TETRAHEDRON:
ASYMMETRY
Tetrahedron: Asymmetry 12 (2001) 959–963 Pergamon
An efficient asymmetric aldol reaction of Chan’s diene promoted
by chiral Ti(IV)–BINOL complex
Annunziata Soriente,* Margherita De Rosa, Marina Stanzione, Rosaria Villano and Arrigo Scettri
Dipartimento di Chimica, Universita ` di Salerno, 84081 Baronissi, Salerno, Italy
Received 22 March 2001; accepted 26 March 2001
Abstract—1,3-Bis-(trimethylsilyloxy)-1-methoxy-buta-1,3-diene (Chan’s diene) can be conveniently used in asymmetric aldol
reaction with aromatic, heteroaromatic, ,-unsaturated and aliphatic aldehydes in the presence of catalytic amounts (2–8% mol)
of chiral Ti(IV)/(R )-BINOL complex. © 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
In recent years the development of new procedures for
the enantioselective addition of enolsilanes to aldehydes
under catalytic conditions has been the main target of
several research groups. From a preparative point of
view notable results have been obtained by using O -
silyldienolates of the type 1, since the formation of
polyfunctional C-(5) key intermediates can be achieved
in high yield and enantioselectivity.
As regards the mechanistic aspects, the reported proce-
dures may alternatively involve either the activation of
the aldehydic functionality by chiral catalysts [Ti(IV)
1–4
and Cu(II)
5,6
complexes] or the formation of a chiral
metalloenolate intermediate.
7
Rather surprisingly, the masked acetoacetic ester 2
(Chan’s diene),
8
easily accessible from commercially
available methyl 3-trimethylsilyloxy-crotonate,
9
has
been used only occasionally in stereoselective aldol
condensations, since the presence of a chelating sub-
stituent situated or to the carbonyl function seemed
to be a strict prerequisite for attaining high levels of
diastereo- and enantioselectivity.
10
2. Results and discussion
In the course of an investigation on the reactivity of
synthetic equivalents of acetoacetate ester dianion
4,11
we have found that the enantioselective addition of
Chan’s diene 2 to aldehydes 3 can be performed very
conveniently in the presence of catalytic amounts of
Ti(OiPr)
4
/(R )-BINOL complex (8 mol%).
As reported in Scheme 1 and Table 1, the reaction
proceeded with good efficiency and enantioselectivity
leading to silylated aldols 4 as the predominant or
exclusive product, while the presence of free aldol 5
could be occasionally detected (entries b, c, e).
Scheme 1.
* Corresponding author. E-mail: titti@chem.unisa.it
0957-4166/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII:S0957-4166(01)00150-1