Lewis hydroxyl function

In the chlorination of 2,4-dichlorophenol it has been found that traces of amine (23), onium salts (24), or triphenylphosphine oxide (25) are excellent catalysts to further chlorination by chlorine ia the ortho position with respect to the hydroxyl function. During chlorination (80°C, without solvent) these catalysts cause traces of 2,4,5-trichlorophenol ( 500 1000 ppm) to be transformed iato tetrachlorophenol. Thus these techniques leave no 2,4,5-trichlorophenol ia the final product, yielding a 2,4,6-trichlorophenol of outstanding quaUty. The possibiUty of chlorination usiag SO2CI2 ia the presence of Lewis catalysts has been discussed (26), but no mention is made of 2,4,5-trichlorophenol formation or content. 

A similar approach was used in the synthesis of the sialyl Lewis X mimetics of type 85 (Scheme 13.22).66 Protein crystallization,67 conformational studies of sLex in solution68 and in bound form to E- and P-selectin69 as well as the study of structure-function relationships70,71 gave information about the functional groups of the sLex-epitope essential for the binding to the selectins. Synthesized mimetics must contain the three essential hydroxyl functions of the fucose. Sialic acid, galactose, and... 

The multitude of hetero Diels-Alder reactions found in the literature clearly demonstrates the importance of this transformation. Thus, this type of cycloaddition is today one of the most important methods for the synthesis of heterocycles. Striking features of this method are the tremendous diversity, excellent efficiency especially in those cases where the reactive dienes and dienophiles are formed in situ, and high stereoselectivity in many cases. There is a broad scope and only little limitation. In recent years the use of Lewis acid, the development of diastereoselective and enantioselective reactions as well as the application of high pressure gave an enormous push. In addition, many of the obtained heterocycles can be transformed into acyclic compounds allowing the stereoselective preparation of e.g. amino and hydroxyl functionalized open chain compounds or even carbocycles to be of interest. Also, for the synthesis of natural products, the hetero Diels-Alder reaction is of great value. Since heterocycles,... 

Lewis acid templation between an epoxide and an adjacent hydroxyl function sets the stage for an intramolecular addition process using an alkyne as a nucleophile <1995TL7145>. Cyclization reactions wherein the regioselectivity of the addition reaction is dictated by a proximal hydroxyl function is the subject of a review <2000T8779>. 

Erythritol 1 is a tetraol with a central eryf/rro-conflgured diol group. In combination with the terminal hydroxyl functions there are several conformations to act as a chelate ligand. In most of its characterized solid-state stmctures, erythritol is coordinated to two central atoms by acting as a bis-l,2-diolato ligand, at which the deprotonation of the hydroxyl groups is a consequence of the synthesis from alkaline solutions and/or complexation of more or less Lewis acidic central atoms. A complex of this type is obtained upon the reaction of erythritol with two equivalents of [(NH3)2Pd(OH)2] in aqueous solution which leads to the centrosymmetric structure of 9 (O Fig. 1) [18],... 

A stronger Lewis acid such as Me2Sn " forces deprotonation of the a-hydroxyl function even at pH 5.5 (60). This coordination mode is also suggested for a copper(II)-D-gluconic acid complex [55]. In more alkaline solution (pH 8), the hydroxyl function at C4 is also deprotonated and Me2Sn is coordinated by the a-alkoxidocarboxylato moiety and 04 of tridentate... 

As mentioned in the previous section, nowadays, readily available and inexpensive cinchona alkaloids with pseudoenantiomeric forms, such as quinine and quinidine or cinchonine and cinchonidine, are among the most privileged chirality inducers in the area of asymmetric catalysis. The key feature responsible for their successful utility in catalysis is that they possess diverse chiral skeletons and are easily tunable for diverse types of reactions (Figure 1.2). The presence of the 1,2-aminoalcohol subunit containing the highly basic and bulky quinuclidine, which complements the proximal Lewis acidic hydroxyl function, is primarily responsible for their catalytic activity. 

The presence of the bidentate chelate in allylation reactions was supported by C NMR studies with the anti halide and phenylselenide substrates in the presence of MgBr2-OEt2 [5]. The introduction of a bulky protecting group such as tert-butyldimethylsilyl ether (TBS) on the hydroxyl function led to the syn allylated product (Scheme 2) [4] and was thus shown to prevent chelation of the bidentate Lewis acid in favor of monodentate complex formation. C NMR studies with TBS ether in the presence of MgBr2 OEt2 validated this finding [5]. 

Marschalk alkylation of unsymmetrical leuco compounds such as 83 under alkaline conditions occurs selectively at the 2-position [69,70], while reaction in the presence of piperidinium acetate (Lewis conditions) [71] occurs at the 3-position [70]. This allows regiospecific syntheses in which the regiochemistry of formation of ring A is controlled by the distant 4-hydroxyl function. This is... 

In order to improve the yield of hydroxyl-functionalized polymers, the effect of addition of THF was investigated. It is known that Lewis bases such as THF reduce or eliminate association of polymeric organolithium chain ends. If the formation of the polystyrene dimer is favored by association of PSLi chain ends, it was expected that the addition of Lewis base would minimize dimer formation. Upon the addition of THF ([THF]/[PSLi] > 15) to PSLi prior to addition of styrene... 

According to Scheme 5, if IL has hydroxyl-alkyl moiety, it can have high activity for CO2 cydoaddition without additives, because of Lewis acidic nature of the OH group. Sun et al. [32] synthesized such task-spedfic ILs as hydroxyl-functionalized imidazolium and ammonium salts (Scheme 7). Among them, l-(2-hydroxyethyl)-3-methylimidazolium bromide ([HEMIm]Br) had the highest activity. It gave almost quantitative yield for the PC synthesis at 120°C and CO2 pressure of 2 MPa for 1 h, while non-modified IL of [EMIm]Br... 

Chiral hydroxyl-functionalized allylic silanes were obtained by the reaction of various aldehydes and chiral double allylating reagent (11) with high enantioselec-tivities (up to 98% ee) and excellent E/Z ratio ( > 25 1 to > 30 1) using 1 equivalent of BP3 OEt2 as a Lewis acid promoter. The resulting chiral allylic silanes can be... 

The most difflcult pharmaceutically relevant oxidation of steroids is the introduction of a 14 -hydroxyl group. This functional group is found in heart-active steroids (cardenolides) such as digitoxigenin, which also contain a 17/J-butenolide substituent. The 14/ -hydroxyl group is easily cleaved off by dehydration and must therefore not be treated with Lewis or... 

Other Organolithium Compounds. Organoddithium compounds have utiHty in anionic polymerization of butadiene and styrene. The lithium chain ends can then be converted to useflil functional groups, eg, carboxyl, hydroxyl, etc (139). Lewis bases are requHed for solubdity in hydrocarbon solvents. 

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