Scheme 2
Scheme 3
4 in the presence of 1-hexene 2a (8 equiv) at 25 °C for 1 h
gave the [(trimethylsilyl)ethynyl]cyclopropane 5a in 67%
yield (Table 1, entry 1). Interestingly the cyclopropanation
took place exclusively at the γ-position of 6a in contrast to
the reaction of R-unsubstituted thioacetals 3, the cyclopro-
panation of which proceeded at the R-position. When an
excess amount of the low-valent titanium 4 was used, the
reduction of the triple bond partly proceeded to give the
corresponding alkenylcyclopropane as a byproduct. The
reaction of organotitanium species generated from several
acetylenic thioacetals 6 with various terminal olefins 2 gave
the alkynylcyclopropanes 5 with excellent regioselectivity.
In all of the reactions examined, formation of the other
possible regioisomer, (trimethylsilyl)cyclopropane 7, was not
observed. When styrene was used as the olefin component,
reduction of the carbon-carbon triple bond partially pro-
ceeded to produce the alkenylcyclopropanes as byproducts.
We found that use of bulky tert-butyldimethylsilyl-substituted
acetylenic thioacetals could eliminate the reduction of the
triple bond, giving [(tert-butyldimethylsilyl)ethynyl]cy-
clopropanes 5 as sole products in good yields (entries 5 and
6). The cyclopropane 5i was regioselectively produced by
both the reactions of R-(trimethylsilyl)-â,γ-acetylenic thio-
acetal 6e and its isomer, γ-(trimethylsilyl)-â,γ-acetylenic
thioacetal 8 (entries 7 and 8). These results indicate that the
regioselectivity of the cyclopropanation depends on the
relative steric bulkiness of R (R1)- and γ (R2)-substituents
of acetylenic thioacetals R2CtCC(SR)2R1.
Table 1. Regioselective Formation of the
(Silylethynyl)cyclopropanes 5
Similar regioselectivity was also observed in the titano-
cene(II)-promoted reaction of the alkynyl thioacetals 6 with
ketones 9 (Scheme 3, Table 2).5 The successive treatment
of the acetylenic thioacetal 6a with the titanocene(II) reagent
4 (3 equiv) and 3-pentanone 9a (1.2 equiv) produced the
conjugated enyne 10a as a major product along with the
vinylsilane 11a (entry 1). The formation of silylacethylene
10a indicates that the process involves a formal allylic
rearrangement. The reaction of more the sterically hindered
tert-butyldimethylsilyl-substituted counterpart 6b gave the
enyne 10b with higher regioselectivity (entry 2). Essentially
complete regioselectivity was observed when less bulky
ketones such as acetone and cyclohexanones were employed.
Furthermore, even when the sterically hindered ketone 9e
was employed, the silylacetylene 10i was produced with
complete regioselectivity by use of the â,γ-acetylenic thio-
acetal 6d bearing a less bulky methyl group at the γ-position
(entry 9). Several enynes 10 possessing the (trialkylsilyl)-
acetylene moiety were protodesilylated to give the terminal
alkynes 12 on treatment with TBAF in THF.
a Carried out under ethylene. b Performed with 3 equiv of 4 and 4 equiv
of styrene.
(5) In contrast to the reaction of 6, a similar reaction of R-unsubstituted
alkynyl thioacetals 3 with ketones 6 was complicated and no carbonyl
olefination product was obtained under the same reaction conditions.
3208
Org. Lett., Vol. 6, No. 18, 2004