K. Tanaka et al. / Tetrahedron Letters 46 (2005) 6429–6432
6431
OSiMe2But
CO2Et
atory Research, No. 14657563, 2002 and by the Naito
Foundation. We thank the Research Center for Mole-
cular Medicine, Faculty of Medicine, Hiroshima Uni-
versity and the Natural Science Center for Basic
Research and Development (N-BARD), Hiroshima
University for the use of their facilities.
tBuMe2SiO
tBuMe2Si
O
KCN
18-crown-6
O
SiMe2But
CN
NCCO2Et
Et2O
0 ˚C, 2 h
9a
H
10a (95%, E/Z = 1/2.8)
SiR3
O
CN
H
4
O
H
Supplementary data
R3Si
11
H
Supplementary data associated with this article can be
details and spectral data.
Scheme 4. Reaction of 9a with ethyl cyanoformate in the presence of
KCN/18-crown-6.
NCCO2Et (1.1 eq)
OSiMe2But
O
KCN (0.1 eq)
References and notes
O
CO2Et
18-crown-6 (0.1 eq)
R3Si
SiMe2But
R3Si
R3SiO
CN
1. Tanaka, K.; Takeda, K. Tetrahedron Lett. 2004, 45, 7859–
7861.
2. Reaction of (2Z,4E)-3 with NCCO2Et resulted in the
formation of an O-carbamoyl derivative.
Et2O, 0 ˚C
2 h
1b,c, 9b,c
H
10b,c
E/Z
E
yield (%)
93
1b
i-Pr3Si
3. For discussions on stereochemistry in the reactions in
which enol silyl ethers are formed from a-silyl alkoxides
bearing a b-leaving group via the Brook rearrangement,
see: (a) Reich, H. J.; Holtan, R. C.; Bolm, C. J. Am. Chem.
Soc. 1990, 112, 5609–5617; (b) Okugawa, S.; Takeda, K.
Org. Lett. 2004, 6, 2973–2975; (c) Clayden, J.; Watson, D.
W.; Chambers, M. Tetrahedron 2005, 61, 3195–3203.
4. To the best of our knowledge, there is no precedent for a
stereochemical discussion concerning Brook rearrange-
ment in a-silyl allylalkoxides.
5. For reviews on the Brook rearrangement, see: (a) Brook,
M. A. Silicon in Organic, Organometallic, and Polymer
Chemistry; John Wiley and Sons, 2000; (b) Brook, A. G.;
Bassindale, A. R. In Rearrangements in Ground and
Excited states; de Mayo, P., Ed.; Academic Press: New
York, 1980, pp 149–221; (c) Brook, A. G. Acc. Chem. Res.
1974, 7, 77–84; For the use of the Brook rearrangement in
tandem bond formation strategies, see: (d) Moser, W. H.
Tetrahedron 2001, 57, 2065–2084; Also, see: (e) Ricci, A.;
DeglÕInnocenti, A. Synthesis 1989, 647–660; (f) Bulman
Page, P. C.; Klair, S. S.; Rosenthal, S. Chem. Soc. Rev.
1990, 19, 147–195; (g) Qi, H.; Curran, D. P. In Compre-
hensive Organic Functional Group Transformations; Katri-
tzky, A. R., Meth-Cohn, O., Rees, C. W., Moody, C. J.,
Eds.; Pergamon: Oxford, 1995, pp 409–431; (h) Cirillo, P.
F.; Panek, J. S. Org. Prep. Proced. Int. 1992, 24, 553–582;
(i) Patrocinio, A. F.; Moran, P. J. S. J. Braz. Chem. Soc.
2001, 12, 7–31.
1c
9b
9c
E
t-BuPh2Si
i-Pr3Si
97
96
99
1/3.3
1/1.5
t-BuPh2Si
Scheme 5. Reaction of 1b,c and 9b,c with ethyl cyanoformate in the
presence of KCN/18-crown-6.
1b,c and 9b,c, which bear electronically and sterically
different silyl groups, with the expectation that in the
case of 9c, the silicate ion 6a would have a sufficiently
long lifetime for conversion to rotamer 6b, leading to
a thermodynamically stable E-isomer, by introduction
of a phenyl group on the silyl group, and consequently
the E/Z ratio in 10 would increase (Scheme 5).15 While
in the case of triisopropylsilyl derivatives 1b and 9b,
more sterically demanding than tert-butyldimethylsilyl
group, essentially the same stereochemical outcome as
that observed for tert-butyldimethylsilyl derivatives 1a
and 9a was observed, tert-butyldiphenylsilyl derivative
9c, more sterically demanding but more silicate ion-sta-
bilizing than the tert-butyldimethylsilyl group, afforded
10c in an increased ratio (E/Z: 1/1.5 vs 1/2.8) of E-iso-
mer to Z-isomer, supporting the above assumption.
In conclusion, we demonstrated the possibility that a
c-anion-induced ring opening in a,b-epoxysilanes pro-
vides an allylsilicate intermediate in a concerted fashion
and demonstrated that the intermediate does not have a
sufficient lifetime to allow it to rotate freely about the
C(a)–C(b) bond and undergoes Brook rearrangement
according to the least motion principle16 to give prod-
ucts in an E/Z ratio depending on the geometry of the
epoxysilanes.
6. X-ray data for (2Z,4E)-3: C18H35NO2Si2; formula weight,
353.65; crystal system, monoclinic; space group, P2(1)/c;
˚
unit cell dimensions, a = 10.745(3) A, a = 90°, b =
˚
˚
7.6785(18) A, b = 94.078(4)°, c = 27.391(6) A, c = 90°;
3
˚
volume, 2254.2(9) A ; Z = 4; R = 0.0702.
7. To the best of our knowledge, this is the first X-ray crystal
structure of a-trialkylsilyl allylic alcohol. For an X-ray
structure of (1-hydroxyprop-2-eny1)silanol, see: Buynak,
J. D.; Strickland, J. B.; Lamb, G. W.; Khasnis, D.; Modi,
S.; Williams, D.; Zhang, H. J. Org. Chem. 1991, 56, 7076–
7083.
8. For reviews of hypervalent silicon compounds, see: (a)
Kost, D.; Kalikhman, I. Hypervalent Silicon Compounds.
In The Chemistry of Organic Silicon Compounds; Rappo-
port, Z., Apeloig, Y., Eds.; Wiley: Chichester, UK, 1998;
Vol. 2, Chapter 23; (b) Chuit, C.; Corriu, R. J. P.; Reye,
C.; Young, J. C. Chem. Rev. 1993, 93, 1371–1448;
Acknowledgements
This research was partially supported by the Ministry of
Education, Science, Sports and Culture, Grant-in-Aid
for Scientific Research (B), No. 15390006, 2003, Explor-