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TIPS
Cl
OH
OH
TIPS
Red Al(3-4 eq.)
TIPS
O
ZnCl
Dry ether
+
TIPS
Dry
ether
TIPS
Cl
78 to
20 °C
Cl
O
OH
30 to
20 °C
yield
OH
36
70%
38
37
TIPS
35
PHOMe
TIPS
H
TIPS
MeOPh
H
MeOPh
H
O
,
10 eq DMSO, 100 ° C
2.2
eq
Mo(CO)
6 toluene
4-methoxybenzaldehyd e
CH
O
TIPS
O
+
TIPS
PTSA,
refluxing
3 Cl
30-40%
O
91%
O
TIPS
O
TIPS
O
O
39
40b (4)
40a (5)
eq
10 6
20 eq DMSO, toluene
53-55 °C, argon, 48 h
65-70%
Mo(CO)
H
MeOPh
TIPS
O
O
O
TIPS
O
41
Scheme 8.7 Studies on the allenic tandem Pauson-Khand reaction.
The synthesis of the bisallene-bisalkyne 38 then took center stage. After a few attempts, 19
the desired (
)-diol 38 was obtained via the procedure illustrated in Scheme 8.7. The tetra-
alkyne 37 was synthesized by treatment of the 1,2-diketone 35 17e,22 with the alkynylide
of propargyl chloride. The propargyl chloride 36 was stirred with butyllithium in diethyl
ether at
±
78 C to produce the alkynylide anion (the lithium anion survived only at
low temperature). 23 Zinc proved to be the best choice as the metal counterpart of the
alkynylide anion with respect to stereoselectivity and yield. A similar effect had been
observed previously by the authors in another system. 17
When lithium was employed
directly (M
Li) in this process, the yield was low and the undesired meso isomer
predominated. When magnesium was employed, this reaction provided the tetrayne 37 in
moderate yield with almost no stereoselectivity. Meso selectivity was also observed by
Diederich et al., 24
=
when trimethylsilyl acetylenyl magnesium was reacted with the same
1,2-diketone 35 .
However, when zinc was employed as the metal counterpart to stabilize the carbanion
(produced by treating the lithium alkynylide of propargyl chloride with anhydrous ZnCl 2
at low temperature), the yield increased to over 80% and the desired dl diastereomer of 37
predominated in a ratio of 6:1 over the meso isomer. To date, the Felkin
Anh model 25 of
addition can be employed to rationalize the observed stereoselectivity of the zinc-mediated
process.
Lithium aluminum hydride has been reported in the literature 26 as the reagent of choice
to convert propargyl chlorides into allenes. Unfortunately, when the tetrayne 37 was treated
under the reported conditions, 26 a complex mixture of products was obtained with the de-
sired bisallene-bisalkyne 38 present in very low yield. A number of other hydride sources
 
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