sequence differed from Donohoe et al. previous oxidative cyclizations in that
stoichiometric pyridine N-oxide was used to oxidize Os(IV) to Os(VI) and that
N,O-carbamates were used as internal oxidants to bring the oxidation state of
osmium to Os(VIII).
The application of the one-pot conditions to (Z,Z)-substrate 199 was not as clean
as the reactions of either 195 or 197 (Scheme 53). By subjecting 199 to a stepwise
sequence where aminohydroxylation was followed by cyclization, the authors were
able to conclude that the problematic step for the use of 199 was the oxidative
cyclization to 201 rather than the aminohydroxylation to 200. Similar problems
were found for the (Z,E)-diene that corresponds to 199.
4.6 Intramolecular Iodoetherification
In 1981, the work of Rychnovsky and Bartlett demonstrated the utility of iodoetherification to build tetrahydrofurans [59]. Others have recently built upon these
studies. For example, Fu, Ma, and coworkers have carried out iodoetherification
of allenyl alcohols [60]. Representative examples are given in Scheme 54 where
204 (available from a Claisen rearrangement of the corresponding enantiomerically
enriched propargyl alcohol) underwent cyclization to give 205 in high yield, with
high diastereoselectivity and with complete preservation of the optical purity of the
starting allene. When substitution was present in the linker between the alcohol and
the allene, 2,3-trans-tetrahydrofuran 203 (R ¼ CH 3 ) was isolated.
5 Furan Reductions
While not as widely used as other methods of generating tetrahydrofurans, furan
reductions can be an effective means of generating saturated rings. Outlined here is
a single example which represents the state of the art in 2012 [61]. In an effort to
O
N
H 3 CO 2 C
H
N
O
O
[Rh(cod)L]BF 4 (10-20 mol %)
THF, H 2 (80 bar), 80 °C
O
N
H 3 CO 2 C
H
N
O
O
L:
S
P
P
R
R
R
R
R
CH 3
CH 2 CH 3
CH(CH 3 ) 2
Loading
20 mol %
10 mol %
10 mol %
Yield
10%
60%
98%
ee
0%
32%
72%
206
207
208
Scheme 55 Furan reductions to deoxynucleosides by Albert et al. [62]
26
J.D. Rainier
stoichiometric pyridine N-oxide was used to oxidize Os(IV) to Os(VI) and that
N,O-carbamates were used as internal oxidants to bring the oxidation state of
osmium to Os(VIII).
The application of the one-pot conditions to (Z,Z)-substrate 199 was not as clean
as the reactions of either 195 or 197 (Scheme 53). By subjecting 199 to a stepwise
sequence where aminohydroxylation was followed by cyclization, the authors were
able to conclude that the problematic step for the use of 199 was the oxidative
cyclization to 201 rather than the aminohydroxylation to 200. Similar problems
were found for the (Z,E)-diene that corresponds to 199.
4.6 Intramolecular Iodoetherification
In 1981, the work of Rychnovsky and Bartlett demonstrated the utility of iodoetherification to build tetrahydrofurans [59]. Others have recently built upon these
studies. For example, Fu, Ma, and coworkers have carried out iodoetherification
of allenyl alcohols [60]. Representative examples are given in Scheme 54 where
204 (available from a Claisen rearrangement of the corresponding enantiomerically
enriched propargyl alcohol) underwent cyclization to give 205 in high yield, with
high diastereoselectivity and with complete preservation of the optical purity of the
starting allene. When substitution was present in the linker between the alcohol and
the allene, 2,3-trans-tetrahydrofuran 203 (R ¼ CH 3 ) was isolated.
5 Furan Reductions
While not as widely used as other methods of generating tetrahydrofurans, furan
reductions can be an effective means of generating saturated rings. Outlined here is
a single example which represents the state of the art in 2012 [61]. In an effort to
O
N
H 3 CO 2 C
H
N
O
O
[Rh(cod)L]BF 4 (10-20 mol %)
THF, H 2 (80 bar), 80 °C
O
N
H 3 CO 2 C
H
N
O
O
L:
S
P
P
R
R
R
R
R
CH 3
CH 2 CH 3
CH(CH 3 ) 2
Loading
20 mol %
10 mol %
10 mol %
Yield
10%
60%
98%
ee
0%
32%
72%
206
207
208
Scheme 55 Furan reductions to deoxynucleosides by Albert et al. [62]
26
J.D. Rainier