Synthetic methodology to prepare polysubstituted 2-aminopyrans. Synthesis of the C32–C38 subunit of immunosuppressant sanglifehrin A

Article abstract of DOI:10.1016/j.tet.2016.06.045

Pyranic hemiaminal synthons are present in natural and unnatural products with biological activity. Here a synthetic pathway to prepare this type of intermediates is presented. The methodology is based on three main steps: a) The [4+3] cycloaddition reaction of an α,α'-dihaloketone and a conveniently protected 2-aminofuran diene. b) Chemical modification of the resulting oxabicyclic cycloadduct and orthogonal protection of organic functions, and c) Reductive ozonolysis of the C6–C7 double bond of the modified cycloadduct to afford a pentasubstituted polyfunctionalized pyranic hemiaminal ester with up to 94% yield. When working with 2,4-dibromo-3-pentanone and 2-tert-butoxycarbonylaminofuran as starting materials, the C32–C38 subunit of immunosuppressant sanglifehrin A was obtained with excellent yield.

Full text of DOI:10.1016/j.tet.2016.06.045

Accepted Manuscript
Synthetic methodology to prepare polysubstituted 2-aminopyrans. Synthesis of the
C32-C38 subunit of immunosuppressant sanglifehrin A
Ángel M. Montaña, Joan Barcia, Albert Corominas
PII:
S0040-4020(16)30566-X
10.1016/j.tet.2016.06.045
TET 27859
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 26 May 2016
Accepted Date: 19 June 2016
Please cite this article as: Montaña EM, Barcia J, Corominas A, Synthetic methodology to prepare
polysubstituted 2-aminopyrans. Synthesis of the C32-C38 subunit of immunosuppressant sanglifehrin A,
Tetrahedron (2016), doi: 10.1016/j.tet.2016.06.045.
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ACCEPTED MANUSCRIPT
Synthetic methodology to prepare polysubstituted 2-aminopyrans. Synthesis of the
C32-C38 subunit of immunosuppressant sanglifehrin A
Ángel M. Montaña*, Joan Barcia and Albert Corominas
O
NHBoc
O
O
R
R
R
R
O
NHBoc
[
4+3]-Cycloaddition
Stereoselective
reduction
OH
OH
O
R
R
Ozonolysis
R
R
NHBoc
NHBoc
O
OH
OH
5

ACCEPTED MANUSCRIPT
SYNTHETIC METHODOLOGY TO PREPARE POLYSUBSTITUTED
2-AMINOPYRANS. SYNTHESIS OF THE C32-C38 SUBUNIT OF
IMMUNOSUPPRESSANT SANGLIFEHRIN A
Ángel M. Montaña*, Joan Barcia and Albert Corominas
Industrial and Applied Organic Chemistry Research Unit, Department of Organic Chemistry,
University of Barcelona, c/ Martí Franquès 1-11, 08028-Barcelona, Spain
*To whom correspondence should be addressed.
ABSTRACT:
Pyranic hemiaminal synthons are present in natural and unnatural products with
biological activity. Here a synthetic pathway to prepare this type of intermediates is
presented. The methodology is based on three main steps: a) The [4+3] cycloaddition
reaction of an α,α’-dihaloketone and a conveniently protected 2-aminofuran diene. b)
Chemical modification of the resulting oxabicyclic cycloadduct and orthogonal
protection of organic functions, and c) Reductive ozonolysis of the C6-C7 double bond
of the modified cycloadduct to afford a pentasusbtituted polyfunctionalized pyranic
hemiaminal ester with up to 94% yield. When working with 2,4-dibromo-3-pentanone
and 2-tert-butoxycarbonylaminofuran as starting materials, the C32-C38 subunit of
immunosuppressant sanglifehrin A was obtained with excellent yield.
Keywords: [4+3]-Cycloaddition, oxyallyl cation, pyran, hemiaminal, ozonolysis.
1
. Introduction
The immunosuppressant agents such as FK506, cyclosporine A and sanglifehrin A
Figure 1) have afforded an important therapeutic tool to make successful organ and
(
bone marrow transplantation and to understand the molecular basis of signal
1
transduction pathways at the cell level. The first two compounds, even though they
have very different structures, form two different drug-protein complexes that inhibit
2
the phosphatase activity of the intracellular signaling molecule calcineurin, blocking T-
cell activation and preventing host rejection of transplants. The importance of
sanglifehrin A is due to its action mechanism because it interferes with signaling
molecules other than calcineurin and binds to cyclophilin inhibiting both T-cell and B-
1

ACCEPTED MANUSCRIPT
3
cell proliferation. Thus, sanglifehrin A, like cyclosporine A, exerts its
immunosuppressive action by binding to cyclophilin A, but at a different site from
cyclosporine A, and unlike the latter, sanglifehrin A does not inhibit calcineurin
activity. Moreover, interesting applications of sanglifehrin family of natural products is
4
been studied for the treatment of hepatitis C virus, with promising results.
The biological relevance of this molecule has propitiated important synthetic
efforts to afford synthetic methodologies that make available derivatives with high
5
molecular diversity in order to carry out structure-activity relationships.
Figure 1. Structure of sanglifehrin A.
In this paper, we present our contribution to this synthetic objective, the
diastereoselective preparation of pyranic hemiaminal synthons, present in natural and
unnatural products with biological activity, in particular in sanglifehrin A molecule. Our
synthetic methodology to prepare this type of intermediates is based on three key steps
(Scheme 1): a) The [4+3] cycloaddition reaction of an oxyallyl cation, generated in situ
from an α,α’-dihaloketone, and a conveniently protected 2-aminofuran diene. b)
Chemical modification of the resulting oxabicyclic cycloadduct and orthogonal
protection of organic functions, and c) Reductive ozonolysis of the C6-C7 double bond
of the modified cycloadduct to afford a pentasusbtituted polyfunctionalized pyranic
hemiaminal with five stereocenters. In the present work we used 2,4-dibromo-3-
pentanone and 2-tert-butoxycarbonylaminofuran as starting materials to afford the C32-
C38 subunit of immunosuppressant Sanglifehrin A with excellent yield.
2

ACCEPTED MANUSCRIPT
NHBoc
H C
O
O
2
O
O
a
3
CH3
H C
CH3
3
H C
CH3
3
O
b
Fe (CO)9
NHBoc
Br Br
2
1
1
'
3
c
OH
OH
O
H C
CH3
NHBoc
H C
CH3
NHBoc
3
3
d
O
OH
OH
4
5
Scheme 1. (a) Generation of oxyallyl cation 1’. (b) [4+3] cycloaddition. (c) Reduction of the carbonyl
group on C3. (d) Reductive ozonolysis.
This methodology is very versatile because: a) Many different haloketones and
heterosubstituted precursors of oxyallyl cations may be used. b) 2-Aminofurans with
different degree of substitution and/or functionalization on C3, C4 or C5 may be used as
dienes. c) Diene and dienophile are easily available from the synthetic point of view. d)
The oxabicyclic cycloadduct could be transformed in very different ways in order to get
molecular diversity. e) The ozonolysis step affords, in the reductive version, a 2-
aminopyran synthon with 2,6-bis(hydroxymethyl) substituents that can be used to
connect it to other subunits in a total synthetic process. Moreover, ozonolysis could be
also performed under oxidative conditions, opening the way to formyl, carboxyl or
epoxide connecting terminal functions.
2
. Results and discussion
2.1. [4+3]-Cycloaddition reaction
The oxabicyclic structures 1-tert-butoxycarbonylamino-8-oxabicyclo[3.2.1]oct-6-en-3-
6
ones 3a-c were prepared by a [4+3]-cycloaddition reaction between a furan derivative,
functionalized at C-2 by a protected amino group, 2, and an oxyallyl cation 1’,
3

