SolutionsRetrosynthetic analysis
Exercise 1
A linear synthesis has eight steps, each in 85% yield. What is the overall yield? And if it is reorganized into two four-step fragments joined in a ninth step, also at 85%?
Linear: 0.858 = 0.27, i.e. 27%. Convergent: the longest sequence has five steps (four plus the coupling), 0.855 = 0.44, i.e. 44%, even though there are nine steps in total. In addition, less valuable material is lost in the final stages of the convergent route.
Exercise 2
Calculate the atom economy of the carbonylation of 1-(4-isobutylphenyl)ethanol (C12H18O, 178 g/mol) with CO (28 g/mol) to ibuprofen (C13H18O2, 206 g/mol).
All the atoms of the reagents end up in the product: 206 / (178 + 28) = 1, i.e. 100%. It is a formal addition of CO across the C–OH bond; that is why this step contributes most to improving the atom economy of the BHC process compared with the Boots route.