SolutionsThe synthetic equivalent
Exercise 1
Which synthons are the following the synthetic equivalents of: a) 1-methoxyvinyllithium, CH2=C(OMe)Li; b) lithium bis(1-methoxyvinyl)cuprate; c) 2-lithio-2-phenyl-1,3-dithiane?
a) Of the acetyl anion, CH3C−=O: it adds to a carbonyl and acid hydrolysis of the enol ether gives the methyl ketone (an α-hydroxy ketone if the electrophile was a ketone). b) Also of the acetyl anion, but in conjugate addition to enones: it gives 1,4-dicarbonyl compounds. c) Of the benzoyl anion, PhC−=O: after reaction with the electrophile and hydrolysis of the dithiane a phenyl ketone is obtained.
Exercise 2
Propose synthetic equivalents for the synthons −CH2COCH3 and +CH2CH2OH, and use them to prepare 5-hydroxypentan-2-one.
−CH2COCH3: the enolate of ethyl acetoacetate (acetoacetic ester synthesis), later hydrolysed and decarboxylated. +CH2CH2OH: ethylene oxide, which opens at the less hindered carbon. The acetoacetate enolate opens the epoxide; hydrolysis and decarboxylation of the keto ester (in practice with intermediate lactonization) gives 5-hydroxypentan-2-one.