Enol trifluoroacetates, from ketones

Nitridomanganese Complexes. Stoichiometric amounts of chiral complexes of type 43 react with silyl enol ethers in the presence of trifluoroacetic or /Moluenesulfonic anhydride to give a-(A-trifluroacetyl)amino- and a-(N-p-tosylamino) ketones, respectively (see Eq. 160).350 - 353 With glycals, the 1-hydroxy-2-( A -trifluoroacetyljamino derivatives are formed (see Eq. 83).354 A mechanism involving approach of the enol ether from the least hindered side of the 43-TFA complex has been proposed.353... 

Scheme 2.12 shows some representative Mannich reactions. Entries 1 and 2 show the preparation of typical Mannich bases from a ketone, formaldehyde, and a dialkylamine following the classical procedure. Alternatively, formaldehyde equivalents may be used, such as l>is-(di methyl ami no)methane in Entry 3. On treatment with trifluoroacetic acid, this aminal generates the iminium trifluoroacetate as a reactive electrophile. lV,A-(Dimethyl)methylene ammonium iodide is commercially available and is known as Eschenmoser s salt.192 This compound is sufficiently electrophilic to react directly with silyl enol ethers in neutral solution.183 The reagent can be added to a solution of an enolate or enolate precursor, which permits the reaction to be carried out under nonacidic conditions. Entries 4 and 5 illustrate the preparation of Mannich bases using Eschenmoser s salt in reactions with preformed enolates. 

Nitration of the potassium enolates of cycloalkanones with pentyl nitrate81 or nitration of silyl enol ethers with nitronium tetrafluoroborate82 provides a method for the preparation of cyclic a-nitro ketones. Trifluoroacetyl nitrate generated from trifluoroacetic anhydride and ammonium nitrate is a mild and effective nitrating reagent for enol acetates (Eq. 2.41).83... 

Similar arguments apply to the six a-carboxy-substituted ketones that have been studied by Kresge and coworkers (entries acetoacetate to oxocyclobutane-2-carboxylate in Table 1). Kresge already noted that the rate constants kucK observed for the uncatalyzed ketonization of some of these compounds would give unrealistically high calculated values for k e near or above 1011 m-1 s-1 using Equation (18). Indeed, these calculated values of k are about two orders of magnitude above those expected from the Marcus relation except that for 4,4,4-trifluoroacetate. The rate constants k c observed for the formation of these a-carboxy-substituted ketones are, however, close to those expected for the protonation of the neutral enols by water, k = kf. 

Other species of general stmcture RSeX, where X = a nonhalide leaving gronp, are also known and often show similar behavior to that of the selenenyl hahdes. " For example, benzeneselenenyl acetate (20), trifluoroacetate (21), and tosylate (22) can be generated in situ from the reactions of the selenenyl halides with silver acetate, trifluoroacetate, or tosylate, respectively (Scheme 10). The former two electrophiles react with enol acetates to produce a-seleno ketones and with alkenes and acetylenes to give 1,2-addition products, while the latter adds similarly to acetylenes. Examples are shown in equations (16) to (18). 

Oxazaborolidenes. Corey has reported the use of a novel oxazaborolidene complex 41 prepared from borane and A-tosyl (5)-tryptophan. This complex functions in a catalytic fashion in enantioselective, Mukaiyama aldol addition reactions (Scheme 8-3) [17]. The addition of ketone-derived enol silanes 42-43 gives adducts in 56-100% yields and up to 93% ee. The use of 1-trimethylsilyloxycyclo-pentene 43 in the addition reactions to benzaldehyde affords adducts 46 as a 94 6 mixture of diastereomers favoring the syn diastereomer in 92% ee. Addition reactions with dienol silanes 44 furnishes products 47 in up to 82% ee. Corey also demonstrated the use of these adducts as important building blocks for the synthesis of corresponding dihydropyrones treatment of 47 with trifluoroacetic acid affords the cyclic product in good yields. 

Arylthallium bis(trifluoroacetate)s are converted by successive treatment with KF and BF3 into aryl fluorides.Thallium(iii) nitrate (TTN) readily oxidizes dialkyl sulphides and selenides to the corresponding sulphoxides or selenoxides, and 2-(alkylthio)-l-arylethanones (37) into compounds (38) in methanolic solution.In a modification of the TTN oxidative conversion of aryl alkyl ketones into arylacetic acids, enol ethers derived from the ketones are used instead of the ketones themselves. This reduces the formation of side products. Cyclic aralkyl ketones (39) may be ring-expanded and alkylated to give compounds (40) via treatment of their Wittig-derived alkenes with TTN/ an extrapolation of the basic reaction discovered previously. 

The synthesis began with Prins qrchzation of the symmetric vinylo-gous ester 273, prepared from heptadienol 272, followed by hydrolysis of the resulting trifluoroacetate and benzylation, to afford the desired 2,6-cis-tetrahydropyran 274 with 92 8 diastereoselection at C5 [113], By this novel desymmetrization, 2,4,6-all-ci5 trisubstituted pyran was efficiently provided. Boron-enolate aldol reaction, as Carreira did, of the methyl ketone 274 with aldehyde 275 gave hydroxy ketone 271 as a single isomer. In contrast to Car-reira s result, samarium-catalyzed intramolecular Tishchenko reduction [114]... 

The total synthesis of ( + /—)-isostemonamide (38) (Scheme 21) proceeded from 213 according to the sequence described for (+ /—)-stemonamide (37) (Scheme 20) in 16 steps and in 3.5% overall yield from 207. In the Cu(I)-mediated addition of Grignard reagent 215 to the Q ,/8-unsaturated ketone 219 only or-attack was observed, and ketone 221 and the corresponding silyl enol ether 220 were formed in 32% and 57% yield, respectively. Both compounds were converted to the a -methylene lactone 222 under treatment with the Mannich reagent iV,iV-dimethylformaldimmonium trifluoroacetate, albeit under different experimental conditions (80). 

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