equivalents of the synthons they produce are not easily prepared. A retrosynthetic step
must lead to readily obtainable starting materials.PROBLEM 36Using bromocyclohexane as a starting material, how could you synthesize the following
compounds?a. c. e.b. d. f.Cl
CH 2 CH 3CH 2 OH CH 2 CH 2 OHC
CH 3CH 3OH COOH++−−retrosynthetic analysisHO CH 3HO CH 3HO CH 3768 CHAPTER 18 Carbonyl Compounds II
SEMISYNTHETIC DRUGS
Taxol is a difficult molecule to synthesize be-
cause of its complicated structure. Chemists have
made the synthesis a lot easier by allowing the yew tree to carry
out the first part of the synthesis. Chemists extract the drug pre-
cursor from the needles of the tree, and the precursor is con-® verted to Taxol in the laboratory. Thus, the precursor is isolat-
ed from a renewable resource, while the drug itself could be ob-
tained only by killing the tree. In this manner, chemists have
learned to synthesize compounds jointly with nature.®SYNTHESIZING ORGANIC
COMPOUNDS
Organic chemists synthesize compounds for
many reasons: to study their properties or to answer a variety
of chemical questions, or because they have unusual shapes or
other unusual structural features or useful properties. One rea-
son chemists synthesize natural products is to provide us with
greater supplies of these compounds than nature can produce.
For example, Taxol —a compound that has been successful in
treating ovarian and breast cancer—is extracted from the bark
of Taxus, the yew tree found in the Pacific Northwest. The sup-
ply of natural Taxol is limited because yew trees are uncom-
mon and grow very slowly and stripping the bark kills the tree.
The bark of one tree provides only one dose of the drug. In ad-
dition,Taxusforests serve as habitats for the spotted owl, an
endangered species, so harvesting the trees would accelerate
the owl’s demise. Once chemists were successful in determin-
ing the structure of Taxol , efforts could be undertaken to syn-
thesize it in order to make it more widely available as an
anticancer drug. Several syntheses have been successful.®®®Once a compound has been synthesized, chemists can study
its properties to learn how it works; then they can design and
synthesize safer or more potent analogs. For example, chemists
have found that the anticancer activity of Taxol is substantial-
ly reduced if its four ester groups are hydrolyzed. This gives
one small clue as to how the molecule functions.®C 6 H 5 CNHC 6 H 5C 6 H 5 COH HOOOO CH^3 COOO OHO OCOCH 3OOHTaxol