Potassium carbonate-18-Crown

Lithium hydride, 165, 240, 257 Methyllithium, 188, 315 Potassium bicarbonate, 253 Potassium carbonate, 233, 254, 322 Potassium carbonate-18-Crown-6, 94 Potassium fluoride-Alumina, 15, 254, 256... 

WITTIG-HORNER REACTION Lithium chlo-ride-Diisopropylethylamine. Potassium carbonate-18-crown-6. Tetramethylethyl-enediamine. 

Cyclization of substituted phenylacetylene sequences afforded functionalized macrocycles that were amenable to subsequent manipulation. For example, transesterification of 42 with octanol in the presence of 18-crown-6 ether and potassium carbonate gave the corresponding ester in 85% yield (Scheme 13). The ester functionalities could be reduced by DIBALH to give the hydroxymethyl-substituted macrocycle (43) in 61 % yield. The low yield of this particular transformation is attributed to mechanical losses during purification, due to the highly polar nature of the product. Macrocycle 43 could then be treated with alkyl bromides to give a group of benzyl ether derivatized PAMs. 

Methyl 3,5-dihydroxybenzoate (25.39 g, 0.151 mol, Aldrich) and potassium carbonate (52.69 g, 0.381 mol) were placed into a 1 L round bottom flask with 200 mL of acetone and a magnetic stirrer. Benzyl bromide (55.04 g, 0.322 mol) was added and washed into the flask with an additional 150 mL of acetone, followed by the addition of a catalytic amount of 18-crown-6 (0.57 g, 2.2 mmol). 

The solubilization of sodium or potassium carbonate into apolar solvents such as benzene or acetonitrile with the aid of 18-crown-6 [3] generates a powerful base that has been used for a variety of preparative transformations (Fedorynski et al., 1978). Mechanistic studies have not been reported thus far. 

Dialkyl N-(benzoxazin-4-yl)methylenemalonates and their optically active forms (1728) were prepared in the reaction of the appropriate pheny-laminomethylenemalonate (1727), triphenylphosphine, and diethyl azodi-carboxylate in THF at -20°C (89EUP3228I5). The hydroxyl group of racemic and optically active phenylaminomethylenemalonates (1727) were tosylated with p-toluenesulfonyl chloride in pyridine, and the products were cyclized by heating in DMF at 80°C in the presence of potassium carbonate and a catalytic amount of 18-crown-6-ether to give 1728 (89EUP322815). 

Reactions.—Aldehydes. The stereochemistry of the alkene produced from ylides generated by using 18-crown-6 complexes of potassium carbonate or butoxide, depends upon the solvent used. In THF typical salt free distributions are obtained whereas in dichloromethane reversal of product distributions is observed.17 A simplified method (Scheme 4) for preparing para-substituted styrenes in high... 

Compound 8 can be reacted with substituted iodoarenes using copper metal and potassium carbonate <1980CB358> or potassium hydroxide <1994MCL(242)127> as a base. The reaction proceeds at 170-180°C and gives good yields. Addition of crown ether 18-Cr-6 allows a lower temperature to be used <2000JCD2105>. 

The construction of the oiganometallic dendron required two synthetic transformations. Selective alkylations of the phenolic hydroxyl groups in the presence of potassium carbonate and I8-crown-6 afforded the ether dimetallic derivative 35. This first generation benzyl alcohol 35 was converted to the benzyl bromide 36 by... 

Sodium 4-hydroxybenzenesulfonate dihydrate (2.37 g, 0.01 mol) was dehydrated by distillation with benzene using Dean-Stark apparatus and then dissolved in 20 mL of acetone in a lOOmL round-bottomed flask with a reflux condenser. Ethyl bromoacetate (2.04 g, 0.012 mol), potassium carbonate (2.79 g, 0.02 mol) and dibenzo-18-crown-6 (74 mg, 0.2 mmol) were added, and the mixture was heated at reflux for 48 h. After cooling to room temperature, the crystals were collected on a Buchner funnel, washed with acetone (2 x 60 mL), and dried under reduced pressure to yield 2 as a white powder (2.68 g, 95 %). m.p. > 300 °C. 

On treatment with a potassium fluoridc-crown ether complex, alkyl 1,2,2,2-tetrachloroethyl carbonates are cleaved at the carbonyl group-oxygen bond to give high yields of alkyl fluoroformates [ 5] (equation 25)... 

Treatment of a hydroxy carbaldehyde precursor with chloroacetone in refluxing THF in the presence of potassium carbonate and 18-crown-6 resulted in the formation of the 2-acetyl furo-chromone (52) in 53% yield (Equation (25)) <84TL2953>. 

Hexaza 18-crown-6 151 (7.8%) and 152 (8.0%) have also been synthesized by treatment of 2,6-fc(chloromethyl)pyridine with jym-dimethylethylenediamine or piperazine in dimethylformamide, utilizing potassium carbonate as the base. The cobalt(II) and copper(II) complexes 153 of 151 have been made and the single crystal X-ray structure determination of each shows that in both cases the metal ion is octahedrally coordinated by the six nitrogen atoms of the ligand. In the case of... 

A synthesis of precursors for trehalostatin analogues from 2-iodo-5-formyl-1,5-lactone has been reported (Scheme 22).94 Reaction of the iodoaldehyde 17395 with potassium fluoride in acetonitrile in the presence of 18-crown-6 gave 174 in 81% yield, together with two other isomers in low yield. Reaction of 174 with potassium carbonate in methanol at -20 °C gave the iododiol 175 in quantitative yield. 

Solutions of 4//-furo- and 4//-thieno[3,2-3]pyrrole derivatives 89 in dioxane were alkylated with chloroacetone or bromoacetophenone under phase-transfer conditions, in the presence of potassium carbonate and 18-crown-6, to afford the kero esters 90 in good yields (60-85%) (Equation 14) <2005EJ04670>. 

Aryl ethers. Phenols are converted into methyl ethers by reaction with 1 in the presence of potassium carbonate and 18-crown-6 at 150°. The reaction involves liberation of chloroform and carbon dioxide (equation 1). If the reaction is conducted in the presence of an alkyl halide, alkyl aryl ethers are formed in generally good yield (equation 11). 

Yield increases in a similar order of magnitude as in the synthesis of the lower homologues were achieved also in the preparation of hexathia[18]crown-6 ( 18S6 , 72) by the cesium effect . Cooper et al. [70] obtained 72 from the dithiol 70 and the dichloro compound 71 in a one-component dilution principle reaction [1] in a yield of 76% using a suspension of cesium carbonate in DMF. When the reaction was carried out under analogous conditions with potassium carbonate, 72 was isolated in a much lower yield of only 38% [70]. 

Z)-OL, -Unsaturated esters,l Wittig-Homer reactions generally show a preference for formation of (E)-alkenes. Thus (E)-a,p-unsaturated esters are obtained preferentially on reaction of aldehydes with trimethyl phosphonoacetate under usual conditions (potassium f-butoxide). Use of a highly dissociated base can favor (Z)-selectivity. The most effective base for this purpose is potassium hexamethyldisilazide, KN[Si(CH3)3]2, in combination with 18-crown-6, although even potassium carbonate with the crown ether is fairly effective. The (Z)-selectivity can be further enhanced by use of 1 as the phos-phonoester. Under these conditions, (Z)-unsaturated esters can be prepared from aliphatic and aromatic aldehydes with Z/E ratios as high as 50 1. The method is also useful for transformation of unsaturated aldehydes to (E,Z)-dienoates and (E,E,Z)-trienoates. 

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