Scheme 1.
Design of Colorimetric Protecting Group for Hydroxyl-Group Protectiona
a Synthesis of the nitrophthalimidobutyric (NPB) acid can be used as protecting group to mask a carbohydrate hydroxyl group. The cleavage reaction
resulted in an orange-colored solution.
mass spectroscopy.8 However, these methods are difficult
to incorporate readily into automated processes. Commercial
peptide synthesizers often rely on colorimetric monitoring
of deprotection cycles using fluorenylmethoxycarbonyl (Fmoc)
that thereby allows continuous feedback during the synthe-
sis.9 The efficiency of a coupling cycle can be inferred from
the amount, monitored by UV-vis spectroscopy, of the
released masking group prior to the next coupling cycle.
The use of Fmoc in the solid-phase synthesis of carbo-
hydrates has been shown.10 However, the extreme base
sensitivity of Fmoc groups render their introduction into
building blocks challenging. In addition, unlike peptides and
nucleic acids, carbohydrates often include branched se-
quences. Therefore, a second less base-sensitive protecting
group that produces a color signature upon removal is
needed. Herein we report the design and synthesis of a new
hydroxyl protecting group for colorimetric monitoring of
reaction cycles and report its use in the solid-phase and
fluorous-phase synthesis of antigenic oligoglucosamines11
associated with infectious Staphylococcus aureus. The
alternative fluorous tag-assisted solution-phase strategy12 has
the advantage of requiring significantly fewer equivalents
of donor building blocks than the solid-phase approach.
The design of colored groups led us first to an investigation
of dye molecules. Unfortunately, many of these compounds
have inconvenient functional groups such as sulfates that
would likely complicate the glycosylation reactions. The
observation that nitrophthalhydrazide (1), a common inter-
mediate in the undergraduate laboratory synthesis of lumi-
nol,13 was a colored solid led us to think of strategies to
generate this compound in the process of deprotecting a
hydroxyl group. Retrosynthetic analysis pointed to opening
of a nitrophthalic anhydride or imide with hydrazine. This
reasoning led us to our final target molecule: nitrophthal-
imidobutyric (NPB) acid (2). The synthesis of NPB was
readily accomplished in 92% or greater yield by melting of
4-nitrophthalic anhydride (3) with inexpensive 3-aminobu-
tyric acid or, more easily, by microwave irradiation of the
neat compounds (Scheme 1). The group can be added to a
(7) (a) Kanemitsu, T.; Wong, C.-H.; Kanie, O. J. Am. Chem. Soc. 2002,
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Scheme 2
.
Synthesis of NPB-Protected Glucosamine Building
Block
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