Tetralones enol acidity

The reaction of 2-iodobenzoic acids derivatives (288) with tetralone enolate ions (284) gives the products 291 which, after acid treatment, afford the ring closure compounds 292 (60-75% yield) (equation 179)328. 

Enol acidity can be increased by simply ensuring that conjugation is present in the enolic form but not the carbonyl form. 2-Indanone (49) has greater enolic acidity than both 2-tetralone (50) and 2-benzosuberone (51) due to more efficient tt-overlap with the adjacent aryl ring . By comparison, conjugation present in both the ketonic and enolic forms, such as in 1-tetralone (52) and 4-chromanone 52 ), increases enol acidity relative to saturated compounds such as acetone (45 ). 

In each of these cases, the stoichiometric achiral acid is kinetically slow, and is also added slowly to the reaction mixture. The commercially available amino acid derivative (12.34) has also been used as an effective enantiomerically pure proton source in the protonation of tetralone enolates such as (12.35). Samarium enolates have also been protonated enantioselectively (up to 93% ee) using catalytic amounts of a C2-symmetric diol. ... 

Electronic effects on enol acidity and keto-enol equilibrium constants for ring-substituted 2-tetralones have been studied. Hammett plots of pKf and against are linear with slopes (p) of -1.66 0.06 and -0.90 0.03, respectively. 

The acid-catalyzed reaction of salicylaldehydes with 2-tetralone 168 affords 12//-bcnzo[ ]xanthcncs 169. The reaction proceeds via an initial condensation of the aromatic aldehyde with the activated methylene group to form the intermediate 170. The extended enolate of the intermediate 170 undergoes electrocyclization, dehydration, and rearomatization to afford 12/7-benzo[ ]xanthenes (Scheme 54) <2004TL8999>. 

The authors had changed the starting material to the substituted tetralone 135 174), which was converted into the enol acetate 136 when treated with acetic anhydride and acid (Scheme 13). Subsequent ozonization afforded aldehyde 137. Chain elongation was achieved by Wittig condensation using Corey-Chaykovsky... 

Asensio and coworkers studied the effects of solvent, temperature, presence of lithium salts and proton acidity on the enantioselective protonation of enolates 230 to yield (/ )-2-methyl-1-tetralone (231, equation 58) with a-sulfinyl alcohols 232 and 1,2-sulhnyl thiols 233 . Stereoselectivity was generally enhanced when lithium bromide was present in the medium during enolization and also with the use of methylene chloride solutions. 

Naphthols are reduced to P-tetralones (which are presumably protected from further reaction as the corresponding enolate anions) even more readily and it is surprising to find that even 6-methoxy-2-naph-thol (65) is converted into ketone (66 Scheme 11). ° Reduction of the equilenin ketal (67) was found to be highly dependent on the choice of metal. Reduction of (67 Scheme 12) or its sodium salt by lithium in the absence of an alcohol afforded (after acid hydrolysis) equilin (68) as the major product, whereas... 

The liquid reagent has high solvent power, particularly for oxygen-containing substances and for aromatic compounds. It is an effective dehydrating and condensing agent comparable to coned, sulfuric acid but less prone to permit secondary reactions such as enolization and aromatic substitution. Treatment with liquid HF at room temperature provides a simple and efficient method for the cyclization of /3-arylpropionic acids and y-arylbutyric acids to 1-indanones and 1-tetralones, as shown by the yields cited under the formulas of typical ketones. It is far superior... 

Enolization of triplet 5,8-dimethyl-1-tetralone by hydrogen transfer effects" ", spin-lattice relaxation of pentacen-6,13-quinone triplets in a benzoic acid host crystal involves proton tunnelling", and photoreduction of benzophenone and thioxanthen-9-one by amines" are other researches of photochemical significance. Interactions of formate ions with triplet states of benzophenone-4-carboxylate and -4-sulphonate, 1,4-naphthoquinone, and anthraquinone-2-sulphonate have been examined systematically by laser flash photolysis". ... 

Screening the conditions for the enantioselective protonation of 2-substituted tetralon-l-eno-lates reveals striking effects as demonstrated by 14157b. The corresponding lithium enolate is complexed with (7 )-13 and then protonated with acetic acid. A drastic solvent effect is observed by which (S)-14 is formed with 6-91 % ee. However, this high enantioselectivity is completely lost if lithium bromide is omitted. 

The reagent was also able to convert carbonyl compounds into enol trifluoroacetates. 3- and 4-phenylalkanoic acids were readily cyclized to 1-indanones and 1-tetralones, respectively, through the intermediary of a mixed anhydride formed with CF3S03H29 (equation 13). 

The enantioselective protonation of silyl enol ethers, such as (12.39), by a catalyst has been achieved using 2 mol% of the proton source (12.40). The acidity of (12.40) is enhanced by coordination to a Lewis acid. The silyloxy group is activated by fluoride ion and up to 99% ee in the asymmetric protonation of a-aryl substituted cyclic silyl enol ethers such as (12.39) has been obtained using a Lewis acidic BINAP. / F complex.In a similar vein, silyl enol ethers of tetralones and indanones undergo asymmetric protonation with moderate to good ee using catalytic quantities of hydrogen fluoride salts of cinchona alkaloids in the presence of acyl fluorides and ethanol, which act as a stoichiometric source of HE 28... 

In the same context, the asymmetric protonation of 2-methyl-1-tetralone-derived trimethylsilyl enol ether was achieved by Levacher et al. in the presence of a cinchona alkaloid [DHQ]2AQN derived from hydroquinine as the organo-catalyst and citric acid as the proton source in DMF. This reaction occurred with a good yield and an enantioselectivity of 73% ee, as shown in Scheme 10.12. 

---