Stereoselective synthesis of D-galactal-derived N-ethoxycarbonyl aziridine, as a new, improved synthetic protocol to glycal-derived N-activated vinyl aziridines

Article abstract of DOI:10.1016/j.tet.2016.12.047

A new protocol for the synthesis of D-galactal-derived N-ethoxycarbonyl vinyl aziridine 1β-CO₂Et, starting from tri-O-acetyl-D-glucal, is described. The new protocol constitutes a simple and fast access, with a satisfactory overall yield (5 steps, 36%), to a D-galactal-derived vinyl aziridine with a clear improvement compared with the previously described procedure leading to the structurally related N-nosyl aziridine 1β-Ns which, starting from the same precursor, proceeded through 13 steps with a low, decidedly unsatisfactory, overall yield (3%).

Full text of DOI:10.1016/j.tet.2016.12.047

Accepted Manuscript
Stereoselective synthesis of D-galactal-derived N-ethoxycarbonyl aziridine, as a new,
improved synthetic protocol to glycal-derived N-activated vinyl aziridines
Salvatore Princiotto, Valeria Di Bussolo, Vittorio Bordoni, Lucilla Favero, Stefano
Crotti, Gloria Uccello Barretta, Federica Balzano, Paolo Crotti
PII:
S0040-4020(16)31334-5
10.1016/j.tet.2016.12.047
TET 28339
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 24 August 2016
Revised Date: 10 December 2016
Accepted Date: 19 December 2016
Please cite this article as: Princiotto S, Di Bussolo V, Bordoni V, Favero L, Crotti S, Barretta GU,
Balzano F, Crotti P, Stereoselective synthesis of D-galactal-derived N-ethoxycarbonyl aziridine, as a
new, improved synthetic protocol to glycal-derived N-activated vinyl aziridines, Tetrahedron (2017), doi:
10.1016/j.tet.2016.12.047.
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ACCEPTED MANUSCRIPT
Stereoselective synthesis of D-galactal-derived N-ethoxycarbonyl aziridine, as
a new, improved synthetic protocol to glycal-derived N-activated vinyl aziridines
a,
b,
a
a
Salvatore Princiotto, * Valeria Di Bussolo, *Vittorio Bordoni, Lucilla Favero,
a
b
b
a,
Stefano Crotti, Gloria Uccello Barretta, Federica Balzano, Paolo Crotti *
Dipartimento di Farmacia, Università di Pisa, 56126 Pisa, Italy
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, 56124 Pisa, Italy
a
b
O
O
O
TBDPSO
MsO
TBDPSO
EtO
AcO
AcO
5 steps
6% yield
t-BuOK
3
N
NHCO Et
C
2
2
OAc

ACCEPTED MANUSCRIPT
Stereoselective synthesis of
D-galactal-derived N-ethoxycarbonyl aziridine,
as a new, improved synthetic protocol to glycal-derived N-activated vinyl aziridines
a,
,
b,
a
a
Salvatore Princiotto * , Valeria Di Bussolo *, Vittorio Bordoni , Lucilla Favero ,
a,
b
b
a,
Stefano Crotti
, Gloria Uccello Barretta , Federica Balzano , Paolo Crotti *
a
Dipartimento di Farmacia, Università di Pisa, Via Bonanno 33, 56126 Pisa, Italy
b
Dipartimento di Chimica e Chimica Industriale, Università di Pisa,
Via G.Moruzzi 3, 56124 Pisa, Italy
_
__________________________________________________________________________________
Abstract: A new protocol for the synthesis of -galactal-derived N-ethoxycarbonyl vinyl aziridine 1β-
D
CO Et, starting from tri-O-acetyl- -glucal, is described. The new protocol constitutes a simple and fast
2
D
access, with a satisfactory overall yield (5 steps, 36%), to a -galactal-derived vinyl aziridine with a
D
clear improvement compared with the previously described procedure leading to the structurally related
N-nosyl aziridine 1β-Ns which, starting from the same precursor, proceeded through 13 steps with a
low, decidedly unsatisfactory, overall yield (3%).
Keywords: Vinyl aziridines, Glycals, Glycosylation, 4-N-(protected)-O-glycosides, Stereoselectivity
_
__________________________________________________________________________________
1
. Introduction
Aminosugars, consisting in carbohydrates bearing a free or N-protected amino functionality in
1
different ring positions of a glycoside scaffold, are structures widely present in nature, either alone or
2
also as constituents of crucial glycoproteins, glycosaminoglycans and GPI anchors. In particular, 4-
aminosugars are found as glycoconjugates, in naturally occurring antibiotics, and their presence has
3
been closely related to the activity of this type of antibiotics. Moreover, following the
glycodiversification approach, the synthesis of naturally occurring antibiotic analogs with the original
4
sugars replaced by synthetic ones, currently represents an interesting challenge.
_
*
________________
Corresponding authors. Tel.: +393472586140.
E-mail addresses: elpincio89@gmail.com (S. Princiotto), valeria.dibussolo@unipi.it (V. Di Bussolo),
paolo.crotti@farm.unipi.it (P. Crotti).
Present address: Dipartimento di Biotecnologie, Chimica e Farmacia, Università degli Studi di Siena,
via Aldo Moro 2, 53100, Siena, Italy.
Present address: Kedrion Biopharma, Gallicano (Lucca), Italy.
1

ACCEPTED MANUSCRIPT
Due to the importance of unusual 4-aminosugars, extensive synthetic studies have been carried
out towards the production of these compounds and a divergent synthesis of aminosugar libraries has
5
, 6
also been proposed.
In view of the obtainment of a new synthetic protocol to introduce an N-protected amino
functionality at C(4)-carbon of a pyranoside system with the simultaneous construction of the
glicosydic bond, the use of an N-protected vinyl aziridine inserted in a glycal system appeared
particularly attractive. Actually, we have recently found that D-allal- and D-galactal-derived N-nosyl
7
aziridines 1α-Ns and 1β-Ns, prepared in situ by cyclization of the corresponding stable precursors
trans N-nosyl-O-mesyl derivatives 2α-Ns and 2β-Ns, respectively, are very effective glycosyl donors,
8
able to glycosylate (under protocol B reaction conditions) O-nucleophiles (alcohols, phenol and
7
partially O-protected monosaccharides) in a completely 1,4-regio- and syn-stereoselective fashion. In
this way, alkyl 4-N-(nosylamino)-2,3-unsaturated-O-glycosides 3α-Ns and 3β-Ns, having the same
configuration as the starting azidridine (syn-1,4-addition products, Scheme 1), are obtained in an
uncatalyzed, directly substrate-dependent, stereospecific glycosylation process with simultaneous,
completely regioselective introduction at C(4)-carbon of an α- or β-directed N-nosylamino group (–
NHNs) from aziridine 1α-Ns or 1β-Ns, respectively, in a completely stereoselective fashion (Scheme
7
,9,10
1
).
Considering that alkyl O-glycosides 3α-Ns and 3β-Ns can be easily deprotected to
corresponding 4-amino-2,3-unsaturated-O-glycosides 4α and 4β with complete retention of
configuration, N-nosyl aziridines 1α-Ns and 1β-Ns can be considered as effective synthetic tools for
the completely regio- and stereoselective introduction of a free amino group at C(4) of a pseudoglycal
7
system (Scheme 1). Moreover, the presence of the unsaturation makes alkyl O-glycosides 3α,β-Ns and
4
α,β susceptible to further elaborations toward more complex structures bearing a free or N-protected-
1
1
amino group with defined regiochemistry and configuration.
2

