Acetate esters cleaved

Treatment of (89) with lead tetraacetate generates the unstable open-ring aldehyde (90) which is quickly converted to a dimethylacetal (91). Following basic hydrolysis of the methyl ester and acetates, the acetal is cleaved with aqueous acid to produce TxB2. A number of other approaches, including one starting from the Corey aldehyde, have been described (58). 

C1O3/ACOH, 25°, 50% yield, [- ROCOPh (- ROH + PhC02H)]. This method was used to remove benzyl ethers from carbohydrates that contain functional groups sensitive to catalytic hydiogenation or dissolving metals. Esters are stable, but glycosides or acetals are cleaved. 

The benzoylformate ester can be prepared from the 3 -hydroxy group in a deoxy-ribonucleotide by reaction with benzoyl chloroformate (anh. Pyr, 20°, 12 h, 86% yield) it is cleaved by aqueous pyridine (20°, 12 h, 31% yield), conditions that do not cleave an acetate ester. ... 

The adamantoate ester is formed selectively from a primary hydroxyl group (e.g., from the 5 -OH in a ribonucleoside) by reaction with adamantoyl chloride, Pyr (20°, 16 h). It is cleaved by alkaline hydrolysis (0.25 N NaOH, 20 min), but is stable to milder alkaline hydrolysis (e.g., NH3, MeOH), conditions that cleave an acetate ester. ... 

RuCl3-nH20, /-BuOH, PhH, 1 h, rt, 46-86% yield. In this case, the acetal is cleaved with simultaneous oxidation to an ethylene glycol ester. 

Related and equally important reactions are the acetoacetic ester synthesis and the eyanoaeetie ester synthesis Here too the initial substituted product can be hydrolyzed and decarboxylated, to yield a ketone 11 (i.e. a substituted acetone) from acetoacetic ester 10, and a substituted acetonitrile 14 from eyanoaeetie ester 13 respectively. Furthermore a substituted acetoacetic ester can be cleaved into a substituted acetic ester 12 and acetate by treatment with strong alkali ... 

The elaboration of 113 to (—)-kinamycins C, F, and J, is shown in Scheme 3.19. To access ( )-kinamycin C (3), the silyl ether function of 113 was cleaved with aqueous hydrochloric acid (95 %). Alternatively, treatment of 113 with lithium hydroxide served to liberate the phenol function and saponify the three acetate esters, to provide ( )-kinamycin F (6) in 92 % yield. Finally, acylation of the tertiary hydroxyl of 113 (acetic anhydride, triethylamine) afforded a tetraacetate. Cleavage of the silyl ether then provided ( )-kinamycin J (10) in 80 % over two steps. 

Trost first introduced the di-fe/7-butylsilylene derivative as a means for protecting 1,2- and 1,3-diols during a synthesis of PiUaromycinone derivatives.213 Di-ferf-butylsilylene derivatives are not as robust as isopropylidene or benzylidene acetals and their use is best reserved for systems requiring deprotection under very mild conditions. Di-isopropylsiiylene derivatives are occasionally used but they usually only survive in highly crowded environments.214 Di-feri-butylsily-lene derivatives survive hydroboration with 9-BBN, mild oxidation (e g the Dess-Martin, ozone), Lewis acids such as trifluoroborane e the rate and titanium tetrachloride, mild acids (pyridinium p-toluenesulfonate). camphorsulfonic acid, strong bases such as feri-butyllithium (THF, -50 °C), DDQ, and sodium meth-oxide in methanol at 0 C — conditions used to cleave acetate esters. 

The lability of esters can be easily enhanced by substituting the a-position with electron-with drawing groups. Thus, me thoxy acetate esters hydrolyse approximately 20 times faster than simple acetates and phenoxyacetates hydrolyse 50 times faster.646 Evans and co-workers used ammonia in methanol to remove a me thoxy acetate in the presence of two secondary acetate functions and two TES ethers from intermediate 341.1 en route to the marine antitumour agent Altohyrtin C [Scheme 4.341 J.647 648 Similarly, they used sodium carbonate in methanol to cleave a secondary methoxyacetate in the presence of two methyl... 

Acetal esters such as methoxymethyl (Mom),P9 (2-methoxyethoxy)methyl (Mem),t l benzyloxymethyl (Bom),P l tetrahydropyranyl,P l 2-(trimethylsilyl)ethoxymethyl (SEM)t l and (methylsulfanyl)methyl (hltm) are sensitive to noild aqueous acids in THE as well as to mild Lewis acids such as magnesium bromide in diethyl ether (2 equiv, rt, l-24h).t l In addition. Bom esters may be cleaved by catalytic hydrogenation.P l In the absence of acid, acetal esters display reactivities typical for unhindered alkyl esters. Surprisingly, these esters... 

Vicinally tri- or tetramethoxy compounds may also be cleaved anodically. Thus 1,1,2-trimethoxycyclohexane obtained on electrooxidation of cyclohexanone enol ether may be oxidized to the acetal ester, (CH30)2CH(CH2)4 COOCH3 [34]. 

Flint The excess Wittig reagent cleaves the acetate ester. 

Winterfeldf s synthesis of brefeldin A (Scheme 1.36) also entailed use of a Michael addition in an anti fashion to a 4-substituted cyclopentenone. Dithiane anion 217 was added in a 1,4 sense to enone 216 to give, after elimination of acetic acid, cyclopentenone 218. Addition of cuprate 219 to 218 in the expected anti manner cleanly afforded the trans cyclopentane 220. Hydride reduction gave the unnatural P-alcohol, which was inverted by a Mitsunobu reaction and the resulting ester cleaved to the desired a-alcohol 221. Protection of the alcohol... 

The reduction step was stereoselective with either Li A1H4 or LiBH4. In both cases, the hydride reducing agent first cleaved the acetate ester and then an intermediate ate complex transferred hydride ion intramolecularly in a cis fashion 235 to the unsaturated nitrile to give the correct stereochemistry 236. Finally cyclisation of the alcohol onto the nitrile to give the trans fused lactone 227 occurred in much the same way that nitriles are converted directly to ethyl esters with ethanol in acid solution. The overall yield of 227 was a respectable 40%. 

In steroids, the carbonyl group at C-3 is generally the most reactive and that on C-ll the least reactive enabling chemoselective reductions of polyoxosteroids5. The following example demonstrates this chemoselectivity, as well as excess formation of the thermodynamically more stable alcohol by the Meerwein-Ponndorf-Verley reduction of a 7,11-diketone 21l82. The acetic ester is cleaved under these conditions. 

---