Ketones acyloxy

The (acyloxy)borane complex 9, readily available from tartaric acid derivative 8, also catalyzes aldol additions of silyl enol ethers34 and silylketene acetals3 5 in an enantioselective manner. Thus,. u -ketones 10 and /Thydroxy esters 12 are available34, as well as a-unsubstituted ketones 1135. 

Analog lassen sich 2- und 4-Hydroxy-, 2,4-Dihydroxy- und 2,4,6-Trihydroxy-aceto-phenon zu den entsprechenden Athyl-phenolen reduzieren [3-(2,4-Dihydroxy-ben-zoyl)-propansaure wird dagegen nicht angegriffen1 2] (vgl. a.S. 273) (Acyloxy-phenyl)-ketone reagieren ahnlich z.B.3 ... 

When an isocyanide is treated with a carboxylic acid and an aldehyde or ketone, an a-acyloxy amide is prepared. This is called the Passerini reaction. The following mechanism has been postulated ... 

Sulfonate esters and halides can also migrate in this reaction. a-Halo and a-acyloxy epoxides undergo ready rearrangement to a-halo and a-acyloxy ketones,... 

The silyl enol ethers of ketones are also oxidized to a-hydroxy ketones by m-chloroperoxybenzoic acid. If the reaction workup includes acylation, a-acyloxy ketones are obtained.250 These reactions proceed by initial epoxidation of the silyl enol ether, which then undergoes ring opening. Subsequent transfer of either the O-acyl or O- l MS substituent occurs, depending on the reaction conditions. 

Acyloxy-l-cyanoalkanes [45, 46], which can be used as precursors for ketones [47], a-hydroxy ketones [48] and 1,4-dicarbonyl compounds [47], are prepared in one pot from the appropriate aldehyde, sodium or potassium cyanide, and the acylating agent under phase-transfer catalytic conditions [47-49]. Attempts to synthesize chiral cyanhydrins using chiral phase-transfer catalysts have been unsuccessful (see Section 12.3). 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

The amide carbonyl vibrational frequencies of A-acyloxy-Af-alkoxyamides are similar to that observed for the twisted l-aza-2-adamantanone (98, 1731 cm ) . It is apparent from the extensive data available for both A-chlorohydroxamic esters (Table 2, Section in.B.2) and Af-acyloxy-A-alkoxyamides that when an amide nitrogen lone pair loses conjugation with the carbonyl (either through twisting/pyramidalization or, in the case of anomeric amides, pyramidalization alone), the configuration is analogous to an ester rather than a ketone. As with esters, acid halides and anhydrides or diacyl peroxides , the carbonyl stretch frequency is higher than that of ketones and aldehydes... 

Again there is a number of interesting syntheses of di- and tetrahydrofuian derivatives. Due to space limitations only a few examples can be mentioned. The transformation of enantio-metically enriched butyn-l-ol monoesters into 2,2,5-ttisubstituted 3-acyloxy-2,5-dihydroftirans with complete stereospecificity was achieved by a Ag(I)-mediated rearrangement via allenic intermediates. The starting materials were prepared by an enantioselective reduction of the corresponding ketone, followed by acylation <99BCJ279>. 

The Conversion of Anhydrides, Carboxylic Esters, or Amides to Ketones with Organometallic Compounds1659 Alkyl-de-acyloxy-substitution... 

Acylation of Aldehydes and Ketones O-Acyl-C-acyloxy-addition... 

Enantioselective condensation of aldehydes and enol silyl ethers is promoted by addition of chiral Lewis acids. Through coordination of aldehyde oxygen to the Lewis acids containing an Al, Eu, or Rh atom (286), the prochiral substrates are endowed with high electrophilicity and chiral environments. Although the optical yields in the early works remained poor to moderate, the use of a chiral (acyloxy)borane complex as catalyst allowed the erythro-selective condensation with high enan-tioselectivity (Scheme 119) (287). This aldol-type reaction may proceed via an extended acyclic transition state rather than a six-membered pericyclic structure (288). Not only ketone enolates but ester enolates... 

Most solid-phase syntheses of pyrazoles are based on the cyclocondensation of hydrazines with suitable 1,3-dielectrophiles. The reported examples include the reaction of hydrazines with support-bound a,(3-unsaturated ketones, 1,3-diketones, 3-keto esters, a-(cyano)carbonyl compounds, and a, 3-unsaturated nitriles (Table 15.19). Pyrazoles have also been prepared from polystyrene-bound 3-(hydrazino)esters, which are generated by the addition of ester enolates to hydrazones (Entry 7, Table 15.19 see also Section 10.3). Benzopyrazoles can be prepared from support-bound hydra-zones using the reaction sequence outlined in Figure 15.11. Oxidation of a polystyrene-bound benzophenone hydrazone yields an a-(acyloxy)azo compound. Upon treatment with a Lewis acid, this intermediate is converted into a 1,2-diazaallyl cation,... 

Lead tetraacetate, 155 a-Acyloxy ketones Benzyltriethylammonium chloride, 239... 

JOC 53 5415 (1988) (a-alkoxy, acyloxy, amino, alkoxycarbonylamino, arenesulfonylamino ketones, 0-keto ester, 0-keto amides)... 

JOC 47 2820 (1982) (/1-diketones to hydroxy ketones) 50 127 (fi-keto carboxylate), 3411 (fi-diketones to hydroxy ketones) (1985) 51 1253 (2,2-dithioalkan-l-ones), 2795 (a-flooro ketones) (1986) 52192 (fi-keto esters), 256,1141,1359 (fi-keto esters), 2036 (/1-diketones to hydroxy ketones), 2086 (a-acetoxy ketone), 2244 (fi-keto ester), 3223 (cyclic /1-diketones to hydroxy ketones), 4363 (y- arid 5-keto acids) (1987) 53 860, 1969, 2589 (x-keto ester), 4405 (a-acyloxy ketone), 4962 (1988) 54 2238 (/S-diketone to fi-hydroxy ketone), 2274, 3221 (a-diketone to a-hydroxy ketone) (1989) 55 3917 (1990)... 

In the classic Passerini reaction (P-3CR), an a-acyloxy carboxamide is formed from the reaction of an isocyanide, an aldehyde (or ketone), and a carboxylic acid. The... 

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