Conjugate enolates

Either pure acetoxyallene, on refluxing with the catalyst for 24 hr, equilibrates to an equimolar mixture of the two isomers. The mixture of allenes, if refluxed with acetic acid, rearranges to the 16,20-conjugated enol acetate ... 

C-H insertion also occurs in the reactions with acetone and acetophenone, presumably through the rearrangement of transient OH-substituted phosphi-ranes [87]. C-C insertions occur for diketones to give 45 and have been postulated to occur via initial 1,2-addition to the conjugated enol 44 [87]. Diimines 46 also undergo C-C insertions [88]. Based on a theoretical evaluation, the products 47 are considered to result from a 2,3-sigmatropic rearrangement of initial formed P,N-ylids. 

The acidifying effect of an adjacent phenyl group outweighs steric effects in the case of l-phenyl-2-propanone, and as a result the conjugated enolate is favored by both kinetic and thermodynamic conditions (Entry 5). 

In 1992, Thornton et al. reported that Mn(salen) (43) catalyzed the asymmetric oxidation of silyl enol ethers to give a mixture of a-siloxy and a-hydroxy ketones, albeit with moderate enantioselectivity (Scheme 28).135 Jacobsen et al. examined the oxidation of enol esters with Mn(salen) (27) and achieved good enantioselectivity.136 Adam et al. also reported that the oxidation of enol ethers with (27) proceeded with moderate to high enantioselectivity.137 Good substrates for these reactions are limited, however, to conjugated enol ethers and esters. Based on the analysis of the stereochemistry,137 enol ethers have been proposed to approach the oxo-Mn center along the N—Mn bond axis (trajectory c, vide supra). 

For substitution reactions with conjugated enol triflates see (a) F. 

Products of ethylation and methylation of enolates of cycloalkane-1,3-diones with ring sizes 7-10 have been studied under a variety of alkylating reagent-solvent systems. Decrease in the 0 C alkylation ratios with increase in ring size is believed to be a consequence of greater steric strain in the conjugated enolate resonance contributor and consequent diminution in the proportion of D-attack. 

The functional group in ring A is a ketone a planar system that takes in carbons adjacent to a ketone is the enol tautomer. We are using acid catalysis here, and that is appropriate for enolization. If we first protonate the carbonyl, then try moving electrons, we shall soon find that the proton at position 6 (standard steroid numbering) can be lost in generating a conjugated enol tautomer. In a reversal of this process back to the ketone, we can pick up a proton at position 6 from either face. In this case, protonation on the upper face allows the methyl substituent to take up the more favourable equatorial position. 

Two consecutive enolate alkylations were utilized to generate the quaternary carbon atom (Scheme 38). Alcohol 238 was transformed into the protected hydroxy enone 244. Regioselective deprotonation at the a-position of the ketone 244 led to a cross-conjugated enolate that was alkylated with the allylic iodide 245. The vinyl silyl moiety in 245 represents a masked keto group [127]. The choice of the TBS protecting group for the hydroxyl group at of 244 was crucial in order to prevent the deprotonation at the y-posi-... 

Conjugated enol ether 18 derived from the corresponding a,/J-unsaturated oxo steroid, gives a 1 1 mixture of 6-fluoro-17/ -hydroxyandrost-4-en-3-one (mixture of 6a- and 6/J-isomers) and 4-fluoro-17/J-hydroxyandrost-5-en-3-one upon treatment with 1-fluoropyridinium triflate (la).52... 

The reagent is extremely useful for the preparation of cross-conjugated enolates of a,(5-un saturated systems ... 

Esters are far less reactive as electrophiles when compared to aldehydes and ketones. Successful tandem vicinal dialkylations are possible using alkyl formates,67 but most esters lack the needed reactivity. More reactive thioesters can serve as electrophiles in these sequences.208 Presence of a potentially electrophilic ester group as a substituent in the conjugate enolate permits very efficient Dieckmann cycliza-tion to take place as the second step of a MIRC sequence (e.g. equations 5118 and 52).24 Ortho esters are far more reactive, giving p-keto esters as adducts when used in sequences that employ enones as substrates.230... 

In addition to the simple one-carbon electrophile formaldehyde, carbon dioxide67 292 and carbonyl sulfide273 also are excellent one-carbon electrophiles for use in conjugate enolate trapping. 

---