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Ether formation, stereoselectivity

Addition of a hydroxy group to alkynes to form enol ethers is possible with Pd(II). Enol ether formation and its hydrolysis mean the hydration of alkynes to ketones. The 5-hydroxyalkyne 249 was converted into the cyclic enol ether 250[124], Stereoselective enol ether formation was applied to the synthesis of prostacyclin[131]. Treatment of the 4-alkynol 251 with a stoichiometric amount of PdCl2, followed by hydrogenolysis with formic acid, gives the cyclic enol ether 253. Alkoxypalladation to give 252 is trans addition, because the Z E ratio of the alkene 253 was 33 1. [Pg.500]

The synthesis was initiated with the stereoselective introduction of two methyl groups onto the tritylated butenolide 1 to give the dimethylated lactone 2 (67%) along with the C-2 epimer (13%). As this stereocenter will be lost in the Stork annulation vide post), both epimers could be used in the total synthesis of 12. Their structures were confirmed by the NOE enhancement in 2. After detritylation, the resulting alcohol was transformed to the dimethyl acetal 3. Reaction with the lithiated MeS02Ph gave the lactol 4, which was silylated to the open chain having the enol silyl ether 5 (91%). These reactions seem to depend on the readiness of the enol silyl ether formation. [Pg.158]

The remaining segment, C-3 to C-8, was constructed by a similar route. Optically active allylic alcohol 229, produced from lithio ethylacetate and methacrolein followed by a second Sharpless kinetic resolution, was hydrolized to the corresponding hydroxy acid. Neutralization followed by iodolactonization then gave 230 in 85% yield. This highly stereoselective cyclization produced a cis-trans ratio of 20 1 via a one-pot procedure. Deprotonation and methylation afforded the expected anti a-methyl compound, contaminated with about 10% of the syn compound but none of the methyl ether. Formation of the silyl ether then produced 231 in 66% yield. Dibal reduction to the aldehyde concomitant... [Pg.71]

The increase in (Z)-stereoselectivity with decreasing solvent coordination to Li (HMPA>THF>ether) indicates that the stereoselectivity is controlled by the extent of internal F-Li coordination in the Li alkoxide intermediate. Thus, it appears likely that the HF elimination proceeds in a trans fashion exclusively through species A in ether, but partially through B in THF and more so in THF/HMPA (Figure 3). Further noteworthy is that a similar (Z) (E) ratio (1 3) to that in entry 5 is observed in the F-enol ether formation from CF3CH(OMEM)CF2CF3 with LDA in THF, where F-Li coordination cannot be operative. [Pg.86]

A stereoselective construction of 1,3-diol systems is based on the reaction of lactol acetates with allylsilanes or silyl enol ethers. Formation of the product is subject to 1,3-asymmetric induction by one or more substituents in the ring. Note that BFj OEt2 is not a suitable catalyst. [Pg.380]

The treatment of an ester (or lactone) with a base and a silyl halide or trillate gives rise to a particular type of sUyl enol ether normally referred to as a silyl ketene acetal. The extent of O- versus C-silylation depends on the structure of the ester and the reaction conditions. The less-bulky methyl or ethyl (or 5-tert-butyl) esters are normally good substrates for O-silylation using LDA as the base. Acyclic esters can give rise to two geometrical isomers of the silyl ketene acetal. Good control of the ratio of these isomers is often possible by careful choice of the conditions. The f-isomer is favoured with LDA in THF, whereas the Z-isomer is formed exclusively by using THF/HMPA (1.24). Methods to effect stereoselective silyl enol ether formation from acyclic ketones are less well documented. ... [Pg.14]

The EF-ring of ciguatoxin 3C has been stereoselectively synthesized by iterative use of a cyclic ether formation process based on chirality-transferring Ireland-Claisen rearrangement and ring-closing alkene metathesis (Scheme 21). ... [Pg.527]

Intramolecular Reactions of Alkynes with Carboxylic Acids, Alcohols, and Amines. Addition of carboxylic acids, alcohols, and amines to alkynes via oxypaUadation and aminopallada-tion proceeds with catalysis by Pd salts. Intramolecular additions are particularly facile. Unsaturated y-lactones are obtained by the treatment of 3-alkynoic acid and 4-alkynoic acid with Pd(PhCN)2Cl2 in THF in the presence of Et3N (eq 54), and -lactones are obtained from5-alkynoic acids. 5-Hydroxyalkynes are converted to the cyclic enol ethers (eq 55). The oxypalla-dation is a irons addition. Thus stereoselective enol ether formation by reaction of the alkynoic alcohol with Pd(PhCN)2Cl2, followed by reduction with ammonium formate, has been applied to the synthesis of prostacyclin (eq 56). Intramolecular addition of amines affords cyclic imines. 3-Alkynylamines are cyclized to 1-pyrrolines while 5-alkynylamines are converted to 2,3,4,5-tetrahydropyridines (eq 57). ... [Pg.505]

