Synthesis and comparative antibacterial activity of verdamicin C2 and C2a. A new oxidation of primary allylic azides in dihydro[2H]pyrans

Article abstract of DOI:10.1021/ol802421d

(Chemical Equation Presented) A synthesis of verdamicin C2 and its congener C2a has been accomplished from sisomicin relying on a novel oxidative transformation of an allylic azide to the corresponding α,β- unsaturated aldehyde, and its stereocontrolled elaboration into the intended 5′ side chain of verdamicin C2 and C2a. In vitro antibacterial testing shows that both C6′ epimers in verdamicin C2 and C2a are equally active against a variety of bacterial strains. Oxidation of allylic primary azides, ethers, and esters of 2-substituted dihydro[2H]pyrans with SeO₂ leads directly to the corresponding aldehydes.

Full text of DOI:10.1021/ol802421d

ORGANIC
LETTERS
2009
Vol. 11, No. 2
429-432
Synthesis and Comparative Antibacterial
Activity of Verdamicin C2 and C2a. A
New Oxidation of Primary Allylic Azides
in Dihydro[2H]pyrans
Stephen Hanessian,* Janek Szychowski, and J. Pablo Maianti
Department of Chemistry, UniVersite´ de Montre´al, C. P. 6128, Succ. Centre-Ville,
Montre´al, P.Q., Canada, H3C 3J7
stephen.hanessian@umontreal.ca
Received October 21, 2008
ABSTRACT
A synthesis of verdamicin C2 and its congener C2a has been accomplished from sisomicin relying on a novel oxidative transformation of an
allylic azide to the corresponding r,ꢀ-unsaturated aldehyde, and its stereocontrolled elaboration into the intended 5′ side chain of verdamicin
C2 and C2a. In vitro antibacterial testing shows that both C6′ epimers in verdamicin C2 and C2a are equally active against a variety of
bacterial strains. Oxidation of allylic primary azides, ethers, and esters of 2-substituted dihydro[2H]pyrans with SeO2 leads directly to the
corresponding aldehydes.
The Gentamicin complex of aminoglycoside antibiotics has
been in clinical use for the treatment of acute Gram-negative
infections since 1963.1 The complex consists of a mixture
of four major congeners (gentamicins C1, C1a, C2, and C2a)
which are all equipotent as antibacterials (Figure 1).2 In
addition to the gentamicins, the genus Micromonospora3can
also produce good quantities of sisomicin,4 verdamicin C2a,5
and lesser amounts of othercomponents like antibiotic G-526
(Figure 1).
These differ from gentamicin C1a, C2a, or C2b (also called
sagamicin) respectively by the presence of a 4′,5′-double
bond in ring I.
Although degradative work has been carried out on
verdamicin, definitive stereochemical assignment of the 6′-
(1) (a) Jao, R. L.; Jackson, G. G. Antimicrob. Agents Chemother. 1963,
161, 148. (b) Jao, R. L.; Jackson, G. G. J. Am. Med. Assoc. 1964, 189,
817.
(2) Sandoval, R. M.; Reilly, J. P.; Running, W.; Campos, S. B.; Santos,
J. R.; Phillips, C. L.; Molitoris, B. A. J. Am. Soc. Nephrol. 2006, 17, 2697.
(3) Weinstein, M. J.; Luedemann, G. M.; Oden, E. M.; Wagman, G. H.;
Rosselet, J. P.; Marquez, J. A.; Coniglio, C. T.; Charney, W.; Herzog, H. L.;
Black, J. J. Med. Chem. 1963, 6, 463.
(4) Weinstein, M. J.; Marquez, J. A.; Testa, R. T.; Wagman, G. H.;
Oden, E. M.; Waitz, J. A. J. Antibiot. 1970, 23, 551.
(5) Weinstein, M. J.; Wagman, G. H.; Testa, R.; Marquez, J. A. German
patent DE 2239964, 1973, CAN 79:51820.
(6) Weinstein, M. J.; Wagman, G. H.; Marquez, J. A. German patent
DE 2334923, 1974, CAN 80:131672.
Figure 1. Structure of related aminoglycosides.
10.1021/ol802421d CCC: $40.75
Published on Web 12/10/2008
2009 American Chemical Society

