Page 919 - Advanced Organic Chemistry Part B - Reactions & Synthesis

cannot be predicted directly from the structure of the reacting haloketone. Instead, 895
the identity of the product is governed by the direction of ring opening of the cyclo-
propanone intermediate. The dominant mode of ring opening is expected to be the SECTION 10.1
one that forms the more stable of the two possible ester enolates. For this reason, a Reactions and
Rearrangement
phenyl substituent favors breaking the bond to the substituted carbon, but an alkyl Involving Carbocation
90
group directs the cleavage to the less-substituted carbon. That both 12 and 13 above Intermediates
give the same ester, 14, is illustrative of the directing effect that the phenyl group has
on the ring-opening step.

O – O – O
CH O OCH 3 –
3
PhCHCH COCH 3
2
Ph Ph

Scheme 10.5 gives some examples of Favorskii rearrangements. Entries 1 and 2
are examples of classical reaction conditions, the latter involving a ring contraction.
Entry 3 is an interesting ring contraction-elimination. The reaction was shown to
be highly stereospecific, with the cis-dibromide giving exclusively the E-double
bond, whereas the trans-dibromide gave mainly the Z-double bond. Entry 4 is a
ring contraction leading to the formation of an interesting strained-cage hydrocarbon
skeleton. Entry 5 is a step in the synthesis of the natural analgesic epibatidine.

10.1.3.2. The Ramberg-Backlund Reaction. -Halosulfones undergo a related
rearrangement known as the Ramberg-Backlund reaction. 91 The carbanion formed by
deprotonation gives an unstable thiirane dioxide that decomposes with elimination of
sulfur dioxide. This elimination step is considered to be a concerted cycloelimination.

O O O O
–
RCHSCH R' RCHSCHR' S RCH CHR'
2
R R'
X O X O H H
The overall transformation is the conversion of the carbon-sulfur bonds to a carbon-
carbon double bond. The original procedure involved halogenation of a sulfide,
followed by oxidation to the sulfone. Recently, the preferred method has reversed the
order of the steps. After the oxidation, which is normally done with a peroxy acid,
halogenation is done under basic conditions by use CBr F or related polyhalomethanes
2 2
for the halogen transfer step. 92 This method was used, for example, to synthesize
1,8-diphenyl-1,3,5,7-octatetraene.

90
C. Rappe, L. Knutsson, N. J. Turro, and R. B. Gagosian, J. Am. Chem. Soc., 92, 2032 (1970).
91 L. A. Paquette, Acc. Chem. Res., 1, 209 (1968); L. A. Paquette, in Mechanism of Molecular Migrations,
Vol. 1, B. S. Thyagarajan, ed., Wiley-Interscience, New York, 1968, Chap. 3; L. A. Paquette, Org.
React., 25, 1 (1977); R. J. K. Taylor, J. Chem. Soc.,Chem. Commun., 217 (1999); R. J. K. Taylor and
G. Casy, Org. React., 62, 357 (2003).
92
T.-L. Chan, S. Fong, Y. Li, T.-O. Mau, and C.-D. Poon, J. Chem. Soc., Chem. Commun., 1771 (1994);
X.-P. Cao, Tetrahedron, 58, 1301 (2002).