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Regioselectively generated

Scheme 32. Stille couplings of regioselectively generated enol triflates with a vinylstannane. Scheme 32. Stille couplings of regioselectively generated enol triflates with a vinylstannane.
A further improvement utilizes the compatibility of hindered lithium dialkylamides with TMSC1 at —78 °C. Deprotonation of ketones and esters with lithium dialkylamides in the presence of TMSC1 leads to enhanced selectivity (3) for the kinetically generated enolate. Lithium t-octyl-t-butyl-amide (4) appears to be superior to LDA for the regioselective generation of enolates and in the stereoselective formation of (E) enolates. [Pg.60]

The stoichiometric hydroamination of unsymmetrically disubstituted alkynes is highly regioselective, generating the azametaUacycle with the larger alkyne substituent a to the metal center [294, 295]. In others words, the enamine or imine formed results from an anti-Markovnikov addition. Unfortunately, this reaction could not be applied to less stericaUy hindered amines. [Pg.125]

Only the introduction of a powerful electron-withdrawing group (R=C02Me) leads to a change in the deprotonation site and the regioselective generation of the corresponding terminal BENA. [Pg.621]

An inner-sphere electron reduction has been proposed as a possible mechanism for the Fe(II)-induced decomposition of 1,2,4-trioxolanes (ozonides) (75) and (76). Benzoic acid was found to be the major product. The nucleophilic Ee(II) species attack the ozonide from the less hindered side of the electrophilic 0-0 a orbital to generate exclusively the Ee(III) oxy-complexed radical (inner-sphere electron transfer). After selective scission of the C-C bond, the resulting carbon-centred radical produced the observed product. The substituent effect determine the regioselective generation of one of the two possible Fe(III)-complexed oxy radicals. The bond scission shown will occur if R is bulkier than R. ... [Pg.176]

Regioselective generation of the organolithium compound benefits from two stabilizing effects. [Pg.183]

The utility of the DePuy fluoride-induced desilylation reaction 38 for the regioselective generation of gas-phase carbanions from trimethylsilyl compounds (equation 13) has... [Pg.1115]

Pd-catalysed decarboxylation-elimination offers a useful method for regioselective generation of conjugated dienes. The polyene system of vitamin A derivative 381 is prepared from the /f-acetoxycarboxylic acid 379 by decarboxypalldation, as shown by 380 [172],... [Pg.149]

Kunishima, M. Hioki, K Kono, K Kato, A. Tani, S. Sml2-Induced 2,3-Wittig rearrangement regioselective generation of a-allyloxy carbanions via 1,5-hydrogen transfer of vinyl radicals./. Org. Chem. 1997, 62, 7542-7543. [Pg.212]

Fig. 13.11. Regioselective generation of ketone enolates, I the effects of different substituents in the Aland appositions. Enolate D is formed in THF at -78 °C with LDA irrespective of whether a substoichiometric amount or an excess of LDA is used. However, if one employs slightly less than the stoichiometric amount of LDA (so that a trace of the neutral ketone is present), then, upon warming, the initially formed enolate D isomerizes quantitatively to enolate C with its more highly substituted C=C double bond. It should be noted that LDA removes an axially oriented a-H from the cyclohexanone this is because only then does the resulting lone pair of electrons receive optimum stabilization by the adjacent C=0 bond. With the kinetically preferred deprotonation leading to the enolate D the axial of-H is transferred to the base (via transition state B), but not the equatorial of-H (via transition state iso-B.)... Fig. 13.11. Regioselective generation of ketone enolates, I the effects of different substituents in the Aland appositions. Enolate D is formed in THF at -78 °C with LDA irrespective of whether a substoichiometric amount or an excess of LDA is used. However, if one employs slightly less than the stoichiometric amount of LDA (so that a trace of the neutral ketone is present), then, upon warming, the initially formed enolate D isomerizes quantitatively to enolate C with its more highly substituted C=C double bond. It should be noted that LDA removes an axially oriented a-H from the cyclohexanone this is because only then does the resulting lone pair of electrons receive optimum stabilization by the adjacent C=0 bond. With the kinetically preferred deprotonation leading to the enolate D the axial of-H is transferred to the base (via transition state B), but not the equatorial of-H (via transition state iso-B.)...
Fig. 10.10. Regioselective generation of ketone enolates II the effects of different substituents in the ft and [V positions. (For the regioselective preparation of enolates C and F, see Figures 10.16 and 10.17, respectively.)... Fig. 10.10. Regioselective generation of ketone enolates II the effects of different substituents in the ft and [V positions. (For the regioselective preparation of enolates C and F, see Figures 10.16 and 10.17, respectively.)...
Kinetic enolates.2 The kinetic enolate of a ketone or ester is generated with enhanced selectivity by a lithium dialkylamide in the presence of chlorotrimethylsilane. In addition, LOBA is superior to LDA for regioselective generation of enolates and for stereoselective formation of (E)-enolates. [Pg.285]