Scheme 1. Preparation of Verdamicin C2 and C2a as a 1:1 Mixture
aminoethyl side chain has not been reported. In fact, both
the 6′-(R)-aminoethyl and 6′-(S)-aminoethyl side chains have
been cited in the literature.7,8
C2 and C2a congeners (7 and 8), which was found to be
equally active as sisomicin against E. coli and S. aureus.
Likewise, addition of EtMgBr and iPrMgBr, followed by
mesylation and azide displacement afforded the ethyl and
isopropyl analogues (S31-32 and S37-38) which were
found to be less active (see the Supporting Information).12
Since the C6′ alcohols 12 and 13 were inseparable, we
deemed it necessary to first confirm our configurational
assignments which were based on a Mosher ester analysis.13
Thus, the mixture containing a 1:1 ratio of (R)- and (S)-
isomers 12 and 13 was deacetylated and O-benzylated to 16
and 17, then this mixture was subjected to ozonolysis to give
the dicarbonyl products 18 and 19 (Scheme 2). Cleavage
The biotransformation of the naturally occurring verdami-
cin to gentamicin C2a by the mutant strain KY11525 from
M. sagamiensis suggested that verdamicin may exist as the
C2a epimer.9 However, the stereochemical identity of
verdamicin has not been definitively established. Further-
more, no study has compared the in vitro antimicrobial
activities of verdamicin C2a and its C6′ epimeric C2
analogue.2
In this letter, we describe a synthesis of verdamicin C2
and its congener C2a from sisomicin relying on a novel
oxidative transformation of an allylic azide to the corre-
sponding R,ꢀ-unsaturated aldehyde, and its stereocontrolled
elaboration into the intended 5′ side chains of verdamicin
C2 and C2a.
Scheme 2. Chemical Degradation for Diastereoselectivity
Determination of the Grignard Addition Products
Sisomicin was converted to the tetraazido analogue 10 by
a known method,10 and the remaining secondary amino group
was acetylated (Scheme 1). Treatment of 10 with stoichio-
metric SeO2 in CH2Cl2 containing 3 equiv of dihydropyran
led to the corresponding aldehyde 11 in quantitative
yield.11,12
Addition of MeMgBr to 11 afforded a 1:1 mixture of
epimers 12 and 13. Initially, the 1:1 mixture was converted
to the 6′-mesylates, then transformed to the corresponding
C6′-azides 14 and 15. This mixture was deacetylated, and
reduction with PMe3 led to verdamicin as a 1:1 mixture of
(7) Weinstein, M. J.; Wagman, G. H.; Marquez, J. A.; Testa, R. T.;
Waitz, J. A. Antimicrob. Agents Chemother. 1975, 7, 246
.
(8) Davies, D. H.; Mallams, A. K. J. Med. Chem. 1978, 21, 189
.
(9) Kase, H.; Shimura, G.; Iida, T.; Nakayama, K. Agric. Biol. Chem.
1982, 46, 515.
(10) (a) Nyffeler, P. T.; Liang, C.-H.; Koeller, K. M.; Wong, C.-H. J. Am.
Chem. Soc. 2002, 124, 10773. (b) Alper, P. B.; Hung, S.-C.; Wong, C.-H.
Tetrahedron Lett. 1996, 37, 6029. (c) Vasella, A.; Witzig, C.; Chiara, J.-
L.; Martin-Lomas, M. HelV. Chim. Acta 1991, 74, 2073. (d) Zaloom, J.;
Roberts, D. C. J. Org. Chem. 1981, 46, 5173. (e) Cavender, C. J.; Shiner,
V. J. J. Org. Chem. 1972, 37, 3567. (f) Fischer, W.; Anselme, J.-P. J. Am.
Chem. Soc. 1967, 89, 5284.
(11) For an example of SeO2 oxidation of 6′-N-(2,4-dinitrophenyl)si-
somicin derivatives into the corresponding aldehydes, see: Nagabhushan,
T. L. U.S. patent 3997524, 1976, CAN 88:51133.
with NaOMe in MeOH afforded the pseudodisaccharide 20
and a mixture of (R)- and (S)-O-benzyl methyl lactates in a
(12) See the Supporting Information for details.
430
Org. Lett., Vol. 11, No. 2, 2009