The nucleophilic addition of bisaldehyde 274 by p-benzy-loxy-phenylmagnesium provided all the necessary carbons for the target molecule. Hydrogenation over catalytic Pd/C then excised the benzyl ethers, and stereoselective formation of trawi-disposed dihydrofuran rings within the resulting polyphenol gave this sophisticated resveratrol tetramer, ampelopsin H (247). [Pg.367]

Macias et al. have developed a stereoselective route to heliannuol A using diene metathesis to generate the requisite oxocine (Scheme 3.26) [38]. The cyclization precursor was obtained from the sequential propargyl ether formation and allyl Grignard addition to the acetophenone derivative 152. Reductive removal of the... [Pg.104]

Alkyl-1,4-dihydropyridines on reaction with peracids undergo either extensive decomposition or biomimetic oxidation to A-alkylpyridinum salts (98JOC10001). However, A-methoxycarbonyl derivatives of 1,4- and 1,2-dihydro-pyridines (74) and (8a) react with m-CPBA to give the methyl tmns-2- 2>-chlorobenzoyloxy)-3-hydroxy-1,2,3,4-tetrahydropyridine-l-carboxylate (75) and methyl rran.s-2-(3-chlorobenzoyloxy)-3-hydroxy-l,2,3,6-tetrahydropyridine-l-carboxylate (76) in 65% and 66% yield, respectively (nonbiomimetic oxidation). The reaction is related to the interaction of peracids with enol ethers and involves the initial formation of an aminoepoxide, which is opened in situ by m-chlorobenzoic acid regio- and stereoselectively (57JA3234, 93JA7593). [Pg.285]

In further modifications of these norprogestins, reaction of norethindrone with acetic anhydride in the presence of p-toluene-sulfonic acid, followed by hydrolysis of the first-formed enol acetate, affords norethindrone acetate (41). This in turn affords, on reaction with excess cyclopentanol in the presence of phosphorus pentoxide, the 3-cyclopentyl enol ether (42) the progestational component of Riglovic . Reduction of norethindrone affords the 3,17-diol. The 33-hydroxy compound is the desired product since reactions at 3 do not show nearly the stereoselectivity of those at 17 by virtue of the relative lack of stereo-directing proximate substituents, the formation of the desired isomer is engendered by use of a bulky reducing agent, lithium aluminum-tri-t-butoxide. Acetylation of the 33,173-diol iffords ethynodiol diacetate, one of the most potent oral proves tins (44). ... [Pg.165]

With the co side chain at C-12 in place, we are now in a position to address the elaboration of the side chain appended to C-8 and the completion of the syntheses. Treatment of lactone 19 with di-isobutylaluminum hydride (Dibal-H) accomplishes partial reduction of the C-6 lactone carbonyl and provides lactol 4. Wittig condensation8 of 4 with nonstabilized phosphorous ylide 5 proceeds smoothly and stereoselectively to give intermediate 20, the bistetra-hydropyranyl ether of ( )-1, in a yield of -80% from 18. The convergent coupling of compounds 4 and 5 is attended by the completely selective formation of the desired cis C5-C6 olefin. [Pg.73]


See other pages where Ether formation, stereoselectivity is mentioned: [Pg.398]    [Pg.398]    [Pg.485]    [Pg.183]    [Pg.416]    [Pg.439]    [Pg.88]    [Pg.33]    [Pg.485]    [Pg.260]    [Pg.309]    [Pg.598]    [Pg.2034]    [Pg.110]    [Pg.153]    [Pg.92]    [Pg.553]    [Pg.553]    [Pg.84]    [Pg.157]    [Pg.439]    [Pg.413]    [Pg.38]    [Pg.321]    [Pg.481]    [Pg.548]    [Pg.719]    [Pg.725]   
See also in sourсe #XX -- [ Pg.2 ]




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Silyl enol ethers stereoselective formation

Stereoselective formation

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