Acid-catalyzed cyclization of vinylsilanes bearing a hydroxy group. Benzyldimethylsilyl group as an effective promoter and novel hydroxy surrogate

Article abstract of DOI:10.1016/S0040-4039(00)00091-5

A benzyldimethylsilyl (BnDMS) group was found to effectively enhance the reactivity of vinylsilanes toward the acid-catalyzed intramolecular addition of a hydroxy group in comparison with a dimethylphenylsilyl group. The BnDMS group of the resultant cyclized products could be easily converted to a hydroxy group by the action of TBAF-H₂O₂-KHCO₃. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00091-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 2129–2132
Acid-catalyzed cyclization of vinylsilanes bearing a hydroxy
group. Benzyldimethylsilyl group as an effective promoter and
1
novel hydroxy surrogate
∗
Katsukiyo Miura, Takeshi Hondo, Tatsuyuki Takahashi and Akira Hosomi
Department of Chemistry and Graduate School of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
Received 1 December 1999; revised 5 January 2000; accepted 7 January 2000
Abstract
A benzyldimethylsilyl (BnDMS) group was found to effectively enhance the reactivity of vinylsilanes toward
the acid-catalyzed intramolecular addition of a hydroxy group in comparison with a dimethylphenylsilyl group.
The BnDMS group of the resultant cyclized products could be easily converted to a hydroxy group by the action
of TBAF–H2O2–KHCO3. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: cyclization; acid catalyst; silicon and compound; oxidation.
It is well recognized that electrophilic substitution of allylsilanes and vinylsilanes proceeds via β-
2
silylcarbenium ion intermediates due to the directing effect of a silyl group. In recent years, much
attention has been paid to carbon–carbon and carbon–hetero atom bond formation by intramolecular
nucleophilic trap of the intermediates because it provides a powerful tool for the stereoselective con-
struction of carbocycles and heterocycles as proved in the Lewis acid-mediated [3+2] cycloaddition of
3
allylsilanes (Eq. (1)). In this process, the silyl group of the employed silicon reagent remains in the
cyclized product. Therefore, from the viewpoint of synthetic utility, the silyl group should be not only
effective in promoting this process but also easily convertible to a hydroxy group by oxidative cleavage
4
of the Si–C bond. Studies on the [3+2] cycloaddition of allylsilanes from this aspect have revealed that
some bulky silyl groups meet these requirements.5
(1)
∗
Corresponding author. Tel: +81 298 53 4237; fax: +81 298 53 6503; e-mail: hosomi@staff.chem.tsukuba.ac.jp (A. Hosomi)
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00091-5
tetl 16377

2
130
Previously, we have reported that the acid-catalyzed 1,2-silyl migrative cyclization of vinylsilanes
6
bearing a hydroxy group is valuable in the stereoselective synthesis of certain cyclic ethers (Eq. (2)). A
dimethylphenylsilyl (DMPS) group is usable for this cyclization via a β-silylcarbenium ion intermediate,
and the resultant product can be transformed to the corresponding alcohol by the known two-step
7
procedure. However, the efficiency of cyclization is not so high. While a t-butyldimethylsilyl (TBDMS)
group effectively promotes the cyclization, it cannot be converted to a hydroxy group. We herein report
that a benzyldimethylsilyl (BnDMS) group imparts high reactivity toward the cyclization to vinylsilanes
like the TBDMS group and serves as an efficient latent hydroxy group.
(2)
6
As reported previously, treatment of vinylsilane (Z)-1b with TiCl for 9.5 h afforded only trans-2b in
4
75% yield (Eq. (3)). In the present study, we initially found that the cyclization of (Z)-1a was completed
in 1.5 h, forming 2a in a comparable yield with reduced trans-selectivity (Eq. (3) and entry 1 in Table 1).
Thus, the BnDMS group considerably accelerated the cyclization. The use of TiCl (O-i-Pr) for (Z)-1a
2
2
achieved a higher yield and trans-selectivity (entry 2) although the less Lewis acidic catalyst was not
suitable for (Z)-1b. The cyclization of (Z)-1a proceeded even at 0°C and exhibited still higher selectivity
8
(
entry 3). AcCl also worked as an efficient catalyst for (Z)-1a (entry 4). Interestingly, in this case, a
9
6
certain formation of 3a was observed. Similar to the result with (E)-1b, the reaction of (E)-1a gave
disappointing results in both yield and stereoselectivity (entry 5).
(3)
Table 1
Acid-catalyzed cyclization of vinylsilanes 1aa
6
Judging from the previous and present results, the ability of a BnDMS group to accelerate the 1,2-
silyl-migrative cyclization exceeds those of DMPS and TMS groups, but it is not as high as that of a
TBDMS group. The low ability of the DMPS group is probably due to the electron-withdrawing character
1
0
2
of the phenyl group, which would decelerate the rate-determining proton transfer to the sp carbon α
6
to the silyl group (Eq. (2)). In the case with the TMS group, desilylation of the substrate to 4 becomes a

