A novel method for inside selective silylation of 1,2-diols

Full text of DOI:10.1021/jo9722358

2422
J . Org. Chem. 1998, 63, 2422-2423
Sch em e 1
A Novel Meth od for In sid e Selective
Silyla tion of 1,2-Diols
Keiji Tanino, Tadashi Shimizu, Michio Kuwahara, and
Isao Kuwajima*
Department of Chemistry, Tokyo Institute of Technology,
Meguro, Tokyo 152, J apan
Received December 10, 1997
Sch em e 2. Regioselective Clea va ge of a Cyclic Silyl
Eth er
Silyl groups are of great importance for protection of a
hydroxy group,1 and site-selective silylation of polyols has
frequently been used in organic synthesis. Since usual
silylation conditions always effect selective protection of the
less hindered hydroxy group,2 inside selective silylation of
a 1,2-alkanediol generally requires multistep transforma-
tions. For example, a 2-siloxy-1-alkanol can be prepared
from the corresponding diol through esterification of the
1-OH group, silylation of the 2-OH group, and hydrolysis of
the ester moiety (Scheme 1). In the present paper, we
describe a novel method for one-pot silylation of the internal
hydroxy group of a 1,2-alkanediol.
There are several reports concerning inside selective
protection of 1,2-alkanediols by benzyl ethers,3 tert-butyl
ethers,4 methoxymethyl ethers,5 or benzoates.6 It is note-
worthy that all of these examples involve regioselective
cleavage of five-membered intermediates, namely, cyclic
acetals, ortho esters, or phospholanes. On the other hand,
in relation to the chemistry of 1-oxa-2-silacyclopentane
derivatives described previously,7 we became intrigued by
ring-cleavage reactions of cyclic silyl ethers induced by
nucleophiles. We envisioned that treatment of a 1,3-dioxa-
2-silacyclopentane derivative with a nucleophile might effect
regioselective ring cleavage to give the corresponding 2-si-
loxy-1-alkanol.8
Sch em e 3
Since five-membered cyclic silyl ethers easily undergo
hydrolysis by silica gel column chromatography,9 it was
desired to carry out both the preparation and ring cleavage
reaction of these ethers in one pot. The conventional
methods for silylation of 1,2-alkanediols by using dialkylsilyl
dichloride10 or ditriflate11 and amines are unsuitable for this
purpose because the resulting ammonium salt would serve
as a proton donor. We found that the reaction of 1,2-
hexanediol with 1 equiv of butyllithium followed by di-tert-
butylchlorosilane in THF affords the desired cyclic silyl ether
with the evolution of hydrogen. Treatment of the solution
with butyllithium at -78 °C resulted in cleavage of the Si-O
bond to yield 2-siloxy-1-hexanol as the major product.12
Interestingly, the isomeric ratio was enhanced in the pres-
ence of N,N,N′,N′-tetramethylethylenediamine (TMEDA),
while use of hexamethylphosphoramide (HMPA) gave some-
what lower selectivity (Scheme 2).
(1) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis,
2nd ed.; J ohn Wiley & Sons: New York, 1991; pp 68-87.
(2) Chaudhary, S. K.; Hernandez, O. Tetrahedron Lett. 1979, 99.
(3) (a) Takano, S.; Akiyama, M.; Sato, S.; Ogasawara, K. Chem. Lett.
1983, 1593. (b) Takano, S.; Akiyama, M.; Ogasawara, K. Chem. Pharm.
Bull. 1984, 32, 791. (c) Takano, S.; Kurotaki, A.; Sekiguchi, Y.; Satoh, S.;
Hirama, M.; Ogasawara, K. Synthesis 1986, 811.
(4) (a) Takano, S.; Ohkawa, T.; Ogasawara, K. Tetrahedron Lett. 1988,
29, 1823. (b) Cheng, W.-L.; Yeh, S.-M.; Luh, T.-Y. J . Org. Chem. 1993, 58,
5576.
(5) (a) Takasu, M.; Naruse, Y.; Yamamoto, H. Tetrahedron Lett. 1988,
29, 1947. (b) Friesen, R. W.; Vanderwal, C. J . Org. Chem. 1996, 61, 9103.
(6) Pautard, A. M.; Evans, S. A., J r. J . Org. Chem. 1988, 53, 2300.
(7) Tanino, K.; Yoshitani, N.; Moriyama, F.; Kuwajima, I. J . Org. Chem.
1997, 62, 4206.
The regioselectivity observed in these reactions would
come from kinetically controlled ring cleavage because the
2-siloxy-1-alkanol seems less stable than the corresponding
1-siloxy-2-alkanol. Indeed, 1a underwent isomerization to
1b under the influence of a catalytic amount of potassium
hydride (eq 1). Therefore, the preferential formation of
2-siloxy-1-alkanols could be attributable to complexation of
lithium at the sterically less hindered oxygen as depicted
in Scheme 3.
(8) It has been reported that treatment of a dimethylsilylene derivative
of a 1,3-diol with tert-butyllithium induced regioselective cleavage of the
less hindered Si-O bond to yield the corresponding siloxy alcohol: Mu-
kaiyama, T.; Shiina, I.; Kimura, K.; Akiyama, Y.; Iwadare, H. Chem. Lett.
1995, 229.
(9) Dialkyl silylene derivatives of 1,2-diols are much less stable than those
of 1,3-diols. See ref 11.
(10) (a) Trost, B. M.; Caldwell, C. G. Tetrahedron Lett. 1981, 22, 4999.
(b) Trost, B. M.; Caldwell, C. G.; Murayama, E.; Heissler, D. J . Org. Chem.
1983, 48, 3252.
(12) Treatment of 2-(butyldi-tert-butylsiloxy)-1-hexanol (1a ) with tet-
rabutylammonium fluoride in THF at room temperature overnight effected
smooth removal of the bulky silyl group.
(11) Corey, E. J .; Hopkins, P. B. Tetrahedron Lett. 1982, 23, 4871.
S0022-3263(97)02235-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/28/1998