Scheme 3. Diastereoselective Reduction of Ketone 23 and Preparation of the Individual Verdamicins C2 or C2a
1:1 ratio as determined by gas chromatography by using
enantiopure reference compounds 21 and 22 which were
independently prepared.14 Enhancement of individual peak
areas by doping with the (R)- and (S)-lactates respectively
confirmed the configurational identities of the 6′-(R)- and
6′-(S)-alcohols in the mixture.
24, which was then deacetylated and reduced to afford 9
(Scheme 4). Treatment of the aldehyde 11 with dimethy-
lamine followed by deacetylation and reduction led, likewise,
to a new congener (26, 6′-N,N-dimethyl sisomicin). Anti-
bacterial activities against a selected panel of strains were
maintained in 9, but significantly reduced in 26.12
Our next goal was to prepare the individual verdamicin
C2 and C2a congeners. Oxidation of the mixture containing
12 and 13 using Dess-Martin periodinane reagent15 gave
the ketone 23 (Scheme 3). Reduction with borane under the
Corey-Bakshi-Shibata conditions16 gave a >15:1 ratio of
the 6′-(R)-alcohol 12 over the epimeric 13 in the presence
of a stoichiometric amount of (S)-CBS. The 6′-(S)-alcohol
13 was obtained in a >15:1 ratio by the use of the (R)-CBS
reagent. These ratios were determined by GC analysis of
the corresponding alcohols 21 and 22 obtained by the above-
described chemical degradation of the CBS reduction prod-
ucts.
Scheme 4. Preparation of Antibiotic G-52 (9) and Compound 26
Transformation of 12 to the mesylate and displacement
with sodium azide gave the (S)-azide 15 (Scheme 3).
Deacetylation and reduction of the azide under Staudinger
conditions led to verdamicin C2a (8) having a 6′-(S)-
aminoethyl configuration. Verdamicin C2 (7), having the 6′-
(R)-aminoethyl configuration, was obtained by using the
same approach starting with the epimeric 6′-(S)-alcohol 13
via the azide 14.
We also prepared the so-called antibiotic G-526 (9, 6′-N-
methyl sisomicin) which is the sisomicin congener related
to gentamicin C2b (5). Treatment of the aldehyde 11 with
methylamine under conditions of reductive amination gave
Verdamicin C2 (7) and C2a (8) exhibited essentially
similar in vitro inhibitory activity compared to gentamicin
complex.12
The venerable SeO2 oxidation of allylic methyl or meth-
ylene groups to the corresponding alcohol or carbonyl
derivative is a time honored and tested reaction.17 The ready
availability of the 6′-azido derivative 10 of sisomicin as
described above gave us the incentive to test its direct
oxidation with SeO2. The successful conversion to aldehyde
(13) Dale, J. A.; Mosher, H. S. J. Am. Chem. Soc. 1973, 95, 512.
(14) Dubost, C.; Leroy, B.; Marko, I. E.; Tinant, B.; Declercq, J.; Bryans,
J. Tetrahedron 2004, 60, 7693.
(15) (a) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155. (b)
Meyer, S. D.; Schreiber, S. L. J. Org. Chem. 1994, 59, 7549.
(16) (a) Corey, E. J.; Helal, C. J. Angew. Chem., Int. Ed. 1998, 37, 1986.
(b) Corey, E. J.; Bakshi, R. K.; Shibata, S.; Chen, C. P.; Singh, V. K. J. Am.
Chem. Soc. 1987, 109, 7925.
(17) Bulman, P. C.; McCarthy, T. J. Comp. Org. Synth. 1991, 7, 83.
Org. Lett., Vol. 11, No. 2, 2009
431

