Acetals conditions

The use of TMOF as a solvent provides strong acetalizing conditions (323 330). This allows the generation of enol ether 331, which on electrophilic attack of hypervalent iodine species [PhI(OMe)2] (83IC1563) gives intermediate 332. Nucleophilic attack of the solvent at the C(4)-position of 332, followed by migration of ring A, results in the formation of 326. The minor product 327 is resulted by a Sn2 attack of methanol at the C(3)-position of 333 (Scheme 85). 

Substituted succinic esters are formed coincidentally with the meth-oxy esters, although in relatively smaller amounts. ds-2-Butene gives a mixture of diesters, meso and d,1-pair, while trans-2-butene yields only the d,l enantiomers. By the addition of sodium acetate, conditions which preclude prior isomerization of the olefins are obtained the diesters are the exclusive products, and the dicarbonylation is stereospecifically cis. The carbonylation of trans-2-butene, which is at least seven times faster than that of the cis isomer, gives exclusively the d,l-succinic ester while m-2-butene gives only the meso diastereomer. These results are consistent with the mechanism shown for cw-2-butene (19) (see top of next page). 

Several aspects of this mechanism deserve comment. The cis-trans isomerization reaction appears to be acid catalyzed as isomerization does not occur in the early stages of the reaction or under buffered (sodium acetate) conditions. The threo-fi-methoxy ester could be readily obtained from the carbonylation of the intermediate product I resulting from trans... 

Fig. 5.1. Separation of chlorophenol acetates (2-0.02 ng). Peaks 1 = 2-chloro 2 = 3-chloro 3 = 4-chloro 4 = 2,6-dichloro 5 = 2,5-dichloro 6 = 2,4-dichloio 7 = 3,4-dichloro 8 = 2,3-dichIoro 9 = 3,5-dichloro 10 = 2,4,6-trichloro 11 = 2,4,5-trichloro 12 = 2,3,4,6-tetrachloro 13 = pentachloro-phenol acetates. Conditions Pyrex glass column (25 m X 0.35 mm I.D.), dynamically coated with SE-30 temperature programme, 3°C/min (95-180°C) helium flow-rate, 2-3 ml/min splitting flow-rate, 0-60 ml/min. (Reproduced from J. Chromatogr. 131 (1977) 412.)...

A remarkable ring enlargement reaction was observed when the diketone, IX/17, (Scheme IX/3) was kept under acetalization conditions (BF3-etherate/ ethyleneglycol) [7]. The seven-membered IX/201 was isolated in 98% yield. Probably this reaction is restricted to five- to seven-membered ring enlarge-ment2). 

Of the possible acetals, benzylidene and isopropylidene have been utilized most frequently in carbohydrate chemistry for simultaneous protection of two hydroxyls. They are introduced by standard acetalization conditions that is, either the aldehyde or the dimethoxy acetal is employed as reagent together with some acid catalyst (Scheme 3.4) [9]. With isopropylidene acetals, the methoxy propenyl ether has been utilized to provide the kinetic product, and formation of benzylidene acetals has been achieved under basic conditions using a,a-dibromotoluene in refluxing pyridine [10]. 

Then, after purification by crystallization, the mixed acetal is cleaved under acetic conditions to provide the hemiacetal 16. 

The condensation of fluorinated 3-oxoesters or 1,3-diketones with benzaldehyde and urea or thiourea result in the diastereoselective formation of 4-fluoroalkyl-4-hydroxy-2-oxo(thioxo)-6-phenylhexahydropyrimidine-5-carboxylates from which by dehydration under acetic conditions the corresponding 6-fluoroalkyl-2-oxo(thioxo)-4-phenyl-l,2,3,4-tetrahydropyrimidine-5-carboxylates were obtained (01ZOR915, 00JFC(103)17) (Scheme 55). 

S, A. Stem and 5. S. Kulknmi, Solubility of Methane in Cellulose Acetate—Conditioning Effect... 

Independently, Harwood also demonstrated the role of chiral-templated isomunchnones in 1,3-dipolar cycloaddition reactions. Thus using the rhodium(II)-catalyzed decomposition of diazo carbonyl compounds, Harwood and co-workers explored cycloadditions of isomunchnone derivatives of (5R)- and (55)-phenyloxazin-2,3-dione. Along with the work of Padwa (vide supra), these reactions appear to represent the first examples of chiraUy templated isomunctmone 1,3-dipolar cycloadditions. For example, reaction of 471 under standard rhodium acetate conditions in the presence of NPM affords a mixture of endo-472 and exo-473 adducts (Fig. 4.146). A-Methylmaleimide and DMAD react with 471 similarly. 

Unexpectedly, the 3,4,6-tri-O-benzyl-D-glucal gave only the methyl a-glycoside 121 under the hydroformylation-acetalization conditions. In fact, there are two electrophilic reagents in competition for the nucleophilic alkene, the rhodium complex and the proton. When the alkene is deactivated (R=Ac) the coordination of rhodium is preferred and the hydroformylation-acetalization takes place. But when it is not deactivated (R=Bn) the acidic proton reacts faster than rhodium and methanol is added. 

Ketones that react slowly under normal acetalization conditions can be converted into dioxolanes at high pressure (eq 12). A procedure has also been developed for the dioxolanation of ketones in the presence of aldehydes using a bis-silyl ether of ethylene glycol (eq 13). The selective acetalization of aldehydes in the presence of ketones has been accomplished using ethylene glycol and alumina or silica gel. ... 

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