ACCEPTED MANUSCRIPT
2 9
generated in situ from 2,4-dibromo-3-pentanone 1, in the presence of Fe (CO) as a
7
reducing agent (Scheme 2). This [4C(4π) + 3C(2π)] cycloaddition reaction has been
8
9
widely studied and optimized by us and other authors and it has been demonstrated to
be an straightforward and scalable method to afford a wide variety of versatile bicyclic
1
0
intermediates with the formation of four chiral stereocenters in only one step.
Scheme 2. Synthesis of 8-oxabicyclo[3.2.1]oct-6-en-3-one by a [4+3] cycloaddition reaction.
Oxyallyl cation 1’ (Scheme 1) was generated in situ during the cycloaddition
reaction by reduction of 2,4-dibromo-3-pentanone with Fe
2 9
(CO) . This haloketone was
easily obtained by bromination of the corresponding 3-pentanone, following well
1
1
described procedures.
The non-commercial 2-(NHBoc)-furan 2, precursor of cycloadducts 3a, 3b and
1
2
3
c, was obtained following the procedure described by Lynch. This procedure
involves a one-pot reaction between the 2-furoyl chloride, sodium azide and an excess
of tert-butyl alcohol. The reaction takes place by a Curtius rearrangement of the
intermediate 2-furoyl azide to obtain, in 82% yield, the desired 2-aminofuran 2.
(Scheme 3).
Scheme 3. Formation of the tert-butyl N-(2-furoyl)-carbamate.
Oxabicyclic ketones 3a-c, were separated by flash column chromatography and
1
3
their stereochemistry was unequivocally established by NMR correlation studies and
confirmed by X-ray diffraction analysis. A certain degree of diastereoselectivity was
4

ACCEPTED MANUSCRIPT
observed in the cycloaddition reaction, with formation of cycloadducts 3a and 3b as
major products and a small amount of stereoisomer 3c. Compound 3d was not detected
even by GC/MS studies of the reaction crude mixture. The low or no formation of 3c
and 3d, respectively, having a 3S* configuration on C2 (with Me-C2 and NHBoc in
close proximity), is due to the high steric hindrance exerted by NHBoc group, which
avoids the approach of the methyl group from the oxyallyl cation in its (E, Z)
7
configuration, along the cycloaddition process. However, the diastereoisomeric ratio
6
may be modified by using more electrophilic oxyallyl cations or taking advantage of
1
4
the coordinating effects of the cationic counterion of the oxyallyl species.
2.2. Stereoselective reduction of the carbonyl group on C3
In the present work, the reduction of the ketone group on C6 was approached by using
different reducing agents and different reaction conditions in order to synthesize the
alcohols with high diastereoselectivity and in high yield. To exemplify this optimization
study, the assays carried out with 3a are quoted in Table 1. In addition, bibliographic
references are included in the table, illustrating precedents of the synthetic
methodologies used in the present work for the different reduction procedures. These
reduction methodologies were evaluated for each substrate and, for preparative
purposes, the most successful or adequate one was selected for each substrate (Scheme
4).
Thus, reduction of 3a was performed with DIBAL-H in anhydrous THF at -78 ºC,
obtaining the alcohol 4a in 90% yield and 5:95 endo /exo diastereoisomeric ratio. The
use of sodium metal in liquid NH may afford also the alcohol 4a, but with the
diastereoselectivity endo / exo, 0:100. On the other hand, 3b was reduced by NaBH in
3
4
anhydrous methanol at 0 ºC obtaining the alcohol 4b in 99 % yield and 99:1 endo/exo
diastereoisomeric ratio (Scheme 4). Finally, ketone 3c was reduced under these last
conditions affording alcohol 4c (exo) in 82% yield and in a stereospecific manner
2
(diastereoselectivity endo/exo: 0:100). In all cases, the diastereoisomeric alcohols were
efficiently separated by flash column chromatography and pure samples were physically
and spectroscopically characterized.
5

ACCEPTED MANUSCRIPT
Table 1. Reduction methodologies evaluated in this work for ketone 3a.
c
a
b
DAS
endo / exo
T
ºC )
t
r
C
Y
Bibliographic
Reference
Entry
Reducing agent
NaBH
Solvent
(
( h )
( % )
100
0
( % )
95
0
(
4a / 4a )
1
2
1
2
3
4
5
6
7
8
9
4
MeOH
MeOH
THF
0
0
2
30 / 70
8l-8o
15
Na(AcO)
3
BH
48
2
-
s
Li( Bu)
3
BH
-78 to 25
-33
100
100
100
78
0
-
16
(L-Selectride®)
0
Na
NH
(l)
0.5
1
99
99
65
80
82
90
0
0 / 100
17
3
LiAlH
THF
THF
-78
67 / 33
8l-8o
18
4
t
Li( BuO) AlH
3
25
7.5
4
49 / 51
(
TTBAL-H)
i
Li( Bu) AlH
2
CH
CH
Cl
Cl
-24 to 25
-78
100
100
100
100
100
0
50 / 50
19
2
2
2
(
DIBAL-H)
i
Li( Bu) AlH
2
2
4
78 / 22
19
(
DIBAL-H)
i
Li( Bu) AlH
2
THF
-78
1.5
1.5
5
5 / 95
20
(
DIBAL-H)
MeO-CH CH O)
Red-Al®)
(
2
2
2 2
AlH
10
11
12
13
14
Toluene
THF
25
-
-
-
-
-
21
(
SmI
2
25
0
22
HSnBu
HSnBu
3
3
/ AIBN
/ AIBN
MeOH
Tolueno
ꢀ
Reflux
5
0
23
ꢀ
Reflux
20
24
100
0
22
HSnBu
3
/ SiO
2
CH
2
Cl
2
25
0
0
24
a
b
c
Conversion of starting material. Yield on isolated product. Diastereoselectivity endo / exo.
6

ACCEPTED MANUSCRIPT
Scheme 4. Reduction of the ketone group in cycloadducts 3a-c.
‒
The results quoted in Scheme 4 confirm the approach of the hydride H ion,
predicted for the reaction as described in Figure 2. When the methyl substituents on C2
and C4 positions are in a cis-exo configuration (3c), the hydride attack took place by the
most accessible face (Si face) of the carbonyl group, giving excellent diastereoisomeric
ratios. When methyl groups adopt a cis-endo configuration (3b), the attack of hydride
takes place by the Re face. However, when methyl groups are in a trans configuration
(
3a), the results indicate that the more hindered face is the Re face flanked by the axial
methyl group on C4 and the NHBoc subunit. This leads the hydride attack preferentially
by the Si face of the ketone and affords a diastereoisomeric ratio slightly lower, 5 : 95.
Thus, we may conclude that the stereoselectivity observed in the reduction of the
carbonyl group is conditioned by the configuration of the substituents at C2 and C4
positions of the cycloadduct at the minimum energy conformation of the oxan-4-one
ring (Figure 2). Thus, methyl groups in axial position for a chair-like conformation of
7

ACCEPTED MANUSCRIPT
the oxan-4-one ring act as umbrellas that block the attack of hydride ions by the Re face
of carbonyl group (Figure 2).
Figure 2. Diastereoselectivity observed in the reduction of carbonyl group in substrates 3a, 3b and 3c, at
their minimum energy conformation.
8

