Chemistry Reference
In-Depth Information
to make the new carbon-carbon bond. There was no great selectivity (with a C
20
chain
there was appreciable insertion into carbons 11-16) but the method gave an indication
of the accessible conformations of the molecule [146]. We later used this method to
determine the conformations of the flexible chains in membranes [147, 148], and
others have adapted it for photo-affinity labeling in proteins.
With inflexible substrates, this photochemical functionalization process showed sig-
nificant selectivity. As our first example, photolysis of steroid ester
70
led to carbonyl
insertion into the C-H bonds at C-7 and C-14 [149, 150]. Other steroid esters afforded
selective functionalizations in various positions [e.g. 151, 152], and compound
71
di-
rectly dehydrogenated the steroid to form the product olefin
72
[150].
Later work gave additional examples, and we were able to elucidate the process by
which
72
is formed [153]. After initial removal of the C-14 hydrogen by the oxygen of
the benzophenone triplet, the resulting C-14 radical inverts to bring C-15 near the
reagent radical; the C-15
hydrogen is then transferred to produce product
72
.
This was demonstrated by use of C-15
a
a
deuterium-labeled steroid. Apparently, in
the more rigid
71
such hydrogen transfer in the radical pair is easier than is coupling
to form a carbon-carbon bond, whose cyclic product would be strained.
We also saw directed functionalizations when a benzophenone reagent (
73
) was not
directly attached to a steroid or long alkyl chain substrate (
74
), but was linked by hy-
drogen bonding [151].
In all the above studies the benzophenone units were singly attached (or with the
previous example, complexed) to the substrates, so that there was intramolecular or
