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A-Lithiated vinyl ethers

TABLE 6. Representative examples of a-lithiated vinyl ethers, generated by deprotonation, and reactions with electrophiles... [Pg.857]

The metalation of vinyl ethers, the reaction of a-lithiated vinyl ethers obtained thereby with electrophiles and the subsequent hydrolysis represent a simple and efficient method for carbonyl umpolung. Thus, lithiated methyl vinyl ether 56 and ethyl vinyl ether 54, available by deprotonation with t- or n-butyllithium, readily react with aldehydes, ketones and alkyl halides. When the enol ether moiety of the adducts formed in this way is submitted to an acid hydrolysis, methyl ketones are obtained as shown in equations 72 and 73 . Thus, the lithiated ethers 56 and 54 function as an acetaldehyde d synthon 177. The reactivity of a-metalated vinyl ethers has been reviewed recently . [Pg.885]

The introduction of umpoled synthons 177 into aldehydes or prochiral ketones leads to the formation of a new stereogenic center. In contrast to the pendant of a-bromo-a-lithio alkenes, an efficient chiral a-lithiated vinyl ether has not been developed so far. Nevertheless, substantial diastereoselectivity is observed in the addition of lithiated vinyl ethers to several chiral carbonyl compounds, in particular cyclic ketones. In these cases, stereocontrol is exhibited by the chirality of the aldehyde or ketone in the sense of substrate-induced stereoselectivity. This is illustrated by the reaction of 1-methoxy-l-lithio ethene 56 with estrone methyl ether, which is attacked by the nucleophilic carbenoid exclusively from the a-face —the typical stereochemical outcome of the nucleophilic addition to H-ketosteroids . Representative examples of various acyclic and cyclic a-lithiated vinyl ethers, generated by deprotonation, and their reactions with electrophiles are given in Table 6. [Pg.885]

Cyclic a-lithiated vinyl ethers can be smoothly generated by deprotonation of the kineticaUy acidic vinyl a-hydrogen of the parent compounds using an alkyUithium base, and their typical use in organic synthesis is as acyl anion equivalents (2005T3139). 2,3-Dihydrofuran, in particular, can be efficiently a-Hthiated with t-BuLi in THF to give anion 57, which can be subsequently alkylated by reaction with functionalized primary alkyl iodide 58 to furnish the substituted 2-alkyl-2,3-dihydrofuran 59. The latter can... [Pg.106]

A review entitled a-heteroatom-substituted 1-alkenyllithium regents carbanions and carbenoids for C-C bond formation has addressed the methods of generation of such species, illustrated the carbenoid reactivity of a-lithiated vinyl halides and vinyl ethers, and emphasized the synthetic potential of the carbanion species in asymmetric synthesis of a-hydroxy- and a-amino-carbonyl compounds. ... [Pg.368]

Being aware of the fact that a hetero-substituted carbon-carbon double bond is convertible into a carbonyl group, one can use a-hetero-substituted lithio-alkenes 2 as nucleophilic acylation reagents 142 and 143, which display the umpoled d reactivity, provided that the carbanionic character is effective. Depending on the hetero-snbstitnent X, the conversion of the vinyl moiety into a carbonyl gronp can be effected either by hydrolysis or by ozonolysis. The former procednre has been applied preferentially in the case of lithiated vinyl ethers, whereas the latter has been nsed in particnlar for cleavage of the double bond in such products that result from the reaction of hthiated vinyl bromides with electrophiles (Scheme 17). [Pg.877]

A number of other systems can be utilized as the synthetic equivalent of a nucleophilic carbonyl group, particularly in Sn2 alkylation reactions. Dithiane derivatives of aldehydes can be converted to nucleophiles by lithiation. The resulting anion is easily alkylated. Hydrolysis leads to a ketone. Lithiated vinyl ethers and... [Pg.419]

Addition of a silyl substituent into a-position of the a-(benzotriazol-l-yl)alkyl ether brings additional possibilities. Thus, lithiation of silyl ether 770 followed by treatment with an aldehyde or ketone gives unstable P-hydroxy-a-silyl-a-(benzotriazol-l-yl)alkyl ether 771 that spontaneously eliminates silanol to give vinyl ether 772 (Scheme 121). Treatment with ZnBr2 followed by hydrolysis with a diluted acid removes both the benzotriazolyl and the methyl groups to furnish carboxylic acid 773. In this way, in a simple manner, aldehydes and ketones are converted to one-carbon homologated carboxylic acid <1996S1425>. [Pg.87]

An ab initio study of the energetics of deprotonation of cyclic vinyl ethers by organolithium reagents has clarified the ring-size-dependent competition between vinylic and allylic deprotonation.The respective transition states involve preequilibrium complexation of lithium to the electron-rich vinyl ether oxygen, prior to deprotonation via a multi-centre process free ions are not formed during the lithiation. [Pg.375]

Metalated vinyl ethers are configurational stable up to —20°C in tetrahydrofuran. H-NMR measurements of 1-ethoxy-1-lithioethene TMEDA did not show any coalescence of the signals for the vinyl protons until the onset of decomposition. Thus, there is no evidence of inversion in this case . Similar configurational stability is displayed by a-lithiated thioethers in tetrahydrofuran no inversion occurs up to 0°C. On the contrary, deprotonated vinyl sulfoxides and sulfones are configurationally less stable . ... [Pg.837]

