COMMUNICATIONS
Table 2. [NiCl2(dppp)]-catalyzed deallylation of diethers.
alcohol 10 in 93% yield [Eq. (f)]. On the other hand, no
reaction occurred with either prenyl and benzyl (Z)-4-
benzyloxy-2-butenyl ethers 7b and 7c, respectively, under
the same conditions (Table 3).
Entry
Substrate R
Solvent
Product Yield
[%][a]
1
2
3
4
5
6
7
8
9
10
3a
3b
3c
3d
3e
3 f
3g
3h
3i
tBuMe2Si
Et2O[b]
Et2O[b]
Et2O[b]
Et2O[b]
Et2O[b]
Et2O[b]
Et2O[b]
4a
4b
4c
4d
4e
4 f
4g
92
95
95
51
80
90
89
73
85
80
4-MeOC6H4
benzyl
allyl
prenyl
MOM
THP
acetyl
pivaloyl
benzoyl
THF/EtOH (4:1)[c] 4h
THF/EtOH (4:1)[c] 4i
THF/EtOH (4:1)[c] 4j
3j
[a] Yield after SiO2 column chromatography. [b] Reaction was carried out
with DIBAL (1.5 equiv) and [NiCl2(dppp)] (1 mol%) at 08C !RT, 2 h.
[c] Reaction was carried out with NaBH4 (4.0 equiv) and [NiCl2(dppp)]
(4 mol%) at 08C !RT.
The present study offers an extremely facile and selective
deallylation procedure for a variety of allyl ethers and also
confirms that the reaction proceeds by nickel-catalyzed
hydroalumination ± elimination. Because of its simplicity
and chemoselectivity, this procedure will undoubtedly extend
the use of allyl groups for the protection of a variety of
hydroxyl compounds.
hemiacetal mixture 6, in which the the acetonide functionality
is intact, by selective deallylation and spontaneous epimeri-
zation [Eq. (d)]. However, the reaction was found to proceed
Experimental Section
To a stirred solution of 1a (100 mg, 0.6 mmol) and [(dppp)NiCl2] (3 mg,
6 mmol) in Et2O[10] (2 mL) under argon was added dropwise DIBAL (1.5m
in toluene, 600 mL, 0.9 mmol) at 08C. The mixture was stirred for 5 min at
the same temperature and then for 2 h at room temperature. The mixture
was diluted with Et2O (3 mL), quenched by addition of H2O (600 mL) and,
after stirring for 1 h, dried directly over MgSO4, filtered through a Celite
pad, and concentrated under reduced pressure to leave the crude product,
which was chromatographed on silica gel (3 g, Et2O/hexane 1/4 v/v) to give
pure 2a (68 mg, 90%).
much better when triethylaluminum was used in place of
DIBAL, because the hemiacetal functionality is more stable
with the former reagent. Although ester functionalities
(acetyl, pivaloyl, and benzoyl) are not incompatible with
DIBAL as a reagent, when the ester substrates 3h ± j were
treated with NaBH4 in THF/EtOH the corresponding hy-
droxy esters 4h ± j were obtained in good yields (Table 2,
entries 8 ± 10).
Chemoselective cleavage of the allyl functionality was also
demonstrated by the reaction of the allyl ethers of allyl
alcohols [Eq. (e), Table 3]. Thus, the reaction of allyl (Z)-4-
benzyloxy-2-butenyl ether (7a) occurred selectively at the
Received: September 26, 1997 [Z10975IE]
German version: Angew. Chem. 1998, 110, 1137 ± 1139
Keywords: allyl ethers ´ aluminum ´ cleavage reactions ´
nickel ´ protecting groups
[1] For a pertinent review, see: F. Guibe, Tetrahedron, 1997, 53, 13509 ±
13556.
[2] For pertinent monographs, see: a) T. W. Greene, P. G. M. Wuts,
Protective Groups in Organic Synthesis, 2nd ed., Wiley, New York,
1991; b) P. J. Kocienski, Protective Groups, Thieme, Stuttgart, 1994.
[3] H. M. Colquhoun, J. Holton, D. J. Thompson, M. V. Twigg, New
Pathways for Organic Synthesis, Plenum, New York, 1988.
[4] K. Fischer, K. Jonas, P. Misbach, R. Stabba, G. Wilke, Angew. Chem.
1973, 85, 1002 ± 1012; Angew. Chem. Int. Ed. Engl. 1973, 12, 943 ± 1026.
[5] A nickel-catalyzed reductive cleavage of cyclic allyl ethers with
DIBAL has been reported recently, see: M. Lautens, P. Chin, S. Ma, T.
Rovis, J. Am. Chem. Soc. 1995, 117, 532 ± 533.
[6] 1H NMR spectrum of propene in CCl4 (60 MHz): d 5.73 (m, 1H),
4.97 (m, 1H), 4.88 (m, 1H), 1.72 (dt, 3H). See: A. A. Bothner-By, C.
Naar-Colin, J. Am. Chem. Soc. 1961, 83, 231 ± 236.
[7] J. Eisch in Comprehensive Organic Synthesis, Vol. 8 (Eds.: B. M. Trost,
I. Fleming), Pergamon, Oxford, 1991, pp. 733 ± 761.
nonsubstituted allyl end to give the corresponding allyl
alcohol 8a in good yield. Facile chemoselective cleavage at
the nonsubstituted allyl end was also observed with the
secondary allylic substrate 9 to give the secondary allylic
[8] For a pertinent review, see: E. Winterfeldt, Synthesis 1975, 617 ± 630.
[9] For examples reported by our group, see: a) S. Takano, M. Akiyama, S.
Sato, K. Ogasawara, Chem. Lett. 1983, 1593 ± 1596; b) S. Takano, M.
Akiyama, K. Ogasawara, Heterocycles 1983, 20, 2237 ± 2238; c) S.
Takano, M. Akiyama, K. Ogasawara, Chem. Pharm. Bull. 1984, 32,
791 ± 794; d) S. Takano, A. Kurotaki, Y. Sekiguchi, S. Satoh, M.
Hirama, K. Ogasawara, Synthesis 1986, 811 ± 817; e) S. Takano, S.
Satoh, K. Ogasawara, J. Chem. Soc. Chem. Commun. 1988, 59 ± 60.
[10] Toluene may be used in place of Et2O; however, a stock solution of
DIBAL and [NiCl2(dppp)] in toluene loses its activity within a week
on standing at 208C.
Table 3. [NiCl2(dppp)]-catalyzed deallylation of diallyl ethers.
Entry
Substrate
R
t [h]
Product
Yield [%][a]
1
2
3
7a
7b
7c
allyl
prenyl
benzyl
2[b]
12[b]
12[b]
8a
8b
8c
71
0[c]
0[c]
[a] Yield after SiO2 column chromatography. [b] Reaction was carried out
in Et2O with DIBAL (1.5 equiv) and [NiCl2(dppp)] (1 mol%) at 08C !RT.
[c] Starting material was recovered unchanged.
Angew. Chem. Int. Ed. 1998, 37, No. 8
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