ACCEPTED MANUSCRIPT
alkyl 4-amino-
2,3-unsaturated-O-glycoside
syn-1,4-addition
product
-
O
O 1 OR
O
OR
O
ROH
3 equiv)
BnO
MsO
BnO
BnO
PhSH BnO
K CO
(
α
4
4
α
2
3
α
MeCN
K CO
NsHN
H N
2
N
2
3
α
α
Ns
NHNs
α
2
2
α-Ns
1α-Ns
3α-Ns
4α
protocol B
O
O
O
1
OR
O
OR
ROH
BnO
BnO
BnO
PhSH BnO
K CO
(
3 equiv)
MeCN
K CO
4
β
4
2
3
β
β
MsO
NsHN
H N
2
N
2
3
β
β
Ns
β
NHNs
β-Ns
1β-Ns
3β-Ns
4β
ROH = MeOH, EtOH, i-PrOH, t-BuOH, PhOH, partially O-protected monosaccharides
Scheme 1. 1,4-Regio- and syn-stereoselective glycosylation of alcohols by N-nosyl aziridines 1α-Ns
and 1β-Ns (protocol B reaction conditions) and stereoselective synthesis of alkyl 4-amino-2,3-
unsaturated-O-glycosides 4α and 4β (see ref. 7).
Although N-nosyl aziridines 1α-Ns and 1β-Ns are interesting and synthetically useful, there are
problems related to their preparation, particularly as aziridine 1β-Ns is concerned (Scheme 2). By using
tri-O-acetyl-D-glucal (5), as a useful and not expensive starting material and chiral source, the synthesis
of aziridine 1β-Ns proceeds through the formation of trans hydroxy mesylate 6β, the stable precursor
12
of
D
-galactal-derived epoxide 7β and then, through
D-gulal-derived trans diol 8, of diastereoisomeric
1
3
trans hydroxy mesylate 6α, a precursor of
D
-allal-derived epoxide 7α. Azidolysis of epoxide 7α to
1
0b
trans azido alcohol 9, followed by the usual protocol (reduction of azido group to amino group, N-
nosylation and O-mesylation) leads to trans N-nosyl-O-mesylate 2β-Ns, the ultimate stable precursor
7
,14,15
of the desired aziridine 1β-Ns.
Aziridine 1β-Ns is not stable and can be prepared only in situ by
cyclization of 2β-Ns under basic conditions (t-BuOK or K CO ) and left to react immediately with a
2
3
nucleophile, as shown in Scheme 2, where MeOH is taken as an example of solvent/nucleophile
8
(
protocol A reaction conditions). Under these conditions, the addition reaction is still completely 1,4-
regioselective, but not stereoselective and a 75:25 mixture of methyl α-O- 10α and β-O-glycoside 10β
7
,9
is obtained. As a whole, the preparation of N-nosyl aziridine 1β-Ns from tri-O-acetyl-
D
-glucal (5) is
1
6
7,10b,12-15
decidedly long with a low overall yield (13 steps, 3%)
improvement (Scheme 2).
and, as a consequence, requires
3

ACCEPTED MANUSCRIPT
O
O
O
O
AcO
AcO
a-e
BnO
f
BnO
BnO
HO
(
ref. 12,14)
(ref. 13,14)
MsO
O
OAc
OH
OH
7
β
5
6β
8
(
only in situ)
g-i
ref. 13,14)
(
O
O
O
O
BnO
MsO
k-m BnO
ref. 7,14)
BnO
j
BnO
(
(ref. 10b,14)
HO
MsO
O
NHNs
N3
OH
2
β-Ns
9
7α
only in situ)
6α
(
MeOH
K CO
2
3
(ref. 7)
O
O
OMe
O
OMe
BnO
BnO
NsHN
BnO
+
NsHN
N
Ns
1
0β (25%)
10α (75%)
1
β-Ns
(
only in situ)
Scheme 2. Synthesis of trans N-nosyl-O-mesylate 2β-Ns from tri-O-acetyl-D-glucal (5) (13 steps, 3%
overall yield) and base-catalyzed cyclization (K CO ) to aziridine 1β-Ns in MeOH (protocol A reaction
2
3
conditions): a) MeONa/MeOH (97%, ref.12); b) 1M LHMDS in THF/BnBr/DMF (39%, ref. 12 and
4); c) TBDMS-Cl/imidazole/DMF (84%, ref. 7); d) MsCl/Py (79%, ref. 12 and 14); e) TBAF/THF
1
+
-
(
(
74%, ref. 12); f) t-BuOK/Bu N Me SiO /THF (85%, ref. 13 and 14); g) TBDMS-Cl/imidazole/DMF
4 3
82%, ref. 13 and 14); h) MsCl/Py (82%, ref. 13); i) TBAF/THF (79%, ref. 13 and 14); j) t-
BuOK/TMGA (74%, ref. 10b and 14); k) Resin supported-PPh /THF-H O (23:1) (82%, ref. 7, 14 and
3
2
1
3 2 2 3 2 2
5); l) NsCl/Et N/CH Cl (80%, ref. 7); m) MsCl/Et N/CH Cl (78%, ref. 7 and 14).
For these reasons, a study was started to find a new, more convenient protocol which, still
starting from tri-O-acetyl- -glucal (5), could lead to a stable -glucal-derived mesylate 2β-PG1,
precursor of a new -galactal-derived aziridine 1β-PG structurally related to trans-N-nosyl-O-
D
D
D
1
mesylate 2β-Ns and N-nosyl aziridine 1β-Ns, respectively. The generic new aziridine 1β-PG and
1
corresponding precursor 2β-PG should have easily removable 6-O-protecting (PG ) and N-
1
2
protecting/activating group (PG
1
) (Scheme 3).
4

ACCEPTED MANUSCRIPT
O
O
PG O
2
AcO
AcO
MsO
NHPG1
2β-PG
OAc
5
1
O
PG O
2
N
PG1
1
β-PG1
Scheme 3. Generic structures of the new D-galactal-derived aziridine 1β-PG and its stable D-glucal-
1
derived precursor 2β-PG , bearing easily removable 6-O-protecting (PG ) and N-protecting/activating
1
2
1
group (PG ).
2
2
. Results and Discussion
.1. Synthesis of 4-O-mesyl-3-deoxy-3-N-(ethoxycarbonylamino)-
D
2
-glucal-derivative 2β-CO Et,
2
the stable precursor of D-galactal-derived N-ethoxycarbonyl vinyl aziridine 1β-CO Et
In the course of literature research into
D-glucal-derivatives useful to our study, D-glucal-derived
urethane 12, structurally related to trans N-nosyl-O-mesylate 2β-Ns, was found to be easily prepared
1
7,18
by reaction with chlorosulfonyl isocyanate (CSI)
of ethyl α-O-glycoside 11α, itself prepared in an
8
5:15 mixture with diastereoisomeric ethyl β-O-glycoside 11β, by Ferrier rearrangement reaction of
.
19
3 2
tri-O-acetyl-D-glucal (5) with EtOH/benzene in the presence of BF Et O (Scheme 4).
5

