Vicinal diketones

Adducts of BF and some organic compounds having labile hydrogen atoms in the vicinity of the atom bonding to the boron atom of BF may form a derivative of BF by splitting out HF. For example, P-diketones such as acetylacetone or benzoylacetone react with BF in benzene (38) ... 

Acids are usually the end products of ketone oxidations (41,42,44) but vicinal diketones and hydroperoxyketones are apparent intermediates (45). Acids are readily produced from vicinal diketones, perhaps through anhydrides (via, eg, a Bayer-ViUiger reaction) (46,47). The hydroperoxyketones reportedly decompose to diketones as well as to aldehydes and acids (45). Similar products are expected from radical— radical reactions of the corresponding peroxy radical precursors. 

In a recent total synthesis of the novel neurotrophic agent merrilactone A (22, Scheme 4) by Inoue and Hirama [24], key intermediate 21 with the cis-bicyclo[3.3.0] octane framework embedded within the caged pentacycle 22 was elaborated from cyclobutane 18 by a sequence of RCM and immediate cleavage of the resulting bicyclic vicinal diol 19 to raeso-diketone 20. Cyclooctenedione 20 then underwent regioselective transannular aldol reaction at low temperature (LHMDS, THF, -100 °C) to produce a 3 1 mixture of isomers in 85% combined yield. The major isomer 21 with the required stereochemistry was then converted into the racemic natural compound ( )-22 in 19 steps. 

Under these solvent-free conditions, the oxidation of primary alcohols (e. g. benzyl alcohol) and secondary alcohols (e.g. 1-phenyl-l-propanol) is rather sluggish and poor and is of little practical utility. Consequently, the process is applicable only to a-hydroxyketones as exemplified by various examples including a mixed benzylic/ali-phatic a-hydroxyketone, 2-hydroxypropiophenone that delivers the corresponding vicinal diketone [106,107]. 

Stereoselective hydride reduction of 1,2,5-thiadiazoline 1,1-dioxides 60 generates unsymmetrical 1,2,5-thiadiazol-idine 1,1-dioxides 61 <1998SL623> that can be readily converted to unsymmetrical vicinal diamines 62 with HBr in the presence of phenol (Scheme 5) <1996TL2859, 1998SL623>. The unsymmetrical thiadiazolidine 1,1-dioxides 63 can also be converted into 1,2-diketones 64 on treatment with selenium dioxide followed by alkaline hydrolysis (Equation 7) <1997SL671>. 

Scheme 9 Synthesis of bis( 1,2-dithiolene) complexes by sulfuration of acyloins or vicinal diketones and complexation in situ...

Figure 5.8 Reaction of a phosphite with a vicinal diketone.

The mechanism [26] of the oxidation of non-terminal aikynes by quaternary ammonium permanganates to give vicinal diketones [e. g. 18, 27] has been studied. Normally, terminal aikynes are totally oxidized in acetic acid to carboxylic acids [13, 27, 28] using a procedure analogous to 10.2.4. 

It is noteworthy to mention that the two water molecules in [Ho(HNIC)3 (0112)2] " cationic complex are occupying trans-positions across the diagonal of a square face (Fig. 21) while they occupy cis (vicinal positions in [M(NIC)3(OH2)2] dimers and in diketonate complexes described earlier. 

These methods are limited in scope. Direct thiation has been reported for the transformation of vicinal diketones to [3,4]-annelated thiophenes <1996CHEC-II(7)49>. Thionations and selenations of vicinal halomethyl ketones with thioacetamide or iV,iV-diethylselenopropionamide as sulfur and selenium donors, respectively, have also been reported <1996CHEC-II(7)49>. In a recent example, thieno[3,4-f]pyrazoles 160 were readily prepared from aryl 5-bromomethyl-l//-l-phenylpyrazole-4-yl ketones 159 by reaction with thioacetamide (Equation 35) <1998JHC71>. Additionally, an example of an amination cyclization by reaction of 161 with benzylamine to afford pyrrolo[3,4-i/ thiazole 162 has been described (Equation 36) <1998JHC71>. 

