Z- and E-enol

Evans [369] and Masamune [370,371] have pioneered the use of thioesters for stereocontrol of the aldol reaction. The accompanying scheme summarizes the reactions of boron and silicon enolates of t-butyl thiopropanoate with aldehydes [372]. Both reactions are stereoconvergent. (Z) and (E)-enolates afford the same diastereoisomer syn with the boron... 

Rearrangement of (a-methyldiphenylsilyl)alkyl ketones.1 These a-silyl ketones rearrange thermally to a mixture of (Z)- and (E)-enol silyl ethers. However, rearrangement in acetonitrile results in only the (Z)-enol silyl ethers (>99 1). These enol silyl ethers are useful precursors to (Z)-lithium enolates. 

Gennari et al. developed a computational model to reproduce the experimental syn/anti setereoselectivity for the aldol reactions of Z and E enol borinates of butanone with acetaldehyde.13 For the reaction of Z-enol borinate 8Z, the chair transition state TS Z-chair A dominates over other three-transition states (Scheme 2.XI). When a Boltzmann distribution was calculated for the competing transition structures, a complete syn/anti selectivity of 99 1 was predicted. The aldol reaction of E-enol borinate 8E with acetaldehyde is, however, calculated to have four transition structures of similar energy (Scheme 2.XII). Although... 

The Z- and E -enolate intermediates should afford a-methylated products of the same absolute configuration. Assuming that Z-enolate gives the product of the opposite absolute configuration to that from E-enolate and the Z/E ratio is 2 1, the theoretical maximum ee of the product is 67% at about 100% conversion or about 100% at <67% conversion. Experimentally, 41 of 81% ee was obtained at about 100 conversion (96% yield) (Table 3.5, entry 1). 

ACETYLENIC ETHERS FROM ALCOHOLS AND THEIR REDUCTION TO Z-AND E-ENOL ETHERS PREPARATION OF 1-MENTHOXY-1-BUTYNE FROM MENTHOL AND CONVERSION TO (Z)-AND (E)-1-MENTHOXY-1 -BUTENE ([Cyclohexane, 2-(1 -butynyloxy)-4-methyl-1 -(1 -methylethyl)- [1S-(1a,2p,4p)]-], end [[[Cyclohexane, 2-(1-butenyloxy)-4-methyl-1-(1-methylethyl)-, [1S-[1a,2P(Z),4p]]- and [lS-[1 ,2p(E),4P]]-)... 

Parts B and C exemplify efficient procedures for the stereoselective reduction of acetylenic ethers to the corresponding Z- and E-enol ethers, synthetically useful intermediates.5 These procedures, which are optimized versions of previously described methods,3 6 also require only common reagents and standard laboratory... 

ACETYLENIC ETHERS FROM ALCOHOLS AND THEIR REDUCTION TO Z- AND E-ENOL ETHERS PREPARATION OF 1-MENTHOXY-1-BUTYNE FROM MENTHOL AND CONVERSION TO (Z)- AND (E)-1 -MENTHOXY-1 -BUTENE... 

Acetylenic Ethers from Alcohols and Their Reduction to Z- and E-Enol Ethers Preparation of 1-Menthoxy-1-butyne from Menthol and Conversion to (Z)- and (E)-1-Menthoxy-1 -butene. 

Aldol condensations with phenyl thiopropionate (2). Reaction of 2 with 1 and a tertiary amine at -7 ° results in a 70 30 mixture of (Z)- and (E)-enolates, whereas reaction at 25° furnishes an 18 82 mixture of these enolatcs. However, both cnolatcs react with aldehydes with high. wn-diastercoselectivity. 

Photolysis of the mixture of the isomeric Z- and E- enol acetates (123, Ar = 3,4(MeO)2CgH2) derived from 4-(3,4-dimethoxypheitylacetyl)-2-methylisoquinolin-l-one gives a... 

There are many cases when formation of an enolate anion from a ketones is accompanied by formation of ( ) and (Z) isomers. Treatment of 2-methyl-3-pentanone with LDA (THF, -78°C), for example, gave a 60 40 mixture of the (Z) and (E) enolates (24 and 25) Subsequent reaction of 24 and 25 with a carbonyl... 

A.iv. The Evans Model. It is known that (Z) enolates are more stereoselective than ( ) enolates even when r1 is not large. The Zimmerman-Traxler model transition states 352-355 do not account for this observation. It has been suggested that the transition states are not chair-like, but skewed, as in 381-384.221 In this representation (Z) enolate 381 leads to the syn aldol. Similarly, (Z) enolate 382 gives the anti aldol, ( ) enolate 383 give the anti aldol and E) enolate 384 is the precursor to the syn aldol. The major steric interactions in this model are those for r1 r3 and r2 - r3. For both (Z) and E) enolates, the r1 r3 interaction favors 381 and 383, respectively. The r2 r3 interaction is more important for the E) enolate and... 

Also, the ketonic isocarbostyryl 4 was converted by Onda et al. into a mixture of Z- and E-enol acetates by reaction with acetic anhydride and potassium acetate. Furthermore, the Z-isomer could be isomerized to the E-isomer by irradiation. The E-isomer was desired since its further irradiation in the presence of iodine was found to yield aromatic benzophenanthridines 5 and 5a " ... 

Because procedures for the selective generation of (Z) and (E) enolates have been elaborated for a variety of carbonyl compounds [45], the kineti-cally controlled aldol addition offers a solution to the problem of simple diastereoselectivity. Representative examples of the Z/syn and E/anti correlation are given in Eqs. (16) and (17) [66, 67]. 

Different transition state models (chair and twist-boat) for (Z) and (E) enolates... 

Similar to the stereocontrol executed by chiral centers at aldehydes, the a- and p-chiral centers of enolates can be used to control the stereochemical outcome of aldol reactions (Scheme 2.106) [7]. Both Z- and E-enolates with o-methyl branches transform under the influence of this chiral center to the corresponding syn- or anti-aldol products. If a p-alcohol capable of performing hydrogen bonding to the aldehyde s proton is present, an 1,5-flnh-asymmetric induction can take place (Scheme 2.106). 

Kobayashi et al. also reported the improved chiral Znp2-catalyzed enantioselective Mannich-type reaction between acylhydrazono ester (113) and silyl enol ethers in water without using any organic cosolvent (Scheme 4.39). Moreover, TfOH was not neces sary in the system, and a cationic surfactant such as cetyltrimethylammonium bromide (CTAB) (2 mol%) effectively increased the yield. In this catalysis with (116), syn and anti adducts (115) with high enantioselectivities were stereospecifically obtained from (Z)- and (E)-enolate, respectively. These Mannich-type reactions under aqueous conditions were based on the double activation of Lewis add and Lewis base (Scheme 4.40) (35). It was thought that a catalytic amount of the fluoride anion provided a high yield of the product fa these reactions, probably due to catalytic turnover of the fluoride anion. 

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