Oxazolines bisoxazolines

Bis-oxazoline ligands can also be produced by oxidative coupling of the copper derivative of diastereoisomerically pure 306 (Scheme 145) . Further lithiations of the product 317, which was produced as single diastereoisomer, occur (as in Scheme 143) at the second site adjacent to the oxazoline, giving, for example, 318, despite the (presumably) less favourable stereochemistry of the lithiation step. Bisoxazolines 318 direct the asymmetric copper-catalysed cyclopropanation of styrene using diazoacetate. 

The enantioselective alkylation of indoles catalyzed by C2-symmetric chiral bisoxazoline-metal complexes 90 encouraged many groups to develop superior asymmetric catalysts which are cheap, accessible, air-stable and water-tolerant. Other analogs of the bisoxazoline-metal complex 90 as chiral catalysts and new Michael acceptors have also been studied. The enantioselective alkylations of indole derivatives with of-hydroxy enones using Cu(II)-bis(oxazoline) catalysts 93 and 94 provided the adducts in good yields... 

The additions of indoles to ethenetricarboxylates as Michael acceptors in the presence of copper(II) complexes (10%) of chiral bisoxazolines (97-100) under mild conditions gave the alkylated products in high yield and up to 96% ee [101]. The observed enantioselectivity could be explained by secondary orbital interaction on approach of indole to the less hindered side of the 102-Cu(II)-ligand complex. The chiral ligands 97-99 of the catalyst gave similar ee%. The phenyl derivative 100 produced inferior results compared to 97-99, while (S,S)-2,6-bis(4-isopropyl-2-oxazoline-2-yl)pyridine (101) gave no reaction (Scheme 29) [56]. The enantioselective alkylation of indoles with arylidene malonates catalyzed by z-Pr-bisoxazoline-Cu(OTf)2 was also reported [102],... 

Finally, two chiral monodentate N-heterocyclic carbene ligands that contain an oxazoline unit have been reported. Glorius et al. reported the synthesis of the imidazolium salts 76 by cyclizing the corresponding bisoxazolines 75 (Scheme 50) [133]. 

Oxazolines 168 and bisoxazolines can be synthesised starting from a prop-2-ynylamide in a reaction catalysed by Pd/C or Pdh in the presence of O2 and CO <02JOC4450>. 4-Amino-2-aryl-2-oxazolines 170 can be synthesised electrochemically starting from jV-(l-amino-2,2-... 

Some interesting applications of oxazoline derivatives in supramolecular chemistry were also described in the literature. Compound 178 was revealed as an efficient receptor for fluorescence sensing of ammonium and organoammonium ions <03OL1419>. Bisoxazoline... 

C2-Symmetric Bisoxazolines as Ligands in Asymmetric Catalysis. Methylenebis(oxazolines) such as (1), (3), and (5) are patterned after the semicorrins, which have been successfully employed as ligands in enantioselective Cu-catalyzed cy-clopropanations and other reactions (see (IS,9S)-I,9-Bis [(t-butyl)dimethylsilyloxy]methyl- 5-cyanosemico rrin). The potential of bisoxazoline ligands of this type, which has been recog-... 

Enantioselective Aziridination of Alkenes. Copper complexes with neutral methylenebis(oxazoline) ligands (1) and (2) have also been employed as enantioselective catalysts for the reaction of alkenes with (Al-tosylimino)phenyliodinane, leading to A-tosylaziridines. The best results have been reported for cinna-mate esters as substrates, using 5 mol % of catalyst prepared from CuOTf and the phenyl-substituted ligand (2) (eq 6). The highest enantiomeric excesses are obtained in benzene, whereas in more polar and Lewis basic solvents, such as acetonitrile, the selectiv-ities are markedly lower. The chemical yield can be substantially improved by addition of 4X molecular sieves. Both Cu - and Cu"-bisoxazoline complexes, prepared from Cu or Cu triflate, respectively, are active catalysts, giving similar results. In contrast to the Cu-catalyzed cyclopropanation reactions discussed above, in which only Cu complexes are catalytically active, here Cu complexes are postulated as the actual catalysts. ... 