ACCEPTED MANUSCRIPT
2
5
3
Scheme 5. Proposed mechanism for the reduction of ketone 3a with Na /NH .
1
7a,b
The use of sodium metal in liquid ammonia
of the carbonyl group of 3a, in a quantitative yield and total diastereoselectivity,
obtaining only the exo diastereoisomer 4a which is, in addition, the
afforded a rapid and clean reduction
2
,
1
7c-g
thermodynamically more stable (Scheme 5).
It is worthwhile to mention that a change of endo / exo diastereoselectivity was
observed when changing the nature of solvent from CH Cl (non-chelating) to THF
2
2
with capability to co-ordinate the Al atom in the case of using DIBAL-H as reducing
agent. These co-ordination effects may modify the energy of the transition states
1 2
leading to products 4a and 4a .
9

ACCEPTED MANUSCRIPT
In order to determine the relative configuration of the reduction products, a
NMR correlation method was used. This method is based on: a) A complete and
1
13
unequivocal assignment of signals from H and C NMR spectra, by performing DEPT,
selective irradiation and 2D COSY and HSQC experiments. b) The careful analysis of
1
13
H and C NMR data and the use of 2D-NOESY experiments (Figure 3). c) The
correlation of the shielding or deshielding effects exerted by the hydroxyl group on C3”
depending on the configuration of the substituents on C1”, C2” and C4” (see Table 2),
and c) By use of the Karplus equation, on the basis of the coupling constants, to find a
consistency between the dihedral angles between vicinal hydrogen atoms, at the
minimum energy conformation, for a certain relative configuration. Next, we exemplify
this methodology of assignment of relative stereochemistry, applied to epimers 4a
a .
1
and
4
2
Thus, on Table 2 and attached bars chart it is possible to appreciate the
1
differences observed between the H NMR spectral data of diastereoisomers 4a
1
and
4
a2. The most significant differences between the two compounds are observed for
hydrogen atoms H2’’, H6’’ and H7’’, which appear more shielded in compound 4a due
to the particular spatial orientation of the hydroxyl group. The olefinic protons H6’’ and
2
2
6
H7’’are deshielded by electric field effect due the proximity of the axial hydroxyl
group in compound 4a , effect that is not possible for compound 4a (Figure 3). The
presence of H2” at lower field in 4a is consistent with the axial disposition of the OH
group, which deshields vicinal axial trans hydrogens between 0.25 and 0.30 ppm. In
both 4a and 4a H9” appears at higher field than H10” because of two concomitant
effects: axial H10” is deshielded by an electric field effect exerted by the oxygen bridge
1,3-dipolar interaction) and, simultaneously, H9” is shielded by the anisotropy cone of
the C6”-C7” double bond.
1
2
1
2
7
1
2
(
1
0

ACCEPTED MANUSCRIPT
1
Table 2. Correlation of H NMR spectral data of epimers 4a
1
and 4a
2
.
δ (ppm)
H2" H3" H4" H5" H6" H7" H9" H10"
Product
4
4
a
a
1
2
2.34
1.75
0.59
3.55 1.85
3.50 1.87
4.65 6.40
4.67 6.15
6.50 1.04
6.22 1.03
0.28 0.01
1.21
1.14
0.07
∆
δ (4a ‒4a )
0.05 -0.02 -0.02 0.25
1
2
0
0
0
0
0
0
0
.7
.6
.5
.4
.3
.2
.1
0
0
.59
0
.28
0
.25
0.07
0
.05
0.01
H2"
H3"
H4"
0.02
H5"
-0.02
H6"
H7"
H9"
H10"
-
-
0.1
Proton
1
1

ACCEPTED MANUSCRIPT
shielding effect
10''
CH3
NOE
0C'' H3
NOE
1
H
H
deshielding effect
O
4''
O
4''
2
''
H
2''
H
OH
BocHN
3''
BocHN
3''
H
shielding effect
6
''
6
''
9''
CH3
shielding effect
9''
CH3
H
H
H
NOE
OH
7
''
7''
NOE
H
H
deshielding effect
4a1
4a
2
Hydroxyl group in equatorial position
Hydroxyl group in axial position
Figure 3. Shielding, deshielding and NOE effects depending on the configuration of the hydroxyl group
for reduction alcohols 4a and 4a
1
2
.
The NOE effects observed in alcohol 4a , from NOESY experiments, are due to
2
the proximity in space of H3” and H
3
C9” and also of H3” and H4”. On the other hand,
clear NOE effects were also observed in alcohol 4a
1
between H3” and H C10” and also
3
between H3” and H2”. Both NOE effects are consistent with the assigned configuration
to C3”.
1
3
If we compare the chemical shifts of C NMR spectra of alcohol 4a
is possible to appreciate important differences (see Table 3 and bars chart attached).
Looking at Figure 4 it is possible to observe that in 4a the olefinic carbons C6” and
C7” are shifted to a lower field by the axial hydroxyl group, due to a δ-effect,
interaction that is not possible in 4a . In addition, the OH group exerts a γ-gauche effect
on both methyl groups in 4a , but only on C9” in 4a , producing an upfield shift due to
1 2
and 4a , it
1
2
2
1
sterically induced polarization of C-H bonds. Contrary to α and β effects, which are
transmitted through the bonds, γ-gauche interaction works through the space. Previous
2
8
studies on this type of effects showed that non-bonding interactions between CH-CH
or CH-OC perturbed the charge density on the atoms. In this sense, that conformation in
which the distance between groups is lower or of the same order than the addition of the
Van der Waals radius, presents an induced polarization effect due to steric hindrance
and it results in a deshielding of protons and a simultaneous shielding of carbon atoms.
On the other hand, the higher difference ꢀδ for C10” is due to the concomitant γ-trans
effect exerted by the axial OH group on C10” in 4a1, due to hyperconjugative
13,29
effects.
1
2

ACCEPTED MANUSCRIPT
1
3
1 2
Table 3. Correlation of C NMR spectral data of 4a and 4a .
OH
OH
1
0''
9''
1
0''
9''
2
'
2
'
4''
6''
4
''
''
2''
2''
O
O
3''
2'
2'
2
'
3''
1''
1''
O
O
5''
5
''
O 1'
1
O 1'
8''
N
H
1
2'
8''
N
H
6
7''
7''
a2
4
4a1
δ(ppm)
C4" C5"
Product
C1" C2"
C3"
75.5
73.5
2.0
C6"
C7"
C9" C10"
4
4
a
1
9
9
4.9 39.9
4.9 41.2
39.6 82.9 135.1 135.0 12.4
34.6 82.9 131.3 131.0 13.5
17.6
11.8
5.8
a2
‒4a
∆
δ (4a
1
2
)
0
-1.3
5.0
0
3.8
4.0
-0.9
7
6
5
4
3
2
1
0
.0
.0
.0
.0
.0
.0
.0
.0
5
.8
5
.0
4
.0
3.8
2
.0
0
.0
0.0
C1"
C2"
1.3
C3"
C4"
C5"
C6"
C7"
C9"
0.9
C10"
-
1.0
-
-
-2.0
Carbon
1
3

ACCEPTED MANUSCRIPT
1 2
Figure 4. δ, γ-gauche and γ-trans effects in 4a and 4a .
3
0
2.3. Reductive ozonolysis
The other key step in the approach to the synthesis of the final 2-aminopyran synthons
3
1
was the reductive ozonolysis of the reduction alcohols 4 (Scheme 6). The same
3
2
reaction conditions and solvent type were used in all cases. The successive addition of
SMe and NaBH as reducing agents, in a solvent system DCM / Methanol (4:1), gave
2
4
very good results, affording always the desired final products with good to excellent
yields. (Table 4). The use of two consecutive reducing agents lead the reaction to the
complete reduction of the ozonide intermediate and the obtention of two hydroxymethyl
side chains on the original positions C6” and C7” (Scheme 6).
1
4

