A-Methoxy ketones

The reaction with a-hydroxy and a-methoxy ketones under these conditions are chelation controlled. 

By the use of chiral oxazolidines derived from a chiral norephedrine and methyl ketones, an asymmetric aldol reaction proceeds in a highly enantioselective manner. In the case of ethyl or a-methoxy ketones, the corresponding anti aldol products were obtained with high diastereo- and enantioselectivities. A chiral titanium reagent, generated from... 

The reactions of enol ester radical cations formed in anodic oxidations were pioneered by Shono [220-225] almost two decades ago. A reaction mode was identified that formally corresponds to that of enol cation radicals. Depending on the electrolysis conditions enol acetates were either converted to a-acetoxy ketones (high concentration of acetate) or to enone products (absence of acetate). Similarly, a-methoxy ketones were obtained through electrolysis in methanol-Et4NOTs. Yields for additional reactions not listed here varied between 29% and 90% [222,223]. 

Maruoka and his colleagues reported that B(CeF5)3 is capable of forming a penta-coordinate complex in the reduction of alkoxy-substituted carbonyl compounds with Bu3SnH [156]. Reduction of an a-methoxy ketone and its deoxy analog (1 1 ratio)... 

In 1913, Kishner observed in one instance that under standard Wolff-Kishner reduction conditions, 2-hydroxy-2,6-dimethyloctan-3-one underwent eliminative reduction upon treatment with hydrazine hydrate and base at elevated temperatures to afford 2,6-dimethyloctan-2-ene (Scheme 7). This same reaction was later found to occur in the case of a-methoxy ketones and has since been referred to as the Kishner eliminative reduction. The reaction entails initial formation of the hydrazone and elimination of the a-substituent to afford the intermediate alkenyldiazene, which subsequently collapses to the desired alkene. Given the facile transformation of ketones into a-halo ketones, these conditions have been used to introduce alkenes regioselectively in the 2a-halocholestan-3-one series as shown in Scheme 8. Yields of 2-cholestene parallel the resistance of the a-halogen to undergo competitive elimination reactions. 

Cr03-H2S04-Hg(II) salts/" and Hg(OAc)2 followed by PdCl2. The reaction has also been accomplished electrochemically. Terminal alkenes react with ceric ammonium nitrate in methanol to give a-methoxy ketones. 

This general procedure has also been extended to allow for the insertion of a methoxy-containing carbon. The uncatalyzed addition of the lithium anion of methoxymethyl enyl sulfone to ketones proceeds readily at low temperature in dimethoxyethane to form the intermediate adduct. Addition of a Lewis acid (ethylaluminum dichloride or diisobutylaluminum diisopropylamide) directly to the reaction mixture effects the rearrangement reaction to produce the ring-expanded a-methoxy ketone. This sequence, illustrated by the example in Scheme 28, is limited to the expansion of four- and five-membered ring ketones. ... 

Electrochemical reduction of 1,3-dibromo ketones in methanol at O C gave cyclo-propanone hemiacetals in excellent yields, e.g. reduction of 29 gave 30 in 90% yield.This conversion can be performed in quantiative yield, thus avoiding formation of the Favorskii ester and an a-methoxy ketone, by working up the reaction mixture at temperatures below 0°C. The intermediacy of cyclopropanones in the reaction was shown by IR monitoring of the controlled-potential electrolysis of l,3-dibromo-3-methylbutan-2-one in dimethylformam-ide in the presence of tetraethylammonium bromide at room temperature. The typical IR absorption at 1825 cm" is direct evidence of the presence of 2,2-dimethylcyclopropanone. ... 

With bulkier alkoxides, e. g. potassium ter/-butoxide in fcrr-butyl alcohol or potassium (4-chlorophenyl)dimethylcarbinolate, a-bromo ketone 16 is converted into rra x-2,3-di-/cr/-butyl-cyclopropanone (17). Addition of methanol or 2-propanol to cyclopropanone 17 at 25 °C gives a fast addition reaction resulting in hemiacetal 21, which slowly undergoes ring opening at 80 °C in methanol to give a-methoxy ketone 22 (R = Me). ... 

LDA is also able to reduce some enolizable -halo and a-methoxy ketones, such as CeHsCOCHsBr and 2,2-dimethoxycyclobutanone, to the corresponding sec-alcohols in surprisingly high yields. ... 

Dicarbonyl compounds are formed from the reaction of silyl enol ethers with methyl vinyl ketones in the presence of BF3 OEt2 and an alcohol (eq 9). a-Methoxy ketones are formed from a-diazo ketones with BF3-OEt2 and methanol, or directly from silyl enol ethers using iodobenzene/BF3-OEt2 in methanol. ... 

Maruoka and coworkers have shown that contrary to popular belief, tris (pentafluo-rophenyl) borane, B(C6E5)3, can adopt a penta-coordinate geometry and thus allow for chelation-controlled reactions. The authors have demonstrated that B(C6F5)3 promoted reductions of a mixture of a-methoxy ketone (93) and its deoxy analogue (94) give primarily the reduction product (95) over (96) from a chelation-controlled pathway (Equation 57) [57]. As a prelude to the next section of this chapter, it may be mentioned that Maruoka and coworkers further verified their hypothesis by the... 

Wender, P. A. and RawUns. D. B., Toward the synthesis of the taxol C,D ring system photolysis of a-methoxy ketones. Tetrahedron, 48, 7033,1992. 

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