beta-scission of the N-O bond in alkyl hydroxamate radicals: a fast radical trap.

Article abstract of DOI:10.1021/ol991392l

[reaction--see text ] The rate of the beta-scission of the N-O bond in the alkyl hydroxamate radical is faster than 2 x 10(8) s(-)(1). This reaction may be useful as a radical trap.

Full text of DOI:10.1021/ol991392l

ORGANIC
LETTERS
2000
Vol. 2, No. 10
1345-1348
â-Scission of the N−O Bond in Alkyl
Hydroxamate Radicals: A Fast Radical
Trap
Min Wu and Tadhg P. Begley*
Department of Chemistry and Chemical Biology, Cornell UniVersity,
Ithaca, New York 14853
tpb2@cornell.edu
Received December 22, 1999
ABSTRACT
The rate of the â-scission of the N−O bond in the alkyl hydroxamate radical is faster than 2 × 108 s-1. This reaction may be useful as a radical
trap.
During our studies on the mechanism of inactivation of
prolyl-4-hydroxylase by 5-oxaproline-containing peptides, we
identified 2 as the product of the enzyme catalyzed oxidation
of 1.1 This suggested that N-O bond cleavage from the
putative radical intermediate 3 had occurred (Scheme 1).
related to 3. In this communication, we describe a small
molecule model system 12 (Scheme 2) in which we have
been able to duplicate the N-O bond fragmentation and
estimate a lower limit for the rate of this reaction.
The synthesis of the model system 12 is outlined in
Scheme 2. Irradiation of 12 with a tungsten lamp in the
presence of tert-butyl thiol resulted in the formation of amide
19 which was isolated in 82% yield and the disulfide 15.
We suggest that these products were formed from radical
14. Decarboxylation of 14 would give radical 16, the
analogue of the putative enzyme-generated radical 3. Cleav-
age of the NO bond of 16 would give 17 which would then
abstract a hydrogen atom from tert-butyl thiol to give amide
19.
Scheme 1
To estimate the rate of the N-O bond cleavage, the
methyl group of 12 was replaced with a cyclopropyl group.
For this compound (20), we expected that ring opening of
(1) Wu, M.; Moon, H.-S.; Begley, T. P.; Myllyharju, J.; Kivirikko, K. I.
J. Am. Chem. Soc. 1999, 121, 587.
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1769. (e) Boivin, J.; Fouquet, E.; Zard, S. Z. Tetrahedron Lett. 1991, 32,
4299.
While N-O bond fragmentation reactions â to a radical
center have been previously described,2 we tested the
plausibility of our proposal on a structure more closely
10.1021/ol991392l CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/26/2000

Scheme 2
the cyclopropyl group of 21 would compete with the N-O
aldehyde 23 and amide 19. The product 25 resulting from
opening of the cyclopropyl ring of 21 was not detected
(detection limit < 10%). The product 22 resulting from direct
hydrogen atom abstraction by 21 was also not detected. The
ring-opening rate of the trimethylsiloxycyclopropylmethyl
radical is 2 × 107s-1.3 If we assume that this is a reasonable
approximate model for the rate of the cyclopropyl ring
opening in 21, we can estimate the rate of the N-O bond
cleavage in 21 to be greater than 2 × 108 s-1.
bond fragmentation and that the ratio of 19 to 25 would
allow us to estimate the N-O bond fragmentation rate
(Scheme 3).
The synthesis of 20 is outlined in Scheme 4. Compound
20 was photolyzed in CD2Cl2 under the same conditions used
for the photolysis of 12. Analysis of the photolysis mixture
by NMR and by GC/MS demonstrated the formation of
(3) (a) Suckling, C. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 537. (b)
Nonhebel, D. C.; Suckling, C. J.; Walton, J. C. Tetrahedron Lett. 1982, 23,
4477.
The methyl group of 12 has also been replaced with a
phenyl-substituted cyclopropyl group (35). Photolysis of this
compound resulted in a more complex reaction mixture than
(4) Newcomb, M.; Choi, S.-Y.; Toy, P. H. Can. J. Chem. 1999, 77, 1123.
(5) (a) Franot, C.; Mabic, S.; Castagnoli, N., Jr. Bioorg. Med. Chem.
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1346
Org. Lett., Vol. 2, No. 10, 2000

Scheme 3
was obtained for the photolysis of 20. NMR analysis of the
crude reaction mixture demonstrated that amide 19 and the
phenyl-substituted analogue of 23 were the major products
of the radical fragmentation reaction. However, several minor
components in the vinylic region that did not correspond with
the expected ring-opened compound were observed and
additional experiments will be needed before this system can
be used to calibrate the rate of the N-O bond fragmentation.
While not allowing for a precise determination, this experi-
ment demonstrates that the NO bond cleavage is competitive
with the ring opening of the (2-phenylcyclopropyl)methyl
radical (k ) 1.6 × 1011 s-1).4
The ring opening of the cyclopropyl carbinyl radical is
the most widely used radical trap in mechanistic enzymology3
and has been used for example to probe for radical
intermediates in the reactions catalyzed by monoamine
oxidase,5 isopenicillin N synthase,6 deacetoxyacetylcephalo-
sporin C synthase,7 methane monooxygenase,8 cytochrome
P450,9 and acyl CoA dehydrogenase.10 However, in many
cases the addition of a cyclopropyl group or a phenyl-
Scheme 4
Org. Lett., Vol. 2, No. 10, 2000
1347

substituted cyclopropyl group to the substrate for an enzy-
matic reaction is too large a perturbation on the structure
and such substrate analogues cannot bind at the active site
of the enzyme.11 In addition, the synthesis involved in the
introduction of the cyclopropyl group into the substrate can
be difficult. For such cases, the alkyl hydroxamate based
radical probe described here may be a useful alternative.
Acknowledgment. This research was supported by a
grant from the American Heart Association.
Supporting Information Available: The synthetic and
photolytic procedures for compounds 12 and 20. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL991392L
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Chem. Soc. 1992, 114, 3160.
(11) (a) Tandon, M.; Wu, M.; Begley, T. P.; Myllyharju, J.; Pirskanen,
A.; Kivirikko, K. I. Bioorg. Med. Chem. Lett. 1998, 8, 1139. (b) Iwata-
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Biochemstry 1994, 33, 10630. (C) Atkinson, J. K.; Ingold, K. U. Biochemstry
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