Dimethyl phosphite mediated hydrogen atom abstraction: A tin-free procedure for the preparation of cyclopentane derivatives

Article abstract of DOI:10.1002/anie.200501309

(Chemical Equation Presented) A radical transformation! An efficient radical cascade reaction leads to functionalized five-membered rings. Alkenyl radicals are generated from readily available terminal alkynes and a dialkyl phosphite. This tin-free proced

Full text of DOI:10.1002/anie.200501309

Angewandte
Chemie
Radical Reactions
the alkenyl radical and its cyclization onto the phenylthio
group, thus leading to benzothiophene derivatives [Eq. (2)].[5]
DOI: 10.1002/anie.200501309
Dimethyl Phosphite Mediated Hydrogen Atom
Abstraction: A Tin-Free Procedure for the
Preparation of Cyclopentane Derivatives**
Florent Beaufils, Fabrice DØn›s, and Philippe Renaud*
Radical reactions have been investigated intensively over the
last two decades.[1] The new synthetic methods that have
emerged from this work are complementary to ionic pro-
cesses and are characterized by mild reaction conditions and
broad functional-group tolerance. The potential of these
reactions is immense, as demonstrated by their recent use in
the synthesis of complexnatural products. However, several
aspects of this chemistry merit further investigation to
increase the synthetic efficiency of the radical approach.
Among these objectives, the development of nontoxic and
environmentally friendly reagents to perform efficient radical
reactions represents a challenging target.
Recent developments in radical reactions involving phos-
phorus reagents have attracted our attention. Hypophospho-
rous acid and its corresponding 1-ethylpiperidine salt have
been used to mediate radical cyclization reactions as an
alternative to tributyltin hydride.[6,7] Diethyl phosphine oxide
(DEPO)[8] and diethyl phosphite[9,10] were also recently used
in radical processes.[11,12]
As diethyl phosphite is known to be a slower reducing
agent than tributyltin hydride and thiophenol,[13] we were
motivated to test the use of a phosphorus reagent in a radical
cascade involving the addition of a P radical onto a triple
bond followed by 1,5-hydrogen abstraction. To the best of our
knowledge, only one radical addition of a dialkyl phosphite to
a terminal triple bond has been reported, which was by Ishii
and co-workers using Mn(OAc)2 as a radical initiator in the
presence of air.[14] Parsons and co-workers reported the
failure of such a radical addition but presented one example
of a clean addition of a thiophosphite to a terminal alkyne.[15]
Herein, we report that dimethyl phosphite is a particularly
efficient reagent for radical additions to terminal alkynes
followed by 1,5-hydrogen transfer/cyclization processes.
Optimization of the reaction conditions were conducted
on substrate 1a [Eq. (3); DLP = dilauroyl peroxide]. Highly
The tin hydride mediated alkenyl radical translocation/
cyclization process developed by Curran et al. represents a
powerful procedure for the preparation of functionalized five-
[2,3]
À
membered rings by selective activation of a C H bond.
Recently, we reported an efficient tin-free version of this
reaction involving thiophenol for the preparation of cyclo-
pentane derivatives through a 1,5-hydrogen transfer/cycliza-
tion sequence [Eq. (1); AIBN = azobisisobutyronitrile].[4]
This method using thiophenol proved to be very efficient
with a wide range of substrates but required high dilution
(10À2 m of substrate) and the slow addition of thiophenol with
a syringe pump [Eq. (1)]. Some limitations were also noticed,
with some substrates undergoing slow hydrogen transfers. For
example, the generation of a nonstabilized acyclic secondary
alkyl radical proved to be difficult because of the reduction of
reproducible results and excellent yields were obtained by
treating a 0.1m solution of the alkyne 1a with five equivalents
of dimethyl phosphite and one equivalent of DLP[16] in
cyclohexane at reflux for 6 h. The desired cyclized product 2a
was obtained in 81% yield, and no trace of the product from
direct reduction was observed. In this particular example,
neither the procedure using tin hydride developed by Curran
and Shen[3] nor our procedure using thiophenol[4] [see Eq. (2)]