ACCEPTED MANUSCRIPT
O
O
OEt
AcO
AcO
EtOH/benzene AcO
.
BF Et O
3
2
AcO
(ref. 19)
OAc
1
1α
5
isolated from 85:15
mixture with β-anomer 11β
CSI/Et O
2
1) EtOH
) KI/NaOH
ref. 17)
2
(
O
AcO
AcO
NHCO Et
2
1
2
Scheme 4. Previously described synthesis of
glycoside 11α with CSI.
D-glucal-derived urethane 12 by reaction of ethyl α-O-
Urethane 12 appeared decidedly interesting to our purpose, provided that some modifications
were necessarily introduced. Actually, the acyl protection of the primary alcohol of 12 had to be
replaced with a different, still easily removable protective group, as t-butyl-diphenylsilyl group (-
OTBDPS), more stable to basic conditions and to nucleophiles, whereas the secondary acetoxy group
had to be substituted with a mesyloxy group. In this way, the obtained trans O-mesyl-N-
(
2
ethoxycarbonylamino)-derivative 2β-CO Et could hopefully act as the ultimate precursor of the new
D
-galactal-derived N-CO Et vinyl aziridine 1β-CO
2
2
Et by base-catalyzed intramolecular cyclization
process, as shown in Scheme 5 (vide infra).
O
O
AcO
TBDPSO
MsO
AcO
NHCO Et
NHCO Et
2
2
1
2
2β-CO Et
2
t-BuOK
O
TBDPSO
N
EtO C
2
1
β-CO Et
2
Scheme 5. Necessary modifications of O-Ac protections in order to make urethane 12 useful for the
preparation of
D
-galactal-derived N-CO
2
2
Et vinyl aziridine 1β-CO Et.
6

ACCEPTED MANUSCRIPT
In this framework, the new protocol to
D-galactal-derived vinyl aziridine 1β-CO Et (see
2
Schemes 4 and 5) was started by repeating the previously described Ferrier reaction consisting in the
1
9
3 2
treatment of a 9:1 benzene/EtOH solution of tri-O-acetyl-D-glucal (5) with BF ⋅Et O. Ethyl α-O-
glycoside 11α was not separated and the obtained 85:15 mixture of ethyl α- and β-O-glycosides 11α
and 11β (90% yield) was directly subjected to the subsequent reaction with CSI following the
previously described protocol (CSI/Et O in the presence of EtOH, followed by reduction with
2
1
7a,20
KI/NaOH), originally applied only to ethyl α-O-glycoside 11α. (Scheme 4).
Contrary to expectations and from the result described by the original authors, only a complex
reaction mixture was obtained and a corresponding unsatisfactory result was achieved also when the
1
7b
same reaction was repeated in MeCN, as the solvent.
Owing to these negative results, the presence in the literature of some related use of CSI, with
possible effective application to our system, constituted by ethyl α- and β-O-glycosides 11α and 11β,
2
1
was checked. In this way, a CSI application in the allyl amination of allyl methyl ether 13,
a
compound functionally very similar to both ethyl O-glycosides 11α and 11β, which essentially are
mixed dialkyl acetals around an allyl carbon, appeared particularly interesting. The new protocol
consisted in the treatment of a solution of methyl allyl ether 13 in CH Cl with CSI/Na CO at –78 °C
2
2
2
3
2 3
followed by reduction with Na SO /KOH. Under these conditions, a 6.2:1 mixture of diastereoisomeric
allyl methoxycarbonylamino-substituted derivatives anti-14 and syn-15 (S 1’ products) had been
N
2
1
obtained, in a completely regioselective fashion (83% yield, Scheme 6).
S 1' products
S
1 products
N
N
NHCO Me
NHCO Me
2
NHCO Me
OMe
2
2
1
) CSI, Na CO
2 3
CH Cl ,-78°C
2
2
+
+
OTBS
anti-14
OTBS
OTBS
OTBS
2) Na
2
SO3, KOH
(ref. 21)
1
3
syn-15
16
not detected
6.2
(83% yield)
1
Scheme 6. Allyl amination of -OTBS-protected methyl allyl ether 13 by CSI (see ref. 21).
7

ACCEPTED MANUSCRIPT
Considering that we were looking for a result corresponding to the one reported in Scheme 6, the
reaction of the mixture of ethyl α- and β-O-glycosides 11α and 11β with CSI was repeated under the
2
0,21
above described new conditions (CSI/Na CO , CH Cl , –78 °C).
However, also in this case a
positive result was not initially obtained and we thought that the described low temperature conditions
–78 °C) could be the reason of the unsuccess and that a warmer temperature was necessary for the
2
3
2
2
(
reaction to occur in our system.
By carrying out the reaction of the 85:15 mixture of ethyl O-glycosides 11α and 11β with
CSI/Na CO in CH Cl at room temperature and by affecting the subsequent reduction/hydrolysis only
2
3
2
2
with Na
2, was obtained as the major reaction product in a highly regio- and complete stereoselective fashion
β-direction of N-CO Et group at C(3) carbon] accompanied by a small amount of corresponding
2 3 N
SO , the desired anti-S 1’ product, D-glucal-derived allyl N-ethoxycarbonylamino derivative
1
[
2
2
2,23
regioisomeric S 1 product(s), pseudoglycal(s) 17 (12 : 17 ratio = 9 : 1, Scheme 7).
Subsequent
N
flash column chromatography afforded pure D-glucal-derived allyl N-ethoxycarbonylamino derivative
1
2 in good yield (61%).
S 1' product
S 1 productc(s)
N
N
OAc
O
OAc
O
O
O
NHCO Et
2
1
) CSI/Na CO3 AcO
1
2
AcO
AcO
:
AcO
CH Cl r.t.
+
AcO
2
2,
+
OEt
2
AcO
:
2) Na
2
SO
3
3
O: Et
NHCO Et
2
1
1α
17
1
1β
12
61% yield)
9 : 1
-
(
-
Scheme 7. Reaction of ethyl O-glycosides 11α and 11β with CSI/Na CO -Na SO protocol at room
2
3
2
3
temperature.
1
7,21
As previously found in similar cases,
the regio- and stereoselectivity observed in this reaction
can be rationalized by the initial coordination of CSI, through its electrophilic carbon, with the acetal
oxygen of the pseudoaxial –OEt group of α-O-glycoside 11α (structure 18, Scheme 8) which
determines the lengthening and subsequent breaking of the anomeric C-OEt bond with the formation of
allyl carbocation 19 and nitrogen-based anion 20, as good leaving group. Subsequent S 1’-type attack
N
by 20, acting as a nucleophile, at C(3) carbon of the delocalized carbocation 19 exclusively occurs from
the less hindered β-face as shown in 21 (route a, Scheme 8), to give, through the intermediate N-
8

ACCEPTED MANUSCRIPT
chlorosulfonyl derivative 22,
D
-glucal-derived urethane 12, as the main reaction product. Actually, a
corresponding alternative/competitive nucleophilic attack by 20 from the α-face (route b in 21, Scheme
) would suffer of torsional tension with the vicinal equatorial C(4)-OAc bond and, as a consequence,
8
is not observed.
−
N
EtO C
SO Cl
2
2
OAc
OAc
O
OAc
O
20
OAc
O
4
4
+
O
AcO
AcO
AcO
1
AcO
+
3
+
H
1
H
2
3
OEt
OEt
2
−
19
11α
ClSO N-C
2
O
ClSO N=C=O
2
COOEt
1
8
−
20
N
a
SO Cl
H
2H
2
1
O
N
H
O
O
AcO
AcO
AcO
AcO
EtO C
+
3
H
4
b
AcO
COOEt
AcO
H
−
NHCO Et
N
2
2
SO Cl
2
20
1
2
SO Cl
2
2
2
21
Scheme 8. Rationalization of the formation of diacetoxy urethane 12 by reaction of ethyl α-O-
glycoside 11α with CSI.
The MeONa/MeOH-catalyzed transesterification of diacetoxy urethane 12 afforded the
corresponding dihydroxy urethane 23 (97% yield) which was protected at the primary hydroxy group
by treatment with t-butyl-diphenylsilyl chloride (TBDPSCl)/imidazole protocol in anhydrous DMF to
give corresponding 6-O-[t-(butyl)-diphenysilyl]- (6-OTBDPS) derivative 24 (78% yield, Scheme 9).
The residual secondary –OH group of mono O-TBDPS derivative 24 was mesylated (MsCl/Py) to give
trans 3-N-(ethoxycarbonylamino)-4-O-mesyl derivative 2β-CO Et (86% yield) the supposed ultimate
2
stable precursor of the corresponding N-ethoxycarbonyl vinyl aziridine 1β-CO Et (Schemes 5 and 9).
2
9

