3
1-Octene
11
(10 mol%)
OH
p-MeO-Ph-
CH(OMe)2
OH
PMP
O
TBDPSO
TBDPSO
O
CH2Cl2, rt
87%, 8 h
4
OH
CSA, CH2Cl2
rt, 2 h, 80%
OH
12
TBDPSO
4
2
13
DIBAL-H, CH2Cl2
-78 oC, 1 h
OPMB
OH
TBDPSO
+
TBDPSO
4
4
78% (combined yield)
OH
14
OPMB
15
Mes
N
Cl
N
Mes
Ph
DMP, CH2Cl2
0 oC-rt, 2 h
80%
DMP, CH2Cl2
0 oC-rt, 2 h
76%
Ru
Cl
PCy3
11
OPMB
O
TBDPSO
TBDPSO
4
4
O
OPMB
16
17
Scheme 3. Regioselective ring opening of p-methoxybenzylidene acetal in case of internal double bond.
was introduced adjacent to 1,2-diol instead of vinyl group. p-
Methoxybezylidene acetal 7j was prepared following standard
protocol and subjected to regioselective reductive opening of the
cyclic acetal group following DIBAL-H conditions. As expected,
the reaction ended up with a mixture of two regioisomers 8j and
8j’ in a ratio of 35:65 by HPLC.
Supporting Information
Supplementary data associated with this article can be found, in
References and notes
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Further to support for this end result drawn based on the
previous approaches,24 we thought about the protection of
internal allylic diol as acetal with anisaldehyde dimethyl acetal
followed by its reductive cleavage with DIBAL-H. To check the
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PMB-acetal protection of compound 12 with p-
methoxybenzylidene acetal furnished acetal 13 as
a
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oxidation in CH2Cl2 to obtain ketones 16 and 17 in 76% and 80%
yield, respectively as shown in Scheme 3.
In conclusion, we report an unusual result which was observed
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during the selective protection of hydroxyl group in terminal
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good to excellent yield, where as in case of internal olefinic
acetal, the reductive cleavage proceeds with less selectivity
giving both allyl and homo allyl alcohol. Though the actual
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The authors thank CSIR, New Delhi, India, for financial
support as part of XII Five Year plan programme under title
ORIGIN (CSC-0108). Y.B. thanks Council of Scientific and
Industrial Research (CSIR-Bhatnagar Fellow Grant), New Delhi,
India for financial assistance in the form of fellowships.