Acetal exchange reactions

Alkynyl anions are more stable = 22) than the more saturated alkyl or alkenyl anions (p/Tj = 40-45). They may be obtained directly from terminal acetylenes by treatment with strong base, e.g. sodium amide (pA, of NH 35). Frequently magnesium acetylides are made in proton-metal exchange reactions with more reactive Grignard reagents. Copper and mercury acetylides are formed directly from the corresponding metal acetates and acetylenes under neutral conditions (G.E. Coates, 1977 R.P. Houghton, 1979). 

The rather unreactive chlorine of vinyl chloride can be displaced with nucleophiles by the catalytic action of PdCb. The conversion of vinyl chloride to vinyl acetate (797) has been studied extensively from an industrial standpoint[665 671]. DMF is a good solvent. 1,2-Diacetoxyethylene (798) is obtained from dichloroethylene[672]. The exchange reaction suffers steric hindrance. The alkenyl chloride 799 is displaced with an acetoxy group whereas 800 and 801 cannot be displaccd[673,674]. Similarly, exchange reactions of vinyl chloride with alcohols and amines have been carried out[668]. 

Carbonates. Basic zirconium carbonate [37356-18-6] is produced in a two-step process in which zirconium is precipitated as a basic sulfate from an oxychloride solution. The carbonate is formed by an exchange reaction between a water slurry of basic zirconium sulfate and sodium carbonate or ammonium carbonate at 80°C (203). The particulate product is easily filtered. Freshly precipitated zirconium hydroxide, dispersed in water under carbon dioxide in a pressure vessel at ca 200—300 kPa (2—3 atm), absorbs carbon dioxide to form the basic zirconium carbonate (204). Washed free of other anions, it can be dissolved in organic acids such as lactic, acetic, citric, oxaUc, and tartaric to form zirconium oxy salts of these acids. 

The earliest attempts to prepare deuterated steroids were carried out by exchange reactions of aliphatic hydrogens with deuterium in the presence of a surface catalyst. Cholesterol, for example, has been treated with platinum in a mixture of deuterium oxide and acetic acid-OD, and was found to yield... 

Selectivity in formation of protective groups may also be achieved by a proper choice of reaction conditions and catalyst. Thus formation of the 3-monothioketal from 3,6-diketones is achieved by dilution of the ethane-dithiol-boron trifluoride reaction mixture with acetic acid. 3-Monocyanohydrins are obtained in good yield from 3,20-diketo-(5a)-pregnanes by diluting the exchange reaction with ethanol. Similarly, dilution of the... 

However, 17a,21-acetonides (103), as well as acetals of other ketones or aldehydes, can be easily prepared by acid-catalyzed exchange reaction with dimethoxypropane or other alkyl acetals in dimethylformamide or benzene. Enol etherification of the A -S-ketone also occurs with the former procedure. 

Hence, enol esters such as isopropenyl acetate are good acylating agents for alcohols. Isopropenyl acetate can also be used to convert other ketones to the corresponding enol acetates in an exchange reaction ... 

Enol esters can also be prepared in the opposite type of exchange reaction, catalyzed by mercuric acetate or Pd(II) chloride, for example. 

The 2+2 cycloadditions of benzyne to cis- and trani-propenyl ether gave cis- and fran -benzocyclobntanes as the main products, respectively [ 117,118], Stereospecific [2+2] cycloaddition reactions were observed between the benzyne species generated by the halogen-Uthium exchange reaction of ort/io-haloaryl triflates and the ketene silyl acetals (Scheme 23) [119],... 

Monoalkylthallium(III) compounds are unstable (73, 79), and very few examples of this class have been isolated. A number of alkylthallium diacetates have been obtained either from oxythallation of olefins with thallium-(III) acetate (see below) or from exchange reactions such as that shown in Eq. (11) (74, 75). Only four alkylthallium dihalides have been isolated so far, namely a neopentylthallium dihalide (60) [Eq. (12)] and the isomeric 2-, 3-, and 4-pyridiomethylthallium dichlorides (20) [Eq. (13)]. Monoaryl-and monovinylthallium(III) derivatives are considerably more stable than... 