Dihydroxylation, the addition of two hydroxy groups across a C = C bond, converts fluorinated alkenes into different products depending on the presence or absence of a fluorine atom at the hydroxylated carbon. Partially fluorinated alkenes with vicinal hydrogen atoms attached to the C = C bond can be hydroxylated to vicinal diols. When the reaction is performed with a sufficiently strong oxidizing agent, the initially formed diols are oxidized to vicinal diketones as the end products. 

Potassium permanganate is a classical reagent for such Wagner-type dihydroxylations. Initially formed vicinal diols 50 immediately eliminate hydrogen fluoride to give x-diketones 51.90... 

Two main synthetic routes are available for the preparation of dithiolenes in the first and most frequently used method, either the free ethylenedithiol or an appropriate salt of the ethylene-dithiolato ligand dianion is reacted with a metal salt to produce anions of the dithiolenes, which may or may not be subsequently oxidized to the neutral species the second one, applied so far only to transition metal dithiolenes, converts vicinal diketones into dithiodiketones and reacts these either with zerovalent metals to form dithiolenes directly or uses metal salts to arrive at cationic species, which are reduced to the neutral dithiolenes either during the reaction or in a subsequent step. 

There are several methods reported in the literature for transforming vicinal diols into ct-diketones while avoiding the risk of C-C bond cleavage.26 Examples include the standard Swem conditions (dimethyl sulfoxide and oxalyl chloride followed by triethylamine), or the use of DMSO activated by acetic anhydride, pyridine-sulfur trioxide complex, or dicyclohexylcarbodiimide (Mq/J-att oxidation). Diones are also obtained by treatment with benzalacetone as a hydride acceptor in the presence of catalytic amounts of tris(triphenylphosphine)ruthenium dichlonde [(PPh RuCFl.27 Recent developments include the use of w-iodoxyben/.oic acid28 or the oxoammonium salt of 4-acctamidoletramethylpipcridine-1-oxyl and y -toluencNulfonic acid.29... 

This explains, for example, the tendency of some 1,2-diols to suffer oxidative carbon-carbon bond breakage under the action of PDC. Thus, although many 1,2-diols can be uneventfully oxidized to a-hydroxyketones with PDC,176 very often a cleavage of a carbon-carbon bond occurs, resulting in two carbonyl functionalities.177 Vicinal tertiary diols, sometimes, are smoothly oxidized to diketones by PDC.178... 

With sodium borohydride and catalytic amounts of titanyl acetoacetonate, a,fi-unsaturated carbonyl compounds give allyl alcohols regioselectively, whereas a-diketones and acyloins are reduced to vicinal diols.325 Enantioselectivities in the reduction of acetophenone, catalysed by 1,3,2-oxazaborolidones, have been examined using the AM1-SCF MO method. The optimized geometries, thermal enthalpies, and entropies of R and S transition states in the stereo-controlling steps of the reduction have been obtained.326... 

In contrast with previous reports, Sodeoka s reaction conditions were applicable to a variety of /8-ketoesters (Scheme 2) [5]. With the Pd aqua complex 3, the Michael adducts were obtained in high yield with high enantioselectivity. For example, the reaction of the /8-ketoester 4 with less reactive /3-substituted enones such as 3-penten-2-one proceeded smoothly to give 5 in 99 % ee. In this reaction, catalytic asymmetric construction of the highly crowded vicinal tertiary and quaternary carbon centers was achieved in one step. Furthermore, the reaction of 1,3-diketones with enones was successfully performed, and the desired triketone 6 was obtained in good yield and with 90 % ee. In contrast, conventional basic conditions (tertiary amines, alkoxides, or ammonium hydroxides) gave complex mixtures, probably because of the instability, under the basic conditions, of the triketone produced. This result clearly indicates that the reaction using the Pd aqua complex 3 is quite mild. 

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