Oxidation reactions dicarbonyl preparation

Rubottom oxidation reactions have been conducted on enolsilanes derived from a number of different carbonyl derivatives including carboxylic acids and esters.15 For example, the Rubottom oxidation of bis(trimethylsilyl)ketene acetal 30 provided a-hydroxy carboxylic acid 31 in 81% yield. Use of alkyl trimethylsilyl ketene acetal substrates generates a-hydroxy esters, as seen in the conversion of 32 to 33.16 The synthesis of 3-hydroxy-a-ketoesters (e.g., 36) has been accomplished via Rubottom oxidation of enolsilanes such as 35 that are prepared via Homer-Wadsworth-Emmons reactions of aldehydes and ketones with 2-silyloxy phosphonoacetate reagent 34.17 The a-hydroxylation of enolsilanes derived from P-dicarbonyl compounds has also been described, although in some cases direct oxidation of the P-dicarbonyl compound is feasible without enolsilane formation.18... 

Several 1,4-dicarbonyl compounds are prepared based on this oxidation. Typically, the 1,4-diketone 10 or the 1,4-keto aldehyde 12 can be prepared by the allylation of a ketone[24] or aldehyde[61,62], followed by oxidation. The reaction is a good annulation method for cyclopentenones (11 and 13). Syntheses of pentalenene[78], laurenene[67], descarboxyquadrone[79], muscone (14 R = Me)[80]) and the coriolin intermediate 15[71] have been carried out by using allyl group as the masked methyl ketone (facing page). 

This 1,3-migration of hydrogen was also observed when 40 reacted with Lawesson s reagent to produce the dithiolactone 41. However, when y-hydroxy-a,P-unsaturated aldehyde 42 was reacted under similar conditions, thiophene 43 was prepared efficiently. These results are not surprising considering that the oxidation state of 42 is equivalent to the traditional saturated 1,4-dicarbonyl substrates of the Paal thiophene reaction via tautomerization of the double bond, and aromaticity is reestablished in the fully conjugated 43. 

Pyridazines 160 were obtained by microwave-assisted reaction of 1,4-dicarbonyl compounds and hydrazine in AcOH and in the presence of DDQ as oxidant in order to obtain the aromatic compound in a one pot reaction [ 105]. The yields reported were relatively low although the method can be applied to the preparation of arrays of trisubstituted pyridazines with high molecular diversity (Scheme 57). 

The majority of nonsteroidal antiinflammatory agents contain an acidic carboxyl group. A series of experimental agents in this class have been prepared in which the acidic proton is supplied by a highly enolizable proton from a function such as a p-dicarbonyl incorporated into a heterocyclic system. As an example, an acylated, highly oxidized isoquinoline moiety can fulfill this function (see also the benzo-thiazines below). Toward this end, reaction of... 

The pyranocoumarin 105 can be prepared via a three-component Diels-Alder reaction between 4-hydroxycoumarin, ethyl vinyl ether and an a-dicarbonyl compound. Similarly to the above, upon treatment of 105 with sulfuric acid in THF, hydrolysis and rearrangement occur to give the furofurochromenone 106. The hemiacetal functionality in 106 may then be oxidized with pyridinium chlorochromate (PCC) to give the lactone 107 <2001EJ03711> (Scheme 28). 

A different route to pyrones is the preparative electrochemical oxidation of enamines in acetonitrile in the presence of tetraethylammonium perchlorate (88MI2) (Scheme 46). The synthesis of 2-pyrone derivatives has been carried out by reaction of /3-dicarbonyl compounds with methyl-a-benzoylamino-/3-dimethylaminoacrylate (96JHC751). Thiapyran derivatives can be obtained by interaction of enamines based on (/3-amino-a-cyanoacryloylmethyl)pyridinium chloride derivatives with carbon disulfide (95M711).The synthesis of pyridine derivatives based on analogous enamines has been described as well (95M711). 

Most known monocyclic 1,2,3-triazines and 1,2,3-benzotriazines are stable at room temperature. No detailed study of the stability of monocyclic 1,2,3-triazines towards water, aqueous acids or bases has been published, but one can assume from the reaction conditions used in the preparation of monocyclic 1,2,3-triazines, by oxidation of (V-aminopyrazoIes, that they are stable to water, aqueous acids or bases at room temperature, at least for a short time. Treatment of triaryl-1,2,3-triazines with aqueous hydrochloric acid at higher temperatures leads to hydrolysis of the ring and formation of 1,3-dicarbonyl compounds (Scheme 3) (60TL(13)19,76UP21800). 

A further general route to the 1,2-dicarbonyl system involves the oxidation of a-ketols (acyloins) (cf. the preparation of benzil from benzoin, Expt 6.143). The acyloins may be prepared from carboxylate esters by a radical coupling reaction involving the use of finely divided sodium metal in anhydrous ether, benzene, or toluene.144... 

In the intermolecular mode, this reaction has been utilized for the preparation of products 28 from various nucleophiles, including C-nucleophiles (e. g. (3-dicarbonyl compounds). A similar reaction in the intramolecular mode provides a powerful synthetic tool for the preparation of various polycyclic compounds via oxidative biaryl coupling [21,27 - 30]. Several examples of these C-C bond forming reactions are shown in Schemes 13-15. Specifically, various dibenzoheterocyclic compounds 30 have been prepared by the oxidation of phenol ether derivatives 29 with [bis(trifluoroacetoxy)iodo]benzene in the presence of BF3-etherate in dichloromethane (Scheme 13) [27-29]. 

The known vanadium carbonyl cations are of two types, namely a tetracarbonyl, [AreneV(CO)4]+, and a dicarbonyl, [Cp2V(CO)2]+. The tetracarbonyl derivatives are readily prepared under mild conditions by the reaction of an arene with vanadium hexacarbonyl. The arenes used include benzene, its methyl derivatives 28, 29), naphthalene, and anisole 29). The cation is probably formed by oxidation of the intermediary arene vanadium tricarbonyl. 

Table I summarizes the application of various low-valent titanium metallocenes as catalysts for olefin hydrogenation. Compounds 10 and 37 are very effective hydrogenation catalysts for C2H4 and cyclohexene. Since different researchers have used widely varying conditions, we can only estimate that the polystyrene-supported (7j-C8H8)2Ti (142) is comparable in activity to compounds 10 and 37. When one recalls that 37 was prepared by a formal oxidation of the Ti centers in 10, it is remarkable that 37 is as good a catalyst as 10. Solutions prepared by reaction of l-methyl-17-allylbiscyclopentadienyltitanium (54) with H2 do appear to be more active hydrogenation (126) catalysts than 10 and 37. The dicarbonyl complex, (17-CsH5)2Ti(CO)2 (39), has been shown to be a catalyst for the hydrogenation of acetylene at —50 atm of H2 (143). It does not catalyze the hydrogenation of simple olefins. However, Floriani and Fachinetti discovered that if...

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