Deprotonation base-mediated

Mesylation of the alcohol 65 followed by deprotonation afforded the sul-fone-stabilized carbanion 66 that underwent a macrocyclization to afford the artificial dolabellane 67 in moderate yield (Scheme 9). Hydrolytic cleavage of the ketal (67) followed by a base-mediated double bond isomerization (into conjugation) afforded an enone containing an exocyclic carbonyl group. Nucleophilic 1,2-addition of methyl lithium introduced the missing... 

This is an example of the first step of an E2 (bimolecular elimination) reaction mechanism. Note the base-mediated deprotonation of the diester converting the ferf-butoxide anion to ferf-butanol. For clarity, the anion was repositioned and the bond was lengthened. Arrow pushing is illustrated below ... 

For more than a century glycosylations were essentially based on methods where the anomeric carbon of the sugar residue to be coupled served as the electrophile (the glycosyl donor) and the alcohol (the glycosyl acceptor) as the nucleophile [O Fig. 1, (A), (B)]. Alternatively, base-mediated deprotonation of the anomeric hydroxy group generating at first an anomeric... 

Remarkable improvements in chiral base-mediated reactions of prochiral ketones under external quench (EQ) conditions with TMS-Cl, furnishing enantiomerically pure enol silanes, were found upon deprotonation in the presence of LiCl. [22, 24] Simpkins et al. studied for instance the conversion of 4-tert-butylcyclohexanone 9 into enol silane 10 by employing the chiral amide base 11 (Scheme 9). [24] Applying the TMS-Cl in situ quench (TMS-Cl-ISQ) protocol a higher level of enantiomeric excess was observed compared to external quench conditions (EQ). However, under external quench conditions in the presence of LiCl (EQ-i-LiCl procedure) significantly higher levels of asymme-... 

You may wonder why the conditions of the Hofmann rearrangement do not lead to amide hydrolysis. The reason is that amidate formation by deprotonation, followed by halogenation, is much faster then nucleophilic attack by the base on the carbonyl function. Conversely, in the base-mediated hydrolysis of primary and secondary amides (those bearing protons on the nitrogen), equilibration with the amidate is a rapid, nonproductive, and reversible dead-end background reaction. 

Michael donors and acceptors are common components in Brmsted base-mediated catalysis. Such transformations offer an uncomplicated route towards all-carbon quaternary stereocenters. In the most basic form, a,P-unsaturated aldehydes are highly reactive templates towards nucleophilic reactions. Under such conditions, mechanistic studies show no polymerization of the unsaturated aldehydes under cinchona alkaloid catalysis [10]. This absence of polymerization is a key mechanistic indicator that the quinucHdine nitrogen of the catalyst does not act as a nucleophilic promoter. Rather, the quinucHdine nitrogen acts, as predicted, in a Bronsted basic deprotonation-activation of various cyclic and acyclic... 

The redox and proton transfer reactions undergone by the flavin prosthetic group are summarized in Scheme 5.2. The vertical reactions are oxidations by Q regenerating P. From the standard potential values (V vs. SCE) of the four flavin redox couples that are involved in Scheme 5.2 and those of the mediators (Table 5.1), all four oxidation steps may be regarded as irreversible. The horizontal reactions are deprotonations by the bases present in the buffer. From the pA values of the various flavin acid-base couples indicated in Scheme 5.2 (over or below the horizontal arrows), reactions H2 and H4 may be regarded as irreversible and reactions HI and... 

In (—)-sparteine-mediated deprotonation and electrophilic substitution reactions, the minor enantiomer is close to the limits of exact determination. Therefore, the influence of the alkyl residue on selectivity was investigated for less efficient (/ ,/ )- ,2-bis(dimethyl-amino)cyclohexane/i-BuLi (equation 11)°. On the base of the isolated corresponding... 

An efficient kinetic resolution was also observed during the (—)-sparteine-mediated deprotonation of the piperidin-2-yhnethyl carbamate rac-112 (equation 25). By treatment of rac-112 with s-BuLi/(—)-sparteine (11), the pro-S proton in (/ )-112 is removed preferentially to form the lithium compound 113, which undergoes intramolecular cyclo-carbolithiation, and the indolizidinyl-benzyllithium intermediate 114 was trapped with several electrophiles. The mismatched combination in the deprotonation of (5 )-112, leading to cp/-113, does not significantly contribute to product formation. Under optimized conditions [0.75 equivalents of s-BuLi, 0.8 equivalents of (—)-sparteine, 22 h at —78°C in diethyl ether] the indolizidine 115 was isolated with 34% yield (based on rac-112), d.r. = 98 2, e.r. = 97 3 optically active (5 )-112 was recovered (46%, 63% ee). 

Nevertheless, whereas the base-promoted isomerization of simple linear oxiranes and cyclohexene oxide occurs via a -deprotonation mechanism, recent denterinm-labeUng experiments demonstrate that the LDA-mediated rearrangement of cyclopentene oxide in nonpolar solvents furnishes the corresponding cyclopentenol through an a-deprotonation route (Scheme 7) . 

Based on this feature, aggregation states of transition-state structures for base-promoted isomerization of oxiranes have been established by kinetic studies of LDA-mediated isomerizations of selected oxiranes in nonpolar media in the presence of variable concentrations of coordinating solvents (ligands). Results reported provide the idealized rate law V = [ligand]" [base] [oxtrane] for a-deprotonation and v = fc[ligand]°[base] / [oxirane]... 

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