Enol trifluoromethanesulfonates

Palladium-Catalyzed Reduction of Enol Trifluoromethanesulfonates to Alkenes Cholesta-3,5-diene... 

Hydrogenolysis of esters to acids can occur provided the R —O bond is weakened, for instance when R is a vinyl, allyl, or aryl group [equation (d)]. Thus, hydrogenolysis of an enol ester is a synthetic method for facile removal of carbonyl oxygens that applies to enol trifluoromethanesulfonates [equation (e)] ... 

Formation of Enol Trifluoromethanesulfonates (Enol Triflates). Kinetic or thermodynamic enolate formation, through reaction of unsymmetrical ketones with properly selected strong bases, followed by treatment with Tf2NPh provides regioselective... 

Formation of Enol Trifluoromethanesulfonates (Enol Triflates). 7V-(2-Pyridyl)his(trifluoromethanesulfonimide) is highly reactive hut easy to handle for the production of vinyl triflates. In most cases 2-PyrNTf2 proved to be substantially more reactive than 7V-phenyltriflimide allowing the majority of vinyl triflates to... 

Considerable efforts have been devoted to the stereoselective introduction of a /(-methyl function in intermediates for the synthesis of 1 jS-methylcarbapenems. While the trimethylsilyl trifluoromethanesulfonate catalyzed reaction of a 4-acetoxyazetidinone derivative with ketene acetals shows no selectivity, ketene thioacetals lead to stereoselective formation of the a-methyl isomer108. The zirconium enolate, however, shows high /(-methyl selectivity. 

As an alternative, tin enolates are very useful in these additions. Usually they are prepared in situ from the amide using tin(II) trifluoromethanesulfonate and a base. They are subsequently reacted with an enone, catalyzed by a Lewis acid47-48 (see Table 3). With triinethylsilyl trifluoromethanesulfonate as a catalyst, in the presence of proline derived diamines anti-adducts are formed exclusively49 (see Section 1.5.2.4.3.1.). 

As an alternative to lithium enolates. silyl enolates or ketene acetals may be used in a complementary route to pentanedioates. The reaction requires Lewis acid catalysis, for example aluminum trifluoromethanesulfonate (modest diastereoselectivity with unsaturated esters)72 74 antimony(V) chloride/tin(II) trifluoromethanesulfonate (predominant formation of anti-adducts with the more reactive a,/5-unsaturated thioesters)75 montmorillonite clay (modest to good yields but poor diastereoselectivity with unsaturated esters)76 or high pressure77. 

High enantioselectivities may be reached using the kinetic controlled Michael addition of achiral tin enolates, prepared in situ, to a,/i-unsaturated carbonyl compounds catalyzed by a chiral amine. The presence of trimethylsilyl trifluoromethanesulfonate as an activator is required in these reactions236. Some typical results, using stoichiometric amounts of chiral amine and various enolates are given below. In the case of the l-(melhylthio)-l-[(trimethylsilyl)thio]ethene it is proposed that metal exchange between the tin(II) trifluoromethanesulfonate and the ketene acetal occurs prior to the 1,4-addition237,395. 

Boron enolates can be prepared by reaction of the ketone with a dialkylboron trifluoromethanesulfonate (triflate) and a tertiary amine.16 Use of boron triflates and a bulky amine favors the Z-enolate. The resulting aldol products are predominantly the syn stereoisomers. 

S. Kobayashi, Lanthanide Trifluoromethanesulfonates as Stable Lewis Acids in Aqueous Media. Yb(OTf)3 Catalyzed Hydroxymethylation Reaction of Silyl Enol Ethers with Commercial Formaldehyde Solution Chem Lett. 1991, 2187-2190. 

The third procedure illustrated by this preparation involves the reaotion of ketones with trifluoromethanesulfonic anhydride in a solvent such as pentane, methylene chloride, or carbon tetrachloride and in the presence of a base such as pyridine, lutidine, or anhydrous sodium carbonate.7-11,15 This procedure, which presumably involves either acid-catalyzed or base-catalyzed enolization of the ketone followed by acylation of the enol with the acid anhydride, has also been used to prepare other vinyl sulfonate esters such as tosylates12 or methanesulfonates.13... 

Asymmetric introduction of azide to the a-position of a carbonyl has been achieved by several methods. These include amine to azide conversion by diazo transfer,2 chiral enolate azidation,3 and displacement of optically active trifluoromethanesulfonates,4 p-nitrobenzenesulfonates,5 or halides.6 Alkyl 2-azidopropionates have been prepared in optically active form by diazo transfer,2 p-nitrobenzenesulfonate displacement,5 and the Mitsunobu displacement using zinc azide.7 The method presented here is the simplest of the displacement methods since alcohol activation and displacement steps occur in the same operation. In cases where the a-hydroxy esters are available, this would be the simplest method to introduce azide. 

Enolization of ketonic substrates can be carried out under far milder conditions if the dialkylboryl trifluoromethanesulfonate esters 59 (eq. [24]) are employed in the presence of hindered tertiary amines (eq. [43]) (6,63). At low temperatures (-78 0°C),... 

Dialkylboron trifluoromethanesulfonates (Inflates) are particularly useful reagents for the preparation of boron enolates from carbonyl compounds, including ketones, thioesters and acyloxazoiidinones. Recentiy, the combination of dicylohexyiboron trifluoromethanesulfonate and triethyiamine was found to effect the enolization of carboxyiic esters. The boron-mediated asymmetric aldoi reaction of carboxyiic esters is particuiariy usefui for the construction of anti p-hydroxy-a-melhyl carbonyl units. The present procedure is a siight modification of that reported by Brown, et ai. ... 

Table 1 summarizes the results of the reactions of ynoates (2a-c) with a silicon enolate (silyl ketene acetal) and a lithium enolate of methyl propionate (Eq. 1). Except for the reaction of 2c, Fe-Mont catalyzed exclusive 1,2-addition of silyl ketene acetal to 2a and 2b to give an adduct of 3 in high yields. However, even trimethyl silyl trifluoromethanesulfonate (TMSOTf), a generally applied homogeneous strong acid, failed to effect the addition reaction. 

Reaction of enolate 5 with methyl trifluoromethanesulfonate diastereoselectively provides 6 as may be expected, alkylation occurs from the less hindered face of the enolate opposite the phosphane ligand87. 

The reaction of 6-methylene-l,3-dioxepanes 89 with trimethylsilyl trifluoromethanesulfonate in the presence of lithium diisopropylamide gave 4-formyltetrahydropyrans 90 as ring-contraction products in 29-77% yield. In the case of 4-phenyl-substituted substrate 89a, trimethylsilyl enol ether 91 was isolated as an intermediate (Scheme 17) <2005BCJ2209>. 

In addition to halides, some pseudohalides undergo facile oxidative addition to Pd and Ni complexes. Trifluoromethanesulfonates (triflates), namely aiyl triflates 3 derived from phenols and enol triflates of carbonyl compounds, are most useful. 

The reaction of 5-methylene-l,3-dioxepanes 386 with trimethylsilyl trifluoromethanesulfonate in the presence of base forms cyclic silyl enol ethers 387 (Scheme 90) <2000TL2171>. 

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