Lithium dienolates, structure

A double ir-face selective aptotic Michael addition of the lithium dienolate, derived from the chiral senecioate (119), to cyclopentenone served for the total synthesis of (-)-kushimone (120 Scheme 45).137-138 The selectivity of the key reaction was rationalized by the assumption of a transition state as shown in (121a). A related approach involves the Michael addition of enolates, derived from chiral propionates, to methyl ( )-crotonate (Scheme 46).139 The formation of the threo isomer (122) as the major product indicates a transition state structure as shown in (121b). This method was utilized in the synthesis of the marine natural product 7,20-diisocyanoadociane (123 Scheme 47).140... 

The synthesis of y-lactones from acetals derived from a tartaric acid dithioester was recently reported552. Quenching the readily obtained lithium dienolate (analyzed as its bis-silylketen acetal, Scheme 111, bottom structure) with a series of aldehydes gave only one diastereomeric lactone, generally in ca 60% yield, along with a variable amount of oxidized diester depending on steric factors (Scheme 111). An intramolecular complexa-tion of the lithium atom is assumed to favor the C2-symmetric Z,Z non-planar chelated... 

Lithium salts of dianions of unsaturated carboxylic acids can adopt the structure of lithium dienolates, 3 or 4 (equation 3), or lithium salts of oxycarbonyl substituted allyl anions, 5 or 6 (equation 4). In any case these formulations provide an oversimplified view of the structures, as these dianions are expected to form ion pairs and aggregates in weakly polar solvents such as diethyl ether or thf. ... 

Aldolization of a,p-unsaturated carbonyl compounds (20) with benzaldehydes in the presence of ATPH is shown in Scheme 6.20. Thus, sequential treatment of a toluene solution of ATPH (3.3 equivalent) with methyl 3-methyl-2-butanoate (20a) (2.0 equivalent) and benzaldehyde (1.0 equivalent) at —78 °C was followed by deprotonation with a THF solution of LTMP (2.3 equivalent). After 30 minutes at the same temperature, a mixture of y-aldol products (21a) and (22a) was obtained in 91% isolated yield with preferential formation of Z isomer (22a) ( -(21a) Z-(21a) = 1 13). In sharp contrast, mesityloxide (20b), senecialdehyde (20c) and the disubstituted a,p-unsaturated carbonyl compounds, methyl crotonate (20d), (T)-3-penten-2-one (20e) and crotonaldehyde (20f) provided the (T)-y-adducts (21b-f), exclusively. The opposite stereoselectivity of the aldol adduct in the reaction of (20a) versus (20b) and (20c) can be explained by the preferable conformation of the extended lithium dienolates confirmed by NMR studies. That is, reaction of ATPH complex of the ester (20a) provided s-cis conformer of the dienolate predominantly, whereas the corresponding dienolates from the ketone (20b) and aldehyde (20c) were found to have s-trans structure. In the cases of the disubstituted substrates (20d-f),... 

The very good yields of especially the bridgehead alkoxy derivatives 231 and the excellent regioselectivity with which they can be transformed to other skeletons plus the fact that a spirocyclopropane moiety is a mimic of and can in fact be considered as a masked gem-dimethyl substituent [28, 112] makes these products versatile precursors of certain natural products. In fact, one may conceive new approaches to the total syntheses of taxol [113-115] and of mediter-raneol [116]. Both strategies rely on the MIMIRC reaction of lithium cycloalka-dienolates [117] with the a-chloro acrylate 1-Me to produce a tricyclic precursor to the appropriate bicyclo [n.2.1]alkanedione derivative, which are key structural units of several diterpenes and their metabolites. 

The particular case of lithium acetylacetonate (acac), a canonic example of /I-diketone enolate, was also examined early. It was shown that its chelated (Z,Z) conformation was almost exclusive in methanol at — 60 °C and that dimers were probably formed in which one of the two lithium cations would be chelated by the two acac anions272. A somewhat similar dimer, obtained from the lithium enolate of ethyl acetoacetate complexed by a 2.1.1 cryptate, was characterized in one of the first 7Li NMR studies of enolates (Scheme 65)273. Note that the structure of the three ft -diketone mono- and dilithium enolates displayed in Scheme 64 has been studied, despite their poor solubility, in both THF-dg and DMSO-d6 by 13C NMR260. The data obtained for the monoenolates are consistent with rapidly equilibrating dimers, while the dimers of dienolates seem to form slowly on the NMR time scale. 

Dietrich et al. (64) have deduced the structure of dilithiodibenzylketone-bis(tmeda), a dienolate. The geometry of this compound is such that the lithium atoms are resting in a nest provided by a phenyl group and the... 

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