Tetralone enolate

The generality of such intermolecular [D, A] complexes is shown in Fig. lc by the exposure of a colorless solution of a-tetralone enol silyl ether to different... 

Fig. 1 Charge-transfer absorption spectra of enol silyl ethers complexes with re-acceptors. (a) Spectral changes accompanying the incremental additions of cyclohexanone enol silyl ether [2] to chloranil in dichloromethane. Inset Benesi-Hildebrand plot, (b) Charge-transfer absorption spectra of chloranil complexes showing the red shift in the absorption maxima with decreasing IP of the enol silyl ethers, (c) Comparative charge-transfer spectra of EDA complexes of a-tetralone enol silyl ether [6] showing the red shift in the absorption maxima with increasing EAs of the acceptors tetracyanoben-zene (TCNB), 2,6-dichlorobenzoquinone (DCBQ), chloranil (CA), and tetracyanoqui-nodimethane (TCNQ). Reproduced with permission from Ref. 37.

When a-tetralone enol silyl ether in dichloromethane is mixed with an equivalent of tetranitromethane (TNM) at room temperature, the solution immediately takes on a bright red coloration owing to formation of the characteristic [D, A] complex37 (equation 12). 

The favourable effect of lithium bromide on facial enantioselective protonation of methyl tetralone enolate by a-sulfinyl alcohols has been attributed to coordination of lithium to both enolate and sulfinyl alcohol followed by competition between diastere-omeric paths involving intramolecular proton transfer the proposed transition-state model is supported by results of PM3 semiempirical calculations. ... 

Recently, Kochi et al. described a novel photochemical synthesis for a-nitration of ketones via enol silyl ethers. Despite the already well-known classical methods, this one uses the photochemical excitation of the intermolecular electron-donor-acceptor complexes between enol silyl ethers and tetranitrometh-ane. In addition to high yields of nitration products, the authors also provided new insights into the mechanism on this nitration reaction via time-resolved spectroscopy, thus providing, for instance, an explanation of the disparate behavior of a- and (3-tetralone enol silyl ethers [75], In contrast to the more reactive cross-conjugated a-isomer, the radical cation of (3-tetralone enol silyl ether is stabilized owing to extensive Tr-delocalization (Scheme 50). 

Benzo[c]phenanthridines are an important class of the isoquinoline alkaloid family. It was possible to synthesize a variety of these compounds based on the S l reactions of appropriate derivatives of o-iodobenzylamines with nucleophiles328. For instance, the pho-tostimulated reaction of 281 (R1 = R2= H, OMe R1 = OPr-/, R2= OMe R R2 = 0CH20) with the appropriate derivative of tetralone enolate ions 284 (R3 = R4 = H, OMe R3 = OMe, R4 = H R3 = OPr-/, R4 = OMe R3R4 = 0CH20) gave the substitution compound, which spontaneously cyclizes to 285 (equation 176). 

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. 

Chiral a-sulfinyl alcohol (S,i s)-ll was also shown to be a promising chiral proton donor in catalytic protonation of 2-methyl tetralone enolate by Asen-sio s group [19]. 

A particularly attractive version of this reaction relies on the action of a catalytic chiral lithium binaphtholate and an excess of water on trimethoxysilylenol ether119. The tetralone enolate thus generated was directly employed in an aldol reaction, which turned out to be poorly diastereoselective but highly enantioselective for both diastereomers (Scheme 27). 

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. ... 

The most recent, and probably most elegant, process for the asymmetric synthesis of (+)-estrone appHes a tandem Claisen rearrangement and intramolecular ene-reaction (Eig. 23). StereochemicaHy pure (185) is synthesized from (2R)-l,2-0-isopropyhdene-3-butanone in an overall yield of 86% in four chemical steps. Heating a toluene solution of (185), enol ether (187), and 2,6-dimethylphenol to 180°C in a sealed tube for 60 h produces (190) in 76% yield after purification. Ozonolysis of (190) followed by base-catalyzed epimerization of the C8a-hydrogen to a C8P-hydrogen (again similar to conversion of (175) to (176)) produces (184) in 46% yield from (190). Aldehyde (184) was converted to 9,11-dehydroestrone methyl ether (177) as discussed above. The overall yield of 9,11-dehydroestrone methyl ether (177) was 17% in five steps from 6-methoxy-l-tetralone (186) and (185) (201). 

