Enols table

Both the Af-fluorosulfonamides and the A -fluoroammonium salts are very effective in the fluormation of enol acetates, enamines, silyl enol ethers, and enolates (Table 2) The reactions are thought to proceed through a mechanism which involves Sf 2 attack on the fluorine atom, but contributions from electron-transfer pathways also exist [65, 68, 73, 75, 76, 79, 80, 81, 82]... 

Xenon difluoride [55], xenon difluoride complexed with dialkyl sulfides [59], and xenon difluoride intercalated with graphite [90] are all effective reagents for the fluonnalion of acids, enolates, or enols (Table 2)... 

From these and many related examples the following generalizations can be made about kinetic stereoselection in aldol additions of lithium enolates. (1) The chair TS model provides a basis for analyzing the stereoselectivity observed in aldol reactions of ketone enolates having one bulky substituent. The preference is Z-enolate syn aldol /(-enolate anti aldol. (2) When the enolate has no bulky substituent, stereoselectivity is low. (3) Z-Enolates are more stereoselective than /(-enolates. Table 2.1 gives some illustrative data. 

Davis et al.111 developed another method for reagent-controlled asymmetric oxidation of enolates to a-hydroxy carbonyl compounds using (+)-camphor-sulfonyl oxaziridine (147) as the oxidant. This method afforded synthetically useful ee (60-95%) for most carbonyl compounds such as acyclic keto esters, amides, and a-oxo ester enolates (Table 4-20). 

Rather, the conformational bias in our substrates is apparently dependent on a very particular relationship between the formyl moiety and unsaturated in the pendant side chain. As a result of our studies, we speculate that the presence of unsaturation at C4-C5 in the aldehyde moiety provides a subtle stabilizing nonbonded interaction between the unsaturation in the aldehyde and the carbonyl of the enolate (Table 2.1,... 

With diketene, intermediates of type (III) were isolated and subsequently cyclized under basic conditions following step (b). In the case of 3-oxo-carboxylic acid esters or 3-acyl Meldrum s acids, cyclization step (b) immediately follows reaction step (a), if a slight excess of amine is employed (85TH1 87TH1). Note that conversion of (III) to (V) involves the (IH)-enol (Table I cf. 75BSF2731). The relatively low yield in the case of malonic acid ester, as well as the failure of the reaction with the non-enolizable diphenyl phosphinylacetic ester and cyanoacetate, points to the participation of an enol structure of (III). 

Preparation and reactivity of magnesium enolates TABLE 12. Stereochemistry of cyclopentanones... 

Preliminary fluorination experiments using optically active A -fluoro compounds ( —)-2b and ( + )-2c show that there is reaction with various metal enolates (Table 16) generated under standard reaction conditions to give the anticipated a-fluoro carbonyl compounds with enantiomeric excesses depending strongly on the structure of the metal enolate.119... 

An unusually strong CD for open-chain ketones 141 was recently reported327. Ketones 141 were formed by hydrogenation of /J,y-unsaturated precursors (As not reported) which were obtained by enantioselective protonation of samarium enolates (Table 8). 

Although methyl 4-oxotetrahydrothiopyran-3-carboxylate and the corresponding 2,3-dihydro-4//-thiopyran-4-one exist as a mixture of keto and enol forms in solution, the 2,3-dihydro sulfoxide is present only as the ketone. However, the sulfone and the tetrahydro sulfoxide and sulfone are present exclusively in the enolic form. The picture is further complicated in the solid state, where the dihydro compound and its sulfoxide exist exclusively as the keto structure but the sulfone is found as the enol (Table 27) <1986J(P2)1887>. The related 2-esters of tetrahydrothiopyran-3-one 268 exist as a ca. 1 2 mixture of keto and enol <1981T2633>. 

More recently, a new molybdenum complex, oxodiperoxymolybdenum-pyridine 3,4,5,6-tetrahydro-1,3-dimcthyl-2-(l //)-pyrimidinonc (MoOs py DMPU = MoOPD) has been introduced in which 1,3-dimethyl-3,4,5,6-tetrahydro-2(lf/>pyrimidinone (DMPU) is used as a non-carcinogenic alternative to HMPA54. This reagent offers comparable results to MoOPH reagent in the oxygenation of enolates (Table 4). 

General and theoretical aspects of the metal enolates TABLE 1. Some significant groups attached to the enolate stmcture 1 and 2... 

