A-Tosyloxylation of ketones

Flavanones - Isoflavones. Although Koser s reagent (1) is known to effect a-tosyloxylation of ketones,1 the reaction with flavanones (2) results in an oxidative 1,2-aryl shift to provide isoflavones (3) in 74-80% yield.2 This conversion has been effected previously with thallium salts. 

The practical value of DMP as a reagent has been extended to various other synthetically useful oxidative transformations, such as the dehydration of primary alcohols under extraordinarily mild conditions [1276], synthesis of various polycyclic heterocycles via the oxidative cascade cyclization of anilides with pendant double bonds [1277], one-pot oxidative allylation of Morita-Baylis-Hillman adducts with allyltrimethylsilane promoted by DMP/Bp3-OEt2 [1278], synthesis of 2-amino-l,4-benzoquinone-4-phenylimides from anilines via DMP oxidation [1279], a-tosyloxylation of ketones using DMP and p-toluenesulfonic acid [1280] and the DMP-mediated oxidative aromatization of 1,3,5-trisubstitutedpyrazolines [1281]. 

As a further extension of the Ochiai s procedure for a-acetoxylation, the catalytic procedure for a-tosyloxylation of ketones using mCPBA as stoichiometric oxidant and iodoarenes as catalysts in the presence of p-toluenesulfonic acid has been developed. Various a-tosyloxyketones 7 can be efficiently prepared in high yields from the reaction of ketones 6 with mCPBA (1.1 equiv) and p-toluenesulfonic acid (1.1 equiv) in the presence of a catalytic amount of iodobenzene with moderate warming (Scheme 4.4) [19]. The mechanism of this reaction involves initial oxidation of Phi by mCPBA in the presence of p-toluenesulfonic acid to generate [hydroxy(tosyloxy)iodo]benzene in situ, which then reacts with the enol form of ketone to give the a-tosyloxyketone. 

A one-pot procedure for the a-tosyloxylation of ketones by the reaction of ketones with mCPBA and TsOH H2O in the presence of catalytic amounts of NH4I and benzene in a mixture of MeCN and trifluo-roethanol (8 2) has been reported (Scheme 4.67) [110]. This method has some advantages, such as mild reaction conditions with a simple procedure and it is suitable for preparing not only a-tosyloxy ketones but also other a-sulfonyloxy ketones. It has been suggested that [hydroxyl(tosyloxy)iodo]benzene, generated by the reaction of iodide anion with mCPBA, benzene and TsOH, serves as the active species in this reaction [110]. 

Poly[hydroxy(tosyloxy)iodo]styrene (25) shows similar reactivity to the monomeric [hydroxy (tosyloxy)iodo]benzene (HTIB) [44] in the a-tosyloxylation of ketones. Moreover, polymeric reagent 25 can be used for the direct conversion of alcohols 26 into a-tosyloxyketones 27 (Scheme 5.14), while HUB has a low reactivity toward alcohols [24,25]. 

The reactivity pattern of compounds 90 and 91 is similar to common hypervalent iodine reagents, as illustrated by their use for the oxidation of sulfides, oxidative iodination of arenes and a-tosyloxylation of ketones (Scheme 5.32) [89]. The products of all these reactions can be conveniently separated from the by-product, 3-iodobenzoic acid, by simple treatment with ion-exchange resin IRA-900 according to... 

Likewise, the a-tosyloxylation of ketones with ion-supported [hydroxy(tosyloxy)iodo]arenes 101 proceeds smoothly in acetonitrile even at 0 °C (Scheme 5.34) [95]. These reactions are faster than those with... 

Polystyrene-supported iodobenzene has been used instead of iodobenzene as a catalyst for the a-tosyloxylation of ketones and alcohols with mCPBA and p-toluenesulfonic acid [111]. In particular, two kinds of polymer-supported Phi, the standard linear poly(4-iodostyrene) and the macroporous crosslinked poly(4-iodostyrene), have been used as recyclable catalysts, which could be recovered from the reaction mixture by simple filtration in 90-100% yield. Recovered poly(4-iodostyrene) could be reused for the same reaction to provide the corresponding a-tosyloxyketone in good yields [111]. Recyclable ionic-Uquid-supported iodoarenes have also been used as catalysts in the a-tosyloxylation of ketones with /nCPBA and p-toluenesulfonic acid [112]. 

