Page 954 - Advanced Organic Chemistry Part B - Reactions & Synthesis

930 Scheme 10.9. (Continued)

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effect in an acyclic system. Not only is the hydroxy group stereodirective, but it also
provides selectivity with respect to the two double bonds. The reaction in Entry 6 was
carried out in the course of synthesis of crenulide derivatives, which are obtained from
seaweed.
Section B gives some examples of metal-catalyzed cyclopropanations. In Entries
7 and 8, Cu(I) salts are used as catalysts for intermolecular cyclopropanation by ethyl
diazoacetate. The exo approach to norbornene is anticipated on steric grounds. In both
cases, the Cu(I) salts were used at a rather high ratio to the reactants. Entry 9 illustrates
use of Rh (O CCH as the catalyst at a much lower ratio. Entry 10 involves ethyl
2 2 3 4
diazopyruvate, with copper acetylacetonate as the catalyst. The stereoselectivity of
this reaction was not determined. Entry 11 shows that Pd(O CCH is also an active
2 3
catalyst for cyclopropanation by diazomethane.
Section C shows cases involving organomercury reagents, which are useful for
introducing functionalized cyclopropane rings when the necessary reagents can be
obtained as mercury compounds. The very vigorous conditions needed for these
reactions indicate the relatively low reactivity of the organomercury compounds toward
-elimination.
Section D illustrates formation of carbenes from halides by -elimination. The
carbene precursors are formed either by deprotonation (Entries 14 and 17) or halogen-
metal exchange (Entries 15 and 16). The carbene additions can take place at low
temperature. Entry 17 is an example of generation of dichlorocarbene from chloroform
under phase transfer conditions.
Intramolecular carbene addition reactions have a special importance in the
synthesis of strained-ring compounds. Because of the high reactivity of carbene or
carbenoid species, the formation of highly strained bonds is possible. The strategy for
synthesis is to construct a potential carbene precursor, such as a diazo compound or
di- or trihalo compound that can undergo intramolecular addition to give the desired
structure. Section E of Scheme 10.9 gives some representative examples. Entries 18
and 19 are cases of formation of strained compounds. The reaction in Entry 20 shows
a preference between the two double bonds, based on proximity, and establishes a ring
system that subsequently undergoes a divinylcyclopropane rearrangement to generate
a nine-membered ring.