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Me
NH 2
Ph
-60 ºC
Me
Ti(O i Pr) 2
35
33
i PrMgCl 2 equiv.
Me
Ph
CN
Ti(O i Pr) 4
Me
32
Me
NH 2
Ti(O i Pr) 2
-20 ºC
Ph
34
36
Scheme 10.11 Titanium-catalyzed [4
+
1] reaction.
This reaction offers a new route for accessing 3-substituted-2-cyclopentenones when 2-
trimethylsilyloxybutadiene 37 is used as the starting diene, using the previously established
conditions, as shown in Scheme 10.12.
1) i PrMgCl 2 equiv.
Ti(O i Pr) 4 ,Et 2 O
-78 °C to -20 °C
Me 3 SiO
Me 3 SiO
O
NH 2
R
R
2) RCN, -20 °C to r.t.
38
39a-d
37
39a R=Ph 75% yield
39b R=Bn 69% yield
39c R= n C 9 H 19 62% yield
39d R=(CH 2 ) 3 Cl 54% yield
Scheme 10.12 Titanium catalyzed [4
+
1] synthesis of cyclopentenones.
1] annulation was developed by Moses in 2006, 22 which
employs stereoselective annulation reactions with silyl vinylketenes derived from Fisher
carbene complexes. As depicted in Scheme 10.13, the Fisher carbene complexes first react
thermically with triisopropyl (TIPS) or tert -butyldimethylsilyl (TBS) substituted alkynes
to form the silyl vinylketenes. Notably, silyl vinylketenes are viable precursors to cyclopen-
tenones or other cycloadducts whereas vinylketenes are highly unstable and exhibit a pref-
erence to undergo dimerization and/or [2
Another example of a [4
+
2] cycloaddition reactions. This difference in
behavior is due to the presence of the silyl substituent that provides significant stabilization
to the ketene moiety and suppresses the dimerization pathway, thereby facilitating its partic-
ipation in [4
+
1] cycloaddition reactions with carbenoid reagents. In the subsequent step, the
silyl vinylketene reacts with a carbenoid reagent such as dimethylsulfonium methylide (as
previously reported by Danheiser). 23 The reaction tolerates dimethylsulfonium methylide
and diazomethane reagents as carbenoids rendering the final cyclopentenones in good
yields.
+
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