TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 377–380
A general strategy to spiro[4.n]alk-2-ene-1,6-diones and
spiro[5.n]alk-2-ene-1,7-diones via intramolecular acylation of
α-sulfinyl carbanions1
Manat Pohmakotr,∗ Tanasri Bunlaksananusorn and Patoomratana Tuchinda
Department of Chemistry, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand
Received 7 September 1999; accepted 6 October 1999
Abstract
A convenient and general synthetic method for spiro[4.n]alk-2-ene-1,6-diones and spiro[5.n]alk-2-ene-1,7-
diones, which involves the intramolecular acylation of an α-sulfinyl carbanion, is described. © 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: acylation; annulation; carbanions; spiro compounds; sulfoxides.
Spirocarbocyclic systems are of interest because they are commonly found as subunits of many natural
products. Synthetic methods directed to these classes of compounds have been extensively developed.2
In contrast to other procedures that are described for obtaining spirocarbocyclic compounds, there are
very few general methods available for the synthesis of spiro[4.n]alkane-1,6-diones and spiro[5.n]alkane-
1,7-diones.3 In the course of our studies on the intramolecular acylation of sulfinyl carbanions for the
preparation of cyclopentenone and cyclohexenone derivatives,4 we investigated a general route for the
construction of spiro[4.n]alk-2-ene-1,6-diones and spiro[5.n]alk-2-ene-1,7-diones. These compounds are
highly functionalized intermediates, which may be useful for further synthetic conversions.
Starting from readily available cyclic β-ketoesters 1a–c, the sulfoxides 2 or 3 could be obtained in good
overall yields by a simple base-catalyzed alkylation using 1,3-dibromopropane or 1,4-dibromobutane
(NaH/DMF, room temperature, overnight) followed by treatment of the resulting bromides with a
tetrahydrofuran solution of thiophenol and triethylamine, at 0°C to room temperature overnight, and
then oxidation of the resulting sulfides with sodium metaperiodate in aqueous methanol at 0°C. Having
considered the structures of the sulfoxides 2 and 3, it was found that the difference in acidities of the
protons alpha to the carbonyl and to the sulfinyl groups should allow selective deprotonation by an
appropriate base. As shown in Scheme 1, effective cyclization of the sulfoxides 2 and 3 to the desired
spiro-diones 7 and 8 required the protection of the carbonyl group in order to avoid the competitive
∗
Corresponding author. Tel: 066-02-2461358-74, ext. 1508; fax: 066-02-2477050; e-mail: scmpk@mahidol.ac.th
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)01897-3