11 led us to study the analogous reaction in a series of simple
dihydropyrans. Surprisingly, no examples of related SeO2
oxidations of allylic azides have been recorded in the
literature. In fact, the only example of an allylic oxidation
of a carbon-nitrogen bond to the carbonyl derivative appears
to be the above-mentioned example by the Schering group.11
Therefore, we undertook a preliminary comparative study
of the oxidation of a series of 6-methyl-substituted
dihydro[2H]pyrans and 1-methyl-substituted cyclohexenes.
As representative model substrates we chose 6-methyl-
dihydro[2H]pyrans containing N3, OMe, and OAc functional
groups on the primary carbon. On the basis of preliminary
observations, we ran the oxidations with and without
additives in the presence of 5 equiv of SeO2.12
A study of the nature of the base on the time and efficiency
of oxidation to the aldehyde showed DMAP to be the most
effective (Table 1, entries 10-12).12 However, addition of
3 equiv of dihydropyran as a scavenger proved to be most
beneficial with regard to yield and reaction time (Table 1,
entries 13-15).12 Preliminary experiments with 1-azidom-
ethyl, 1-methoxymethyl, and 1-acetoxymethyl 1-cyclo-
hexenes under the same oxidation conditions, with or without
additives, produced mixtures of the expected aldehydes and
respective allylic endocyclic alcohols.12
Clearly, the oxidation of 6-methyl-substituted dihydro-
[2H]pyrans is highly regioselective and influenced by the
nature of the substituent on the primary carbon atom with
respect to reaction rate. We defer from speculative mecha-
nistic discussions favoring a concerted or stepwise pro-
cess,19,20 although the involvement of oxocarbenium ion
intermediates may be highly favored.
The methyl ether was oxidized much faster to the
aldehyde, although the yield was modest compared to the
azide (Table 1, entries 1-3). Addition of Bu4NN3 (TBAA)
In conclusion, we have devised a practical synthesis of
verdamicin as a mixture of 5′-aminoethyl substituents and
as the individual 6′-(R)- and 6′-(S)-isomers which correspond
to the analogous components in the gentamicin complex.
Although there is biogenetic evidence that verdamicin is
related to gentamicin C2a,2 the results of the present work
demonstrate that the stereochemistry at C6′ is not critical
for antibacterial activity in this series.
Table 1. Allylic Oxidation of 6-Methyl-Substituted
Dihydro[2H]pyrans
With the exception of gentamicin/tobramycin resistant P.
aeruginosa, and the gentamicin acetylating strains, verdami-
cin has been shown to possess potent in Vitro and in ViVo
(mice) activity.7 Comparative acute toxicity administered via
parenteral (intravenous or subcutaneous) routes showed an
improved performance for verdamicin compared to gentami-
cin or tobramycin.7 With the facile access to the natural
verdamicin C2 and its congener C2a by semisynthesis from
the readily available sisomicin, more detailed studies can be
carried out for broader antibacterial testing, and to evaluate
levels of toxicity of individual epimers.
entry
R
additive (equiv)
TBAA (1)
time
3 h
45 min
6 h
isolated yield (%)
1
2
3
N3
OMe
OAc
70
47
41
4
5
6
N3
1 h
<30 min
4 h
70
65
60
OMe TBAA (1)
OAc
N3
TBAA (1)
7
8
9
TBACN (1)
1.5 h
45 min
5 h
71
65
69
OMe TBACN (1)
OAc
N3
TBACN (1)
DMAP (1)
Acknowledgment. We thank NSERC, FQRNT, FRSQ
fellowships, and Achaogen Inc. for financial assistance and
antibacterial testing.
10
11
12
4 h
1.5 h
16 h
81
80
77
OMe DMAP (1)
OAc
DMAP (1)
Supporting Information Available: Experimental pro-
cedures, full spectroscopic data for all new compounds, and
antimicrobial activities. This material is available free of
charge via the Internet at http://pubs.acs.org.
13
14
15
N3
DHP (3)
2.5 h
30 min
17 h
98
82
86
OMe DHP (3)
OAc DHP (3)
OL802421D
rendered the reaction mixture homogeneous with improved
yields (Table 1, entries 4-9). On a preparative scale, addition
of 1 equiv of DMAP was beneficial, although reaction times
were somewhat longer.12,18
(18) Internal salts between SeO2 and amines have been reported: Touzin,
J.; Neplechova, K.; Zak, Z.; Cernik, M. Collect. Czech. Chem. Commun.
2002, 67, 577.
(19) Stephenson, L. M.; Speth, D. R. J. Org. Chem. 1979, 44, 4683
.
(20) Singleton, D. A.; Hang, C. J. Org. Chem. 2000, 65, 7554
.
432
Org. Lett., Vol. 11, No. 2, 2009