Communications
Ta ble 1. On e-P ot Silyla tion of 1,2-Diols
J . Org. Chem., Vol. 63, No. 8, 1998 2423
sponding R-siloxy aldehydes by Swern oxidation. A variety
of R-siloxy aldehydes, including optically active ones that
are important building blocks in organic synthesis, can be
easily prepared by this methodology. For example, (S)-1-
phenyl-1,2-ethanediol was converted into (S)-2-(butyldi-tert-
butylsiloxy)-2-phenylacetaldehyde (>95% ee, 80% overall
yield) by two-step transformations.14
Furthermore, R-siloxy aldehydes with a functionalized
silyl group show promise for stereoselective C-C bond
formations. For example, in the presence of Me2AlCl,
R-(alkynylsiloxy) aldehydes 12 and 13 underwent rearrange-
ment of the alkynyl group to yield an anti diol predominantly
(eq 4).15 It is noteworthy that a similar non-chelation-type
entry
R
R′
C4H9
product
% yielda
a /bb
1
2
3
4
5
6
7
8
C4H9
1
2
3
4
5
6
7
8
84
89
96
94
76
83
85
89
94:6
94:6
93:7
99:1
99:1
95:5
99:1
98:2
C4H9
C6H5
C4H9
CtCC6H13
C4H9
CtCC4H9
C4H9
c-C6H11
c-C6H11
BnOCH2
C6H5
C4H9
C6H5
C6H5
a
b
Combined isolated yield of a and b. Determined by GLC
analysis.
F igu r e 1.
This method is applicable to various kinds of 1,2-diols as
shown in Table 1. Use of phenyllithium or an alkynyl-
lithium as a nucleophile also gave the corresponding 2-si-
loxy-1-alkanols in high selectivity (entries 2 and 3). Steric
hindrance around the inside hydroxy group seems to en-
hance the regioselectivity (entries 4 and 7), and the effect
was typically observed in the reaction of 1-(hydroxymethyl)-
cyclohexanol, which has an inside tertiary hydroxy group
(eq 2). Distinction between a tertiary hydroxy group and a
secondary hydroxy group by the one-pot silylation method
was also examined (eq 3). Although formation of the cyclic
silyl ether required higher temperature and a prolonged
reaction period in this case, the cyclic ether underwent
smooth ring cleavage to give siloxy alcohol 10a as a single
isomer.13
adduct was obtained, albeit in lower selectivity, by the
intermolecular reaction of 2-(butyldi-tert-butylsiloxy)hexanal
and the corresponding alkynyllithium (eq 5).16 The high
stereoselectivity of the intramolecular reaction can be
rationalized by transition-state models in which the siloxy
group is perpendicular to the carbonyl group (Figure 1).17
The reaction would proceed mainly through TS-1 because
TS-2 suffers from steric repulsion between the alkyl sub-
stituent and the carbonyl oxygen.
In conclusion, a novel method for selective silylation of
an internal hydroxy group of 1,2-diols was developed. To
our knowledge, the present method is the first example of
this type of transformation that is feasible in one pot. We
are currently investigating synthetic reactions using 2-si-
loxy-1-alkanols or R-siloxy aldehydes having a functionalized
silyl group.
Ack n ow led gm en t. This work was partially supported
by grants from the Ministry of Education, Science, Sports,
and Culture of the J apanese Government.
Su p p or tin g In for m a tion Ava ila ble: Experimental proce-
dures and characterization data for 1-17 and the R-siloxy alde-
hydes (8 pages).
J O9722358
(15) Diastereoselective intramolecular allylation reaction of a â-(allylsi-
loxy)aldehyde promoted by a Lewis acid has been reported: Reetz, M. T.;
J ung, A.; Bolm, C. Tetrahedron 1988, 44, 3889.
As shown in eq 2, the position of the silyl group was
confirmed by converting these silyl ethers into the corre-
(16) It has been reported that the reaction of [2-(trimethylsilyl)ethynyl-
]lithium and 2-(tert-butyldiphenylsiloxy)heptanal afforded the corresponding
adduct as an 84:16 mixture of anti and syn isomers: Alami, M.; Crousse,
B.; Linstrumelle, G.; Mambu, L, Larcheveˆque, M. Synlett 1993, 217.
(17) Crossover experiments were performed by using a 1:1 mixture of
R-(alkynylsiloxy) aldehydes 12 and 13, and no product arising from an
intermolecular addition reaction was observed. See the Supporting Informa-
tion.
(13) The same product was also obtained by treating aldehyde 11 with
methyllithium.
(14) Treatment of the aldehyde with DIBAL afforded alcohol 6a , which
was then converted into the corresponding MTPA ester. HPLC analysis of
the MTPA ester indicated that the diastereomeric excess is >95%.