gave satisfactory results because of the slow rate of the
hydrogen-abstraction step.
[*] F. Beaufils, Dr. F. DØn›s, Prof. P. Renaud
University of Berne
Department of Chemistry and Biochemistry
Freiestrasse 3, 3000 Bern 9 (Switzerland)
Fax: (+41)316-313-426
E-mail: philippe.renaud@ioc.unibe.ch
[**] We thank the Swiss National Science Foundation (Grant 20-
103627), the Roche Foundation (postdoctoral fellowship to F.D.),
and the University of Berne for support of this study.
Propargylmalonates 1b–e were then tested according to
Equation (4). The generation of a tertiary alkyl radical from
1b is very efficient with dimethyl phosphite (Table 1, entry 1),
Supporting information for this article is available on the WWW
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Table 1: Dimethyl phosphite mediated hydrogen abstraction [cyclization
according to Eq. (4)].
derivative.[18] However, this method suffers from the use of tin
hydride and the difficulty of preparing the starting alkenyl
iodides. On the other hand, substrates 5a–c are readily
prepared by alkylation of the corresponding b-keto ester
followed by decarboxylation and are efficiently converted
[Eq. (7)] into the spiro ketone 6a (97%, d.r. 70:30), 6b (77%,
d.r. 53:47), and 6c (64%, d.r. 79:21).
Entry
Substrate
Product
d.r.[a]
Yield [%][b]
R
Y
Z
1
2
3
4
1b
1c
1d
1e
H
iPr
H
Me
CO2Et
Ph
Me
H
H
2b
2c
2d
2e
–
91
91
55
79
82:18
92:8
73:27
H
OTBS
H
[a] Determined by GC or NMRanalysis of the crude reaction mixture.
[b] Yield of isolated product. TBS=tert-butyldimethylsilyl.
and the cyclic product 2b is obtained in 91% yield. Substrates
1c and 1d, which bear an ester and a phenyl substituent,
respectively, undergo a radical translocation/cyclization pro-
cess to the desired substituted cyclopentanes 2c and 2d in 91
and 55% yield (entries 2 and 3), respectively. Finally, a radical
reaction with substrate 1e gave the cyclopentane derivative in
79% yield[17] (entry 4).
On the basis of these initial results, a method for the rapid
assembly of fused bicyclic compounds was investigated
[Eq. (5)]. Conjugate addition of dimethyl propargylmalonate
In conclusion, we have reported herein that dialkyl
phosphites are powerful reagents for radical translocation/
cyclization reactions that start from readily available terminal
alkynes. A simple one-pot procedure has been developed.
The excess reagent is easily removed by evaporation, and
filtration through a short pad of silica gel affords a clean
reaction product. The cyclic phosphonates produced by this
reaction cascade are particularly attractive for further trans-
formations.[11,19,20] Application of this process to the synthesis
of natural products is currently underway.
Experimental Section
to cyclopentenone and cyclohexenone affords the radical
precursors 3a and 3b, respectively. Reaction of 3a and 3b
with dimethyl phosphite and diethyl phosphite proceeds
[Eq. (5)] with complete regioselectivity and affords the fused
bicycles 4a and 4b in 71 and 84% yield as a mixture of two
diastereomers (d.r. 85:15 and 61:39, respectively). Radical
stabilization by the ketone explains the regioselectivity.
Interestingly, the deoxygenated compound 3c gives the
expected bicycloalkanes 4c in 93% yield (d.r. 90:10) under
similar reaction conditions, thus demonstrating that stabiliza-
tion of the translocated radical is not a prerequisite for the
formation of fused bicyclic systems [Eq. (6)].
A mixture of 1b (1.0 mmol, 226 mg), DLP (1.0 mmol, 398 mg), and
dimethyl phosphite (5 mmol, 550 mg) in cyclohexane (10 mL) was
heated at 808C for 6 h. After completion (monitoring by GC), the
solution was cooled and the cyclohexane evaporated under reduced
pressure. CH3CN (10 mL) was added to precipitate the by-products
derived from DLP. After filtration and evaporation, the residue was
purified by flash chromatography (EtOAc/cyclohexane, 80:20) to
afford the phosphonate 2b (306 mg, 91%).
Received: April 14, 2005
Published online: July 20, 2005
The synthesis of spiro compounds through 1,5-hydrogen
transfer has been reported recently employing an iodoalkenyl
Keywords: alkynes · cyclopentanes · fused-ring systems ·
hydrogen transfer · radical reactions
.
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