Convergent dendrimers, with their versatile three-dimensional scaffold, may be tailored to mimic, perhaps crudely, some elements of enzymatic structures. Numerous catalytic moieties, including manganese porphyrins,253,254 bis(oxazoline) copper complexes,304 305 tertiary amines,306 binaphthol titanium complexes,285 307 titanium taddolates,292,308 thiazolio-cyclophanes,309 and fullerene-bound bisoxazoline copper complexes,310 have been incorporated at the core of dendritic molecules to determine the effect of dendritic encapsulation on their catalytic activity. 

The versatile oxazoline ring has been utilized in reactions with novolacs. Oxazolines polymerize readily to give thermoplastics of various molecular weights [see Eq. (10)]. Reaction of a bisoxazoline with a novolac gives a cross-linked system that has properties dependent on the relative amounts of bisoxazoline to novolac... 

Prop-2-ynylamides 108 and 109 have been carbonylated under oxidative conditions to give oxazolines 110 and bisoxazolines 111 bearing an (alkoxycarbonyl)methylene chain at the 5-position in good yields. The cyclization-alkoxycarbonylation process was carried out in alcoholic media at 50-70°C and under 24bar pressure of 3 1 CO/air in the presence of catalytic amounts of 10% Pd/C or Pdl, in conjunction with KI [62]. 

Although many different bisoxazolines and other semicorrin-type ligands have been prepared [53,54], the bis(ferf-butyl)oxazoline 11 is still the most versatile ligand for cyclopropanation. However, there are certain applications which give better results with other ligands. For the cyclopropanation of trisubstituted and 1,2-disubstituted (Z)-olefins, Lowenthal and Masamune found the bisoxa-zoline 12 to be superior to the bis(ferf-butyloxazoline) 11 [56]. This is illustrated by the reaction of 2,5-dimethyl-2,4-hexadiene leading to chrysanthemates (Scheme 9). Again, the best diastereo- and enantioselectivities were obtained with bulky diazoacetates. Both the trans/cis ratios and ees were similar to those reported for Aratani s catalyst (Scheme 5). 

The palladium-catalysed Suzuki coupling reaction of aryl halides with arylboronic acids has proved to be a general and convenient synthetic tool employed in organic chemistry to prepare biaryl compounds.[1] The discovery and development of active and efficient palladium-catalyst systems have been the focus of great interest recently. New catalytic systems based on paUadium-oxazolines, such as 2-aryl oxazolines and 2,2 -(l,3-phenylene)bisoxazoline (Figure 4.1) have been developed for the coupling reaction. These catalytic systems have the potential to overcome... 

Some new combinations of chiral ligands with different Lewis acids have been lately evaluated in catalytic asymmetric 1,3-DC reactions of nitrones. When the complex derived from copper(II) triflate and bis(oxazoline) 72 was used as chiral catalyst in the cycloaddition of nitrone 66 and crotonate 68, both endo and exo isomers were obtained with very high enantioselectivities (7 3 dr > 99% ee). In this reaction, the presence of molecular sieves 4 A (MS) was crucial as in their absence the nitrone decomposed and almost no cycloadduct was obtained <04TL9581>. Sibi et al. found that square planar complexes derived from copper triflate and some chiral bisoxazolines favour the COZ-exo approach in the 1,3-DC of nitrone... 

Allylpalladium complexes with BOX-type ligands and glucopyrano-oxazoline-palladium catalysts were used as catalysts for enantioselective allylic substitution (277). A chiral bisoxazoline ligand (BOXBZ) developed by Pfaltz has been used for asymmetric carbo- and heteroannulation reactions (278). An axial binaphthyl-based ligand possessing oxazolyl substituents (BOXAX) was developed by Hayashi and co-workers and successfully applied for the asymmetric Wacker-type cyclization (279). 

An enantioselective process of this reaction has been developed by replacing the phenanthroline ligand with a chiral bisoxazoline or pyridine-oxazoline ligand (30,47,50). For example, the reaction of diene 35 catalyzed by palladium... 

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