A-Iodo ketone

Alkenes have also been converted to more highly oxidized products. Examples are (1) Treatment with KMn04 in aqueous acetone containing acetic acid gives a-hydroxy ketones. (2) 1,2-Disubstituted and trisubstituted alkenes give a-chloro ketones when oxidized with chromyl chloride in acetone RCH=CR R"—> RCOCCIR R". (3) a-Iodo ketones can be prepared by treating alkenes with... 

A mixture of 1,4-dioxane and water is often used as the solvent for the conversion of aldehydes and ketones by H2Se03 to a-dicarbonyl compounds in one step (Eq. 8.117).331 Dehydrogenation of carbonyl compounds with selenium dioxide generates the a, (i-unsaturated carbonyl compounds in aqueous acetic acid.332 Using water as the reaction medium, ketones can be transformed into a-iodo ketones upon treatment with sodium iodide, hydrogen peroxide, and an acid.333 Interestingly, a-iodo ketones can be also obtained from secondary alcohol through a metal-free tandem oxidation-iodination approach. 

Iodine may be employed as a mediator to achieve a-hydroxylation of carbonyl compounds. In basic methanolic solution containing iodide, oxidation was reported to lead to a-iodo ketones, which further reacted to give a-hydroxy ketals [171]. The electrolysis of alkylidenemalonates in the presence of iodide as a mediator has been shown to yield cyclopropane derivatives [172]. 

Different protocols have been tested to prepare enolates from /1-aryl-a-iodoketones. Reactions nsing EtsB/PhsSnH in benzene, EtsB in benzene or ether and w-BuLi in ether failed to provide the corresponding enolates. Alternatively, the use of EtMgBr succeeds in generating reactive magnesium enolates from a-iodo ketones. In most cases, the formation of the enolate in THF is cleaner than that in EtiO (equation 8). 

Nal in refluxing dioxane-water is used. Hindered bromo ketones are not reduced. In the absence of acid, conversion to the a-iodo ketone is the only observed reaction (1, 1087). 

After florisil (magnesium-silicate) filtration and concentration, crude 18 was treated with a THF solution of a 1 1 mixture of sodium iodide and mcte-chloroperoxybenzoic acid (MCPBA) yielding in 67 % (from 5) the a-iodo ketones 6.8 The ratio of diastereomers in this mixture was not described further. Mechanistically MCPBA oxidizes the iodide ion to an iodoniumion-species, which reacts with the double bond, generating intermediate 21. After TMS is removed the tricyclic iodonium ion collapses to desired 6. In contrast to this transformation the researchers observed no useful yields by direct treatment of the silyl enol ether with molecular iodine. 

P-Hydroxy ketones. a-Bromo ketones undergo a Reformatsky-type reaction with aldehydes which is promoted by R3Sb but which is only possible with iodine as catalyst. The function of I2 is probably for a Br/I exchange, since I2 is not required in reactions with a-iodo ketones. 

Diketo diester 512 has played a key role in one of Paquette s approaches to the pentagonal dodecahedrane. Direct hydroboration-oxidation of502 provided as the principal product the unwanted isomer 511 (49 %) rather than 512 (30 %). This complication was circumvented by the improvisation of the cross-corner oxygenation sequence outlined in Scheme 75.420,42 The iodolactonization of diacid 510 proceeded with high efficiency to give 513 which underwent cleavage to 514 in the presence of methanolic sodium methoxide at room temperature. Oxidation to a-iodo ketone 515 followed by reductive deiodination with zinc-copper couple and ammonium chloride in methanol solution furnished isomerically pure 512. 

Dehalogenation of ot-halo ketones. This combination converts a-halo ketones into dithioketals of the parent ketone. Although any halo ketone undergoes this reaction, yields are generally higher with a-bromo or a-iodo ketones. 

Asymmetric reduction of a-halo ketones. An a-halo substituent increases the rate and cnantioselectivity of reduction of aryl ketonc by neat 1. Chemical yields of chloro-and bromohydrins thus obtained arc usually >90 optical yields are usually also >90%. Chemical yields in reduction of a-iodo ketones. ire lower because of dciodination. The absolute configuration is (R) when 1 is derived fiom ( + )-a-pincnc. As expected, optical yields are lower when the reduction is conducted with aliphatic a-halo ketones. 

ShaCK, Jean TS, Wang DC (1990) Intramolecular Radical Cyclization of Silylacetylenic or Olefinic a-Iodo Ketones Application to the Total Synthesis of ( )-Modhephene. Tetrahedron Lett 31 3745... 

Silver chromate, Ag2Cr04, a reddish-brown precipitate prepared by adding an aqueous solution of silver nitrate to an aqueous solution of potassium chromate, is used with iodine for the preparation of a-iodo-ketones from alkenes [616]. 

The exchange reactions of a-iodo ketones and a,a-dibromo ketones have been re-... 

Treatment of a-iodo ketone and aldehyde with an equimolar amount of Et3B yielded the Reformatsky type adduct in the absence of PhaSnH (Scheme 21), unlike ot-bromo ketone as shown in Scheme 15 [22], Ethyl radical abstracts iodine to pro-duee carbonylmethyl radical, which would be trapped by EtsB to give the corresponding boron enolate and regenerate an ethyl radical. The boron enolate reacts with aldehyde to afford the adduct. The three-component coupling reaction of tert-butyl iodide, methyl vinyl ketone and benzaldehyde proceeded to give the corresponding adduct 38, with contamination by the ethyl radical addition product 39. The order of stability of carbon-centered radical is carbonylmethyl radical > Bu > Pr > Ef > Me . 

If enol acetates contain no extra C=C bond, a>iodo ketones can also be prepared by means of IC1 in CHC13 if the acetyl chloride formed is simultaneously removed.709... 

Iodo 20-oxo steroids are important intermediates for synthesis of 21-hydroxy 20-oxo steroids, e.g., dehydrocorticosterone and cortisone,711 also deoxycorticosterone 712 so the preparation of these a>iodo ketones by the following very interesting reaction with iodine should be noted ... 

Desoxybenzoins. Benzoins are reduced to desoxybenzoins by treatment with red phosphorus and iodine in carbon disulfide containing pyridine (3.5 hr., 25°). Presumably an a-iodo ketone is formed as an intermediate. This deoxygenation has been carried out also with hydriodic acid (1,449). 

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