ACCEPTED MANUSCRIPT
3
3
Scheme 6. Mechanism of ozonolysis of 4 to obtain 2-aminopyrans 5.
It is worth nothing that very high yields were obtained with the use of SMe
and also a great functional group compatibility was observed. Thus, both
functional groups present on the substrates (OH and NHBoc) resulted unchanged.
2
and
NaBH
4
1
5

ACCEPTED MANUSCRIPT
Table 4. Results from the reductive ozonolysis reaction of substrates 4 for the obtention of THP final
products, 5.
a, b, c
Yield
Entry
Substrate
Products
(
%)
1
88
4
4
a
a
1
5a
5a
1
2
3
4
5
75
70
73
94
2
2
4
4
b
1
2
5b
1
b
5b
2
4
c
2
5c
2
a
Reaction conditions used: 1) O
3
2 4
, ‒78 ºC, 0.5 h. 2) SMe , (2.3 eq.), ‒78 ºC, 1 h. 3) NaBH , (2.3 eq.), r.t.,
b
c
3
h. Solvent system: CH
2
Cl
2
/ MeOH (4:1). Yield on isolated product. In all cases, the conversion of
the starting material was complete.
3
. Conclusions
By the herein describe synthetic methodology it is possible to obtain 2-aminopyrans
with high stereochemical diversity and in good yield. Selecting the type of substitution
pattern and electrophilicity of the oxyallyl cation, in the [4+3]-cycloaddition reaction,
1
6

ACCEPTED MANUSCRIPT
we may design the type of substituents in the final 2-aminopyran and also their relative
stereochemistry. The configuration of the alcohol on C4 of pyrans may be also selected
by using the adequate reduction methodology and reaction conditions. All stereoisomers
of precursors, intermediates and final products are easily separable by chromatography,
and pure samples of them were obtained and physically and spectroscopically
characterized. Finally the objective of synthesizing the C32-C38 subunit of
immunosuppressant sanglifehrin A has been fulfilled by the obtention of 2-aminopyran
2
5c . Derivatives having formyl, hydroperoxy-hemiacetals or epoxides at C2 and C6 of
pyran, instead of hydroxymethyl groups, may be obtained by oxidative epoxidation;
results from these studies will be published at due course. These derivatives may
facilitate the connection of the 2-aminopyran synthons to other molecular subunits for
the total synthesis of complex natural products.
4
. Author information
Corresponding author
Phone: +34-93.402.16.81. Fax: +34-93.339.78.78. E-mail: angel.montana@ub.edu
Notes
The authors declare neither competing financial interest nor any other conflict of
interest.
5
. Acknowledgements
We thank the Spanish Ministry of Economics and Competitiveness (MINECO) for
financial support (Grant: CTQ2015-65040-P). In addition, financial support from the
University of Barcelona (Grant: UB-VRR-2012/AR000126) and from the Generalitat de
Catalunya (Grant: AGAUR 2014-SGR-1658), is also gratefully acknowledged.
6
. Experimental section
6.1. General procedures
NMR spectra were recorded on a Varian Inova 200 or 300 MHz, on a Varian Mercury
00 MHz and/or in a Bruker DMX 500 MHz apparatus. Chemical shifts (δ) are
4
1
7

ACCEPTED MANUSCRIPT
expressed in ppm versus tetramethylsilane as an internal standard. IR spectra were
recorded on a NICOLET 6700 FT-IR by film, KBr pellet or ATR (Attenuated Total
Reflectance) methods. Mass spectrometry was performed on a Hewlett-Packard 5890
3 4
apparatus, generally under a CI (Chemical Ionization) method by using NH or CH or
by direct insertion under Electron Impact at 70 eV and 150 ºC. The elemental analyses
were obtained in a FISONS Elemental Analyzer, Model Na-1500. The samples were
previously pyrolized al 1000 ºC in the presence of a catalyst, under oxygen atmosphere,
and the content of carbon, hydrogen and nitrogen were determined by evaluation of the
combustion gases by gas chromatography using a FID detector. Solvents were dried,
according to standard procedures, and distilled prior to use. All other major chemicals
were obtained from commercial sources and used without further purification. Gas
chromatography was performed by using a Shimatzu AOC-20i apparatus with a
capillary column (HP-5 Crosslinked 5% Phe-Me-Siloxane, 0.25 ꢁm film thickness, 30
m length and 0.32 mm diameter). Used carrier gas brands and pressures were: He = 5.5
bar (Linde, Helium 5.0), Air = 3 bar (Linde, synthetic air), H = 3 bar (Linde, Hydrogen
2
5
.0). The experimental conditions are specified in each case. Ozonolysis reaction was
carried out using an ozone-generator Fischer Ozone-500 apparatus, under the following
conditions: Intensity = 0.25-0.40 A, PO2 = 0.25 bar, O flow = 50-100 mL/min.
3
6.2. Molecular computer calculations
Geometry and energy minimization of molecules were preoptimized by molecular
3
4
mechanics MM2 followed by semiempirical quantum mechanical PM7 algorithm,
3
5
implemented in the MOPAC-2012 software. This software was also used to calculate
the formation enthalpy. Density functional theory (DFT) based methods at the B3LYP
3
6,37
functional level
were used for subsequent full refinements, within the Gaussian-
3
8
0
3W (Revision E.01, version 6.1) software package and using the 6–31++G(d, p)
3
9
basis set. All calculations were performed on isolated molecules (gas phase).
6.4. Synthetic procedures
6
.4.1. Synthesis of 2,4-dibromo-3-pentanone, (1)
1
8

ACCEPTED MANUSCRIPT
In an oven-dried 250 mL three neck round-bottom flask, equipped with a stirring bar, a
Dimroth condenser, a calcium chloride trap, a thermometer and a pressure equalized
addition funnel, 3-pentanone (54 mL, 0.52 mmol) and phosphorous tribromide (1 mL,
0
.01 mmol) were added. The mixture was cooled to 0 ºC and bromine (54 mL, 0.53
mmol) was slowly added along 5 hours, maintaining the reaction mixture between 5 ºC
and 10 ºC. During the addition of bromine, the release of hydrogen bromide was
observed. One hour after the complete addition of bromine, the conversion of 3-
pentanone was complete as observed by GC. Then, the mixture was submitted to
vacuum (20 torr) for 30 min at rt to eliminate the dissolved hydrogen bromide. The
reaction crude was purified by fractional distillation under vacuum (10 mmHg),
collecting the fraction between 65 ºC and 70 ºC. This fraction was redistilled to obtain
chemically pure 2,4-dibromopentan-3-one (71.96 g, 79% yield) as a colourless oily
diastereoisomeric mixture: meso / dl pair 4/1.
-1
3
IR (film) ῡ (cm ): 3445 (C=O), 2980, 2920, 2875 (H─Csp ), 1730 (C=O), 1440, 1380,
1
1
350 (C─C), 1200, 1120, 1070, 1050, 1020 (C─O). H NMR (200 MHz, CDCl
3
) δ
(ppm): pair dl 1.88 (2H, d,
J
= 6.9 Hz, H2, H4), 4.77 (6H, q,
J
= 6.9 Hz, H1, H5); meso:
13
1
.81 (2H, d,
J
= 6.7 Hz, H2, H4), 4.99 (6H, q, J = 6.7 Hz, H1, H5). C NMR (50 MHz,
CDCl
3
) δ (ppm): pair dl 21.61 (C1, C5), 43.95 (C2, C4), 195.77 (C3); meso: 19.42
(
C1, C5), 43.75 (C2, C4), 195.77 (C3). MS (GC-MS, CI, NH , 70 eV, 150 ºC, t = 6.16-
3
R
7
.03 min) m/z (%): 279 (12, M+N
2
H
7
), 262 (100, M+NH
4
), 246 (1, M+2H), 244 (1, M),
= 200 ºC, t = 30
1
82 (6, C Br). GC (T = 100 ºC, t
5
H
10
O
2
i
i
= 2 min., r = 10 ºC/min., T
f
f
min.) meso: t = 5.7 min.; pair dl: t = 6.4 min.
R
R
6
.4.2. Synthesis of tert-butyl furan-2-yl-carbamate, (2)
1
9

