Enones stabilized carbon

It was envisaged that the enones were produced following abstraction of H-1 (a process facilitated by the ability of sulfur atoms to stabilize radicals on bonded carbon centers), radical bromination, elimination of hydrogen bromide to give substituted glycals, allylic bromination at C-3, and loss of acetyl bromide. In the formation of compound 6, hydrogen abstraction from C-5 was deemed to compete with that from C-1, and to lead to substitution at the former site with the formation of a relatively stable product. 

There is also a frontier orbital effect that assists conjugate addition over the aldol reaction, You will reeall that the carbonyl carbon is a relatively hard centre, whereas the 3 carbon of an enone is soft, As the nucleophilic enolate becomes more stabilized with extra electron-withdrawing groups, it beeomes increasingly soft and hence more likely to attack the [J carbon,... 

The formation of carbon-carbon bonds by conjugate addition of carbonucleophiles to a,/3-unsaturated systems has been studied intensively and reviewed over the past few years . Interestingly, applications with simple, unstabilized lithium enolates are relatively rare. Most reported examples are limited to the addition of stabilized enolates, such as those derived from malonates or acetoacetates. Nevertheless, some diastereo- and enantioselective versions of the conjugate addition, even with unstabilized lithium enolates, are well known. In 2004, Tomioka and coworkers studied the influence of a chiral diether (191) on the 1,4-addition of lithium ester enolates (189) to a,-unsaturated ketones (equation 51) . Their investigations showed that good enantioselectivities were obtained with cyclic enones, like 2-cyclopentenone (190) addition to a mixture of 189 and 191 gave the desired 1,4-adduct (R)-192 with 74% ee, but only 47% yield. Unfortunately, also the Peterson product 193 was formed in a yield of 22% by initial 1,2-addition of the enolate to the Michael acceptor. 

Selenium-stabilized carbanions behave as excellent nucleophiles and react with primary alkyl bromides or iodides, allylic and benzylic bromides, epoxides, oxeta-nes, disulfides, trialkylsilyl chlorides, aldehydes, ketones, carbon dioxide, dime-thylformamide, acid chlorides or alkyl chloroformates. With conjugated enones, in the presence of HMPA as cosolvent, the 1,4-addition product is essentially obtained. 

Lewis acid complexes of -substituted a, 3-unsaturated ketones and aldehydes are unreactive toward alkenes. Crotonaldehyde and 3-penten-2-one cannot be induced to undergo ene reactions like acrolein and methyl vinyl ketone. The presence of a substituent on the -carbon stabilizes the enal- or enone-Lewis acid complex and stericdly retards the approach of an alkene to the -carbon. However, Snider et al. have found that a complex of these ketones and aldehydes with 2 equiv. of EtAlCk reacts reversibly with alkenes to give a zwitterion (22). This zwitterion, which is formed in the absence of a nucleophile, reacts reversibly to give a cyclobutane (23) or undergoes two 1,2-hydride or alkyl shifts to generate irreversibly a p, -disubstituted-a,P-unsaturated carbon compound (24). 

The similarities between organic tin and silicon compounds, particularly their abilities to stabilize positive charge on a -carbon atom, suggest that vinylic and allylic tin compounds should be useful substrates for Friedel-Crafts acylation reactions. However, very few examples of the acylation of stannanes under the action of Lewis acids have been reported, and this remains a field of Friedel-Crafts reactions not yet fully exploited. The acylation of 1,2-bis(tri-n-butylstannyl)ethylene in the presence of aluminum chloride offers a moderately yielding route to tributylstannyl-substituted enones, useful as precursors to 4,5-dialky lcyclopent-2-enones. 

In this case, diethylenetriamine acts as a trinucleophile its interaction with amino enone 71 involves double nucleophilic addition at the /1-carbon, forming ammonia, and an attack at the carbonyl group, liberating water. The reaction takes place when the compound contains a polyfluoroalkyl substituent, not only enhancing the electrophilicity of the /1-carbon atom, but also stabilizing the imidazolidine ring. 

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