in each C2 isomer (e.g., 2, δ 1.66 m, 2H) appeared as a
complex second-order multiplet owing to the fact that the
H4 protons were chemical-shift equivalent but magnetically
inequivalent. Conversely, the 4-CH2 protons in the meso
isomer 3 are both chemical shift inequivalent and magneti-
cally inequivalent and appear as diastereotopic protons
exhibiting a first-order ABX2 pattern (δ 1.79 dt 1H, J )
14.4, 8.4 Hz; δ 1.84, dt, J ) 14.4, 4.7 Hz, 1H). Analogous
patterns were observed for C2-symmetrical 7 and meso-6.
Interestingly, the 1H NMR signal of the corresponding C12
methylene group in 1 also exhibited a complex second-order
pattern (400, 500, and 600 MHz) similar to those of 2 and
7, but dissimilar to the 4-CH2 signals of 3 and 6, suggesting
that the spin systems in 1, 2, and 7 reflected local C2 or
pseudo-C2 symmetry, largely dictated by an anti relationship
of the C11 and C13 OH groups.
A highly suggests that the C14 shares the same configuration
as L-serine.3 Zwittermicin A is synthesized by a hybrid
polyketide synthase-nonribosomal peptide synthase (PKS-
NRPS) that comprises nine open reading frames including a
loading domain for the starter unit that is homologous with
serine adenylation domains found in gene clusters for
biosynthesis of iturin A and mycosubtilin. Since the proposed
gene sequence for production of 1 shows C13-C15 origi-
nating from L-serine and epimerase domains are absent, it
is highly likely that C14 is L and the absolute stereochemistry
for C8-C14 in 1 is as depicted.
The absolute configuration at the remaining C4 stereo-
center in 1 was determined as 4S by Marfey’s analysis.5 Acid
hydrolysis of authentic 1 (6 N HCl, 24 h, 110 °C) and
derivatization of the products with 2,4-dinitrophenyl-5-fluoro-
L-alaninamide (Marfey’s reagent) under standard conditions,
followed by analysis (C18 HPLC-MS) gave one peak that
matched the peak (coinjection, MS spectrum) obtained by
similar treatment of commercially available (-)-(S)-N3-
ureido-2,3-diaminopropionic acid (S-albizziin).
An unequivocal assignment of relative configuration for
the diaminotetraol segement in 1 was made by pairwise
comparisons of the differences in the 13C chemical shifts (∆δ)
for C10-C15 of 1 and model compounds (Figure 1).11 There
are only six diastereomers of the symmetrically substituted
diaminoheptanetetraol models but eight diastereomers of the
C10-C15 segment in 1. To complete the comparison, the
13C δ assignments of C1 isomers 4 and 5 were reversed to
give the remaining two isomerssvirtual compounds “4b”
and “5b”. The C2V symmetric 2 is the only model compound
with a close match to 1 for every carbon (Figure 1) except
C9, which is the point of difference between 1 and the models
and expected to show an “outlying” ∆δ in every case.
In conclusion, we have assigned the configuration of 1 as
(4S,8S,9R,10R,11R,13R,14S). using an integrated approach
based on synthesis and pairwise comparisons with model
compounds, Marfey’s analysis, and published data. This sets
the stage for completion of 1 by chain extension of a suitably
protected derivative of 2 and attachment of the N3-ureido-
2,3-diaminopropionamide side chain, which is the subject
of current research in our laboratories.
Acknowledgment. We thank Mark Zabriskie (Oregon
State University) for helpful discussions, and Entotech, Inc.
(Davis, CA) for a sample of authentic zwittermicin A. X-ray
analysis was carried out by A. Rheingold (UCSD). HRMS
measurements were carried out by R. New (UC Riverside
MS Facility), Y. Su (UC San Diego MS Facility), and the
Scripps Center for Mass Spectrometry (La Jolla, CA). The
UC Davis 400 MHz NMR and LCMS were purchased with
funds provided by instrument grants NSF CHE-9808183 and
RR14701-01, respectively. This work was supported by a
grant from the NIH (to T.F.M., RO1 AI39987) and a
fellowship from the Ecotoxicology Lead Campus Program
(to E.W.R., UC Davis).
Importantly, the other C2V isomer 7 had the largest
mismatch, which secures confidence for assignment of
erythro relationships in each of the C10,11 and C13,14 diads.
Elimination of the mismatched meso isomers 3 and 6, as
suggested by 1H NMR and stereotopicity analysis of the 12-
CH2 signal (above), is now corroborated by 13C NMR.
Therefore, compounds 1 and 2 share the same relative
configuration at the stereogenic centers corresponding to C10,
C11, C13, and C14 of 1. The data in Figure 1, in conjunction
with the relative configurations at C8-C10,1 now allow us
to extend the assignment of relative configuration of 1 to
C8-C15.
Although no direct evidence for the absolute configuration
of C8-C15 is yet available, analysis of the published
sequence of the gene cluster for biosynthesis of zwittermicin
Supporting Information Available: Experimental pro-
1
cedures, X-ray data for 22, and selected H and 13C NMR.
This material is available free of charge via the Internet at
(11) 13C NMR measurements of the hydrochloride salts of 1 and models
2-7 were carried out under essentially identical conditions (D2O, 25 °C)
with internal CH3CN as reference.
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Org. Lett., Vol. 9, No. 3, 2007