INTERMOLECULAR ADDITION OF α-ALKOXYCARBONYL-β-STANNYLVINYL RADICALS TO ELECTRON-DEFICIENT ALKENES. SIMPLE ADDITION AND ANNULATION REACTIONS

Article abstract of DOI:10.1016/S0040-4039(00)93438-5

Regio- and stereocontrolled synthesis of vinylstannanes was achieved by intermolecular addition reaction of α-alkoxycarbonyl-β-stannylvinyl radicals to activated alkenes.Under high dilution conditions cyclohexene derivatives were obtained via <2+2+2> radical annulation reaction.

Full text of DOI:10.1016/S0040-4039(00)93438-5

Tetrahedron Lcttcrs, Vol.32, No.38, pp 5101-5102, 1991
Printed in Great Britain
0040-4039/91 $3.00 + .00
Pergamon Press ple
INTERMOLECULAR
RADICALS
SIMPLE ADDITION
ADDITION
OF
~-ALKOXYCARBONYL-/~--STANNYLVINYL
TO
ELECTRON-DEFICIENT
ALKENES.
AND [2+2+2] ANNULATION
REACTIONS
Euu Lee* and Chang Uk Hur
Department of Chemistry, College of Natural Sciences,
Seoul National University, Seoul 151-742, Korea
Abstract:
Regio- and stereocontrolled synthesis of vinylstannanes was achieved by intermolecular
addition reaction of a~-alkoxycarbonyl-fl-stannylvinyl radicals to activated alkenes. Under high dilution
conditions cyclohexene derivatives were obtained via [2+2+2] radical annulation reaction.
Recently we reported facile synthesis of ~--methylenelactones and a,--methylenecycloalkanones via
intramolecular cyclization of a--alkoxycarbonyl-fl--stannylvinyl radicals9
To further define the scope of
the reaction of these vinyl radicals, their intermolecular addition reaction with substituted alkenes was
studied.
Reaction
tributylstannane (leq.) and AIBN (0.1eq.) in benzene (0.2M) under reflux resulted in the formation of
hydrostannation products 3 (E and isomeric mixture) and intermolecular addition products in
moderate yields (Scheme 1). In agreement with observations made in the study of intramolecular
of ethyl
propiolate (1)
with
various
alkenes 2 (3eq.)
in
the
presence of
Z
4
cyclization reaction of a'-alkoxycarbonyl-fl--stannylvinyl radicals, reaction with electron-deficient alkenes
provided relatively high yields of the intermolecular addition products 4. Electron-rich alkenes were
inactive towards a--alkoxycarbonyl-fl--stannylvinyl radicals. It is also worth noting that only Z isomers of
vinyl stannanes 4 were obtained.
(Scheme 1)
CO2Et
(I)
+
~ E Benzene(0.2M~
+
(2)
2h,]L
(3)
(4
(3eq)
E
a)
~)
e)
~ k C O 2 M e
62 %(Z:E=3:I)
40 %(Z:E=3:I)
51%(Z:E~3.2:I)
31%
~ C N
52
38
%
%
I
~ C O 2 M e
O
d)
e)
f)
O
75 %(Z:E=2.7:I)
91%(Z:E=2.9:I)
85 %(Z:E-3:I)
Ii
%
~ O A c
~ k O B u
....
....
Vinylstannanes are of increasing importance as intermediates in synthetic organic chemistry.2
Owing to their great versatility as building blocks for synthesis, considerable effort has been devoted to
the regio- and stereocontrolled synthesis of vinylstannanes.3
The above method should provide an
alternative way for their production, which should lead to stereoselective synthesis of trisubstituted
olefins.
Slow addition of tributylstannane (0.heq.) to a more diluted solution (0.1M) of ethyl propiolate (1)
and electron-deficient alkenes 2 (heq.) provided cyclohexene derivatives in moderate yields (Scheme
2).
5101

5102
The new products are believed to be obtained by the [2+2+2] triple radical Michael addition reactions.
Recently, cyclopentane ring annulation by a [3+2] radical cycloaddition reaction has been reported.4 The
unprecedented [2+2+2] triple radical Michael annulation represents a novel radical cycloaddition scheme
for preparation of substituted cyclohexene rings (Scheme 2).
(Scheme 2)
~C02Et
___ CO2Et
(I)
+
+
~ C O 2 M e
Bu~SnH,AIBN
Slow add'n,4hv
Benzene(0.1M)
M e O 2 C ~
(4a)
28%
+
(2a)
(5a). CO2Me
(Seq)
43%
CO2Et
~ C N
(4b)
30%
N C - - ~
(2b)
~
+
(5eq)
(5b) CN
48%"
C02Et
+
4~'c%Me
.~
(4c)
+
Me02~
(2c)
28%
(5c) I C02Me
(5eq)
46%*
*
NMoRnsSP~sCtr ~ ~n~ GmCx~nalys~st~heO~ot~t ~nn~l@itioOnrPrOduct 5a and 5b
The product 5c was obtained as single isomer.
a
Mechanistically, these annulation products are considered to be formed via the following reaction
sequence (Scheme 3); i) generation of a-alkoxyearbonyl-~-stannylvinyl radical via intermolecular
a
addition of a stannyl radical to the propiolate, 2) intermolecular addition of the vinyl radical to an
activated alkene, 3) subsequent intermolecular addition of the alkyl radical to another molecule of the
same alkene, 4) intramolecular 6-endo cyclization, 5) ~-elimination of the stannyl radical (Scheme 3).
(Scheme 3)
~'E
(2)
B u 3 S n C O 2 E t ~'E
--~_ CO2Et Bu3Sn; Bu3 Sn~_~/CO2 Et
~
~
(2)
.<
(1)
E
CO2Et
Bu3Sn CO2Et
. ~
Bu3Sn CO2Et
6- endo
~
~
-Bu~Sn-
~- elimination
E
E - ~ /
cyclizatio~
E
- - A
E
E
(5)
E
In conclusion, it is apparent that intermolecular addition reaction of a--alkoxycarbonyl-~-~
stannylvinyl radicals with activated alkenes affords a useful method for carbon-carbon bond formation.
Acknowledgements:
(KOSEF).
The present study was supported by the Organic Chemistry Research Center
References and Notes
(a) Lee, E.; Ko, S.B.; Jung, K.W.; Chang, M.H. Tetrahedron Lett. 1989, 30, 827. (b) Lee, E.;
Hur, C.U.; Park, J.H. Ibid. 1989, 30, 7219. (c) Lee, E.; Hur, C.U.; Park, C.M. ibid. 1990, 31, 5039.
Zhang, H.X.; Guibe, F.; Balavoine, G. J.Org.Chem. 1990, 55, 1857, and references cited therein.
a) Wulff, W.D.; Peterson, G.A.; Bauta, E.W. Ibid. 1986, 51, 277. (b) Ichinose, Y,; Oda, H.;
shima, K.; Utimoto, K. Bull.Chem.Soc.Jpn. 1987, 60, 3468. (c) Kikuhaxa, K.; Umekawa, F.;
Wada, G.; Matsuda, T. Chem.Lett. 1988, 881.
(a) Salcic, R.N.; Cekovic. Z. Tetrahedron Lett. 1990, 31, 4203, and references cited therein. (b)
Feldman, K.S.; Vong, A.K.K. Ibid. 1990, 31, 823. (c) Curran, D.P.; Shen, W.; Zhang, J.;
Heffner, T.A.J.Am.Chem.Soc. 1990, 112, 6738.
4.
(Received in Japan 29 June 1991)