Sodium hydride reduction of quinoline in HMPA leads to a 2 3 mixture of 1,2-dihydroquinoline (82) and 1,4-dihydroquinoline (83) isolated as the A-methoxycarbonyl derivatives. In situ produced copper hydride reagents react with pyridinium species with high regioselectivity generating 1,4-dihydropyridine... [Pg.588]

While the abstraction of protons adjacent to the carbon-nitrogen double bond of imines/imine derivatives has been utilized for tiie regioselective generation of azaallyl anions (which are useful in asymmetric ketone synthesis), it competes with and often prevents the addition of nucleophiles to imines. For this reason, imine additions often involve azomethines (e.g. benzylidineanilines) which are not capable of enolization. Many potentially useful additions, however, involve substrates capable of proton abstraction. By avoiding in certain instances some of the structural features of imines/imine derivatives and the reaction conditions responsible for proton abstraction, products resulting from this serious side reaction can be minimized. [Pg.357]

Regioselectively generated silyl enol ethers react with methyllithium to afford regio-chemically pure lithium enolates. Treatment of these enolates with reactive electrophiles leads to regiospecifically alkylated ketones. [Pg.229]

The reactions of crotylboronic esters with aldehydes is regioselective, generating two new stereochemical relationships and potentially four possible stereoisomeric products. Thus, there are two stereochemical aspects enantioselection Re- vs. 5i-face addition) and diastereoselection (syn vs. anti). [Pg.310]

Sulfonium methylides can also be prepared from sulfides by treatment with Me3SiCH20Tf followed by desilylation with CsF. ° This method allows the regioselective generation of sulfonium ylides (i.e. 140), which would not form under equilibrium deprotonation conditions (c/. 139 141 Scheme 21). ... [Pg.893]

In the laboratory of S.F. Martin, the vinylogous Mannich reaction (VMR) of a 2-silyloxyfuran with a regioselectively generated iminium ion was utilized as the key step in the enantioselective construction of (+)-croomine. " The carboxylic acid moiety of the starting material was converted to the acid chloride which spontaneously underwent decarbonylation to give the corresponding iminium ion. Reaction of this iminium ion with the 2-silyloxyfuran afforded the desired threo butenolide isomer as the major product. [Pg.275]

Yamada, K., Tanaka, S., Kohmoto, S., Yamamoto, M. Novel regioselective generation of nitroalkane dianions. J. Chem. Soc., Chem. Commun. 1989,110-111. [Pg.598]

Scheme 5.18 Regioselective generation of an N-acyliminium ion pool by oxidative C—Si bond dissociation... Scheme 5.18 Regioselective generation of an N-acyliminium ion pool by oxidative C—Si bond dissociation...

See other pages where Regioselectively generated is mentioned: [Pg.361]    [Pg.6]    [Pg.140]    [Pg.249]    [Pg.116]    [Pg.567]    [Pg.133]    [Pg.8]    [Pg.128]    [Pg.468]    [Pg.1116]    [Pg.107]    [Pg.365]    [Pg.82]    [Pg.249]    [Pg.879]    [Pg.781]    [Pg.41]    [Pg.879]    [Pg.350]    [Pg.848]    [Pg.415]    [Pg.601]    [Pg.88]    [Pg.601]   


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