Guanidines chiral

The wide applicability of the PK reaction is apparent in the synthesis of pyrroles, for example, 45, en route to novel chiral guanidine bases, levuglandin-derived pyrrole 46, lipoxygenase inhibitor precursors such as 47, pyrrole-containing zirconium complexesand iV-aminopyrroles 48 from 1,4-dicarbonyl compounds and hydrazine derivatives. The latter study also utilized Yb(OTf)3 and acetic acid as pyrrole-forming catalysts, in addition to pyridinium p-toluenesulfonate (PPTS). 

Ishikawa and co-workers reported that treatment of chiral nonracemic guanidin-ium bromide 66 (Scheme 3.22) with aldehyde 67, in the presence of tetramethyl-... 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 3, 84 Borane, thiocyanato-halogenohydro-, 3,88 Borane, trialkoxy-amine complexes, 3, 88 Borane, triaryl-guanidine complexes, 2,283 Borane, trifluoro-complexes Lewis acids, 3,87 van der Waals complexes, 3, 84 Borane complexes aminecarboxy-, 3,84 aminehalogeno-, 3, 84 amines, 3, 82, 101 B-N bond polarity, 3, 82 preparation, 3, 83 reactions, 3, 83 bonds B-N, 3, 88 B-O, 3, 88 B-S, 3, 88 Jt bonds, 3, 82 carbon monoxide, 3, 84 chiral boron, 3, 84 dimethyl sulfide, 3, 84 enthalpy of dissociation, 3, 82... 

Corey, E. J. Grogan, M. J. Enantioselective Synthesis of a-Amino Nitriles from N-Benzhydryl Imines and HCN with a Chiral Bicyclic Guanidine as Catalyst Org Lett. 1999,1,157-160. 

Krai has recently reported a related system in which porphyrins bearing bicyclic guanidine substituents (such as the tetra-substituted species 70 shown is Scheme 35) form highly ordered chiral assemblies in aqueous solution [92]. 

FIGURE 4. Molecular structure of nitric salt of chiral bicyclic guanidine (13)78. Reproduced by per-mission of the Royal Society of Chemistry... 

Jacobsen (1998), Corey (1999) chiral ureas/thioureas and guanidine Enamine Cataiysis... 

Keywords Asymmetric organocatalysis Bifunctional catalyst Brpnsted base Chiral scaffold Cinchona akaloid Cyclohexane-diamine Guanidine... 

While the significance of the bifunctional Brpnsted base catalysts has been illustrated in the previous sections, few examples rely solely on a Brpnsted base interaction for asymmetric catalysis. However, in the past few decades, a novel catalyst system has emerged as a powerful promoter of chiral transformations. The guanidines have gained the reputation as super bases in organic transformations. 

It was not until 1994 that Chinchilla and co-workers identified a synthesized a chiral guanidine for asymmetric catalysis [115],... 

The authors reported the first chiral guanidine catalyzed addition of nitro-olefms to aldehydes (Scheme 62, Table 3). While reactivity and selectivity were not optimal, the discovery led to great developments in the field of asymmetric Brpnsted base catalysis. 

The chiral guanidine s role as a strong Brpnsted base for the reactions of protic substrates has been proposed. In 1999, Corey developed a C -symmetric chiral guanidine catalyst to promote the asymmetric Strecker reaction [117]. The addition of HCN to imines was promoted high yields and high enantioselectivities for both electron-withdrawing and electron-donating aromatic imines (Scheme 64). 

Fig. 9 Corey s proposed catalytic cycle for chiral guanidine promoted hydrocyanation...

Ma and co-workers extended use of chiral guanidine catalysts to the addition of glycine derivatives to acrylates [121], Addition products were achieved in high yield with modest enantioselectivity (Scheme 67). The ferf-butyl glycinate benzophenone imines generally provided better enantiomeric ratios than the ethyl glycinate benzophenone imines. Based on this observation, the authors hypothesized that an imine-catalyst complex determines the stereochemical outcome of the product. 

Another structurally modified guanidine was reported by Ishikawa et al. as a chiral superbase for asymmetric silylation of secondary alcohols [122]. Soon after, Ishikawa discovered that the same catalyst promoted asymmetric Michael additions of glycine imines to acrylates [123]. The additions were promoted in good yield and great asymmetric induction under neat reaction conditions with guanidine catalyst 250 (Scheme 68). The authors deduced that the high conversion and selectivity were due to the relative configuration of the three chiral centers of the catalyst in... 

Ishikawa and co-workers also reported a class of structurally modified guanidines for promotion of the asymmetric Michael reaction of ierf-butyl-diphenylimino-acetate to ethyl acrylate [124,125]. In addition to a polymer support design (Scheme 69), an optical resolution was developed to achieve chiral 1,2-substituted ethylene-l,2-di-amines, a new chiral framework for guanidine catalysis. The authors discovered that incorporating steric bulk and aryl substituents in the catalyst did improve stereoselec-tivitity, although the reactivity did suffer (Scheme 70, Table 4). 

Terada and co-workers reported a novel guanidine catalyst with a chiral binaphthol backbone for the asymmetric addition of dicarbonyl compounds to nitro-olefins [126]. Substitution on the binaphthol backbone dramatically increased enantioselectivity. 

Recently, Tan reported related bicyclic guanidine 20 as a chiral Br0nsted base to promote the highly enantioselective Diels-Alder reaction of various anthrones and maleimides (Scheme 5.41) [75]. Interestingly, use of dithranol led to the exclusive formation of the enantio-enriched Michael adducts. 

Ooi has recently reported application of chiral P-spiro tetraaminophosphonium salt 37 as a catalyst for the highly enantio- and diasterioselective direct Henry reaction of a variety of aliphatic and aromatic aldehydes with nitroalkanes (Scheme 5.51) [92]. Addihon of the strong base KO Bu generates in situ the corresponding catalyhcally active triaminoiminophosphorane base A. Ensuing formation of a doubly hydrogen-bonded ion pair B positions the nitronate for stereoselective addition to the aldehyde. This catalyst system bears many similarities to guanidine base catalysis. 

The power of cyclic chiral nitrones in synthesis was demonstrated by Nagasawa et al. (52) by the synthesis of the enantiomerically pure pentacyclic guanidine derivative 38 (Scheme 12.14). The 1,3-dipolar cycloaddition of 27b with 34 took... 

Taylor and co-workers <99SL795> repotted some promising results with DBU-like cyclic guanidine derivatives of type 44, equipped with the appropriate chiral auxiliary. Both yields and ee s were modest, but the ready availability of the amine, and the flexibility of substitution on the... 

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