Cleavage of enol ethers

NOC1 and an alcohol 9-9 From ozonides 9-10 Oxidative cleavage of enol ethers 9-13 Reaction between carboxylic acids and lead tetraacetate 9-18 Oxidation of ethers 9-22 Oxidation of primary alcohols or aldehydes... 

Dipolar cycloaddition reactions are most commonly applied for the synthesis of five-membered heterocyclic compounds.86 87 [3+2] cycloaddition reactions of transition-metal propargyl complexes have been reviewed.88 Addition of diazomethane to carbene complexes (CO)5Cr= C(OEt)R results in cleavage of the M = C bond with formation of enol ethers H2C = C(OEt)R,3 89 but (l-alkynyl)carbene complexes undergo 1,3-dipolar cycloaddition reactions at the M = C as well as at the C=C bond. Compound lb (M = W, R = Ph) affords a mixture of pyrazole derivatives 61 and 62 with 1 eq diazomethane,90 but compound 62 is obtained as sole... 

The anodic oxidation of enol ethers in methanol yields a-methoxylated carbonyl conqKxinds, which are useful intermediates for the synthesis of carbonyl compounds utilizing the technique of oxidative cleavage of glycols (equation 4S). ... 

For some functionalized alkenylboranes, cleavage with methanol may be preferable to cleavage with a carboxylic acid. Relatively unhindered alkenyldialkylboranes, such as 9-alkenyl-9-BBNs, are readily cleaved by 1 equiv. of methanol under gentle heating. More hindered derivatives may require catalysis by 2,2-dimethylpropanoic acid, but these mild, almost neutral conditions may be beneficial for acid-labile functionalities. For example, methanolysis has been used for production of enol ethers e.g. equation 61). ... 

Cleavage of enol and dienol methyl ethers.2 Treatment of these, substrates in aqueous acetonitrile at 20° for 5 minutes gives the aldehydes or ketones in quantitative yield after aqueous worjt-up or treatment with alumina. 

Some of the early uses [13] of MCPBA indicate the initial objectives that one would consider when evaluating new potential substitutes. The epoxidation of alkenes such as 1-octene (Eq. 1) and ethyl crotonate (Eq. 2), and the epoxidation of enol ethers followed by cleavage (Eq. 3) [14] are illustrative. 

Oxidation of enol ethers. The procedure of Sheng and Zajacek (2, 287) when applied to 5,6,7,8-tetrahydrochromane (1) leads to cleavage of the double bond to give a dicarbonyl compound, 6-ketononanolide (2), in 50% yield.1 The reaction... 

Substitution of an amino for the alkoxyl group of simple aliphatic ethers has little preparative importance the ether linkage must be weakened by a carbonyl, carboxyl, or nitrile group in the -position if the reaction is to occur under not too drastic conditions. Certain /8-methoxy ketones of type (17) can be converted into / -amino ketones below 100°.1081 Cleavage of the ether group in activated enol ethers is still easier for instance, 2-(acetoxymethylene)-... 

Enantioselective protonation. Cleavage of enol silyl ethers and ketene bis(tri-alkylsilyl) acetals by the complex leads to chiral ketones and esters. 

Ether cleavage. Silica gel on which oxalic acid is deposited hydrolyzes enol ethers. This reagent is useful for converting 3-methoxy-2,5(10)-diene steroids (Birch reduction products of A-aromatic steroids) into the corresponding 5(10)-en-3-ones. For the cleavage of other ethers in methanol, the silica-alumina gel (prepared by the sol-gel method) is very valuable. The ease of deprotection follows the order TMS > 1 -methyl-1 -methoxyethyl 1 -ethoxyethyl > THP methoxymethyl. 

Cleavage of allyl ethers (1, 300 2, 158 3, 246-247 5, 736). Allyl ethers are cleaved when heated at 60-80 with Pd on activated charcoal in the presence of an acid ( >-TsOH) to effect isomerization to an enol ether. ... 

Cleavage of enol trimethylsilyl ethers or enol acetates by methyllithium (entries 1 and 2, Scheme 1.3) as a route to specific enolate formation is limited by the availability of these materials. Preparation of the enol trimethylsilyl ethers and enol acetates from the corresponding ketones usually affords a mixture of the two possible derivatives, which must be then separated. It is sometimes possible to find conditions that favor the formation of one isomer for example, reaction of 2-methyl-cyclohexanone with lithium diisopropylamide and trimethylchlorosilane affords the less highly substituted enol ether preferentially by 99 1 over the more highly substituted one (kinetically controlled conditions). ... 

We investigated an interaction of 2-acyl( clohexane-l,3-diones la-c containing a perfluoroalkylated side chain with o-phenylenediamine. A direct reaction of the latter with o-phenylenediamine produees a mixture of acid cleavage products, as reported for their aeyelie analogues [7], As an alternative approach to the synthesis of perfluoroalkylated dibenzo[b,e][l,4]diazepinones, an interaction of enol ethers of 2-perfluoroalkanoylcyclohexane-l,3-diones la- c with o-phenylenediamine is proposed, beeause of an advanced reactivity of enol ethers to nucleophilic reagents versus the initial cyclic P,P -triketones [8],... 

Alkali Metal Enolates by Cleavage of Enol Acetates or Silyl Enol Ethers... 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

---