Oxidation of the steroidal olefin (XXVII) with thallium(III) acetate gives mainly the allylic acetates (XXXI)-(XXXIII) (Scheme 15), again indicating that trans oxythallation is the preferred reaction course (19). Addition of the electrophile takes place from the less-hindered a-side of the molecule to give the thallinium ion (XXVIII), which by loss of a proton from C-4 would give the alkylthallium diacetate (XXIX). Decomposition of this intermediate by a Type 5 process is probably favorable, as it leads to the resonance-stabilized allylic carbonium ion (XXX), from which the observed products can be derived. Evidence in support of the decomposition process shown in Scheme 15 has been obtained from a study of the exchange reaction between frawr-crotylmercuric acetate and thallium(III) acetate in acetic acid (Scheme 16) (142). 

Baker et al, have obtained some kinetic data on the exchange reactions between the 1,10-phenanthroline, 2,2 -dipyridine and 2,2, 2"-tripyridine complexes of Co(III) and Co(ll) in both H2O and D2O solution. The isotopic method ( Co) and separations involving either extraction of Co(II) with organic solvents (n-hexanol-ether or saturated sodium acetate in n-hexanol) or precipitation of Co(III) as Co(phen)3(l3)3 were used. For the 1,10-phenanthroline system, a rate law... 

A. Claisen Rerrangements of Ketene Aminats and Imidates. A reaction that is related to the orthoester Claisen rearrangement utilizes an amide acetal, such as dimethylacetamide dimethyl acetal, in the exchange reaction with allylic alcohols.257 The products are y, 8-unsaturated amides. The stereochemistry of the reaction is analogous to the other variants of the Claisen rearrangement.258... 

The reactions are accelerated by bromide salts, which are thought to exchange for acetate in the ir-allylic complex. The reactions of acyclic compounds occur with minimal E Z isomerization. This result implies that the TT-allyl intermediate is captured by carbonylation faster than E Z isomerization occurs. 

The effectiveness of zinc is related to its associated anion (Table 7) the chloride, acetate, or benzoate salts were the most effective. The maximum effectiveness of the metal would be expected if the exchange reaction with the pigment surface was... 

Methoxymethylation of alcohols is generally achieved through alkylation with chloromethyl methyl ether. The procedure described here for the preparation of Bu3SnCH20CH20CH3 avoids the use of the highly toxic chloromethyl ether by employing an acid-catalyzed acetal exchange reaction with dimethoxymethane for the... 

A mixture of rhodium II) acetate (228 mg, 0.516 mmol), the imidazolidinone (1.70 g, 6.15 mmol), and dry chlorobenzene (20 mL) is heated under reflux for 18 h in a flask fitted with a Soxhlet extraction apparatus into which a thimble is placed containing an oven-dried mixture of sodium carbonate and sand (2 1, 5 g). The progress of the ligand-exchange reaction can be monitored by HPLC (p-Bondapak-CN column, methanol). The resulting blue solution is concentrated under reduced pressure, and the residue is purified by column chromatography (reversed phase silica, Bakerbond Cyano 40 mm prep. LC packing, methanol). 

Vinyl ethers are important raw materials in the production of glutaraldehyde, as well as of vinyl polymer materials which contain oxygen and are expected to degrade easily in Nature. The [IrCl(cod)]2 catalyzes an efficient exchange reaction between vinyl acetate 57 and alcohols or phenols 58, leading to the corresponding vinyl ethers 59 (Equation 10.11) [27]. Usually, the acid-catalyzed exchange reaction between alcohols and vinyl acetate results in alkyl acetates 60, and also to vinyl alcohol 61 which is readily isomerized to acetaldehyde 62. 

Telluro(seleno)ketene acetals have been synthesized by the Al/Te exchange reaction, as shown in the following scheme, starting from hydroalumination of acetylenic selenides. ... 

Treatment with water gives the corresponding Z tellurides. Reaction with BuSeBr, iodine and A -bromosuccinimide (NBS) gives respectively Te-Se ketene acetals, a-iodo- and a-bromo vinyl tellurides as mixtures of Z and E stereoisomers (in contrast with the total retention of configuration of the above-discussed Zr/Te exchange reactions). The acylation was effected with acylchlorides in the presence of Cul. ... 

However, the hemiaminal 17 is unstable as a free base and readily undergoes exchange reactions. Since the hydroxy moiety of 17 is more easily displaced than the amine moiety, a highly reactive cyclopropyliminium salt 18 is formed, which then reacts with weak nucleophiles such as ethanol, to give e. g., 19. Otherwise in water solution 17 can also probably eliminate ammonia to form the highly reactive cyclopropanone 20, which is in equilibrium with its hydrate 21 and hemi-acetal 22, Eq. (8) [20]. It has been reported that hydrate 21 is also a potent inhibitor of ALDH [20,21]. 

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