Maruoka-type ammonium salt catalysts

In 1999, in consideration of the readily structural modifications and fine-tuning of catalysts to attain sufficient reactivity and selectivity, Maruoka and coworkers designed and prepared the structurally rigid, chiral spiro ammonium salts of type 1 derived from commercially available (S)- or (R)-1,1 -bi-2-naphthol as a new C2-symmetric chiral phase-transfer catalyst, and successfully applied this to the highly efficient, catalytic enantioselective alkylation of N-(diphenylmethylene)glycine tert-butyl ester under mild phase-transfer conditions (Scheme 5.1) [7]. 

On the other hand, Maruoka and coworkers were intrigued with the preparation of symmetrical N-spiro-type catalysts to avoid the independent synthesis of two different binaphthyl-modified subunits required for 1. Along this line, 4,4, 6,6 -tetra-arylbinaphthyl-substituted ammonium bromide (S, S)-13 was assembled through the reaction of aqueous ammonia with bis-bromide (S)-14 on the basis of previous studies on the substituent effect of this type of salt. The evaluation of (S,S)-13 as a chiral phase-transfer catalyst in the alkylation of 2 uncovered its high catalytic and chiral efficiency (Scheme 5.9) [9]. 

Maruoka and coworkers designed a new and highly efficient chiral N-spiro-type quaternary ammonium salt (S)-70 with dual functions for the asymmetric epoxidation of various enone substrates (Scheme 5.44) [45]. The exceedingly high asymmetric induction is ascribable to the molecular recognition ability of the catalyst toward enone substrates by virtue of the appropriately aligned hydroxy functionality, as well as the chiral molecular cavity. Indeed, the observed enantioselectivity depends heavily... 

A new class of suitable optically active organocatalyst for enantioselective alkylations has recently been developed by Maruoka and co-workers [le, 33-37]. This catalyst is not based on an alkaloid-related quaternary ammonium salt but consists of a C2 -symmetric compound of type 29 (or derivatives thereof bearing other types of substituent on the 3,3 positions of the binaphthyl unit) [33, 34], In the presence... 

The Maruoka group recently reported an alternative concept based on a one-pot double alkylation of the aldimine of glycine butyl ester, 44a, in the presence of the chiral ammonium salt 29 as chiral phase-transfer catalyst (the principal concept of this reaction is illustrated in Scheme 3.18, route 2) [58], Under optimized reaction conditions products of type 43 were obtained in yields of up to 80% and with high enantioselectivity (up to 98% ee). A selected example is shown in Scheme 3.20. 

Asymmetric phase-transfer catalysis is a method that has for almost three decades proven its high utility. Although its typical application is for (non-natural) amino acid synthesis, over the years other types of applications have been reported. The unique capability of quaternary ammonium salts to form chiral ion pairs with anionic intermediates gives access to stereoselective transformations that are otherwise very difficult to conduct using metal catalysts or other organocatalysts. Thus, this catalytic principle has created its own very powerful niche within the field of asymmetric catalysis. As can be seen in Table 5 below, the privileged catalyst structures are mostly Cinchona alkaloid-based, whereas the highly potent Maruoka-type catalysts have so far not been applied routinely to complex natural product total synthesis. 

Benzylic bromination of atropisomers has proven important in the preparation of a variety of dinapththazepine catalysts, like Maruoka s chiral, quaternary ammonium salt used for PTC. Amine 69 is considered a key building block for the preparation of this type of chiral catalyst. Bis-bromination of 67 provided 68 in 54% yield using NBS and catalytic AIBN in cyclohexane. Notably, the pure, bis-bromide 68 precipitates out of solution. Again, the benzyl bromide system was utilized as a leaving group. 

Aldol and Related Condensations As an elegant extension of the PTC-alkylation reaction, quaternary ammonium catalysts have been efficiently utilized in asymmetric aldol (Scheme 11.17a)" and nitroaldol reactions (Scheme ll.lTb) for the constmction of optically active p-hydroxy-a-amino acids. In most cases, Mukaiyama-aldol-type reactions were performed, in which the coupling of sUyl enol ethers with aldehydes was catalyzed by chiral ammonium fluoride salts, thus avoiding the need of additional bases, and allowing the reaction to be performed under homogeneous conditions. " It is important to note that salts derived from cinchona alkaloids provided preferentially iyw-diastereomers, while Maruoka s catalysts afforded awh-diastereomers. 

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