Ketones, addition derivatives reactions

A unique method to generate the pyridine ring employed a transition metal-mediated 6-endo-dig cyclization of A-propargylamine derivative 120. The reaction proceeds in 5-12 h with yields of 22-74%. Gold (HI) salts are required to catalyze the reaction, but copper salts are sufficient with reactive ketones. A proposed reaction mechanism involves activation of the alkyne by transition metal complexation. This lowers the activation energy for the enamine addition to the alkyne that generates 121. The transition metal also behaves as a Lewis acid and facilitates formation of 120 from 118 and 119. Subsequent aromatization of 121 affords pyridine 122. 

The addition-elimination reaction of hetero-atom-substituted nitroalkenes provides functionalized derivatives of unsaturated nitro compounds.26 Nitroenamines are generally prepared from a-nitro ketones and amines (see Chapter 5 regarding acylation of nitro compounds).26... 

Isotellurazoles 4 were obtained in low yields (3-11%) by the one-pot reaction of alkynyl ketones with hydroxylamino-O-sulfonic acid and K2Te in aqueous solution containing sodium acetate (83S824 87H1587). A plausible mechanism of the reaction includes formation of the oxime derivative and subsequent nucleophilic addition of telluride anion to the triple bond followed by cyclization to 4. The reaction is accompanied by the formation of telluro bis(alkenyl ketones) 5 in yields approximately equal to those of 4. When alkynyl aldehydes are used instead of ketones, the single reaction products are the tellurobis(alkenyl nitriles) 6 (83S824). 

Organocerium derivatives possess low basicity. 5-Pyrimidylcerium dichlorides (e.g., 904) are available via low-temperature metalmetal exchange using cerium trichloride and 5-lithiated pyrimidines. The 5-pyrimidinylcerium dichloride is superior to its 5-lithio analogue in addition reactions to aldehydes and ketones, especially in reactions when the carbonyl compound is enolizable. 

At an early date it was already recognized that the ketone (IX) derived from an oxidation of the C-18 carbinol function of methyl reserpate could be of considerable utility for further transformation of the reserpine pentacyclic ring system, but early attempts at the preparation of the desired compound by conventional oxidation, e.g., by Oppenauer s method, AAchlorosuceinimide, sodium dichromate, or chromic oxide in pyridine, were unsuccessful with both methyl reserpate and methyl 18-epireserpate. The ketone was finally obtained by heating methyl reserpate p-bromobenzene sulfonate with dimethyl sulfoxide in the presence of triethylamine (162), a method successfully used for simpler compounds (163). Subsequently, it was found that this oxidation could also be realized with other benzene sulfonate esters of methyl reserpate and 18-epireserpate. That the stereochemistry of the molecule was unaffected was proved by sodium borohydride reduction of the ketone, which gave equal amounts of methyl reserpate and its 18-epimer. This and other simple reactions of the ketone are sketched in Chart III, and additional observations will be given. 

In contrast to the -enolates derived from cyclic ketones, addition of propiophenone trichlorosilyl enolate (Z)-21 to aldehydes requires longer reaction times and higher loadings of catalyst. Although the yields of the aldol products remain high, both the diastereo- and enantioselectivities are attenuated as compared to their E counterparts (eq 9, Table 8). 

Similar information is available for other bases. Lithium phenoxide (LiOPh) is a tetramer in THF. Lithium 3,5-dimethylphenoxide is a tetramer in ether, but addition of HMPA leads to dissociation to a monomer. Enolate anions are nucleophiles in reactions with alkyl halides (reaction 10-68), with aldehydes and ketones (reactions 16-34, 16-36) and with acid derivatives (reaction 16-85). Enolate anions are also bases, reacting with water, alcohols and other protic solvents, and even the carbonyl precursor to the enolate anion. Enolate anions exist as aggregates, and the effect of solvent on aggregation and reactivity of lithium enolate anions has been studied. The influence of alkyl substitution on the energetics of enolate anions has been studied. ... 

Replacement in vinylic derivatives is, however, facilitated by electron-withdrawing groups to the site of substitution and addition-elimination reactions of this type have been reviewed by Rappoport and by Patai and Rappoport Such reactions have been applied in the direct synthesis of vinyl azides. For example the synthesis in 81 % yield of the )5-azidovinyl ketone (25), of unspecified stereochemistry, from the chloro derivative and azide ion in dimethylformamide at 40° has been described by Maiorana . 

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