The pyrrolidine enamine of 2-tetralone (177) was converted to l-cyano-2-tetralone, which exists almost entirely in the enolic form (178), by reaction with cyanogen chloride (J23). Reaction of 177 with cyanogen bromide gave N-naphthylpyrrolidine (179), presumably via the unstable bromoenamine (180). The latter observation is in accord with the mode of reaction of the heterocyclic enamine (126) with cyanogen bromide, which resulted in the... 

In the alkylation of enolate anions, a mixture of mono- and polyalky lation produets is usually obtained, and when enolization of a di-a-methylene ketone is possible toward both sides, a mixture of di-a- and a,a -dialkylation products ean be expeeted. Thus the enamine alkylation sequenee beeomes partieularly attractive when eontrolled monoalkylation is imperative beeause of difficulties in separation of a mixture of alkylation produets. One of its first synthetie applications was in the reaetions of /8-tetralones with alkyl halides. Yields in exeess of 80% were usually found 238-243) in these reaetions, which make valuable intermediates for steroid and diterpene syntheses more aecessible. 

This method fails, however, with bicyclic ketones such as 1-tetralones even in the presence of TsOH, affording only enol trimethylsilyl ethers such as 107 a [114, 115]. A subsequent investigation revealed that cyclohexanone reacts with equivalent amounts of N-trimethylsilyldimefhylamine 463 in the presence of TMSOTf 20 at -30 °C to give the enol silyl ether 107 a, whereas reaction of cyclohexanone, benzaldehyde, and chlorodimethyl ether with 463 and TMSOTf 20 or TCS 14 at 1-20 °C afforded the iminium salts 547, 548, and 549 in high yield [116-118]. Analogously, N-trimethylsilylpyrrolidine 550 and N-trimethylsilylmorphoHne 294 convert aldehydes such as benzaldehyde, at ambient temperature in the presence... 

The effect of chelating polyamines on the rate and yield of benzylation of the lithium enolate of 1-tetralone was compared with HMPA and DMPU. The triamine... 

The facile nitration of a wide variety of ketones with TNM in Table 2 is illustrative of the synthetic utility of enol silyl ethers in facilitating a-substitution of carbonyl derivatives. It is necessary to emphasize here that the development of a strong charge-transfer (orange to red) coloration immediately upon the mixing of various ESEs with TNM invariably precedes the actual production of a-nitroketones in the thermal nitration (in the dark). The increasing conversion based on the time/yields listed in Table 2 qualitatively follows a trend in which electron-rich ESE from 6-methoxy-a-tetralone reacts faster than the relatively electron-poor ESE from cyclohexanone. 

The first promising asymmetric aldol reactions through phase transfer mode will be the coupling of silyl enol ethers with aldehydes utilizing chiral non-racemic quaternary ammonium fluorides,1371 a chiral version of tetra-n-butylammonium fluoride (TBAF). Various ammonium and phosphonium catalysts were tried138391 in the reaction of the silyl enol ether 41 of 2-methyl-l-tetralone with benzaldehyde, and the best result was obtained by use of the ammonium fluoride 7 (R=H, X=F) derived from cinchonine,1371 as shown in Scheme 14. 

Fluorination of the sodium enolate of 2-methyl-1-tetralone by (-)-A-tluoro-2,10-(3,3-dichlorocamphorsultam) gives (5 )-(- -)-2-iluoro-2-methyl-1-tetralone in 70% ee, which corresponds to the opposite asymmetric induction to that achieved using non-racemic (camphorsulfonyl)oxaziridines as closely related hydroxylation reagents. ... 

Asymmetric fluorination of the lithium enolate derived from tetralone 307 has been achieved with several new chiral A-fluoro-l,2-benzothiazine reagents (Scheme 44) <2000JOC7583, 2000CPB1954, see also 1999JFC(97)65>. For instance, sulfonamides 149 and 150 afford the opposite enantiomers of fluoroketone 308 in up to 79% yield and 62-70% ee. 

The enol acetates from acetophenone, benzylphenyl ketone, isopropylphenyl ketone, benzylmethyl ketone, and tetralone gave the corresponding a-fluoroketones, in a similar fashion. 

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