ETAL ENOLATES

Scheme 19. Titanium-mediated metathesis strategy for the conversion of olefinic esters to 6- and 7-mem -bered cyclic enol ethers, (a) 4.0 equiv of Tebbe reagent (93), THF, 25°C, 20 min then reflux, 2-8 h, 64% (129), 45% (131a), 32% (131b), 45% (133) (Nicolaou etal.) [34a]...

Fluoral hydrate and hemiacetals are industrial products. They are stable liquids that are easy to handle, and they react as fluoral itself in many reactions. Thus, in the presence of Lewis acids, they react in Friedel-Crafts reactions. They also react very well with organometallics (indium and zinc derivatives) and with silyl enol ethers.Proline-catalyzed direct asymmetric aldol reaction of fluoral ethyl hemiac-etal with ketones produced jS-hydroxy-jS-trifluoromethylated ketones with good to excellent diastereo- (up to 96% de) and enantioselectivities. With imine reagents, the reaction proceeds without Lewis acid activation. The use of chiral imines affords the corresponding 8-hydroxy ketones with a 60-80% de (Figure 2.49). ° ... 

They demonstrated the effectiveness of the alkylation of enolates (160) + (161) -> (162) for preparing precursors of penam and cepham derivatives. The penam synthesis by Schutz and Ugi, as well as die cepham synthesis131 (168) -> (173) profited from Just etal. immensely.130... 

Carbonate of bisphenol-A Single Tgi FTIR 1 had 54 or 63 mol% vinyl[ enol Li etal. (1999a)... 

Ca HX)lactone Carbonate of bis[ enol-A Single Tg - Shah etal. (1986)... 

Carbonate of tetramethyIbis[ enol-A Methyl methacrylate-co-styrene Single Tg III had 4.5-58.5 wt% MMA Kim etal. (1994a)... 

Historical perspective C. H. Heathcock, Comp. Org. Syn. 2, 133-179 (1991). General review T. Mukaiyama, Org. React. 28,203-331 (1982). Application of lithium and magnesium enolates C. H. Heathcock, Comp. Org. Syn. 2, 181-238 (1991) of boron enolates B. M. Kim etal, ibid. 239-275 of transition metal enolates I. Paterson, ibid. 301-319. Stereoselective reactions of ester and thioester enolates M. Braun, H. Sacha, J. Prakt. Chem. 335,653-668 (1993). Review of asymmetric methodology A. S. Franklin, I. Paterson, Contemp. Org. Syn. 1,317-338 (1994). Cf. Claisen-Schmidt Condensation Henry Reaction Ivanov Reaction Knoevenagel Condensation Reformatskv Reaction Robinson Annulation. 

The same group also reports an alternative method of generating the intermediate ( )-vinyloxyboranes from diazo-ketones and trialkylboranes. Isomerization of (E) - to (Z) -intermediates can be effected simply, and both isomers are again used in stereoselective aldol syntheses. Interestingly, hydrolysis of the intermediates leads directly to ketones, and hence constitutes a synthesis of ketones from carboxylic acid derivatives (Scheme 77). Evans etal. have reported similar results from the use of boron enolates in the stereochemical control of aldol reactions. ... 

The y-extended enolate anion derived from (40) can in principle undergo intramolecular alkylation at either the a- or the y-position to yield (41) or (42) respectively. Piers etal. have shown that careful choice of the reaction conditions can lead to almost exclusive formation of the desired isomer. ... 

In a study of the intramolecular alkylation which the enone (153) undergoes when treated with base, Piers etal. have found that careful choice of the reaction medium can ensure almost exclusive formation of either (154), the product derived from the kinetic enolate, or (155), the product derived from the thermodynamic enolate. An intramolecular alkylation [(156) (157)] is also a key... 

Kobayashi etal. found that aldimines were preferentially activatedby Yb(OTf)3 in the comparative reaction of aldehydes and aldimines with enol silyl ethers in sharp contrast to conventional Lewis acids, which activated aldehyde and promoted aldol reactionexclusively (Table 13.6) [10]. NMR-analyses revealed selective formation... 

Electron transmission spectroscopy (ETS) results due to Aflatooni etal. (1998). ForG, VEA was estimated for keto tautomer from enol tautomer experimental value (-0.46 eV) plus the calculated difference in total energies between the two tautomers (0.28 eV). 

Aside this confusion, there is a principal argument, not to use Evans convention, because the hard descriptors E andZ must not be redefined. The soft descriptors cis and trans, however, can be used without violation of the strict definitions of the unequivocal E and Z. Therefore, in this book, the recommendation of Eliel etal. [18] is followed using the soft descriptors cis and raKS, if a series or a class of enolates are addressed [19]. Thereby, cis means that the OM substituent is on the same side as the higher-priority group at the a-carbon atom, and trans means that the OM substituent is on the opposite side. Only in those cases, where an individual enolate is concerned, EtZ nomenclature is used according to its strict definition. 

Sasaki etal have utilized RCM to generate the seven-membered ciguatoxin A-ring (Scheme 3.41) [50]. From the intact B-D subunit 212 that was synthesized using a novel enol phosphonate alkyl Suzuki coupling sequence, RCM using [Ru]-I catalyst gave tetracycle 213. 

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