Handbook of Functionalized Organometallics Applications in S

10.5 Cycloadditions
affords cyclohexadienes in 98% yield [118]. The reaction has been shown to occur
with retention of stereochemistry and is not significantly influenced by electronic
effects [119]. This process provides convenient access to 6,6- and 6,5-fused ring
systems including nitrogen heterocycle [118±120]. Extension of this cycloaddition
reaction to diene-allene has been executed. Cycloadducts are obtained in a 2:1 ratio
in excellent yield [121]. It is interesting to note that 6,5-fused ring is obtained
when [Rh(COD)Cl] is used as the catalyst. A complex of mixture of products is
obtained when the reaction is carried out at 185 C without metal catalysts [122].
10.5.3
[4+ 4] Cycloaddition
Oligomerization of 1,3-dienes selectively yields four-, six- eight-, or twelve-membered
rings,depending on the nature of catalysts and reaction conditions [4,123].
Ni(COD) 2-PPh 3 catalyst has been shown to efficiently promote intramolecular
cyclooctadiene formation in good yield from the corresponding bisdienes [124].
The product distribution depends on the nickel±phosphine ratio and the best yield
of A (70% cis/trans= 19/1) is obtained when bisdiene is treated in toluene at 60 C
with 11 mol% Ni(COD) 2 and 33 mol% Ph 3P [124]. When a phosphite ligand is
used,cyclohexene B or cyclopentane C derivatives are the major products. Functional
groups such as esters,ketones,or ethers remain intact under the reaction
conditions. The reaction is stereoselective and has been used for the synthesis of
polycyclic natural products [125±128]. For example,the key intermediate for the
enantioselective synthesis (+)-asteriscanolide is obtained from a similar synthetic
route [128]. The use of this strategy for the construction of a taxan skeleton has
been briefly explored [129].
E
E
E=CO 2Et
H
O
O
TBDMSO
Ni(COD)
PPh 3
E
E
Ni(COD) 2
PPh 3
+
+ E
H
E
E
E
A B C
O
H H
H
O
Ni(COD) 2/P(O-o-BiPh) 3
74% (7/1)
TBDMSO
O
H
O
H
H
H
O
(+)-asteriscanolide
415