ACCEPTED MANUSCRIPT
O
O
AcO
AcO
HO
HO
MeONa
MeOH
9
7%
NHCO Et
NHCO Et
2
2
1
2
23
DMF
imidazole
TBDPSCl
8
1%
O
O
TBDPSO
MsO
TBDPSO
MsCl
Py
HO
8
6%
NHCO Et
NHCO Et
2
2
2β-CO Et
2
24
Scheme 9. Transformation of diacetoxy urethane 12 into aziridine precursor 2β-CO Et.
2
2
.2. Formation in situ of vinyl aziridine 1β-CO Et and regio- and stereoselectivity of its reaction
2
with MeOH under protocol A and protocol B reaction conditions
Considering that the expected N-CO Et vinyl aziridine 1β-CO Et (Scheme 5) could reasonably
2
2
be unstable and, as a consequence, only be obtained in situ, as was the case of the previously studied N-
7
,10
nosyl aziridine 1β-Ns, the only way to check its formation in situ was to make aziridine 1β-CO Et
2
react with a nucleophile, such as MeOH, immediately after its formation. As a consequence, a solution
of trans-N-(ethoxycarbonyl)-O-mesyl derivative 2β-CO Et in MeOH (protocol A reaction conditions)
2
was treated with t-BuOK (1 equiv) in order to determine the deprotonation of the residual acid N-H
bond. Subsequent intramolecular S 2 substitution on the vicinal C(4) carbon bearing the good leaving
N
group (-OMs) in the required trans-diaxial relationship, necessary for the cyclization to occur and
2 2
possible in conformer 2β”-CO Et, could have led to the desired aziridine 1β-CO Et (Scheme 10).
1
0

ACCEPTED MANUSCRIPT
TBDPSO
-
OTBDPS
O
NCO Et
2
TBDPSO
MsO
t-BuOK
O
MsO
EtOCON
-
O
MeOH
4
MsO
3
NHCO Et
2
2
β'-CO Et
2β"-CO Et
2
2
2
β-CO Et
2
H6
H6'
H6 H6'
O
OMe
O
OMe
O
TBDPSO
TBDPSO
TBDPSO
H5
H5
H
H
+
1
1
MeOH
EtOCONH
EtOCONH
H2
H2
H4
H4
N
H3
5α
H3
EtO C
2
2
1
:
1
25β
1
β-CO Et
2
2 2
Scheme 10. Cyclization of ultimate stable precursor 2β-CO Et to aziridine 1β-CO Et and
glycosylation of MeOH (protocol A reaction conditions) with the formation of methyl α- and β-O-
glycosides 25α and 25β.
A few minutes after the addition of the base, TLC analysis of the reaction mixture showed the
gradual disappearance of the starting material [trans-N-(ethoxycarbonyl)-O-mesylate 2β-CO Et] and
2
the contemporary formation of a product with a higher R . After 16 h stirring at room temperature,
f
NMR analysis of the crude reaction product, by combined use of scalar and dipolar 1D and 2D
techniques, indicated the presence of an almost 1:1 mixture of anti- and syn-1,4-addition products,
methyl α- 25α and β-O-glycoside 25β, respectively (94%) which turned out to be not separable by any
2
4
chromatographic conditions tried (Scheme 10). The formation of methyl O-glycosides 25α and 25β
clearly indicated that, under the reaction conditions tried, the desired -galactal-derived N-CO Et
D
2
aziridine 1β-CO Et was formed, with trans N-methoxycarbonyl-O-mesyl- derivative 2β-CO Et as the
2
2
ultimate stable precursor.
The complete 1,4-regioselectivity observed in the reaction of aziridine 1β-CO Et with MeOH
2
under protocol A reaction conditions is in accordance with what observed with the previously examined
7
,10
N-nosyl aziridine 1β-Ns in the corresponding reaction with MeOH under the same conditions.
Typically, under protocol A reaction conditions, nucleophilic attack by MeOH, solvent/nucleophile of
the reaction, exclusively occurs at C(1) vinyl carbon of aziridine 1β-CO Et which, in the presently
adopted non-catalyzed conditions and in the presence of a weak nucleophile (MeOH), is the more
2
1
1

ACCEPTED MANUSCRIPT
reactive site in the molecule. In this way, only corresponding anti- 25α, by route a, and syn-1,4-
addition product 25β, reasonably by route b and aziridine nitrogen-nucleophile coordinate route c, are
’
obtained in a completely regioselective way, with aziridine reacting through conformer 1β -CO Me
2
2
5
(
Scheme 11). The observed lack of stereoselectivity (a 1:1 mixture of α- and β-anomer is obtained) is
the consequence of the large excess of MeOH, solvent of the reaction, which can attack C(1) carbon
2
6
indifferently from the α and β side of the molecule. Significantly, corresponding anti-1,2-addition
product (route d, Scheme 11) or any other reaction products turned out to be completely absent.
The demonstrated formation in situ of aziridine 1β-CO Et in the methanolysis carried out under
2
protocol A reaction conditions, made a preliminary examination possible of the regio- and
stereoselective behavior under the synthetically more interesting protocol B reaction conditions, that is
under conditions which had shown for N-nosyl aziridine 1β-Ns a completely 1,4-syn-stereoselective
7
7
result (Scheme 1). As a consequence, following the common protocol, a solution of trans N-CO Et-
2
1
O-mesylate 2β-CO
2
Et in anhydrous MeCN, containing MeOH (3 equiv), was treated with t-BuOK. H
NMR examination of the crude reaction product showed the presence of methyl β-O-glycoside 25β,
having the same configuration as the starting aziridine (syn-1,4-addition product), as the only reaction
product (Scheme 11, route c). This result, superposable to that previously obtained with N-nosyl
7
aziridine 1β-Ns under the same conditions (Scheme 1), permits to consider aziridine 1β-CO
2
Et an
effective, easy to prepare, glycosyl donor potentially able to glycosylate alcohols in a completely
stereoselective, directly substrate-dependent fashion and, thus, a valid synthetic tool to be preferred to
the corresponding aziridine 1β-Ns.
.
1
2

