Chemistry Reference
In-Depth Information
moving by the time the rather tight transition state for hydrogen atom abstraction oc-
curs. As with the enzymatic processes, the advantage is entropy.
We used the radical relay process, chlorinating C-9 and then generating the 9(11)
double bond, in a synthesis of cortisone
91
[158]. This is a substitute for manufacturing
processes in which C-9 or C-11 are hydroxylated by biological fermentation. Also, with
templates that directed the chlorination to C-17 of 3
-cholestanol, such as that in
90
,
we were able to remove the steroid sidechain [159-162]. Using an electrochemical
oxidation process, we could direct chlorination by simple chloride ion with an iodo-
phenyl template [163]. A general review of the processes with iodophenyl templates
has been published [164].
The radical relay process also works with other template types. Thus, the thioether
unit in
92
directed chlorination of C-14 by SO
2
Cl
2
[165]. Also, the sulfur in the thiox-
anthone template of
93
directed the radical relay process to C-9 [166]. The thiophene
sulfur in
94
was able to direct chlorination to C-9 in all three attached steroids [167]. In
all these cases, an intermediate is formed with a chlorine atom bonded to sulfur.
A chlorine atom could also coordinate to the nitrogen of the pyridine template in
compound
95
, directing chlorination to C-9 in a radical relay process [168]. Spectro-
a
