Regioselective conjugate additions, lithium

Conjugate additions. Enals undergo selective allylation with diallylcerium chloride in the presence of ATPH. Regioselective Robinson annulations can be initiated by the conjugated addition of lithium enolates to enones. - Remarkably, silyllithium reagents add to the p-position of aromatic carbonyl compounds in the presence of ATPH. ... 

The simplest products are formed when Nu=H, but this poses a problem of regioselectivity in the nucleophilic attack step a nucleophilic hydride equivalent that selectively undergoes conjugate addition to the enone is required. This is usually achieved with extremely bulky hydride reagents such as lithium or potassium tri(sec-butyl)borohydride (often known by the trade names of L- or K-Selectride, respectively). In this example, K-Selectride reduces the enone to an enolate that is alkylated by methyl iodide to give a single regioisomer. 

Asymmetric induction is also observed in the conjugate addition of the chiral acetone imines (32) to cycloalkenones to provide 3-acetonyl-cycloalkanones (33) in reasonable yields after lithiation and copper catalysis. Alternatively, the masked 1,5-dicarbonyl compounds (34) are obtained when lithium methyl dithioacetate (35) adds in a regioselective manner to cycloalkenones, although no stereoselectivity is observed. ... 

Extra selectivity is needed in the synthesis of the open chain compound 96. This is clearly an aldol, but analysis reveals the need for two forms of regioselectivity in that an a extended enolate 97 is required to add 1,2 to a conjugated aldehyde 35. Fortunately aldehydes tend to prefer 1,2 to 1,4-addition (chapter 9), especially with hard nucleophiles such as lithium enolates, and this synthesis works as planned.26... 

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