Protection zinc-acetic acid

Hydrolysis of 2-haloethyl esters. 2,2,2-Trichloroethyl esters have been used for protection of carboxyl groups because they are readily cleaved by zinc-acetic acid at 0° (3, 295-296). Ho has reported that 2-haloethyl esters can be cleaved by sodium trithiocarbonate in refluxing acetonitrUe in 75-85% yield. The suggested mechanism is formulated. ... 

Hydrogen per oxide/formic acid-zinc acetic acid Oxidative degradation of spirostanes with protection of carbon-carbon double bonds as 1,2-formoxychlorides... 

C—C double bonds may be protected against electrophiles by epoxidation and subsequent removal of the oxygen atom by treatment with zinc and sodium iodide in acetic acid (J.A. Edwards, 1972 W. Kndll, 1975). Halogenation has often been used for protection, too. The C—C double bond is here also easily regenerated with zinc (see p. 138, D.H.R. Barton, 1976). 

Trichloro- and 2,2,2-tribromoethoxycarbonyl (Tceoc and Tbeoc) protecting groups are introduced with the commercially available 2,2,2-trihaloethyl chloroformates. These derivatives are stable towards CrOj and acids, but can smoothly be cleaved by reduction with zinc in acetic acid at 20 °C to yield 1,1-dihaloethene and CO. Several examples in lipid (F.R. Pfeiffer, 1968, 1970) and nucleotide syntheses (A.F. Cook, 1968) have been described. 

The blocking and deblocking of carboxyl groups occurs by reactions similar to those described for hydroxyl and amino groups. The most important protected derivatives are /-butyl, benzyl, and methyl esters. These may be cleaved in this order by trifluoroacetic acid, hydrogenolysis, and strong acid or base (J.F.W. McOmie, 1973). 2,2,2-Trihaloethyl esters are cleaved electro-lytically (M.F. Semmelhack, 1972) or by zinc in acetic acid like the Tbeoc- and Tceoc-protected hydroxyl and amino groups. 

Many carbamates have been used as protective groups. They are arranged in this chapter in order of increasing complexity of stmcture. The most useful compounds do not necessarily have the simplest stmctures, but are /-butyl (BOC), readily cleaved by acidic hydrolysis benzyl (Cbz or Z), cleaved by catalytic hy-drogenolysis 2,4-dichlorobenzyl, stable to the acid-catalyzed hydrolysis of benzyl and /-butyl carbamates 2-(biphenylyl)isopropyl, cleaved more easily than /-butyl carbamate by dilute acetic acid 9-fluorenylmethyl, cleaved by /3-elimination with base isonicotinyl, cleaved by reduction with zinc in acetic acid 1-adamantyl, readily cleaved by trifluoroacetic acid and ally], readily cleaved by Pd-catalyzed isomerisation. 

Dibromo adducts have been used for many years for purifying olefins. Reduction with zinc or iodide has been the traditional way of regenerating the olefin, although occasionally other reagents, ferrous chloride, for example, have been employed. Zinc is usually effective and may be used in acetic acid in favorable cases. Milder conditions are necessary if sensitive groups are present (ref. 185, p. 51). A well known application of the protective function... 

A carboxylic acid (not the salt) can be the nucleophile if F is present. Mesylates are readily displaced, for example, by benzoic acid/CsF. Dihalides have been converted to diesters by this method. A COOH group can be conveniently protected by reaction of its ion with a phenacyl bromide (ArCOCH2Br). The resulting ester is easily cleaved when desired with zinc and acetic acid. Dialkyl carbonates can be prepared without phosgene (see 10-21) by phase-transfer catalyzed treatment of primary alkyl halides with dry KHCO3 and K2C03- ... 

The commercial process for the production of vinyl acetate monomer (VAM) has evolved over the years. In the 1930s, Wacker developed a process based upon the gas-phase conversion of acetylene and acetic acid over a zinc acetate carbon-supported catalyst. This chemistry and process eventually gave way in the late 1960s to a more economically favorable gas-phase conversion of ethylene and acetic acid over a palladium-based silica-supported catalyst. Today, most of the world s vinyl acetate is derived from the ethylene-based process. The end uses of vinyl acetate are diverse and range from die protective laminate film used in automotive safety glass to polymer-based paints and adhesives. 

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