2
B. Bodnár et al. / Bioorganic & Medicinal Chemistry Letters xxx (2017) xxx–xxx
Some of the most abundant monosaccharides in the nature,
D-
glucose, -mannose, -galactose and -ribose were chosen to pre-
D
D
D
pare these building blocks. We aimed at synthesizing their azide
derivatives in which the azide group is built into their glycosidic
position or in place of their primary hydroxy groups (position 6
in hexopyranoses and position 5 in ribofuranose). The most com-
mon ways for the preparation of glycosyl azides are either an SN2
substitution of a protected glycosyl halide by sodium azide at high
temperature in DMF18 or an SN1 substitution of a peracetylated
carbohydrate under mild conditions using a Lewis acid catalyst
and trimethylsilyl azide.19–21 We have chosen the latter method
as all of our sugars contained a neighbouring participation group
at position 2 (O-acetyl or O-benzoyl) that can ensure the desired
stereoselectivity. The primary hydroxy groups of the monosaccha-
rides can be replaced to azides in a two-step procedure22 involving
the introduction of a good leaving group (e.g. a tosyl) to the pri-
mary hydroxy and a subsequent azide substitution of the tosylate
by sodium azide in DMF.
First, the hexoses studied were peracetylated according to a lit-
erature method23 to yield compounds 6–8 (Scheme 2). Next, the
glycosidic O-acetyl groups were replaced with the azide group
using tin tetrachloride as a Lewis acid catalyst and trimethylsilyl
azide as the source of the nucleophilic azide ion to afford com-
pounds 9–11.
The SN1 type substitution resulted in only 1,2-trans products
due to the neighbouring group participation of the O-acyl group
at position 2. For
D-glucose and D-galactose the b-anomer, for D-
mannose the -anomer formed in this way in 72–79% yield. The
a
purity of the azide products and their quantities were sufficient
for the subsequent conjugation reactions.
In order to introduce the azide group to positions 6 (hexopyra-
noses) and 5 (pentofuranose), first the methyl glycosides of
D-glu-
cose, -mannose and -ribose (12–14, Scheme 3) were selectively
D
D
tosylated in pyridine on their primary hydroxy groups without
the protection of the secondary hydroxy groups. This distinction
was allowed by the higher reactivity of the primary hydroxy
groups over the secondary ones.
Unfortunately, the direct replacement of the tosyl group with
azide in compounds 15–17 was not successful probably due to sol-
ubility reasons, therefore the secondary hydroxy groups were ben-
zoylated first, then the tosyl–azide exchange has successfully
occurred in all the fully protected monosaccharides 18–20 and
resulted in the fully protected 6-azido-6-deoxy- and 5-azido-5-
deoxymonosaccharides (21–23).
The azide group-containing monosaccharides (9–11 and 21–23)
were coupled to the alkyne-containing D-secoalcohol (4a) and D-
secooxime (4b) using similar CuAAC conditions that we used pre-
viously (Scheme 4) applying copper(I) iodide, triphenylphosphine
and DIPEA as a base with a slight access of propargyl-D-secosteroid
in toluene at boiling temperature until TLC showed quantitative
conversions.24
Scheme 1. The synthesis of estrone derivatives 2–5 obtained earlier.
In case of two glucose-containing bioconjugates (24a and 24b),
which showed the best biological activities, the acetyl protecting
groups were removed by the Zemplén’s method25 using sodium
methylate in methanol to obtain their unprotected derivatives
30a and 30b.
modulate both the cytotoxic properties and the Na+/K+-ATPase
inhibitory properties of cardiac glycosides.17 In this vein, we
planned to perform CuAAC reactions of steroidal alkynes (4a and
4b) with protected monosaccharide azides and to investigate the
in vitro antiproliferative activities of these bioconjugates by means
of MTT assays against a panel of human adherent cancer cell lines
(HeLa, MCF-7 and A2780).
The antiproliferative properties of the D-secoestrone–carbohy-
drate conjugates (24–30) were characterized in vitro on a panel
of human adherent cancer cell lines (HeLa, A2780 and MCF-7) by
means of MTT assays (Table 1).
The antiproliferative properties of some of the presented com-
pounds proved to comparable to that of reference agent cisplatin
that is utilized clinically in the treatment of certain gynaecological
malignancies.26,27 The most potent compounds (24b, 25b and 26b)
exhibited remarkable activities with IC50 values in the range 5.3–
As our aim was to synthesize carbohydrate–
conjugates from our previously reported 3-O-propargyl
D
-secoestrone bio-
-secoe-
D
strones using CuAAC, this conjugation reaction required the
synthesis of azide-containing carbohydrate building blocks and
their CuAAC reaction with propargylated D-secoestrones (4a and
4b, Scheme 1).
20.5 lM, exerting their best effects against A2780 cells. Among