ACCEPTED MANUSCRIPT
In a round-bottom flask equipped with a stirring bar and a Dimroth condenser, 2-furyl
chloride (10 g, 0.07 mol), tert-butyl alcohol (80 mL, 0.85 mol) and sodium azide (5.1g,
0
.08 mol) were placed, under argon atmosphere. The mixture was stirred for 20 h at
room temperature and the formation of the intermediate 2-furylazide as a white solid
was observed. The mixture was heated to reflux (100 ºC) and maintained under these
conditions for additional 15 h. Once 2-furylazide was completely converted (control by
TLC eluted with hexane/ethyl acetate 4:6), the solvent was removed in vacuo, obtaining
a white solid, which was submitted to a flash column chromatography on silica gel,
eluting with a mixture of hexane and ethyl acetate of increasing polarity. The elution
with a mixture of hexane and EtOAc (9:1), afforded the desired product as a white solid
(10.8 g, 82% yield). The product has to be stored in a cold place and away from light, in
order to avoid its photochemical decomposition.
MP = 98-99 ºC (ethyl acetate). TLC: Rf = 0.66 (SiO , hexane/ethyl acetate 4:6,
2
-1
developed by ninhydrin). IR (film) ῡ (cm ): 3267 (N─H, st), 2980, 1700 (C=O, st),
t
1
1
546 (N─H, def), 1250 ( Bu). H NMR (300 MHz, CDCl
.04 (1H, brs, H4"), 6.63 (1H, brs, H3"), 7.00-7.12 (1H, m, H5"). C NMR (50 MHz,
(ppm): 28.2 (C2'), 81.3 (C1'), 95.1 (C4"), 111.2 (C3'), 136.0 (C5'), 145.4
3
) δ (ppm): 1.50 (9H, s, H2'),
1
3
6
CDCl
3
) δ
3 4
(C2'), 151.9 (C1). MS (CI, NH , 70 eV, 150 ºC) m/z (%): 285 (13, M+NH ), 268 (100,
t
t
M+1H), 212 (22, M+2H‒ Bu), 167 (36, M+1H‒COO Bu).
6
.4.3. Synthesis of tert-butyl
N-{2,4-dimethyl-3-oxo-8-oxabicyclo[3.2.1]oct-6-en-1-
yl}carbamate, (3)
2
0

ACCEPTED MANUSCRIPT
[
2 9
4+3] Cycloaddition procedure using Fe CO
In a round bottomed flask fitted with a magnetic stirrer, under argon atmosphere,
compound 2 was placed. Afterwards, inside an Atmosbag® filled with argon, the
pyrophoric diironnonacarbonyl was added to the reactor, (in a molar ratio Fe
furan = 1.75 : 1), as a bright yellow solid. Then, anhydrous acetonitrile (in a ratio of
.82 mL of ACN : 1 mol Fe CO ) was added and the mixture was stirred for 5 min.
2 9
CO :
0
2
9
Dibromoketone 1, freshly filtered through neutral alumina, was added dropwise at ‒10
ºC, (in a molar ratio of 1.2 : 1; dibromoketone : furan). The reaction mixture was stirred
at room temperature for 6.5 h. The crude was concentrated to dryness and the residue
was dissolved in acetone. Cerium ammonium nitrate (in a molar ratio CAN : Fe
2 9
CO =
1
: 1) was added and the reaction mixture was stirred for 5 min. Afterwards, the solvent
was evaporated under vacuum and the residue was filtered through a short path of silica
gel, and then it was submitted to a flash column chromatography on silica gel, using
mixtures of hexane and diethyl ether of increasing polarity to isolate cycloadduct 3 in a
7
6% yield and diastereoselectivity: 3a/3b/3c = 55/40/5.
tert-Butyl N-{(1S*, 2R*, 4R*, 5R*)-2,4-dimethyl-3-oxo-8-oxabicyclo[3.2.1]oct-6-en-1-
yl}carbamate, (3a)
Colourless oil. TLC:
KMnO ). IR (film) ῡ (cm ): 3341 (N‒H, st), 2977, 2936, 1709 (C=O, st), 1503 (N‒H,
def), 1460, 1369, 1331, 1246 ( Bu), 1167 (C‒O‒C, st), 1055 (C‒O‒C, st as). H NMR
400 MHz, CDCl (ppm): 1.08 (3H, d, 9,2 = 7 Hz, H9"), 1.35 (3H, d,
H10"), 1.46 (9H, s, H2'), 2.31 (1H, q, 4,10 = 7.6 Hz, H4"), 3.01 (1H, q, 2,9 = 7 Hz, H2"),
.74 (1H, s, H5"), 5.19 (1H, s, NH), 6.28 (2H, s, H6" and H7"). C NMR (100 MHz,
CDCl ) δ (ppm): 9.8 (C9"), 16.4 (C10"), 28.4 (C2'), 48.2 (C4"), 52.9 (C2"), 80.9 (C1'),
f 2
R = 0.60 (SiO , eluted with hexane /EtOAc 3:7, developed with
-1
4
t
1
(
3
)
δ
J
J10,4 = 7.6 Hz,
J
J
1
3
4
3
8
1.3 (C5"), 95.3 (C1"), 132.8 (C6"), 134.1 (C7"), 153.9 (C1), 210.7 (C3"). MS (CI,
t
NH
3
, 70 eV, 150ºC) m/z (%): 285 (13, M+NH
4
), 268 (100, M+H), 212 (22, M+2‒ Bu),
t
1
67 (36, M+H‒COO Bu). GC (T = 100 ºC, t = 1 min, r = 10 ºC/min, T = 250 ºC, t = 20
i
i
f
f
-1
min): t
R
= 9.57 min. Anal. Calcd for C14
H
4
21NO (267.32 g·mol ): C, 62.90; H, 7.92; N,
5
.24. Found: C, 62.95; H, 7.96; N, 5.22.
tert-Butyl N-{(1S*, 2R*, 4S*, 5R*)-2,4-dimethyl-3-oxo-8-oxabicyclo[3.2.1]oct-6-en-1-
yl}carbamate, (3b)
2
1