Whereas carbenoid character is definitely present in metalated alkyl vinyl ethers, lithiated alkyl and aryl vinyl sulfides and thioesters, which are easily available by hydrogen-lithium exchange, do not display carbenoid-typical reactions . They rather behave like nucleophilic reagents, so that their discussion is beyond the scope of this overview despite their utility in synthesis The same appiies to various derivatives of enamines, deprotonated in the vinyiic a-nitrogen position - . [Pg.856]

The effect of lithiating various unsaturated ethers, including 2,3-dihydrofuran, has been examined by means of 13C NMR spectroscopy and from the results (Table 24) the degree of s character in the unsaturated carbon atoms has been estimated. It differs but little amongst vinyl ethers and is somewhat more than the 33.3% of a formal sp2 hybrid (80JOC4959). [Pg.572]

Formation of vinylic (trigonal C-Li), allylic or benzylic organolithiums a to O is more favourable. The useful acyl anion equivalent methoxyvinyllithium 11, for example, is formed on treatment of methyl vinyl ether with f-BuLi at -65 °C,13 and furan is lithiated by BuLi in Et20 at 0 °C to give 12.14 More complex carbohydrate-derived vinyl ethers such as 13 also lithiate readily.15... [Pg.12]

S-Substituted a-lithiated silyl enol ether 557 has been prepared by reductive lithia-tion of vinyl tellurides834 and sulfides835,836 with lithium 1 -(dimethylamino)naphthalenide (LDMAN). This intermediate 557 gave, after inverse Brook rearrangement, the enolate 558 and after hydrolysis the corresponding acylsilane (Scheme 151). [Pg.233]

The a-lithiated cyclic vinyl ethers 599 and 603 have been allowed to react with alkyl iodides881,886 and compounds 599, 600909,910 and 606910 with carbonyl compounds. [Pg.240]

This procedure consists of the synthesis of a precursor, methoxymethyl vinyl ether, an a-hydroxy enol ether, and the intramolecular hydrosilylatlon of the latter followed by oxidative cleavage of the silicon-carbon bonds. The first step, methoxymethylation of 2-bromoethanol, is based on Fujita s method.7 The second and third steps are modifications of results reported by McDougal and his co-workers. Dehydrobromination of 2-bromoethyl methoxymethyl ether to methoxymethyl vinyl ether was achieved most efficiently with potassium hydroxide pellets -9 rather than with potassium tert-butoxide as originally reported for dehydrobromination of the tetrahydropyranyl analog.10 Potassium tert-butoxide was effective for the dehydrobromination, but formed an adduct of tert-butyl alcohol with the vinyl ether as a by-product in substantial amounts. Methoxymethyl vinyl ether is lithiated efficiently with sec-butyllithium in THF and, somewhat less efficiently, with n-butyllithium in tetrahydrofuran. Since lithiation of simple vinyl ethers such as ethyl vinyl ether requires tert-butyllithium,11 metalation may be assisted by the methoxymethoxy group in the present case. [Pg.104]

Molecular oxygen inserts into substituted cyclopentadienes under acidic conditions producing pyrylium salts a hydroperoxide rearrangement is proposed <05JOC5768>. In a further development of the use of benzotriazole (Bt) in synthesis, P-lithiation of the vinyl ether 16 and quenching of the anion with chalcone affords an enol ether. Cyclisation with PCI5 produces 2,4,6-triarylpyrylium salts <05S245>. [Pg.383]

Vinyl Ethers. a-Staimylated enol ethers provide a general and convenient substrate for introduction of acyl groups into azines (Scheme 94). The stannanes are available from enol ethers by a-lithiation and quenching with trialkylstannyl chloride. Mild acid hydrolysis of the a-pyrimidinylethenyl ethers yields the acyl-substituted pyrimidines such as the methyl pyrimidinyl ketone 259. The masked acyl group is introduced into the electrophilic 4-position in the 4,5-dichloro derivative 258. In 5-bromo-2-chloropy-rimidine, chemoselectivity leads to a masked acyl group in the 5-position 260. When the... [Pg.478]

Cyclic vinyl ethers such as (9) are lithiated to give acyl carbanion equivalents (10), whilst a-substituted vinyl-lithiums of type (11) react with aldehydes and ketones, ArCOR, to give substituted 2-butenolides (12). ... [Pg.172]


See other pages where A-Lithiated vinyl ethers is mentioned: [Pg.857]    [Pg.858]    [Pg.860]    [Pg.871]    [Pg.857]    [Pg.858]    [Pg.860]    [Pg.871]    [Pg.856]    [Pg.867]    [Pg.229]    [Pg.26]    [Pg.156]    [Pg.41]    [Pg.628]    [Pg.84]    [Pg.469]    [Pg.252]    [Pg.82]    [Pg.53]    [Pg.96]    [Pg.420]    [Pg.676]    [Pg.321]    [Pg.168]   


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Vinyl lithiation

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