Acetal selective

Generation of Alkylideneallyl Cations from Alkylidenecyclopropanone Acetals Selectivity of Reaction with Nucleophiles... 

Holm PK, Eker P, Sandvig K, van Deurs B. Phorbol myristate acetate selectively stimulates apical endocytosis via protein kinase C in polarized cells. Exp Cell Res 1995 217(1) 157-168. 

Methyl acetate is the principal by-product in the reductive carbonylation of methanol. As indicated in Table I, decreasing the Hp/CO increases the methyl acetate selectivity. In the limit of pure CO, methyl acetate is obtained in 90-95% selectivity. 

The spectral data for nitroso acetals arising from intramolecular nitronate cycloadditions mirror those of the previously presented nitroso acetals. Selected examples are collected in Tables 2.27 and 2.28, as well as in Figure 2.9 (85-87). 

Conjugate reduction. Silicon hydrides and a Pd(0) catalyst reductively cleave allylic acetates selectively (equation I).1... 

PS-PB-PS M = 89,17% CDCI3, toluene (non-selective) Dimethoxyethane (selective for PB) Ethyl acetate (selective for PS)... 

An important polyalcohol for regioselective acetalizations is the triol of Figure 9.18 because it can be easily obtained from S-malic acid. This substrate contains both a 1,2-diol and a 1,3-diol. Each of these subunits can be incorporated into an acetal selectively—depending on the carbonyl compound with which the acetalization is carried out ... 

The liquid phase processes resembled Wacker-Hoechst s acetaldehyde process, i.e., acetic acid solutions of PdCl2 and CuCl2 are used as catalysts. The water produced from the oxidation of Cu(I) to Cu(II) (Figure 27) forms acetaldehyde in a secondary reaction with ethylene. The ratio of acetaldehyde to vinyl acetate can be regulated by changing the operating conditions. The reaction takes place at 110-130°C and 30-40 bar. The vinyl acetate selectivity reaches 93% (based on acetic acid). The net selectivity to acetaldehyde and vinyl acetate is about 83% (based on ethylene), the by-products being CO2, formic acid, oxalic acid, butene and chlorinated compounds. The reaction solution is very corrosive, so that titanium must be used for many plant components. After a few years of operation, in 1969-1970 both ICI and Celanese shut down their plants due to corrosion and economic problems. 

UButylmethoxyphenylsilyl ethers (r-BMPSi ethers). In DMF in the presence of NfCjHj), this bromosilane reacts with primary, secondary, and tertiary alcohols to form silyl ethers in good yield, and also with some enolizable ketones to form enol silyl acetals. Selective silylation of primary alcohols is possible by use of CHjClj as solvent. The hydrolytic stability of these ethers is intermediate between that of t-butyldimethylsilyl ethers and that of t-butyidiphenylsilyl ethers. The most useful feature of this new protecting group is the selective cleavage by fluoride ion in the presence of other silyl ethers. 

Hydration with boranes proved difficult due to extensive reductive cleavage of the epoxide (Scheme 45). The best results (54%) were achieved with borane in THF. The subsequent oxidation with basic hydrogen peroxide led to the primary alcohol as well as to saponification of the acetate. Selective protection of the primary alcohol 397 as trichloroethyl carbonate was followed by oxidation with RuCl3/NaI04, thereby producing not only the y-lactone but also the cyclic ketone. 

Acetate esters are common by-products of LTA decarboxylation procedures. The yield of diese products, derived from further oxidation of the alkyl radical and quenching of the subsequent carbocation by acetate ions, can be improved by working in acetic acid in die presence of potassium acetate. Selective monodecarboxylation of 1,3- and 1,4-dicarboxyIic acids leads, via an analogous mechanism, to y- and 8-lactones in mo rate to good yields, as illustrate in equation (39). 

Dialkyl acetals and ketals can easily be formed from carbonyl compounds with alcohols under acidic conditions. Some representative examples for the great variety of methods available for this transformation are given in Scheme 77. As is demonstrated, both simple alcohols themselves or formic acid ortho esters can be used for acetal formation in the presence of hydrochloric acid, toluenesulfonic acid °° or activated alumina (Montmorrilonite clay K-10). ° Owing to different carbonyl reactivities, regio- and chemo-selective differentiation is often realizable, as has been shown, for example, on androstane-3,17-dione (78). Acid-catalyzed acetalization selectively delivers the 3-ketal, whereas the sterically hindered 17-carbonyl function remains unaffected. Under neutral conditions the reactions are promoted by cata-... 

The separation of diastereomeric cyclohexanols on a preparative scale is often tedious. Sometimes the diastereomers are barely resolved by GC, or recourse to HPLC must be taken. Thus, the separation of the above cis- and /ran.s-alcohols on a preparative scale is of practical interest. Treatment of the mixture of the alcohols with chloroacetyl chloride, followed by two fractional crystallizations, gives the Pwi.v-acetate selectively. Saponification affords the diastereo- and enantiomerically pure /rrm-alcohol on a 10-g scale12. 

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