ACCEPTED MANUSCRIPT
aziridine nitrogen-nucleophile
coordination (hydrogen bond)
O
C
Me
δ+
N
H
O
EtO
MeOH
O
O
OMe
TBDPSO
OTBDPS
O
protocol B
route c
TBDPSO
c
β
b
O
EtOCONH
1
N
3
H
EtO C
2
1β'-CO Et
β
2
a
25β
β
routes b,c
Me
1
β-CO Et
route d
d
syn-1,4-addition product
route a = pseudoequatorial attack
2
protocol A
route a
O
1
: 1
O
BnO
EtOCONH
OMe
OMe
TBDPSO
(
anti-1,2-addition product )
not obtained
EtOCONH
α
β
2
5α
anti-1,4-addition product
route a = pseudoaxial attack
Scheme 11. Regio- and stereoselectivity of the glycosylation of MeOH by vinyl aziridine 1β-
CO Et under protocol A and B reaction conditions.
2
7
As previously admitted for aziridine 1β-Ns, the completely 1,4-regio- and syn-stereoselectivity
observed in the reaction of aziridine 1β-CO Et with MeOH under protocol B can be rationalized by the
2
occurrence of a coordination between MeOH and the aziridine nitrogen in the form of an hydrogen
bond, as shown in Scheme 11. In this way, the nucleophile is appropriately disposed for an
entropically-favored attack on C(1) carbon from the same side of the heterocyclic functionality through
a pathway (route c) which, due to the small amount of MeOH present, is the only active addition
process with consequent exclusive formation of methyl β-O-glycoside 25β in a completely regio- and
2
7,28
stereoselective fashion, as experimentally found.
2
.3. Conclusion
The results obtained in the present study are of considerable synthetic importance because a new,
simple and fast synthetic protocol to access -galactal-derived N-activated vinyl aziridines, as 1β-
CO Et, has been found.
D
2
1
3

ACCEPTED MANUSCRIPT
By using tri-O-acetyl-
D
-glucal (5) as the starting material and the chiral source, and by noting, as
7
previously done in the case of 1β-Ns, that the base-catalyzed cyclization of the stable precursor to the
corresponding aziridine is quantitative, the new protocol to N-ethoxycarbonyl-O-mesylate 2β-CO Et,
2
(
the ultimate precursor of aziridine 1β-CO Et) requires only 5 steps (Schemes 4, 7 and 9) with a
2
overall yield of 36%, this particularly pleasing if compared with the yield previously obtained in the
case of the structurally related N-nosyl aziridine 1β-Ns and corresponding stable precursor 2β-Ns (13
7
,10b,12-14
steps, 3% overal yield, Scheme 2).
The preliminary results obtained with the new aziridine 1β-
CO Et in the glycosylation of MeOH, as a model O-nucleophile, are identical to those previously
2
obtained with the corresponding N-nosyl azidine 1β-Ns. In particular, the exclusive formation of the
corresponding syn-1,4-addition product (methyl β-O-glycoside 25β) observed under protocol B
2
reaction conditions, indicated that aziridine 1β-CO Et is an effective, completely 1,4-regio- and syn-
stereoselective glycosyl donor.
1
4

ACCEPTED MANUSCRIPT
3
3
. Experimental
.1.General.
All reactions were performed in a flame-dried modified Schlenk (Kjeldahl shape) flask fitted with
a glass stopper or rubber septa under a positive pressure of argon. Flash column chromatography was
performed employing 230-400 mesh silica gel (Macherey-Nagel). Analytical TLC were performed on
Alugram SIL G/UV254 silica gel sheets (Macherey-Nagel) with detection by 0.5% phosphomolybdic
acid solution in 95% EtOH. Benzene and Et
2
O were distilled from sodium/benzophenone. HPLC grade
MeOH, EtOH and CH Cl were used without any further purification.
2
2
3
.2. Synthesis of mesylate 2β-CO Et, stable precursor of N-CO Et vinyl aziridine 1β-CO Et
2
2
2
1
9
3
.2.1 Ethyl
α
- and
β
-O-glycosides 11
α
and 11
β
. Following a previously described procedure,
a
solution of tri-O-acetyl-
D-glucal (5) (3.0 g, 11.03 mmol) in a 9:1 anhydrous benzene: EtOH mixture
.
(
13.30 mL) was treated at 0 °C with BF Et O (0.60 mL, 4.74 mmol). After 2 h stirring at the same
3
2
temperature, the reaction mixture was diluted with Et O. Evaporation of the washed (saturated aqueous
2
NaHCO ) organic solution afforded a crude product consisting of an 85:15 mixture of ethyl α- and β-
3
1
19
O-glycosides 11α and 11β (2.56 g, 90% yield) ( H NMR) which was directly used in the next step,
without any further purification.
3
.2.2. 4,6-Di-O-acetyl-3-deoxy-3-N-(ethoxycarbonylamino)-D-glucal (12). Following a previously
2
1
described procedure, a suspension of Na CO (0.183 g, 1.73 mmol) in anhydrous CH Cl (2.0 mL)
2
3
2
2
containing chlorosulfonylisocyanate (0.24 g, 0.15 mL, 1.70 mmol) was treated at room temperature
with a solution of the mixture of ethyl α- and β- O-glycosides 11α and 11β (0.20 g, 0.77 mmol) in
anhydrous CH
Water (3 mL) was added and the reaction mixture was extracted with AcOEt (3 mL x 2). The combined
organic extracts were added to an aqueous solution of Na SO (25%) and KOH (10%) (2.5 mL) and
2 2
Cl (2.0 mL) and the reaction mixture was stirred at the same temperature for 90 min.
2
3
the reaction mixture was stirred at room temperature for 18 h. Evaporation of the separated organic
phase afforded a crude mixture (0.204 g) consisting of a 90:10 mixture of ethyl carbamate 12 and 1,4-
1
addition product(s) 17 ( H NMR) which was subjected to flash chromatography. Elution with a 6:4
1
7a
hexane/AcOEt mixture afforded pure diacetoxy ethyl carbamate 12 (0.142 g, 61%), as a solid, m.p.
1
7a
-1 1
1
13-115 °C (lit. , m.p. 113-115 °C): FTIR(Nujol) ν
3430, 1740, 1720, 1650 cm . H NMR (250
max
1
5

