst featuring a thiourea moiety to greatly increase the acidity of an intramolecular carboxylic acid was shown to be competent for this reactivity [120].

A potential alternative to the use of stronger chiral acids to access unstabilized oxocarbeniums was demonstrated in 2015 by the Luo group (Scheme 4.36) [121]. In this example, a trityl cation/chiral phosphate salt was shown to activate α‐ketoesters toward asymmetric reactivity with a variety of coupling partners. Numbers of transformations were compatible with this system, such as 1,4‐addition with indoles, Diels‐Alder with cyclopentadiene, and 1,2‐addition with α‐methyl styrene, which occurred in high yields and selectivities.