ACCEPTED MANUSCRIPT
Colourless oil. TLC:
KMnO ). IR (film) ῡ (cm ): 3347 (N‒H, st), 2979, 2936, 1715 (C=O, st), 1522 (N‒H,
def), 1456, 1368, 1348, 1250 ( Bu), 1157 (C‒O‒C, st), 1038 (C‒O‒C, st as). H NMR
400 MHz, CDCl (ppm): 0.97 (3H, d, 10,4 = 7 Hz, H10"), 1.08 (3H, d,
H9"), 1.46 (9H, s, H2'), 2.79 (1H, dq, 4,5 = 4.6 Hz, H4"), 3.02 (1H, q,
Hz, H2"), 4.91 (1H, dd, 5,6 = 1.2 Hz, H5"), 5.27(1H, s, NH), 6.29 (2H, s,
H6" and H7"). C NMR (100 MHz, CDCl ) δ (ppm): 10.0 (C9"), 10.6 (C10"), 28.5
f 2
R = 0.60 (SiO , eluted with hexane /EtOAc 3:7, developed with
-1
4
t
1
(
3
)
δ
J
J9,2 = 7 Hz,
J4,10 = 7 Hz,
J
2,9
J =
7
J5,4 = 4.6 Hz, J
13
3
(C2'), 48.9 (C4"), 54.2 (C2"), 80.9 (C5"), 80.9 (C1'), 95.7 (C1"), 132.8 (C6"), 134.1
(C7"), 154.0 (C1), 208.0 (C3"). MS (CI, NH
3
, 70 eV, 150ºC) m/z (%): 285 (13,
t
t
M+NH ), 268 (100, M+H), 212 (22, M+2‒ Bu), 167 (36, M+H‒COO Bu). GC (T =100
4
i
ºC, t
i
= 1 min, r = 10 ºC/min, T
f
= 250 ºC, t
f
= 20 min): t
R
= 9.87 min. Anal. Calcd for
-1
14 4
C H21NO (267.32 g·mol ): C, 62.90; H, 7.92; N, 5.24. Found: C, 62.88; H, 7.90; N,
5
.25.
tert-Butyl N-{(1S*, 2S*, 4R*, 5R*)-2,4-dimethyl-3-oxo-8-oxabicyclo[3.2.1]oct-6-en-1-
yl}carbamate, (3c)
Colourless oil. TLC:
KMnO ). IR (film) ῡ (cm ): 3341 (N‒H, st), 2977, 2936, 1709 (C=O, st), 1503 (N‒H,
def), 1460, 1369, 1331, 1246 ( Bu), 1167 (C‒O‒C, st), 1055 (C‒O‒C, st as). H NMR
400 MHz, CDCl (ppm): 1.29 (3H, d, 9,2 = 7.6 Hz, H9"), 1.33 (3H, d,
H10"), 1.46 (9H, s, H2'), 2.28 (1H, q, 4,10 = 7.6 Hz, H4"), 2.65 (1H, q,
H2"), 4.73 (1H, d, 5,6 = 2 Hz, H5"), 5.17 (1H, s, NH), 6.21 (1H, ddd, 6,7 = 6.1 Hz,
Hz, 6,4 = 0.5 Hz, H6"), 6.38 (1H, d, 7,6 = 6.1 Hz, H7"). C NMR (100 MHz, CDCl
δ (ppm): 13.8 (C9"), 17.8 (C10"), 28.5 (C2'), 48.6 (C4"), 53.0 (C2"), 80.9 (C1'), 81.1
f 2
R = 0.60 (SiO , eluted with hexane /EtOAc 3:7, developed with
-1
4
t
1
(
3
)
δ
J
J
10,4 = 7.6 Hz,
J
J2,9 = 7.6 Hz,
J
J
6,5
J =
13
2
J
J
3
)
3
(C5"), 95.3 (C1"), 132.2 (C6"), 135.4 (C7"), 153.9 (C1), 213.0 (C3"). MS (CI, NH , 70
t
eV, 150ºC) m/z (%): 285 (13, M+NH ), 268 (100, M+H), 212 (22, M+2‒ Bu), 167 (36,
4
t
M+H‒COO Bu). GC (T
i
= 100 ºC, t
i
= 1 min, r = 10 ºC/min, T
f
= 250 ºC, t
f
= 20 min):
-1
t
R
= 9.20 min. Anal. Calcd for C14
H21NO (267.32 g·mol ): C, 62.90; H, 7.92; N, 5.24.
4
Found: C, 62.93; H, 7.89; N. 5,27.
2
2

ACCEPTED MANUSCRIPT
6
.4.4. General methods for the reduction of the carbonyl group in (3). Synthesis of
epimeric alcohols (4)
4
a) Use of NaBH (applied to 3b and 3c)
In a 50 mL oven-dried round-bottomed flask, equipped with a stirring bar and fitted
with septa, NaBH (567.4 mg, 15 mmol) and anhydrous MeOH (8 mL) were placed,
4
under argon atmosphere. The system was cooled by an ice-water bath and the
cycloadduct 3 (1 g, 3.74 mmol), dissolved in anhydrous MeOH (8 mL), was slowly
added to the reaction flask via syringe. Once the reaction was complete after 6 h
(
control by TLC: SiO
2
, chloroform/methanol 95:5, two elutions, developed with
was quenched with distilled water (2 mL) and the
ninhydrin), the excess of NaBH
4
resulting mixture was stirred at room temperature for 15 min at 0 ºC. Then, the reaction
mixture was concentrated to dryness in vacuo. The resulting crude product was
3
dissolved in anhydrous CHCl (20 mL) and the organic solution was filtered out via
cannula, washing the solid residue (3 x 5 mL of CHCl ). The organic solution was
3
concentrated to dryness and the resulting crude oil was submitted to a flash column
chromatography on silica gel, eluting with mixtures of hexane and ethyl acetate of
increasing polarity. The elution with hexane / EtOAc (7:3), allowed the separation of
both diastereoisomeric alcohols 4, as white solids, in excellent yields (99% for 3b and
8
1 2 1 2
2% for 3c). Diastereoselectivity: 4b (endo)/4b (exo) = 99/1 and 4c (endo)/4c (exo) =
0
/100.
b) Use of DIBAL-H (applied to 3a
)
To a solution of ketone 3a (300 mg, 1.12 mmol) in anhydrous THF (3 mL), a solution 1
M in hexane of DIBAL-H (3.36 mmol) was added at ‒78 ºC and stirred along 75 min
under argon atmosphere (control by TLC). At the end of the reaction a saturated
4
aqueous solution of NH Cl (12 mL) was added , at ‒78 ºC, to quench the excess of
reducing agent and the mixture was stirred for 10 min. The organic solvent (THF) was
evaporated in vacuo at room temperature, water was added (10 mL) and the system
2
3

ACCEPTED MANUSCRIPT
extracted with diethyl ether (4 x 10 mL). The organic layers were combined together,
4
dried over anhydrous MgSO , filtered and concentrated to dryness. The resulting
residue was submitted to a flash column chromatography on silica gel, eluting with
mixtures of chloroform and ethyl acetate of increasing polarity, isolating pure
diastereoisomeric compounds 4a
2 1
(0.258 g) and 4a (0.014 g). Yield = 90%.
Diastereoselectivity: 4a1 (endo) /4a2 (exo) = 5/95.
3
c) Use of Na / NH (applied to 3a)
Dry ammonia was liquefied (Pe = ‒33 ºC) by using a gas condenser (cooled by dry ice
and acetone) fitted to an oven-dried heart-shaped three-necked flask, equipped with
magnetic stirring and a cooling bath of dry ice /acetone. 40 mL of liquid ammonia were
generated and the bicyclic ketone 3a (0.135 g, 0.505 mmol) was added dissolved in
diethyl ether (10 mL) by a syringe. Afterwards, Na (46 mg, 2.010 mmol) was added in
small pieces, observing the formation of a blue colour that disappears in a few seconds.
The reaction mixture was stirred under these conditions for 30 min (control by TLC:
2
SiO , chloroform / methanol 95:5, two elutions, developed with ninhydrin). The cooling
bath was removed and the ammonia was evaporated by an argon stream, obtaining a
yellowish solid, which was lixiviated with diethyl ether (5 x 5 mL). The organic extracts
were combined together and concentrated to dryness, obtaining a colourless oil that was
submitted to a flash column chromatography on silica gel, eluting with mixtures of
chloroform and ethyl acetate of increasing polarity, isolating pure diastereoisomer
compounds 4a
/100. Control by GC of the reaction crude prior to the column chromatography showed
no presence of diastereoisomer 4a
2 1 2
(134.6 mg). Yield = 99%. Diastereoselectivity: 4a (endo) /4a (exo) =
0
1
.
tert-Butyl N-{(1S*,2S*,3S*,4S*,5R*)-3-hydroxy-2,4-dimethyl-8-oxabicyclo[3.2.1]oct-
-en-1-yl}carbamate, (4a
6
1
)
2
4