ACCEPTED MANUSCRIPT
MHz, CDCl
3
) δ 6.41 (dd, 1H, J = 5.9, 2.1 Hz), 5.08 (dd, 1H, J = 10.2, 8.4 Hz), 4.73-4.78 (m, 1H), 4.74
(
(
dd, 1H, J = 5.9, 2.3 Hz), 4.47-4.58 (m, 1H), 4.43 (dd, 1H, J = 11.7, 4.7 Hz), 4.17-4.24 (m, 2H), 4.12
1
3
q, 2H, J = 7.1 Hz), 2.11 (s, 6H), 1.26 (t, 3H, J = 7.1 Hz); C NMR (62.5 MHz, CDCl ) δ 171.0, 169.1,
3
1
55.8, 146.6, 100.2, 77.2, 68.5, 62.8, 61.6, 49.5, 29.9, 20.9, 14.7. Anal. Calcd for C13
7
H19NO : C, 51.82;
H, 6.36; N, 4.65. Found: C, 51.99; H, 6.57; N, 4.41.
3
.2.3. 6-O-(t-Butyldiphenylsilyl)-3-deoxy-3-N-(ethoxycarbonylamino)-D-glucal (24). A solution of
diacetoxy ethyl carbamate 12 (0.136 g, 0.45 mmol) in MeOH (2.0 mL) was treated with MeONa (0.016
g, 0.30 mmol) and the reaction mixture was stirred for 18 h at room temperature. Evaporation of the
filtered organic solvent afforded a crude product consisting of practically pure trans diol 23, as a pale
yellow oil (0.095 g, 0.437 mmol, 97%): FTIR(liquid film) νmax 3398, 1695, 1531, 1232, 1126, 1050
-1 1
cm ; H NMR (250 MHz, MeOD) δ 6.37 (dd, 1H, J = 6.0, 2.0 Hz), 4.95 (broad s, 1H), 4.57 (dd, 1H, J
=
6.0, 2.0 Hz), 4.09 (q, 2H, J = 7.1 Hz), 3.90-3.71 (m, 5H), 1.23 (t, 3H, J = 7.1 Hz).
Crude diol 23 was dissolved in anhydrous DMF (2.5 mL) and treated at 0 °C with TBDPSCl
(
0.126 g, 0.12 mL, 0.46 mmol) in the presence of imidazole (0.063 g, 0.92 mmol). After 18 h stirring at
room temperature, the reaction mixture was diluted with AcOEt. Evaporation of the washed (saturated
aqueous NaHCO and brine) organic solution afforded a crude product (0.20 g) consisting of O-TBDPS
3
derivative 24 which was subjected to flash chromatography. Elution with an 8:2 hexane/AcOEt mixture
afforded mono O-TBDPS derivative 24 (0.155 g, 78%), pure as a colourless oil, R = 0.13 (8:2
f
-1 1
hexane/AcOEt): FTIR(liquid film) νmax 3387, 1695, 1529, 1233, 1048, 701 cm . H NMR (250 MHz,
CDCl ) δ 7.62-7.75 (m, 4H), 7.30-7.48 (m, 6H), 6.42 (dd, 1H, J = 5.9, 2.2 Hz), 4.74-4.83 (m, 1H), 4.48
3
(
dd, 1H, J = 5.9, 2.2 Hz), 4.23-4.31 (m, 1H), 4.14 (q, 2H, J = 7.1 Hz), 4.12 (d, 1H, J = 4.5 Hz), 3.97-
13
4
1
.01(m, 1H), 3.78-3.88 (m, 2H), 1.24 (t, 3H, J = 7.1 Hz), 1.06 (s, 9H). C NMR (62.5 MHz, CDCl
55.9, 145.1, 135.9, 129.9, 129.8, 127.7, 99.4, 79.1, 70.7, 63.1, 61.7, 52.5, 26.9, 19.5, 14.6. Anal. Calcd
Si: C, 65.90; H, 7.30; N, 3.07. Found: C, 65.79; H, 7.18; N, 2.89.
3
) δ
for C25H33NO
5
3
.2.4. 6-O-(t-Butyldiphenylsilyl)-3-deoxy-3-N-(ethoxycarbonylamino)-4-O-mesyl-D-glucal (2β-CO Et).
2
A solution of mono O-TBDPS derivative 24 (0.091 g, 0.20 mmol) in a 1:1 CH Cl /pyridine mixture
2
2
(
2.0 mL) was treated with MsCl (0.060 mL, 0.30 mmol) at 0°C and the resulting reaction mixture was
stirred at the same temperature for 18 h. After dilution with CH Cl , evaporation of the washed
2
2
(
saturated aqueous NaHCO and brine) and filtered organic solution afforded a liquid residue which
3
was co-evaporated at the rotary evaporator (40 °C) with toluene (3 mL x 2) to give a crude product
1
6

ACCEPTED MANUSCRIPT
consisting of practically pure O-mesyl derivative 2β-CO
.12 (8:2 hexane/AcOEt): FTIR(liquid film) νmax 3413, 1653, 1360, 1234, 1176, 1100 cm . H NMR
250 MHz, CDCl ) δ 7.67-7.78 (m, 4H), 7.31-7.51 (m, 6H), 6.40 (dd, 1H, J = 6.1, 1.8 Hz), 5.08 (t, 1H,
2 f
Et (0.092 g, 86%), as a pale yellow oil: R =
-1 1
0
(
3
J = 6.5 Hz), 4.93-5.05 (m, 1H), 4.69 (dd, 1H, J = 6.1, 3.1 Hz), 4.41-4.51 (m, 1H), 4.05-4.20 (m, 3H),
1
3
3
1
.70-3.93 (m, 2H), 3.08 (s, 3H), 1.23 (t, 3H, J = 7.1 Hz), 1.07 (s, 9H). C NMR (62.5 MHz, CDCl
3
) δ
56.1, 145.6, 135.8, 130.0, 129.8, 127.9, 99.2, 76.4, 75.5, 61.6, 61.4, 48.2, 38.8, 26.8, 19.3, 14.7. Anal.
Calcd for C H NO SSi: C, 58.51; H, 6.61; N, 2.62. Found: C, 58.72; H, 6.85; N, 2.85.
2
6
35
7
3
2
.2.5. Treatment of mesylate 2β-CO Et with t-BuOK in MeOH (protocol A). A solution of trans N-
CO Et-O-mesyl derivative 2β-CO Et (0.080 g, 0.15 mmol) in MeOH (2.5 mL) was treated with t-
2
2
BuOK (0.018 g, 0.16 mmol) and the reaction mixture was stirred for 18 h at room temperature.
Dilution with Et
2
O and evaporation of the filtered organic solution afforded a crude product (0.066 g,
1
9
4%) consisting of an 1:1 mixture of methyl α- and β-O-glycosides 25α and 25β ( H NMR) which
1
turned out to be not separable under any TLC conditions tried. H NMR (600 MHz, CD CN, 25 °C)
3
data for the mixture of methyl α- and β-O-glycosides 25α and 25β: δ 5.98 (dd, 1H, J = 9.9 Hz, J = 5.8
Hz, H(3), 25α], 5.96 [dd, 1H, J = 10.1 Hz, J = 5.7 Hz, H(3), 25β), 5.85 (dd, 1H, J = 9.9 Hz, J = 2.3 Hz,
H(2), 25α), 5.77 (d, 1H, J = 10.1 Hz, H(2), 25β), 5.48 (d, 1H, J = 8.7 Hz, NH, 25β), 5.41 (d, 1H, J =
9
.5 Hz, NH, 25α), 4.99 (s, 1H, H(1), 25β), 4.87 (d, 1H, J = 2.3 Hz, H(1), 25α), 4.18-3.90 (m, 6H,
CH (Et), H(4) and H(5), 25α and 25β), 3.82 and 3.80 (m, 2H, H(6), 25α and 25β), 3.73 (dd, 1H, J =
0.9 Hz, J = 6.7 Hz, H(6’), 25β), 3.69 (dd, 1H, J = 10.9 Hz, J = 7.4 Hz, H(6’), 25α), 3.40 (s, 3H, OMe,
2
1
24
2
5β), 3.39 (s, 3H, OMe, 25α), 1.17 (t, 3H, J = 7.0 Hz, Me, 25α and 25β).
3
.2.6. Reaction of aziridine 1β-CO Et with MeOH in anhydrous MeCN (protocol B). A solution of
2
trans N-CO
2
Et-O-mesyl derivative 2β-CO Et (0.040 g, 0.075 mmol) in anhydrous MeCN (0.90 mL)
2
containing MeOH (9.1 µL, 0.225 mmol, 3 equiv) was treated with t-BuOK (0.009 g, 0.080 mmol) and
the reaction mixture was stirred for 18 h at room temperature. Dilution with Et O and evaporation of
2
the filtered organic solution afforded a crude product consisting of practically pure methyl β-O-
1
glycoside 25β ( H NMR) (0.032 g, 95% yield) which was subjected to flash chromatography. Elution
with an 1:1 hexane/AcOEt mixture afforded pure methyl 6-O-(t-butyldiphenylsilyl)-2,3,4-trideoxy-4-N-
(
ethoxycarbonylamino)-β-D-threo-hex-2-enopyranoside (25β) (0.022 g, 62%), as a pale yellow liquid:
-1 1
R
f
= 0.29 (1:1 hexane/AcOEt); FTIR(liquid film) νmax 3359, 1660, 1540, 1417, 1120, 1058 cm . H
1
7