2
131
serious problem. The side reaction also diminishes the accelerating ability of the BnDMS group to some
extent.
The cyclization of (Z)- and (E)-vinylsilanes 5a were also examined (Eq. (4) and Table 2). In both
6
systems using TiCl and TsOH, (Z)-5a was smoothly cyclized to trans-6a unlike (Z)-5b. In addition, the
use of (E)-5a provided cis-6a in a reasonable yield with high stereoselectivity.
4
11
(4)
Table 2
Acid-catalyzed cyclization of vinylsilanes 5aa
Vinylsilanes 8 and 10 bearing no α-substituent underwent the cyclization without 1,2-silyl migration
to turn to 2-(silylmethyl)tetrahydrofurans 9 and 11, respectively, in accordance with the cyclization of
1
2
their DMPS analogues (Eqs. (5) and (6)). In this case also, the BnDMS group was more effective in
accelerating the cyclization than the DMPS group. The TiCl -catalyzed cyclization of 10 resulted in a
4
slightly lower diastereoselectivity than that of its DMPS analogue at rt. However, the high reactivity of
10 enabled the cyclization at −20°C, which effected high trans-selectivity.
(
5)
6)
(
We have already shown that the DMPS group of trans-2b or trans-6b can be converted to a hydroxy
group by treatment with t-BuOK/DMSO followed by TBAF–H O –KHCO /MeOH–THF (12a from
2
2
3
6
,7
trans-2b, 88%; 12b from trans-6b, 89%). In the present study, it turned out that the cyclized products
bearing a BnDMS group were smoothly oxidized to the corresponding alcohols in one-step using only
the latter set of reagents (Eqs. (7) and (8)). The yields of alcohols 12 from trans-2a and trans-6a were
comparable to those from trans-2b and trans-6b. The oxidation of 11 gave a much better result than that
of its DMPS analogue (59% by the two-step procedure).

2
132
(
7)
8)
(
In conclusion, we have succeeded in enhancing the synthetic utility of the acid-catalyzed cyclization
of vinylsilanes by the introduction of a BnDMS group. The silyl group works as an effective promoter
of the cyclization and a novel hydroxy surrogate. We are further studying the stereoselective synthesis of
cyclic ethers utilizing the directing effect of the BnDMS group, and the results will be reported in due
course.
Acknowledgements
Financial support for our work is provided by Grants-in-Aid for Scientific Research on Priority Areas
from the Ministry of Education, Japan. We thank Dow Corning Toray Silicone Co. Ltd and Shin-Etsu
Chemical Co. Ltd for a gift of organosilicon compounds. T.H. acknowledges support from the Research
Fellowships of the Japan Society for the Promotion of Science for Young Scientists.
References
1
2
. Studies on Organosilicon Chemistry. No. 149.
. (a) Fleming, I.; Barbero, A.; Walter, D. Chem. Rev. 1997, 97, 2063. (b) Panek, J. S. In Comprehensive Organic Synthesis;
Trost, B. M., Ed.; Pergamon Press: Oxford, 1991; Vol. 1, p. 579. (c) Fleming, I.; Dunoguès, J.; Smithers, R. H. Org. React.
1
989, 37, 57. (d) Bassindale, A. R.; Taylor, P. G. In The Chemistry of Organic Silicon Compounds; Patai, S.; Rappoport,
Z., Ed.; Wiley: Chichester, 1989; Part 2, p. 893. (e) Hosomi, A. Acc. Chem. Res. 1988, 21, 200. (f) Colvin, E. W. Silicon in
Organic Synthesis; Butterworths: London, 1981.
3
4
. (a) Knölker, H.-J. J. Prakt. Chem. 1997, 339, 304. (b) Masse, C. E.; Panek, J. S. Chem. Rev. 1995, 95, 1293.
. (a) Colvin, E. W. In Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon Press: Oxford, 1991; Vol. 7, p. 641. (b)
Jones, G. R.; Landais, Y. Tetrahedron 1996, 52, 7599.
5
. (a) Akiyama, T.; Hoshi, E.; Fujiyoshi, S. J. Chem. Soc., Perkin Trans. 1 1998, 2121. (b) Groaning, M. D.; Brengel, G. P.;
Meyers, A. I. J. Org. Chem. 1998, 63, 5517. (c) Knölker, H.-J.; Jones, P. G.; Wanzl, G. Synlett 1998, 613. (d) Suginome, M.;
Matsunaga, S.; Ito, Y. Synlett 1995, 941.
6
7
. Miura, K.; Hondo, T.; Saito, H.; Ito, H.; Hosomi, A. J. Org. Chem. 1997, 62, 8292.
. (a) Murakami, M.; Suginome, M.; Fujimoto, K.; Nakamura, H.; Andersson, P. G.; Ito, Y. J. Am. Chem. Soc. 1993, 115, 6487.
(
b) Tamao, K.; Ishida, N. J. Organomet. Chem. 1984, 269, C 37.
. The actual catalyst would be HCl generated by the reaction of the hydroxy group of the substrate with AcCl. See Ref. 6.
. The cyclized product 3a can be isomerized to 2a by the action of TiCl . The AcCl-catalyzed cyclization of 1a followed by
a brief treatment (10 min) with 5 molTiCl gave 2a in 89% (trans:cis=98:2) along with less than 1% of 3a.
8
9
4
4
1
1
0. Mayr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 938.
1. The reason for the stereospecific cyclization can be explained by the reaction mechanism previously proposed by us. See
Ref. 6.
12. (a) Miura, K.; Okajima, S.; Hondo, T.; Hosomi, A. Tetrahedron Lett. 1995, 36, 1483. (b) Miura, K.; Hondo, T.; Okajima,
S.; Hosomi, A. Tetrahedron Lett. 1996, 37, 487.