ACCEPTED MANUSCRIPT
Colourless oil. TLC:
f 2
R = 0.47 (SiO , chloroform/methanol 95:5, two elutions,
-1
developed by ninhydrin). IR (film) ῡ (cm ): 3330 (O‒H, st), 2973, 2932, 1717 (C=O,
t
1
st), 1526 (N‒H, def), 1366, 1250 ( Bu), 1161 (C‒O‒C, st), 995. H NMR (400 MHz,
CDCl (ppm):1.04 (3H, d, 9,2 = 7.4 Hz, H9"), 1.21 (3H, d, 10,4 = 7.6 Hz, H10"), 1.44
9H, s, H2'), 1.85 (1H, q, 4,10 = 7.6 Hz, H4"), 2.03 (1H, d, = 10.6 Hz, OH), 2.34 (1H,
dq, 2,3 = 5.6 Hz, H2"), 3.55 (1H, brs, H3"), 4.65 (1H, d,
H5"), 5.10 (1H, s, NH), 6.40 (1H, dd, J6,5 = 1.6 Hz, J6,7 = 5.6 Hz, H6"), 6.50 (1H, d, J7,6
3
)
δ
J
J
(
J
J
J
2,9 = 7.4 Hz,
J
J5,6 = 1.6 Hz,
1
3
=
3
5.6 Hz, H7"). C NMR (100 MHz, CDCl ) δ (ppm): 12.4 (C10"), 17.6 (C9"), 28.5
(C2'), 39.6 (C4"), 39.9 (C2"), 75.7 (C3"), 82.9 (C5"), 94.9 (C1"), 135.0 (C7"), 135.1
(
C6"), 154.2 (C1). MS (CI, NH , 70 eV, 150ºC) m/z (%): 270 (43, M+H), 214 (89,
3
t
t
t
M+2‒ Bu), 196 (96, M‒O Bu ), 170 (100, M+2‒COO Bu), 152 (83, M‒NH
2
Boc). Anal.
-1
Calcd for C14
.59; N, 5.23.
H23NO (269.34 g·mol ): C, 62.43; H, 8.61; N, 5.20. Found: C, 62.40; H,
4
8
tert-Butyl N-{(1S*,2S*,3R*,4S*,5R*)-3-hydroxy-2,4-dimethyl-8-oxabicyclo[3.2.1]oct-
-en-1-yl}carbamate, (4a
6
2
)
Colourless oil. TLC: Rf = 0.32 (SiO , chloroform/methanol 95:5, two elutions,
2
-1
developed by ninhydrin). IR (film) ῡ (cm ): 3342 (N‒H, st), 3336 (O‒H, st), 2975,
t
2
932, 1717 (C=O, st), 1526 (N‒H, def), 1324, 1248 ( Bu), 1161 (C‒O‒C, st), 1094,
1
1
053 (C‒O‒C, st as), 1014, 982. H NMR (400 MHz, CDCl
3
)
δ
(ppm): 1.03 (3H, d,
J
9,2
=
6.7 Hz, H9"), 1.14 (3H, d, J10,4 = 6.9 Hz, H10"), 1.44 (9H, s, H2'), 1.48 (1H, d,
Hz, OH), 1.75 (1H, dq, 2,3 = 8.0 Hz, H2"), 1.87 (1H, ddq,
4,10 = 6.9 Hz, H4"), 3.50 (1H, dd, 3,4 = 6.9 Hz, 3,2 = 8.0 Hz, H3"), 4.67
1H, brs, H5"), 5.08 (1H, s, NH), 6.15 (1H, dd, J6,5 = 1.7 Hz, J6,7 = 5.9 Hz, H6"), 6.22
J
= 7.0
J
2,9 = 6.7 Hz,
J
J4,5 = 1.5 Hz,
J
4,3 = 6.9 Hz,
J
J
J
(
13
(
3
1H, d, J7,6 = 5.9 Hz, H7"). C NMR (100 MHz, CDCl ) δ (ppm): 11.8 (C10"), 13.5
(C9"), 28.5 (C2'), 34.6 (C4"), 41.2 (C2"), 73.5 (C3"), 82.9 (C5"), 94.9 (C1"), 131.0
(
C7"), 131.3 (C6"), 153.9 (C1). MS (CI, NH , 70 eV, 150ºC) m/z (%): 270 (28, M+H),
3
t
t
2
2
14 (51, M+2‒ Bu), 170 (100, M+2‒COO Bu), 152 (63, M‒NH Boc). Anal. Calcd for
2
5

ACCEPTED MANUSCRIPT
-1
14 4
C H23NO (269.34 g·mol ): C, 62.43; H, 8.61; N, 5.20. Found: C, 62.46; H, 8.63; N,
5
.19.
tert-Butyl N-{(1S*,2S*,3S*,4R*,5R*)-3-hydroxy-2,4-dimethyl-8-oxabicyclo[3.2.1]oct-
-en-1-yl}carbamate, (4b
6
1
)
White solid. MP = 137-138 ºC (CCl ). TLC: Rf = 0.47 (SiO , chloroform/methanol
4
2
-
1
9
2
5:5, two elutions, developed by ninhydrin). IR (film) ῡ (cm ): 3334 (O‒H, st), 2973,
t
930, 1723 (C=O, st), 1624, 1520 (N‒H, def), 1456, 1364, 1250 ( Bu), 1165 (C‒O‒C,
1
st), 1084, 1016. H NMR (400 MHz, CDCl
3
)
δ
(ppm): 0.98 (3H, d,
J10,4 = 7.6 Hz,
H10"), 1.08 (3H, d, J9,2 = 7.6 Hz, H9"), 1.44 (9H, s, H2'), 1.62 (1H, brs, OH), 2.25-2.35
2H, m, H2" y H4"), 3.73-3.79 (1H, m, H3"), 4.62 (1H, ddd,
(
J
5,6 = 1.6 Hz,
6,5 = 1.6 Hz, 6,7 = 6.2 Hz, H6"),
7,6 = 6.2 Hz, H7"). C NMR (100 MHz, CDCl ) δ (ppm): 12.8 (C10"),
2.9 (C9"), 28.5 (C2'), 38.7 (C2"), 41.9 (C4"), 73.1 (C3"), 82.8 (C5"), 94.7 (C1"), 134.7
J5,3 = 1.6 Hz,
J
6
1
5,4 = 3.2 Hz, H5"), 5.18 (1H, s, NH), 6.47 (1H, dd,
J
J
13
.50 (1H, d,
J
3
(C6"), 136.5 (C7"), 154.2 (C1). MS (CI, NH
3
, 70 eV, 150ºC) m/z (%): 214 (40, M+2‒
t
t
t
Bu), 196 (53, M‒O Bu), 170 (100, M+2‒COO Bu), 152 (63, M‒NH
2
Boc), 110 (93).
-1
Anal. Calcd for C14
4
H23NO (269.34 g·mol ): C, 62.43; H, 8.61; N, 5.20. Found: C,
6
2.45; H, 8.62; N, 5.23.
tert-Butyl N-{(1S*,2S*,3S*,4R*,5R*)-3-hydroxy-2,4-dimethyl-8-oxabicyclo[3.2.1]oct-
-en-1-yl}carbamate, (4b
6
2
)
4 f 2
White solid. MP = 58-59 ºC (CCl ). TLC: R = 0.27 (SiO , chloroform/methanol 95:5,
-1
two elutions, developed by ninhydrin). IR (film) ῡ (cm ): 3334 (O‒H, st), 2973, 2930,
t
1
1
713 (C=O, st), 1526 (N‒H, def), 1456, 1393, 1366, 1250 ( Bu), 1163 (C‒O‒C, st),
1
024, 993, 955. H NMR (400 MHz, CDCl
3
)
δ
(ppm): 0.95 (3H, d,
J10,4 = 7.0 Hz,
2
6

