Allenic ketone

The isomerization of acetylenic oxiranes cis- and trows-91 to allenic ketone 94 has recently been described (Scheme 5.18). It is proposed that the rearrangement proceeds via a dilithium ynenolate [33]. 

Quantitative yields of allenes were obtained from keten and the stable phosphoranes (48). Addition of similar phosphoranes occurred at the j3-position of the allenic ketones (49) to give the phosphoranes (50). The compound (50 = COPh) eliminated phosphine oxide under the... 

Few furan 3-carbanions are stable except at low temperatures and only recently have they been extensively used. Ring opening is common. Gilchrist and Pearson218 report that the fate of the unusually stable carbanion 84 depends upon both solvent and temperature. At room temperature and in benzene it partly opens giving the allenic ketone 85 in ether it is fairly stable. In hexane, the 3-lithiofuran is precipitated unless heated to 65 C when it isomerizes to acetylenic salts giving the allenic ketone 85 with water. 

Accordingly, the cyclopropenylidene anthrones 190/198 were converted by ferric chloride in hydroxylic solvents to the allene ketal 466, whose hydrolysis gives the allenic ketone 46 7288. The dioxolane 468 was obtained from the alkyl-substituted quinocyclopropene 190 in glycol and the ketone 467 in methanol. Apparently FeCl3 served not only as an oxidant, but also as a Lewis acid assisting solvent addition to C1 2 of the triafulvene. 

Butatrienyl cations 61 were also implied in the solvolysis of the w-propynyl vinyl triflate 6243. The vinyl triflate solvolyzes 35-70 times faster than the corresponding simple vinyl analogues and gives products resulting mainly from the a-alkynyl vinyl cation 63. A small amount of the allenic ketone 64 derived from the butatrienyl cation is also detected (equation 22). 

Scheme 2.48 Synthesis of allenic ketones with the aid of aluminum-based Lewis acids.

The intramolecular dipolar cycloaddition of a nitrone with an unactivated allene was also studied [76], Treatment of 5,6-heptadien-2-one with N-methylhydroxyl-amine in refluxing ethanol yielded allenyl nitrone 78, which cyclized with the terminal allenic C=C bond to give an unsaturated bicyclic isoxazolidine. On the other hand, the site selectivity decreased with an allenic ketone having a trimethylene tether. 

Allenic ketones undergo a thermal cycloaddition reaction with 1,3-dienes. The carbon-carbon double bond proximal to the carbonyl group reacts exclusively as in the case of allenic esters [105]. 

Cyclic dienes such as cyclopentadiene and furan react with allenic ketones at a lower temperature (Table 12.7) [105, 106], Preferential formation of the endo adducts was observed for the both dienes. 

Table 12.7 Reactions of cyclic dienes with allenic ketones.

Martin and Martin 51) have reported similar small (AJ= -0.2 to -0.45 Hz) changes for VHH in a series of alkyl substituted vinyl bromides. The same authors si> have repeated a 1.5 Hz decrease with increasing solvent polarity for the negative geminal coupling constant in allenic ketones of the form RCOCH = C=CH2 where R is methyl or ethyl. Reaction field interactions are suggested. 

The dispersity or homogeneity of the reductant in a reaction system sometimes plays a decisive role. It is also important for synthetic practice. Crandall and Mualla (1986) compared reduction of 7-methylocta-5,6-diene-2-one [H3C-C(CH3)=C=CH-CH2-CH2-C(0)-CH3] in THF by the action of naphthalene-sodium, on the one hand and, by sonically activated sodium on the other. In both the cases, one-electron transfer yields the anion-radical salt of the allenic ketone with sodium. However, only in the case of sonicated sodium is this salt stabilized, eventually giving H3C-C(CH3)=C=CH-CH2-CH2-C(0H)-CH3 along with cyclic products (l-methyl-2-isopropylidene cyclopentanol and l-methyl-2-isopropylcyclopent-2-enol). If naphthalene-sodium is used, only the cyclic alcohols are obtained as mentioned earlier. 

The treatment of furfuryl phenyl selenides (178) with n-butyllithium or metallic lithium affords furfuryllithium reagents which undergo ring opening to furnish initially the allenic ketones (180) (80TL3209). Two equivalents of n-butyllithium are required for complete conversion due to the acidic character of the allenic hydrogen. The allenic ketones are readily isomerized to the dienones (181) on p-toluenesulfonic acid treatment (Scheme 38). 

The thiophilic addition of Grignard reagents was also carried out with S-allylic and propargylic dithioates. In these cases a [2, 3] sigmatropic rearrangement of the magnesiocarbanion occurred, and alkylation at sulfur yielded the dithioacetals of (i-unsaturated ketones [334] or of allenic ketones [335]. 

For the most part, alkynic and allenic ketones have found limited use in conjugate addition-enolate trapping sequences 69,81-83 their analogous esters have been used with far greater frequency (vide infra). Alkynic ketones, in particular, have found use in development of a new anionic polycyclizadon method consisting of intramolecular Michael addition followed by intramolecular alkylation (equation 15).84... 

Allenic ketones and esters are found to react with azides by addition, preferentially to the activated double bond in the a, -position to the ester or keto group, although no regioselectivity is observed.254 Allenic nitriles undergo similar additions, and unstable methylene triazolines are obtained (Scheme 78).301... 

Allenic imines, silver-catalyzed cycloisomerizations, 9, 559 Allenic ketones, silver-catalyzed cycloisomerizations, 9, 558 Allenic natural products, via copper-catalyzed substitutions,... 

Similarly, ester-activated methylene groups react with propiolaldehydes (Scheme 139) <2004TL9197> and 1,2-allenic ketones (Scheme 140) <20020L505, 2003JOC8996> to afford 2//-pyran-2-ones. 

In the majority of cases alkyl hydrazines give single pyrazoles.160, 328, 336,337 In 1958 Bertrand established that allenic ketones give pyrazoles quantitatively with hydrazine.338 The reaction may be formulated as follows, the second stage being stabilization by aromatization. Acetylenic nitriles give aminopyrazoles with hydrazine.339... 

As isomerase substrates, XV and XVI would be enzymatically isomerized to A4>5-allenic ketones. Since allenic ketones react with nucleophiles by addition across C=C, it was hoped that such a process would occur at the isomerase catalytic site. The desired reaction sequence is given in Figure 11. A crystallographic determination of the structures of Compound XV, its allenic isomer, and the analogous normal substrate androst-5-ene-3,17-dione has been conducted by Carrell and co-workers... 

The reaction can be extended to allene-aldehydes. Kang et al. [16] reported on the Ru3(CO)12-catalyzed cyclocarbonylation of allene-aldehydes or allene-ketones leading to a-methylene-y-butyrolactones (Eq. 6). The fact that a ketone moiety also functions as a two-atom unit indicates that the reaction includes metallacycle 5 as an intermediate. 

The same products are accessible by silver-catalyzed cycloisomerization of allenic ketones. Marshall and Bartley327 used AgNOs/silica gel in hexane to convert the allenic ketones 384 into the furans 386 with excellent yields (Scheme 112). Deuterium labeling experiments were interpreted in terms of the intermediate 385 which seems to arise from the coordination of silver catalyst to the allenic double bond distal to the carbonyl group. Again, gold precatalysts can be used with much lower catalyst loadings than their silver counterparts (see Section 9.12.4.3). 

Various applications of the method have been reported324,325,325 1 and include the efficient formation of the functionalized furan 388, a precursor of the agylcon 389 of the naturally occurring antibiotic eleutherobin, from the allenic ketone 387 (Scheme 113).328... 

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