Chemistry Reference
In-Depth Information
Me
Me
Me
O
S
H
H
Zn, aq. NH
4
Cl
R
R
O
O
THF, rt
R
R
36a-c
38a-c
(> 96% ee)
38a
(R = H): 92%
38b
(R = Me): 96%
38c
(R = CO
2
Et): 95%
Scheme 5.32
the triple bond, gave the tetracyclic enones
40a
-
c
in good yields and with poor to good
diastereoselectivities (Scheme 5.33)
H
Co
2
(CO)
8
Me
3
Si
O
O
O
Me
3
NO, rt, 12-16 h
O
R
O
H
H
39a
(R = Me)
39b
(R = Ph)
39c
(R =
c
-Hex)
Me
3
Si
R
R
R
H
H
(Major isomer)
40a
(R = Me): 63%, 2:1 dr
40b
(R = Ph): 69%, 5:1 dr
40c
(R=
c
-Hex):76%,6.4:1 dr
Scheme 5.33
The diastereomers of
40b
and
40c
could be separated by column chromatography.
The major isomer of
40b
was transformed into the tricyclic diketone
41
, whose circular
dichroism established an (
R
) configuration for the newly created stereogenic center in
40b
(Scheme 5.34). No attempts were apparently made to quantify the recovery of the
(
S,S
)-1,2-diphenylethane-1,2-diol auxiliary in the acetal cleavage step.
Several years later, Undheim and co-workers described the enantioselective synthesis of
the bicyclic
-amino esters
43
and
44
by the intramolecular PKR of an enyne derived from
the Schollkopf bislactim auxiliary
42
(Scheme 5.35).
34
It is worth noting that the stereogenic
centers in the bislactim ring did not exert any significant degree of stereocontrol in the
bicyclization, but simply allowed the chromatographic separation of the 1:1 diastereomer
mixture obtained. The absolute stereochemistry of
43
and
44
was ascertained by single-
crystal X-ray diffraction analysis of their immediate precursors.