ACCEPTED MANUSCRIPT
H10"), 1.04 (3H, d,
and OH), 2.85 (1H, dd,
.6, H5"), 5.14 (1H, s, NH), 6.19 (1H, dd,
J
9,2 = 7.0 Hz, H9"), 1.44 (9H, s, H2'), 1.71-1.79 (3H, m, H2",H4"
J3,2 = 8.6 Hz,
J
3,4 = 8.6 Hz, H3"), 4.63 (1H, d,
6,5 = 1.8 Hz, 6,7 = 6.4 Hz, H6"), 6.24 (1H, d,
7,6 = 6.4 Hz, H7"). C NMR (100 MHz, CDCl ) δ (ppm): 13.8 (C9"), 14.3 (C10"),
8.5 (C2'), 40.8 (C4"), 44.4 (C2"), 78.5 (C3"), 81.9 (C5"), 95.3 (C1"), 130.0 (C7"),
5,4
J5,6 = 1.8 Hz, J =
3
J
J
1
3
J
3
2
1
1
C
t
32.0 (C6"), 154.0 (C1). MS (CI, NH , 70 eV, 150ºC) m/z (%): 214 (34, M+2‒ Bu),
3
t
t
96 (90, M‒O Bu), 170 (100, M+2‒COO Bu), 152 (55, M‒NH
2
Boc). Anal. Calcd for
-1
14 4
H23NO (269.34 g·mol ): C, 62.43; H, 8.61; N, 5.20. Found: C, 62.39; H, 8.64; N,
5
.23.
tert-Butyl N-{(1S*,2R*,3R*,4S*,5R*)-3-hydroxy-2,4-dimethyl-8-oxabicyclo[3.2.1]oct-
-en-1-yl}carbamate, (4c
6
2
)
White solid. MP = 109-110 ºC (CCl
4
). TLC:
f 2
R = 0.50 (SiO , hexane/diethyl ether
-1
(
3:7), three elutions, developed by ninhydrin). IR (film) ῡ (cm ): 3332 (O‒H, st), 2977,
t
2
1
1
1
934, 1699 (C=O, st), 1651, 1559 (N‒H, def), 1505, 1458, 1368, 1323, 1246 ( Bu),
1
167 (C‒O‒C, st), 1051 (C‒O‒C, st as), 995. H NMR (400 MHz, CDCl )
δ (ppm):
3
.10 (3H, d,
.70-1.74 (1H, m, OH), 1.77 (1H, dq, J4,3 = 6.4 Hz, J4,10 = 6.8 Hz, H4"), 2.17 (1H, dq,
2,3 = 6.8 Hz, H2"), 4.13 (1H, dd, 3,4 = 6.8 Hz, H3"), 4.64 (1H, brs, H5"),
.25 (1H, s, NH), 6.09 (1H, dd, 6,5 = 1.6 Hz, 6,7 = 6.0 Hz, H6"), 6.24 (1H, d, J7,6=6.0
J9,2 = 6.8 Hz, H9"), 1.12 (3H, d, J10,4 = 6.8 Hz, H10"), 1.44 (9H, s, H2'),
J
2,9
=
J
3,2
J = J
5
J
J
13
Hz, H7"). C NMR (100 MHz, CDCl ) δ (ppm): 8.9 (C9"), 13.7 (C10"), 28.5 (C2'),
3
3
1
4.4 (C4"), 38.7 (C2"), 67.5 (C3"), 82.9 (C5"), 94.2 (C1"), 129.8 (C6"), 132.9 (C7"),
t
53.8 (C1). MS (CI, NH
3
, 70 eV, 150ºC) m/z (%): 214 (29, M+2‒ Bu), 196 (100, M‒
t
t
O Bu), 170 (49, M+2‒COO Bu), 152 (69, M‒NH Boc). Anal. Calcd for C H NO
2
14 23
4
-1
(
269.34 g·mol ): C, 62.43; H, 8.61; N, 5.20. Found: C, 62.41; H, 8.65; N, 5.24.
6
.4.5. General method for the reductive ozonolysis reaction: use of SMe
2
and
NaBH as reducing agents. Synthesis of (5)
4
2
7

ACCEPTED MANUSCRIPT
In a 25 mL two-neck heart-shaped oven-dried flask, equipped with a Dimroth condenser
connected to a CaCl
NaHCO (136 mg, 1.62 mmol), anhydrous CH
mL) were placed. The mixture was cooled with a solid CO
Then, O was bubbled inside the solution for 0.5 h, by a diffusor, until the reaction
mixture was saturated of ozone (adopting a blue colour). After reaction completion
control by TLC), the system was purged with N , in order to remove the excess of
2
trap tube, the corresponding alcohol (4) (269 mg, 1 mmol),
Cl (10 mL) and anhydrous MeOH (2.5
/acetone bath at ‒78 ºC.
3
2
2
2
3
(
2
ozone. Then, a stirring bar was placed in the flask; the Dimroth condenser removed and
two septa were adapted to the necks of the flask. Afterwards, SMe (0.17 mL, 2.3 mmol)
2
was added at –78 ºC and the mixture was stirred for 20 minutes. Then, the mixture was
warmed to room temperature, NaBH
system was stirred for 3 h. After reaction completion (control by TLC), the reaction
mixture was cooled to 0 ºC and the NaBH excess was quenched with water (0.5 mL)
4
(91 mg, 2.3 mmol) was added and the reaction
4
and the mixture was stirred for 15 minutes. Then, MeOH (5 mL) was added to dissolve
and quench the formed boron byproducts and the mixture was concentrated to dryness
under vacuum at room temperature. The obtained crude was lixiviated with ethyl acetate
(8 x 10 mL), and filtered by cannula. The organic phases were combined together and
the solvent was evaporated in vacuo. The resulting residue was submitted to a flash
column chromatography on silica gel eluting with mixtures of hexane and ethyl acetate
of increasing polarity to isolate pure 2-aminopyran 5.
tert-Butyl
N-[(2R*,3S*,4S*,5S*,6S*)-4-hydroxy-2,6-bis(hydroxymethyl)-3,5-
1
dimethyltetrahydropyran-2-yl]carbamate, (5a )
OH
9
''
8''
2
'
4
''
5
''
3''
2''
1'
2'
NH
7''
1
O
6
''
1
''
1
0''
O
2'
O
OH
OH
White solid, 269 mg, 88% yield. MP = 181-182 ºC (CCl ). TLC: Rf = 0.26 (SiO2,
4
-1
EtOAc, developed by ninhydrin). IR (film) ῡ (cm ): 3345 (O‒H, st), 2975, 2930, 2456,
t
1
1
6
699 (C=O, st), 1514 (N‒H), 1460, 1396, 1371, 1250 ( Bu), 1167 (C‒O‒C, st), 1099,
1
065 (C‒O‒C, st as), 1011. H NMR (400 MHz, CD
3
OD)
δ
(ppm): 0.94 (3H, d,
J
9,5
=
.8 Hz, H9"), 0.97 (3H, d,
5,4 = 10.3 Hz, H5"), 2.20 (1H, dq,
1H, m, H6"), 3.53 (1H, d, 10a,10b = 11.2 Hz, H10a"), 3.57 (1H, dd,
J
8,3 = 7.0 Hz, H8"), 1.44 (9H, s, H2'), 1.66 (1H, ddq,
J5,9 = 6.8
Hz,
J
5,6
=
J
J
3,4 = 5.0 Hz, 3,8 = 7.0 Hz, H3"), 3.30-331
J
(
J
J7a,OH = 6.0 Hz,
J
7a,7b
2
8