ACCEPTED MANUSCRIPT
NMR (600 MHz, CD
3
CN) δ 7.65-7.82 (m, 3H), 7.38-7.58 (m, 7H), 5.96 (dd, 1H, J = 10.1, 5.7 Hz),
5
.77 (d, 1H, J = 10.1, 5.7 Hz), 5.48 (d, 1H, J = 8.7 Hz, NH), 4.99 (broad s, 1H), 4.20 (q, 2H, J = 7.1
Hz), 4.12-4.18 (m, 1H), 3.91-3.97 (m, 1H), 3.75-3.82 (m, 1H), 3.73 (dd, 1H, J = 10.9, 6.7 Hz), 3.40 (s,
1
3
3
H), 1.27 (t, 3H, J = 7.1), 1.03 (s, 9H); C NMR δ (150 MHz CD
3
CN) 150.8, 145.8, 135.4, 130.0,
1
29.9, 128.1, 127.7, 98.4, 74.8, 63.7, 63.6, 54.7, 45.0, 26.3, 19.2, 13.4. Anal. Calcd for C H NO Si:
26
35
5
C, 66.49; H, 7.51; N, 2.98. Found: C, 66.35; H, 7.29; N, 2.73.
3
.3. NMR Measurements
NMR measurements were performed on a spectrometer operating at 600 MHz, for H nuclei. The
1
temperature was controlled to ± 0.1 °C. The 2D NMR spectra were obtained by using standard
sequences with the minimum spectral width required. Proton 2D gCOSY (gradient COrrelated
SpectroscopY) spectra were recorded with 128-200 increments of 4-16 scans and 2K data points. The
relaxation delay was 1 s. 2D TOCSY (TOtal Correlation SpectroscopY) spectra were recorded by
employing a mixing time of 80-120 ms. The pulse delay was 1 s; 256-512 increments of 16-32 scans
and 2K data points each were collected. 1D TOCSY spectra were acquired with 1024-4096 scans in
3
2K data points with a 1 s relaxation delay and a 100 ms mixing time. The 2D NOESY (Nuclear
Overhauser Enhancement SpectroscopY) experiments were performed by employing a 1 s mixing time.
The pulse delay was 1 s; 200 increments of 16scans and 2K data points each were collected. 1D
NOESY spectra were recorded using the selective pulse SEDUCE generated by means of the Agilent
Pandora Software. The selective 1D NOESY spectra were acquired with 1024 scans in 32K data points
with a 1-5 s relaxation delay and a 1 s mixing time. gHSQC (gradient Heteronuclear Single Quantum
Coherence) and HMBC (Heteronuclear Multiple Bond Correlation) spectra were recorded by
employing a pulse delay of 1.2 s and 128 increments of 64 scans. HMBC experiments were optimized
1
13
for a long-range H- C coupling constant of 8 Hz.
Acknowledgments
This work was supported by the University of Pisa (University Funds 2013-2015).
Supplementary data
Supplementary data associated with this article can be found, in the online version, at…...
1
8

ACCEPTED MANUSCRIPT
References and notes
1
. (a) Banoub, J.; Boullanger, P.; Lafont, D. Chem. Rev. 1992, 92, 1167–1195. (b) van den Bos, L.
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003, 1, 4160–4165. (c) Bernfield, M.; Gotte, M.; Park, P. W.; Reizes, O.; Fitzgerald, M. L.;
Lincecum, J.; Zako, M. Annu. Rev. Biochem. 1999, 68, 729–777. (d) Dwek, R. A. Chem. Rev.
996, 96, 683–720.
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2
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. (a) Werz, D. B.; Ranzinger, R.; Herget, S.; Adibekian, A.; von der Lieth, C. W.; Seeberger, P. H.
ACS Chem. Biol. 2007, 2, 685–691. (b) Bertozzi, C. R.; Kiessling, L. L. Science 2001, 291,
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357–2364; (c) Helenius, A.; Aebi, M. Science 2001, 291, 2364–2369; (d) Seeberger, P. H.;
Werz, D. B. Nature 2007, 446, 1046–1051.
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47.
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. Rai, R.; McAlexander, I.; Chang, C.-W. Org. Prep. Proced. Int. 2005, 37, 337-375.
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Chang, C.-W. T. J. Org. Chem. 2004, 69, 1513-1523. (b) Georges, M.; Mackay, D.; Fraser-Reid,
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2
89-906. (d) Ichikawa, Y.; Hirata, K.; Ohbayashi, M.; Isobe, M. Chem. Eur. J. 2004, 10, 3241-
251. (e) Kallinen, A.; Tois, J.; Sjöholm, R.; Franzén, R. Tetrahedron: Asymmetry 2010, 21,
367-2371. (f) Pedersen, C. M.; Olsen, J.; Brka, A.B.; Bols, M. Chem. Eur. J. 2011, 17, 7080-
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7
. Y. Hui, Y.; Chang, C.-W. T. Org. Lett. 2002, 4, 2245-2248.
. Di Bussolo, V.; Romano, M. R.; Pineschi, M.; Crotti, P. Tetrahedron 2007, 63, 2482-2489.
8
. Protocol A reaction conditions: the aziridine stable precursor, trans N-nosyl-O-mesylate 2α-Ns or
2β-Ns as an example, is dissolved in the glycosyl acceptor (low boiling alcohol as MeOH, EtOH,
i-PrOH or t-BuOH) and treated with a base (t-BuOK or K CO , 1 equiv). Under these conditions,
2
3
the glycosylation occurs in the presence of a large excess of nucleophile, solvent of the reaction.
Protocol B reaction conditions: the aziridine stable precursor, trans N-nosyl-O-mesylate 2α-Ns or
2
β-Ns as an example, is dissolved in a non-nucleophilic solvent (MeCN, THF, or toluene)
containing the glycosyl acceptor (alcohols, also complex, phenol or partially O-protected
monosaccharide, 3 equiv) and the reaction mixture is treated with a base (t-BuOK or K CO , 1
2
3
equiv). Under these conditions, the glycosylation occurs in the presence of only a small excess of
O-nucleophile.
9
. Glycosylation of alcohols by aziridines 1α-Ns, 1β-Ns, 1α-Ms and 1β-Ms under protocol A
reaction conditions are always completely 1,4-regioselective (exclusive formation of syn- and
anti-1,4-addition products) but not stereoselective and mixtures of corresponding α- and β-
anomers are commonly obtained. Corresponding 1,2-addition products are never observed under
these conditions.
1
1
0. Similar results had been previously obtained with the corresponding N-mesyl aziridines. See: a)
Di Bussolo, V.; Romano, M. R; Pineschi, M.; Crotti, P. Org. Lett. 2005, 7, 1299-1302. b) Di
Bussolo, V.; Romano, M. R.; Favero, L.; Pineschi, M.; Crotti, P. J. Org. Chem. 2006, 71, 1696-
1
699.
1. a) Di Bussolo, V.; Checchia, L.; Romano, M. R.; Pineschi, M.; Crotti, P. Org. Lett. 2008, 10,
493-2496. b) Checchia, L. PhD Thesis, University of Pisa, School of Graduate Studies “Scienza
2
del Farmaco e delle Sostanze Bioattive”, 2009, p. 90-91.
1
1
2. Di Bussolo, V.; Caselli, M.; Pineschi, M.; Crotti, P. Org. Lett. 2003, 5, 2173-2176.
3. Di Bussolo, V.; Caselli, M.; Romano, M. R.; Pineschi, M.; Crotti, P. J. Org. Chem. 2004, 69,
2
0

