Lateral lithiation regioselectivity

For cases where the regioselectivity of lithiation cannot be biased in the required direction, silylation can be used to block acidic sites while further lithiations are carried out. The blocking silyl group is later readily removed with fluoride . This method was used to avoid a competing lateral lithiation in the synthesis of the aldehyde 665 (Scheme 251... 

Lateral lithiation of 244 and intramolecular electrophilic substitution with iminium ions derived from the partly reduced phthalimide 247 in acidic solution leads to the polycyclic derivatives37 such as 248. The 4-Me compound cyclises with some regioselectivity favouring para to OMe. 

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