A diaryIprolinol silyl ether induces ee < 99% for aldehyde a-alkylation on 1,4-addition to acridinium salts. Computational chemistry has been used to explain stereoinduction by a family of iodooxazoline catalysts developed for enantioselective a-tosyloxylation of ketones by m-CPBA/TsOH. a-Benzoyloxylation of ketones (cyclohexanones and 1-indanones) by dibenzoyl peroxide occurs enantioselectively in the presence of a mixture of 9-amino-(9-deoxy)epi-dihydroquinidine and salicylic acid. ° The scope and limitations of asymmetric a-oxyacylation of cyclic ketones by chiral Ai-alkyl-O-acyl hydroxylamines have also been reported. ... 

The catalytic enantioselective a-tosyloxylation of ketones using iodoaryloxazoline catalysts has been reported. A unique mode of stereoinduction from the chiral oxazoline moiety, where the stereogenic centre alpha to the oxazoline oxygen atom is significant, has been disclosed. Computational chemistry was used to rationalize the stereoinduction process. The catalysts presented promote currently among the best levels of activity and selectivity for this transformation. 

R = I(OAc)2, I(02C-CF3)2, and I(OH)OTs, respectively]. In addition to being useful for general oxidations (alcohol to ketone, hydroquinone to quinone, etc.), the recyclable reagents catalyse a-tosyloxylation of methyl alkyl ketones on the more hindered side, an ostensibly unexpected result for such a bulky reagent. 

Higher levels of enantioselectivities in the a-oxytosylation of ketones were achieved in several recent works. Chi Zhang and coworkers have evaluated spirobiindane-based chiral iodoarenes as catalyst and were able to obtain a-tosyloxylated ketones in up to 58% enantiomeric excess using catalyst 12 (Figure 4.1) [28]. Moran and Rodriguez have prepared several chiral aryl iodides (e.g., structures 13 and 14, Figure 4.1) and... 

The polymeric silica materials derived from silylated aryl iodides have been prepared via sol-gel processes, either by the hydrolytic polycondensation of a bis-silylated monomer or by the co-gelification of a monosilylated precursor with tetraethyl orthosilicate [113]. These silica-supported aryl iodides have been successfully applied as supported catalysts in the a-tosyloxylation of aliphatic ketones in the presence of m-chloroperbenzoic acid as an oxidant, with the corresponding a-tosyloxyketones obtained in moderate to good isolated yields. The supported catalysts can be recycled by a simple filtration [113]. 

Another analogue of HTI which was used with either ketones or silyl enol ethers was [hydroxy(mesyloxy)iodo]benzene, PhI(OH)OS02Me [25]. A related reagent formed in situ from iodosylbenzene and trimethylsilyl triflate, probably PhI(OSiMe3)OTf, reacted similarly with silyl enol ethers to afford a-ketotriflates (see Table 5.3). /1-Diketones and /1-ketoesters underwent tosyloxylation by HTI the reaction was very effective in substrates with a perfluoroalkyl moiety and gave their hydrates [26] ... 

Based on the oxidation-tosyloxylation sequence, Togo and coworkers have developed the preparation of a-tosyloxy ketones and aldehydes 146 in good yields from alcohols 145 by treatment with iodosylbenzene and p-toluenesulfonic acid monohydrate (Scheme 3.61) [206]. This method can also be used for the direct preparation of thiazoles (147, X = S), imidazoles (147, X = NH) and imidazo[l,2-a]pyridines 148 from alcohols in good to moderate yields by successive treatment with iodosylbenzene and p-toluenesulfonic acid monohydrate, followed by thioamides, benzamidine and 2-aminopyridine, respectively (Scheme 3.61) [206]. 

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