ACCEPTED MANUSCRIPT
7
383-7386.
1
4. Previously described particularly low yielding steps [b (32%, ref. 12), d (72%, ref. 12), f (68%,
ref. 13), g (70%, ref. 13), i (68%, ref. 13), j (65%, ref. 10b), m (65%, ref. 7)] were re-examined
and the corresponding more satisfactory yields obtained are indicated in Scheme 2. Moreover, the
previously reported protocol (SnCl /PhSH/CH Cl , 70% yield, k step, ref. 10b) for the reduction
2
2
2
of trans azido alcohol 9 to the corresponding trans amino alcohol has been substituted by the
simpler, higher yielding resin supported-PPh
ref.15).
3 2
/THF/H O protocol, as reported in Scheme 2 (see
1
1
5. Di Bussolo, V.; Fiasella, A.; Favero, L.; Bertolini, F.; Crotti, P. Org. Lett. 2009, 11, 2675-2678.
6. The overall yield (3%) has been calculated on the obtainment of the ultimate stable precursor,
trans N-nosyl-O-mesylate 2β-Ns, on the assumption that its cyclyzation to aziridine 1β-Ns, not
isolable, is quantitative.
1
1
7. a) Hall, R. H.; Jordaan, A.; Lourens , G. J. J. Chem. Soc. Perkin 1 1973, 38-44. b) Hall, R. H.;
Jordaan, A.; de Villiers, O. G. J. Chem. Soc. Perkin 1 1975, 626-629.
8. Fer a review on CSI, see: a) Rasmussen, J.; Hassner, A. Chem. Rev. 1976, 76, 389-408. b) Graf. A
Angev. Chem. Int. Ed. 1968, 7, 172-182.
1
2
9. Ferrier, R. J.; Prasad, N. J. Chem. Soc. (C) 1969, 570-575.
0. The use of the 85:15 mixture of ethyl α- and β-O-glycosides 11α and 11β was justified by the
1
good quality of the crude reaction product [only 11α and 11β were present ( H NMR)] and by
considering that a separation/purification of 11α from the low amount of diastereoisomeric 11β
would have lowered the yield. Moreover, the possibility that 11β reacts with CSI as 11α, making
the separation not necessary, could not be excluded. In the contrary case, unreacted 11β or an
undesired side product deriving from its reaction with CSI could be separated subsequently or, in
the most favorable case, could be lost in the work-up.
2
1

ACCEPTED MANUSCRIPT
2
2
1. Kim, J. D.; Zee, O. P.; Jung, Y. H. J. Org. Chem. 2003, 68, 3721-3724. The formation of a small
amount of undesired corresponding regioisomeric S 1 product(s) 16 (9%, Scheme 6), when the
N
3
same reaction was carried out in CHCl , is also reported.
N
2. S 1 product(s) 17 was not separated pure from the reaction mixture. However, its formation has
been tentatively admitted in accordance with a previously observed behavior of CSI in similar
1
systems (see Scheme 6, ref. 21) and by the presence in the H NMR spectrum of the crude
reaction mixture of a multiplet at δ 5.75-5.95 corresponding to the vinyl protons of a
pseudoglycal system as 17 actually is.
2
3. The strict correspondence between the starting 85:15 mixture of ethyl α- and β-O-glycosides 11α
and 11β and the final 9:1 mixture of allyl N-ethoxycarbonylamino derivative 12 (S
and the supposed S 1 product(s) 17 could indicate the completely regioselective behavior of α-
and β-O-glycosides 11α and 11β: 11α (pseudoaxial-oriented –OEt group) leads to S 1’ product
N-ethoxycarbonylamino derivative 12), whereas 11β (pseudoequatorial-oriented –OEt group)
leads to S 1 product(s) 17 (Schemes 7 and 8).
N
1’ product)
N
N
(
N
2
4. Accurate NMR analysis of the crude reaction product made the structural identification of the two
compounds present in a 1:1 mixture possible. In particular the absence in both compounds of O-
mesyl group and the presence of: a) an anomeric H(1) proton, b) a methoxy group at C(1)
anomeric carbon, c) a N-(ethoxycarbonyl)-amino group (-NHCO Et) at C(4) carbon, and d) two
2
vinyl proton signals at δ 5.90-6.30 ppm clearly indicated their nature of 1,4-addition products.
Subsequent appropriate 1D-NOESY experiments made the exact regio- and stereochemical
assignement to both compounds possible indicating a structure corresponding to anomer 25α
(
anti-1,4-addition product) and 25β (syni-1,4-addition product) (Schemes 10 and 11). In
particular, spatial proximity between H(1) and H(5) protons was confirmed for β-O-glycoside
5β by detecting their mutual dipolar interaction, as expected for protons in a syn 1,3-diaxial
2
arrangement, possible in the corresponding more stable conformer 25β’ with –CH OTBDPS side
2
chain equatorial, as here shown:
2
2

ACCEPTED MANUSCRIPT
TBDPSO
EtOCONH
OTBDPS
O
OMe
4
EtOCONH
H
5
OMe
O
1
H4
4
1
H
1
H4
H5
NOE
H1
2
5β'
25β"
2
5. Theoretical conformational studies in a vacuum, carried out, for simplicity, on the simplified 6-
OSiMe - and N-CO Me substituted aziridine 1β-CO Me-OTMS, structurally related to aziridine
3
2
2
1
β-CO Et, indicated that aziridine model 1β-CO Me-OTMS exists only as the corresponding
2
2
conformer 1β’-CO Me-OTMS with the –CH OTMS side chain equatorial and N-CO Me group
2
2
2
in the exo position (see Supplementary data).
O
Me SiO
3
=
N
MeO C
2
1β-CO
2
Me-OTMS
2
1β'-CO Me-OTMS
The results obtained with aziridine 1β-CO Me-OTMS, reasonably extended to aziridine 1β-
2
CO Et, indicate that also the presently studied aziridine exists only as the corresponding
2
conformer 1β’-CO Et with the –CH OTBDPS side chain equatorial and the exo-directed N-
2
2
2
CO Et group as shown in Scheme 11 (see Supplementary data).
2
6. In addition to the presence of a large excess of MeOH, the absence of stereoselectivity found in
the glycosylation of MeOH by aziridine 1β-CO Et, under protocol A reaction conditions, could
2
be the consequence of a complete balance between two opposite directing effects. Actually, if on
one hand, the formation of 25α through a stereoelectronically favored pseudoaxial attack by
MeOH (route a) should be preferred with respect to 25β which is formed by a less favored
pseudoequatorial attack by MeOH (routes b and c), on the other hand, the formation of 25β, at
least through route c, appears to be entropically favored by the occurrence of an aziridine
nitrogen-nucleophile (MeOH) coordination (hydrogen bond), as shown in Scheme 11.
2
7. Because arising from a coordination process, methyl β-O-glycoside 25β can be called also
coordination product (see ref. 7)
2
3

ACCEPTED MANUSCRIPT
2
8. It is interesting to note that the presence of the bulky –OTBDPS group in 1β-CO Et has no
2
influence on the completely 1,4-regio- and syn-stereoselective result observed in the methanolysis
under protocol B reaction conditions. However, appropriate studies will be carried out in order to
have a complete examination of the regio- and stereoselective behavior of aziridine 1β-CO Et
2
with different types of O-nucleophiles under different reaction conditions.
2
4