ORGANIC
LETTERS
2006
Vol. 8, No. 23
5295-5298
Site-Specific Preparation of
2-Carboalkoxy-4-substituted
Naphthalenes and 9-Alkylphenanthrenes
and Evidence for an Allene Intermediate
in the Novel Base-Catalyzed Cyclization
of 2-Alkynylbiphenyls
Yi Wang and Donald J. Burton*
Department of Chemistry, UniVersity of Iowa, Iowa City, Iowa 52242
Received August 23, 2006
ABSTRACT
A site-specific preparation of 2-carboalkoxy-4-substituted naphthalenes and 9-alkylphenanthrenes is described. The successful cyclization of
an allene intermediate provides supportive evidence for the previously proposed mechanism.
Polysubstituted naphthalene derivatives are important build-
ing blocks for the synthesis of pharmaceuticals1 and poly-
cyclic aromatic electronic materials.2 A variety of methods
have been developed for their preparation including elec-
trophilic substitution of naphthalenes,3 coupling of halonaph-
thalenes with organolithium or Grignard reagents,4 annula-
tions via Fischer carbenes,5 palladium-catalyzed cyclization
of alkynes with benzynes6 and cyclization of alkynes,7 [3+3]
benzannulation of benzenoid ring systems,8 reaction of
1-methoxybenzocyclobutene and alkynes,9 TiCl4-mediated
annulations of R-aryl-substituted carbonyl compounds with
alkynes,10 and benzotriazole-assisted aromatic ring annula-
tion.11 However, these methods involve expensive catalysts
or multistep synthesis. In some cases, regioselectivity was
difficult to achieve. Therefore, a new and efficient methodol-
ogy for the site-specific synthesis of polysubstituted naph-
thalenes was of interest to us. Herein, we wish to report a
novel base-catalyzed cyclization to prepare polysubstituted
naphthalene derivatives.
In our previous reported work in the base-catalyzed
cyclization of fluorinated enynes, we found that the presence
of a vinylic fluorine in the enyne successfully promoted
cyclization (with a DABCO or DBU base system) and
provided a site-specific synthesis of 1-alkyl-3-fluoronaph-
thalenes.12 In contrast, (Z)-non-1-en-3-yn-1-ylbenzene gave
no reaction with DABCO or DBU.12 We proposed that the
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10.1021/ol0620850 CCC: $33.50
© 2006 American Chemical Society
Published on Web 10/17/2006