Molecular structure of metal enolates TABLE 2. IR data for uranyl fi-ketoenolates [U02(dik)2l... 

The preferred formation of the kinetically favored (Z)-silylketene acetal with amide bases in THF can be rationalized by a cyclic transition state model (128) that enables a close interaction between Li cation, carbonyl oxygen and base (Scheme 23). The presence of additives such as HMPA or DMPU results in a greater degree of solvation of the lithium cation and a weakened Li -caibonyl oxygen interaction. Accordingly, the association between base and ester is diminished and the 1,3-diaxial strain in transition state (129) is reduced, whereas transition state (128) is still destabilized by A -strain." In the presence of a slight excess of ester in the enolization mixture, a kinetic resolution process accounts for an additional increase in the ratio of the ( )- vj. the (Z)-lithium enolate (Table 3). ° ... 

Ireland s deprotonation model is widely used to rationalize the stereochemistry with various ethyl ketones and bases. " In the absence of additives that solvate the lithium cation such as HMPA, proton transfer occurs via a chair-hke closed transition state. Under these conditions, the (Z)-enolate is disfavored because of the 1,3-diaxial interaction between the Me and the i-Pr group on nitrogen. As the steric requirement of the R group increases, so does the A strain between the R and Me groups in forming the double bond, thus destabilizing the ( )-(0)- relative to the (Z)-(0)-enolate (Table 6.2). 

The presence of a benzyloxymethyl substituent at the allylic position of ester (202) leads to a predominance of the E)-syn rearranged product with Hf and Ti enolates (Table 18, entries 10 and 11). In these cases favorable chelation between the ester enolate and the benzyloxy oxygen stabilizes transition state (X Scheme 15). When the benzyloxy substituent is more remote (Table 18, entries 12-14) such chelation is less favorable and the reaction proceeds through transition state (U). 

The use of vinylselenium derivatives for the synthesis of functionalized cyclopropanes has also been studied. Cyclopropylcarbonyl compounds were formed during the reaction of vinyl se-lenoxides with lithium enolates (Table 29). 

Chiral acetate (204) shows excellent diastereofacial selectivity and has obvious utility as a reagent for asymmetric aldol reactions. As shown in equation (122), reaction of (204) with benzaldehyde provides diastereomers (205) and (206). As shown in Table 23, entry 1, the diastereoselectivity is 83% if the lithium enolate is formed in the conventional manner and the aldol reaction is carried out in THF at -78 C. A significant improvement is obtained by using the magnesium enolate (Table 23, entry 5), and diastereoselectivity of up to 98% is obtained by the use of very low reaction temperatures (Table 23, entries 10-13). 

Van Horn and Masamune have shown that it is possible to prepare either boryl enolate stereoisomer from the same ketone by changing the steric demand of the dialkylboryl triflate (Scheme Evans et al. also carried out independent studies on the stereochemical course of these aldol reactions and demonstrated that dialkylboryl enolates were kinetically stable even at elevated temperatures. The results obtained from the Evans group also established that exceptionally high levels of aldol dia-stereoselection are governed by the stereochemistry of the dialkylboryl enolates (Table 1). 

Although formation of enolizable N-silylimines is usually accompanied by isomerization to the en-amine and by deprotonation of the aldehyde precursor, N-silylimines derived from enolizable aldehydes have nevertheless been reported by Cainelli, Panunzio and coworkers to condense with enolates (Table 20 entries 15-17). The temperature at which the N-silylimine is generated, -30 C, may be critical since nonenonolizable N-silylimines are normally generated at -70 °C. Condensations with enolizable N-silylimines, like those with nonenolizable N-silylimines, are stereoselective and produce mainly cis 3-lactams. The yields (30-40%), however, are lower. 

Addition of a Lewis acid has been noted to improve the stereoselectivity (Table 12, entries 1-3). High stereocontrol is observed for products which can equilibrate via enolization (Table 12, entries 6-10) and for the sterically demanding sulfone derivative (Table 12, entry 4). Application of this methodology to EPC synthesis is illustrated by the use of lactones prepared in 8 to 9 steps from mannose (Table 12, entries 11-18)1 4. The stereochemical information within the reactive allyl ether moiety is faithfully transmitted and, remarkably, the Z geometry of the olefins is retained during formation of the vinylcyclopentanes. These more sterically hindered substrates may give mixtures of 5- and 7-membered rings (Table 12, entries 11,12, and 14-16). 

Ester Enolates (Table 20, Entries3 and 4) General Procedure ... 

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