Mitsunobu dehydration of N-Boc neomycin B

Article abstract of DOI:10.1039/b513113h

Recation of hexa-N-boc neomycin B with triphenylphosphine (TPP), p-nitrobenzoic acid, and diisopropyl azodicarboxylate (DIAD) in toluene, which results in the formation of an epoxide in ring IV, was analyzed. Epoxide formation was favored by the trans-dia

Full text of DOI:10.1039/b513113h

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Mitsunobu dehydration of N-Boc neomycin B†
Sabina Quader,a Sue E. Boyd,a Ian D. Jenkins*a,b and Todd A. Houston*a
Received 16th September 2005, Accepted 8th November 2005
First published as an Advance Article on the web 22nd November 2005
DOI: 10.1039/b513113h
Reaction of hexa-N-Boc neomycin B with TPP and DIAD in
toluene results in the formation of an epoxide in ring IV, not
an aziridine or azetidine as previously reported.
2 (29%), dehydrated products of this ester and of neomycin were
obtained as by-products. When this reaction was attempted with
lauric acid and double the amount of DIAD and TPP (4 equiv. ea.),
more substantial amounts of these by-products were observed. In
order to better identify these dehydration products, the Mitsunobu
reaction was repeated in the absence of any carboxylic acid. Reac-
tion of 1 with TPP (2 equiv.) and DIAD (2 equiv.) in toluene at RT
for 20 h afforded a 40% yield of a major monodehydration product
(M + Na+ = 1219.60032, HRMS), following purification by
column chromatography. Acetylation of this product produced a
pentaacetate (M + H+ = 1407.67615, HRMS) indicating that two
hydroxyl groups of the parent neomycin had reacted under the
Mitsunobu conditions, consistent with formation of an epoxide
(3).
Aminoglycosides were used as the first successful treatment of
the devastating effects of tuberculosis over 60 years ago. In
spite of drug resistance to these compounds, they are still an
important component of the chemotherapeutic arsenal against
this disease. Today, Mycobacterium tuberculosis continues to kill
more people than any other single microorganism due primarily
to its virulence in HIV-infected individuals.1 The waxy, mycolate-
laden exterior of the mycobacterium presents a formidable barrier
to most drug molecules. Accordingly, we sought to modify
aminoglycosides with fatty acids to improve their efficacy against
this microorganism. Based on previous success with mycolic acids
in the Mitsunobu esterification of the primary hydroxyl groups
of trehalose, for the synthesis of cord factor analogues,2 we set
out to apply this chemistry to hexa-N-Boc neomycin B. This
compound has a single primary hydroxyl group that should be
the most reactive site for this reaction, obviating the need for
selective protection. However, two recent reports have indicated
that it is preferable to protect all secondary hydroxyl groups as
acetates to maximise the yield of Mitsunobu displacement at the
5III-position with thioacetic acid (84%)3 and N3-benzoylthymine
(78%),4 respectively. Similar chemistry has been carried out on
tobramycin analogues by Hanessian, et al.5
Rigorous characterisation of these compounds by NMR ini-
tially proved quite difficult. The spectrum for the parent compound
as its peracetate (1-heptaacetate) in CDCl3 at room temperature
appears broadened by exchange processes, presumably due to re-
stricted rotation and steric congestion. Significantly more spectral
dispersion is observed in d5-pyridine solution, however there is
still significant broadening of many resonances in the spectrum
and it is not readily assigned. Both 1-heptaacate and the epoxide
1
3-pentaacetate, ho◦wever, gave well-resolved H-NMR signals in
1
d5-pyridine at 90 C (Fig. 2). All further H-NMR studies on
the neomycin derivatives were, therefore, undertaken using the
peracetylated compounds at elevated temperatures.
Analysis of the COSY, HMQC and HMBC and 1D NOE
spectra obtained for 1-heptaacetate at 363 K enabled the complete
assignment of the 1H-NMR signals of the four sugar units (I–IV).
The chair conformation of the ido ring (IV) is unchanged from
the unprotected parent species,6 as confirmed by the small values
3
observed for the JHH vicinal coupling constants of the ido ring
protons (3J2IV,3IV, 3J3IV,4IV, 3J4IV,5IV = 2–3 Hz). These small couplings
attest to the mutually equatorial disposition of H2IV, H3IV and
H4IV in contrast to the trans-diaxial disposition of the H2I, H3I,
H4I sets of the gluco ring (I) residue (3J2I,3I, J3I,4I, J4I,5I = 9.5–11
Hz). Comparison of the COSY spectra for 1-heptaacetate and 3-
pentaacetate confirms that the gluco ring, streptamine, and ribose
rings remain unchanged as a result of the reaction. The resonances
of the ido ring, however, are significantly different in the anhydro
3
3
Fig. 1 Neomycin B derivatives.
As a model reaction, we treated hexa-N-Boc neomycin B 1
(Fig. 1, 1 equiv.) with p-nitrobenzoic acid (2 equiv.), triphenylphos-
phine (TPP) (2 equiv.), and diisopropyl azodicarboxylate (DIAD)
(2 equiv.) in various anhydrous solvents. While no significant con-
version was observed in DMF or THF, reaction in toluene did pro-
duce the desired 5III-ester. In addition to the anticipated product
1
derivative 3 in comparison to 1 (Fig. 2). The shielding of H-
NMR signals at H3IV and H4IV in 3 is consistent with formation of
an epoxide at C3IV and C4IV (d H3IV = 3.48, d H4IV = 3.30, d C3IV
=
52.4, d C4IV = 52.7 ppm; cf. compound 1: d H3IV = 5.52, d H4IV
=
5.24, d C3IV = 70.0, d C4IV = 67.5 ppm). The loss of a distinctly
