TETRAHEDRON
LETTERS
Tetrahedron Letters 44 (2003) 8471–8474 Pergamon
Intramolecular hetero Diels–Alder reactions of vinyl allenes and
aldehydes
David Rega ´s, Juan M. Ruiz, Marı ´a M. Afonso, Antonio Galindo and J. Antonio Palenzuela*
Instituto Universitario de Bio -Orga ´nica ‘Antonio Gonza ´lez’, Departamento de Quı ´mica Orga ´nica, Universidad de La Laguna,
38206 La Laguna, Tenerife, Spain
Received 16 July 2003; revised 3 September 2003; accepted 12 September 2003
Abstract—Compounds containing a vinyl allene moiety linked to an aldehyde by a tether have been synthesised and shown to
undergo the intramolecular hetero Diels–Alder reaction. Only one compound with stereochemistry consistent with an exo
approach of the dienophile to the dienic portion of the vinyl allene was obtained in each case. Depending on the length of the
tether and on the substitution pattern on the allene the reactions were spontaneous at room temperature or required heating or
the presence of a Lewis acid to proceed.
© 2003 Published by Elsevier Ltd.
Vinyl allenes have been successfully used as dienes in
the intramolecular Diels–Alder reaction leading to
polycyclic compounds, which can be used to synthesise
complex natural products.
1–4
A classification scheme
has been suggested for these reactions based on the
location of the tether on the vinyl allene.
5
Of all the
possible types included in that classification, only two,
Types I
1–3
and V
E
4
(Fig. 1), have been experimentally
carried out, an important difference being observed
between these two types in the stereochemical outcome
of the reaction.
In Type V
E
the system is more flexible and gives
mixtures of compounds resulting from the endo or exo
approach of the dienophile to the vinyl allene. In Type
I systems, the structural features of the allene moiety
confer a high degree of rigidity to the system, making
the intramolecular Diels–Alder reaction of this type of
compounds a highly selective process. The length of the
tether also plays a role in the reactivity of the system.
5
In Type I systems, it was observed that when the
bicyclic compound formed was of the hydrindane type,
the reaction proceeded at room temperature or below
depending on the substituents on the allene. When
decalin-type systems were prepared, higher tempera-
tures were required to effect the cyclization.
We have recently shown that semicyclic vinyl allenes
can act as dienes in the hetero Diels–Alder reaction
with aldehydes as heterodienophiles in the presence of
Lewis acids, and that the reactivity observed was simi-
lar to that of dienes substituted with alkoxy or silyloxy
groups.
6
Following our interest in the pericyclic reactivity of
vinyl allenes, we decided to test the feasibility of carry-
ing out the intramolecular variant of this reaction.
Although only a few examples of intramolecular hetero
Diels–Alder reactions in which the heterodienophile is a
carbonyl compound exist,
7
we were intrigued by the
effect of the allene on the reactivity. For this task, we
needed to devise a procedure to prepare compounds
containing the vinyl allene system with different sub-
stituents and an aldehyde group, linked by a tether of
variable length. Based on the above considerations for
the intramolecular Diels–Alder reaction of vinyl allenes,
we decided to place the tether on the terminus of the
Figure 1.
Keywords : vinyl allene; intramolecular hetero Diels–Alder; cycloaddi-
tion.
Supplementary data associated with this article can be found at
doi:10.1016/j.tetlet.2003.09.091
* Corresponding author. Tel.: +34-922-318443; fax: +34-922-318442;
e-mail: jpalenz@ull.es
0040-4039/$ - see front matter © 2003 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2003.09.091