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Chiral guanidine bases

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). [Pg.85]

Kohn, U., Klopfleisch, M., Gorls, H. and Anders, E. (2006) Synthesis of hindered chiral guanidine bases starting from (5)-(7/,7/-dialkylaminomethyl)pyrrolidines and BrCN. Tetrahedron Asymmetry, 17, 811-818. [Pg.136]

Surprisingly, there have been only few synthetic studies on polymer-supported asymmetric superbase reagents. Recently, Wannaporn and Ishikawa prepared a new chiral guanidine based polymer catalyst and applied it to the asymmetric Michael addition reaction of iminoacetate with methyl vinyl ketone [39] (Scheme 6.7). Although the catalyst shows only moderate levels of reactivity and enantioselectivity, the result demonstrates the possibility of expanding an exciting field of asymmetric synthesis using polymer-supported chiral superbase catalysts. [Pg.192]

The axially chiral guanidine base (231) has been employed to catalyse the viny-logous Michael addition of a-thio substituted fiiranones (229) to nitroalkenes. The resulting a,K-functionalized butenolides (230) were obtained with <99 1 dr and ... [Pg.421]

M. Terada, K. Ando, Enantioselective direct vinylogous Michael addition of functionalized furanones to nitroalkenes catalyzed by an axially chiral guanidine base, Org. Lett. 13 (2011) 2026-2029. [Pg.290]

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. [Pg.186]

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). [Pg.186]

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. [Pg.189]

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. [Pg.102]

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. [Pg.109]

A further development, by the Grigg group, was the use of menthyl acetal 48 26 This chiral acetal reacted with aromatic iminoesters 43c-g in the presence of silver acetate (1.5 equiv) and guanidine base 47 (1.2 equiv) in acetonitrile, to give cycloadducts 49 in good yields and as a single diastereoisomer in each case (Scheme 2.13). In contrast, toluene was the preferred solvents for aliphatic iminoesters 43e and 43m-p. [Pg.52]

TBDMSCl, DMAP, EtjN, DMF, 25°C, 12 h. These conditions were used to silylate selectively a primary over a secondary alcohol. In the silylation of carbohydrates, it was shown that these conditions inhibit silyl migration whereas the use of imidazole as base causes migration." Besides DMAP, other catalysts such as 1,1,3,3-tetramethylguanidine, l,8-diazahicyclo[5.4.0]undec-7-ene (83-99%), l,5-diazabicyclo[4.3.0]non-5-ene, and ethyldiisopropyl-amine have also been used. A chiral guanidine has been used to give modest kinetic resolution of chiral secondary alcohols with TBDMSCl and TIPSCl. ... [Pg.190]

A series of chiral guanidines, either symmetrical or nonsymmetrical, was newly synthesized from commercial amino alcohols using a concise and efficient aziridine-based synthetic methodology [25]. [Pg.98]

Terada, M., Ube, H. and Yaguchi, Y. (2006) Axially chiral guanidine as enantioselective base catalyst for 1,4-addition reaction of 1,3-dicarbonyl compounds with conjugated nitroalkenes. Journal of the American Chemical Society, 128, 1454-1455. [Pg.140]

Basavaiah,D., Rao,K.V.and Reddy,B.S.(2006) (55)-l,3-Diaza-2-imino-3-phenylbicyclo[3.3.0] octane first example of guanidine based in situ recyclable chiral catal3dic source for borane-mediated as3munetric reduction of prochiral ketones. Tetrahedron Asymmetry, 17, 1036-1040. [Pg.142]

Corey and Kania reported an enantioselective Claisen rearrangement reaction of macrocychc lactone for the synthesis of (-l-)-dollabellatrienone (56) [13], Reaction of the lactone 53 with chiral (5,5)-diazaborolidine L2BBr 54 and Barton s base (6) resulted in Claisen rearrangement to give carboxylic acid 55 in 86% yield with >98% ee (diastereos-electivity >98 2) (Scheme 7.10). In this reaction, rapid deprotonation by sterically hindered guanidine base is the key to suppress side reactions. [Pg.217]

Some bifunctional hydrogen-bond donor/Brpnsted base catalysts are shown in Figures 2.39 and 2.40. They comprise chiral amino alcohols and amino phenols, chiral amine-thiourea derivatives, and chiral guanidines, among others. In the absence of detailed experimental NMR or kinetic studies [179], most of our... [Pg.63]

More recently, Tan and co-workers reported the reaction of p,7-unsaturated thioesters with di-tert-butyl azodicarboxylate promoted by the chiral guanidine 32 as the Br0nsted base catalyst (Scheme 11.16) [57]. By judicious choice of the double bond geometry of the (3,y-unsaturated thioester compound, the reaction can deliver either enantiomeric product with excellent enantioselectivity. [Pg.397]

To overcome the difficulty associated with the design of chiral guanidines as a catalyst, the binaphthyl-derived 22 was initially developed with the expectation that substituents at the 3,3 -positions of the binaphthyl backbone would break the planar symmetry of the guanidine skeleton to create an attractive chiral environment. This type of axially chiral organic base catalyst, particularly 22a, could indeed... [Pg.177]

In 1994 Chinchilla and coworkers [34] identified and synthesized a chiral guanidine for the asymmetric catalytic addition of nitroolefins to aldehydes. Since this initial discovery, numerous developments in the field of asymmetric Brmsted base catalysis have utilized chiral guanidines. [Pg.357]

Reviews have featured recent applications of organocatalysts to asymmetric aldol reactions, including particular focus on catalysis by small molecules. The effects of introduction of a diaryl (oxy)methyl group into chiral auxiliaries, catalysts, and dopants have been discussed and applications of amidine-, isothiourea-, and guanidine-based nucleophilic catalysts for a range of reactions of carbonyl compounds have been highlighted. " ... [Pg.15]


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See also in sourсe #XX -- [ Pg.85 ]




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