separated AX spin system observed for the H6IV protons and the
inversion of resonances for the H2IV and H5IV protons is indicative
of significant conformational change in the ido ring.
aEskitis Institute, Griffith University, Nathan, Brisbane, QLD, Australia
4111. E-mail: T.Houston@griffith.edu.au; Tel: +617 3735 4115
bNatural Product Discovery, Griffith University, Nathan, Brisbane, QLD,
Australia 4111, I.Jenkins@griffith.edu.au; Tel: +617 3735 6025
Further evidence in support of the epoxide structure was
obtained by reaction with sodium azide. A monoazide product
† Electronic supplementary information (ESI) available: 1D and 2D NMR
data for the peracetates of 1, 3, 4 and 5. See DOI: 10.1039/b513113h
3 6 | Org. Biomol. Chem., 2006, 4, 36–37
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Fig. 2 Expansions of the 1H NMR spectra (400 MHz, 363 K, C5D5N) obtained for 1-heptaacetate (bottom) and 3-pentaacetate (top), with suppression
of residual water present in the solvent. The changes observed in the resonances assigned to ring IV are highlighted.
1
4 was obtained the H-NMR data of which are consistent with
the stereochemistry shown. Such an arrangement would arise
from 3 through reaction at C3IV with inversion of configuration
(overall retention of neomycin B configuration) by the azide anion.
Formation of the (talo) epoxide 3 under Mitsunobu conditions is
consistent with generation of the (triphenylphosphonium) leaving
group on the less hindered O-3 position of the ido ring. Formation
of the alternative (altro) epoxide would require generation of
the (triphenylphosphonium) leaving group on the more hindered
O-4 position which would be subject to a severe 1,3-diaxial
interaction with the 2-N-Boc group. In addition, the transition
state for (altro) epoxide formation would be subject to a dipolar
repulsion7 (field effect) between the leaving group and the 2-N-Boc
group.
In conclusion, we have presented evidence for the formation
of a new epoxide derivative of neomycin B. Epoxide formation is
favoured by the trans-diaxial arrangement of the vicinal diol in the
ido ring6 and this competes effectively with reaction at the primary
hydroxyl group at C5III.8 Although aziridines can be formed by
Mitsunobu reaction of N-Boc–amino alcohols,9 we have seen no
evidence for aziridine or azetidine ring formation within ring IV of
neomycin10 under these reaction conditions, or those of Fourrey
et al.11 This epoxide may have interesting biological properties in its
own right and serve as a versatile intermediate for the preparation
of modified neomycins or neomycin conjugates.
References
1 J. D. McKinney and J. E. Gomez, Nat. Med., 2003, 9, 1356.
2 S. Bottle and I. D. Jenkins, Chem. Commun., 1984, 385; I. D. Jenkins
and M. B. Goren, Chem. Phys. Lipids, 1986, 41, 225.
3 M. Fridman, V. Belakhov, L. V. Lee, F.-S. Liang, C. H. Wong and T.
Baasov, Angew. Chem., Int. Ed., 2005, 44, 447.
4 A. Hernandez Linares, D. Fourmy, J.-L. Fourrey and A. Loukaci, Org.
Biomol. Chem., 2005, 3, 2064.
5 S. Hanessian, M. Tremblay and E. E. Swayze, Tetrahedron, 2003, 59,
983.
6 J. L. Asensio, A. Hidalgo, I. Cuesta, C. Gonzalez, J. Can˜ada, C. Vicent,
J. L. Chiara, G. Cuevas and J. Jimenez-Barbero, Chem. Commun., 2002,
2232.
Fourrey, et al. have recently reported that in the Mitsunobu
reaction of hexa-N-Boc neomycin B with N3-benzoylthymine
(a reaction analogous to the reaction reported here with p-
nitrobenzoic acid) a highly strained tricyclic derivative of ring IV of
hexa-N-BOC neomycin B is formed, in addition to the expected
displacement at the 5III-position by N3-benzoylthymine.4 Their
proposed structure contains both aziridine and azetidine ring
systems fused to ring IV. We believe that epoxide formation is
much more likely than aziridine or azetidine formation based on
literature precedent and the additional evidence reported here. It is
noteworthy that the mass spectral data reported by Fourrey, et al.
is actually consistent with an epoxide, not the aziridine–azetidine
structureproposed, andthere is aconsiderabledegree of agreement
7 A. C. Richardson, Carbohydr. Res., 1969, 10, 395.
8 R. D. Guthrie and I. D. Jenkins, Aust. J. Chem., 1981, 34, 1997.
9 A. M. Kawamoto and M. Wills, J. Chem. Soc., Perkin Trans. 1, 2001,
1916.
10 For aziridine formation involving a streptamine system, see: D. Ikeda,
Y. Horiuchi, S. Kondo and H. Umezawa, J. Antibiot., 1980, 33, 1281.
11 At the suggestion of both a referee and Professor Fourrey, we
have repeated their experiment and isolated a compound identical
by 13C-NMR to their compound 8. However, the 1H-NMR in d5-
pyridine at 90 ◦C shows 6 NH peaks (compound 5, refer to the
electronic supplementary information), consistent with an epoxide, but
not the aziridine–azetidine structure assigned. Note added in Proof.
In a personal communication, Professor Fourrey has accepted our
interpretation and agreed that the structure assigned to 8 in their paper
should be revised.
in the H- and 13C-NMR data reported for their compound and
1
epoxide 3. It is clear that their reported structure does not fit
our data, for several reasons, but particularly because we observe
carbamate proton (NH) resonances for both of the nitrogens at the
C2IV and C6IV positions (Fig. 2, see also ref. 11). Furthermore,
aziridine opening by azide in their putative compound would not
generate a product in the parent conformation of the ido ring as
was observed here.
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