Tc~~nhedronLettcrs.Vo131.No.3O,pp4341~342 1990 PriitedinGrcatBritain

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ALKYLATION AND SPIROANNULATION OF

3-BROMO-2-CYCLOALKEN-1aNES VIA RADICAL INTERMEDIATES

Eun Lee* and Dae Sung Lee

Department of Chemistry, College of Natural Sciences

Seoul National University, Seoul 151-7-/Z’, Korea

Abstract: Reaction of 3-bromo2+ycloalken-l-ones with various alkenes in the presence of tributyl-

stannane resulted in the formation of 3-alkylated and 3-spiroannulated products.

Carbon<arbon bond formation via radical intermediates is the focus of much current interest in organic

synthesis.1 Chemistry of vinyl radicals has been amply discussed* and we recently demonstrated the utility of

cl-carbonylvinyl radicals generated by the addition of stannyl radicals to propiolates and ynones.3 The

reactions of /J-carbonylvinyl radicals have scarcely been studied4 and we wish to report here results of our

initial studies on these intriguing intermediates generated from 3-bromw2<ycloalken-l-ones.

Cl

=kx+pEj Bu3SnH,AIBN

X la. n-2, R-Me z. n-2, R-H 2 3 - _ %. n-l, R-Me -

Scheme

Reaction of 3-bromo-2-methyl-2-cyclohexen-l-one(&) with various alkenes (5 eq.) in the presence of

tributylstannane (1.5 eq.) and AIBN (0.1 eq.) in benzene (substrate+&:! M) under reflux resulted in the

formation of 3-alkylated products 2 and S-spiroannulated products 3 in moderate yield (Scheme). Methyl

acrylate and vinyl acetate were both amenable to alkylation and spiroannulation and the yield of the Spiro

product increased with higher concentration of alkenes. Methyl acrylate reacted with the enone radical faster

than vinyl acetate as ascertained in the mixed alkene experiment. Vinyl sulfone gave only the alkylation

product. Methyl substituent at C-2 is essential for spiroannulation, as 3-bromo-2_cyclohexen-l-one (lb)

formed only 3-alkylated products with methyl acrylate, vinyl acetate, and acrylonitrile. The size of the ring

is also important: 3-brom+2-methyl-2_cyclopenten-l-one (h) afforded much lower yield of spiroannulation

product with methyl acrylate (Table).

4341

4342

Table

Substrate Olefin Bu$SnH

X Addition Time (h)

Yield(

2 3

Ia CO2Me

COzMe

COzMeb

S02Ph

OAc

OAcb

OBub

COsMe [OAc

lb COsMe

COzMe

OAc

CN

Ic COsMe

2

4

10

1.5

3

7

10

5

5

10

5

5

2

74

49

44

35d

72

32

36

59f

8

27

3oc

13

32c

5e

27s 14s

61

45

65

88

71 2

a) Isolated yield based on the recovered starting material. b) 10 eq. of olefin was employed.

c) Only two stereoisomers were obtained. d) Accompanied by the saturated ketone (56%).

e) Mixture of three stereoisomers. f) X=COzMe. g) X=COsMe. h) X=COzMe, OAc.

The scope and the utility of the aikylation and the unprecedented [2+2+1] triple radical Michael

spiroannulation reaction will be the subject of further studies in these laboratories.

Acknowledgement: We thank the Ministry of Education, Korea (basic science grant) and Yukong

Corporation for generous financial support.

(a) D.P.Curran, Synthesis 1988,417,489. (b) B.Giese, Radicals in Organic Synthesis: Formation of

Carbon-Carbon Bonds; Pergamon Press: Oxford, 1986. (c) MRamaiah, Tetrahedron 1987,43,3541.

(a) D.P.Curran, D.S.Kim, H.T.Liu, WShen, J.Am.Chem.Soc. 1988,110,5900. (b) T.M.Dietz, R.J.von

Trebra, B.J.Swanson, T.H.Koch, 1987,109,1793. (c) G.Stork, R.Mook, Jr., Tetrahedron Lett.

1986,27,4529, (d) D.P.Curran, S.C.Kuo, J.Am.Chem.Soc. 1986,108,1106.

(a) E.Lee, S.B.Ko, K.W.Jung, Tetrahedron Lett. 1989,30,827. (b) E.Lee, C.U.Hur, J.H.Park, ibid

1989,30,7219.

Generation of a @-carbonylvinyl radical intermediate by photolysis was reported: I.Saito, H.Ikehira,

T.Matsuura, Tetrahedron Lett. 1985,26,1993.

(Received in Japan 15 May 1990)