4.2 Palladium-Catalyzed Cyclizations
Palladium has also been used to carry out oxidative cyclizations of hydroxyolefins.
One advantage to the use of Pd is that it is amenable to tandem cyclization–coupling
sequences. To this end, Nicolai and Waser have examined oxyalkynylation reactions
between hydroxyolefins and bromoacetylenes where 2,5-trans-tetrahydrofurans
were generated from simple starting materials [42]. Illustrated in Scheme 42 is
their use of this chemistry to generate tetrahydrofuran 160 in 59 % yield as a single
stereoisomer. Mechanistically, they proposed a Pd(0)/Pd(II) catalytic cycle which
contrasts their previously proposed Pd(II)/Pd(IV) cycle that was invoked for related
lactone cyclizations [43].
4.3 Ruthenium-Catalyzed Cyclizations
RuO 4 was first shown to be effective at carrying out the oxidative cyclization of
1,5-dienes to give 2,5-cis-tetrahydrofurans by Sharpless and coworkers [44]. The
Stark group has recently been able to optimize these reactions. For example,
Stark and Cheng have described the oxidative conversion of 5,6-dihydroxy olefins
into enantiomerically pure tetrahydrofurans using RuO 4 (Scheme 43) [45]. One of
their more impressive examples involved the use of diene 161 to generate tetrahydrofuran 162 in 68 % yield as a single diastereomer. Over the course of their
work, the authors demonstrated that the formation of Ru diester 163 was critical for
successful cyclizations.
O
O
OH
OH
TPAP (5 mol %)
NMO (1.2 equiv)
CH 2 Cl 2 , H 2 O, rt
68%
O
O
HO
O
OH
H
H
R
O Ru
O
O
O
O
via:
162 (dr >95:5)
161
163
Scheme 43 Dihydroxy olefin cyclizations by Cheng and Stark [45]
O
HO
H
H OH
OTBDPS
TBDPSO
RuCl 3 (0.2 mol %)
NaIO 4 on wet SiO 2
THF, 0 °C
83%
TBDPSO
OTBDPS
lipase Amano AK
vinyl acetate, hexane, 60 °C
81%
O
AcO
H
H OH
TBDPSO
OTBDPS
O
HO
H
H OH
10
O
O
164
(+)-cis-Solamin
166 (ee >99%)
165 (dr>92:2)
Scheme 44 Oxidative cyclization approach to cis-solamin by Go ¨ksel and Stark [46]
20
J.D. Rainier
Palladium has also been used to carry out oxidative cyclizations of hydroxyolefins.
One advantage to the use of Pd is that it is amenable to tandem cyclization–coupling
sequences. To this end, Nicolai and Waser have examined oxyalkynylation reactions
between hydroxyolefins and bromoacetylenes where 2,5-trans-tetrahydrofurans
were generated from simple starting materials [42]. Illustrated in Scheme 42 is
their use of this chemistry to generate tetrahydrofuran 160 in 59 % yield as a single
stereoisomer. Mechanistically, they proposed a Pd(0)/Pd(II) catalytic cycle which
contrasts their previously proposed Pd(II)/Pd(IV) cycle that was invoked for related
lactone cyclizations [43].
4.3 Ruthenium-Catalyzed Cyclizations
RuO 4 was first shown to be effective at carrying out the oxidative cyclization of
1,5-dienes to give 2,5-cis-tetrahydrofurans by Sharpless and coworkers [44]. The
Stark group has recently been able to optimize these reactions. For example,
Stark and Cheng have described the oxidative conversion of 5,6-dihydroxy olefins
into enantiomerically pure tetrahydrofurans using RuO 4 (Scheme 43) [45]. One of
their more impressive examples involved the use of diene 161 to generate tetrahydrofuran 162 in 68 % yield as a single diastereomer. Over the course of their
work, the authors demonstrated that the formation of Ru diester 163 was critical for
successful cyclizations.
O
O
OH
OH
TPAP (5 mol %)
NMO (1.2 equiv)
CH 2 Cl 2 , H 2 O, rt
68%
O
O
HO
O
OH
H
H
R
O Ru
O
O
O
O
via:
162 (dr >95:5)
161
163
Scheme 43 Dihydroxy olefin cyclizations by Cheng and Stark [45]
O
HO
H
H OH
OTBDPS
TBDPSO
RuCl 3 (0.2 mol %)
NaIO 4 on wet SiO 2
THF, 0 °C
83%
TBDPSO
OTBDPS
lipase Amano AK
vinyl acetate, hexane, 60 °C
81%
O
AcO
H
H OH
TBDPSO
OTBDPS
O
HO
H
H OH
10
O
O
164
(+)-cis-Solamin
166 (ee >99%)
165 (dr>92:2)
Scheme 44 Oxidative cyclization approach to cis-solamin by Go ¨ksel and Stark [46]
20
J.D. Rainier