[3,3]-Sigmatropic rearrangement/allylboration/cyclization sequence: Enantioenriched seven-membered-ring carbamates and ring contraction to pyrrolidines

Article abstract of DOI:10.1002/anie.201509824

The combination of in situ generated α-isocyanato allylboronic esters and aldehydes afforded seven-membered-ring enecarbamates with high levels of diastereo- and enantiocontrol. They were easily converted into diversely substituted 1,3-oxazepan-2-ones. An unprecedented rearrangement of 5-acetoxy-7-aryl or styryl derivatives led to tetrasubstituted pyrrolidines. A computational study provides evidence on the feasibility of the proposed mechanism of this unusual ring contraction. Honey I shrunk the rings: A new protocol for the diastereo- and enantiocontrolled synthesis of seven-membered-ring ene carbamates is described. These compounds were easily converted into diversely substituted 1,3-oxazepan-2-ones. Tetrasubstituted pyrrolidines were obtained by an unprecedented ring contraction rearrangement of a 5-acetoxy derivative in the presence of trifluoroboron etherate.

Full text of DOI:10.1002/anie.201509824

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International Edition: DOI: 10.1002/anie.201509824
German Edition: DOI: 10.1002/ange.201509824
Heterocycles
[3,3]-Sigmatropic Rearrangement/Allylboration/Cyclization Sequence:
Enantioenriched Seven-Membered-Ring Carbamates and Ring
Contraction to Pyrrolidines
AurØlie MacØ, Sabrina Touchet, Patricia Andres, Fernando Cossío, Vincent Dorcet,
FranÅois Carreaux,* and Bertrand Carboni*
Abstract: The combination of in situ generated a-isocyanato
allylboronic esters and aldehydes afforded seven-membered-
ring enecarbamates with high levels of diastereo- and enantio-
control. They were easily converted into diversely substituted
1,3-oxazepan-2-ones. An unprecedented rearrangement of 5-
acetoxy-7-aryl or styryl derivatives led to tetrasubstituted
pyrrolidines. A computational study provides evidence on the
feasibility of the proposed mechanism of this unusual ring
contraction.
followed by direct addition of aldehydes to the crude reaction
mixture, and then aqueous NaHCO3 15 hours later, afforded
the cyclic ene carbamates 5 in good to moderate yields (three-
step process; Table 1). The structure of 5 was assigned on the
basis of NMR data and X-ray crystallographic analysis of
5a.[8]
The following mechanism can be proposed. After dehy-
dration of 1, the resulting allylcyanate 2 undergoes a [3,3]-
sigmatropic rearrangement which affords the a-isocyanato
allylboronate 3 (Scheme 1).[5d,9,10] A chairlike transition state
N
itrogen heterocycles are widespread in a plethora of
molecules of interest, either in materials science, electronics,
optics, or biology.[1] Among these, seven-membered-ring
carbamates are relatively under-represented despite their
interesting biological activities.[2] Several approaches are
available for their synthesis, and the most common ones are
based on intramolecular cyclization of 1,4-aminoalcohols or
derivatives.[3] Others are restricted to isolated examples or
very particular structures.[4] Our interest in the use of boronic
esters in cascade reactions[5] led us to consider their utility for
the synthesis of enantioenriched 4,5-dihydro-1,3-oxazepin-2-
ones. These versatile intermediates led, through an unprece-
dented ring contraction reaction, to tetrasubstituted pyrroli-
dines, another family of major biological significance.
Table 1: Synthesis of the enecarbamates 5.[a]
Access to the key carbamates 1 was first achieved from
optically active 1-alkyn-3-ols.[6,7] Dehydration with phosphine
and carbon tetrabromide in the presence of triethylamine,[5d]
[*] Dr. A. MacØ, Dr. S. Touchet,[+] P. Andres,[++] V. Dorcet, Dr. F. Carreaux,
Dr. B. Carboni
Institut des Sciences Chimiques de Rennes
UMR 6226 CNRS-UniversitØ de Rennes 1
Campus de Beaulieu, 35042 Rennes Cedex (France)
E-mail: francois.carreaux@univ-rennes1.fr
bertrand.carboni@univ-rennes1.fr
Prof. F. Cossío
Departamento de Química Orgµnica I, Facultad de Química,
Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/
EHU) and Donostia International Physics Center (DIPC)
P8 Manuel Lardizabal 3, 20018 San Sebastiµn/Donostia (Spain)
[a] The reactions were carried out by addition of a solution of CBr4
(0.56 mmol, 2.8 equiv) in CH2Cl2 (0.5 mL) to a mixture of 1 (0.2 mmol),
polymer-supported phosphine (2.5 equiv), and Et3N (0.4 mmol, 2 equiv)
in dry CH2Cl2 (2 mL). The aldehyde (0.24 mmol, 1.2 equiv) was added
after 6 h at 08C. The mixture was maintained at RT for 15 h before
quenching with aqueous NaHCO3. [b] Yield of product isolated after
silica gel chromatography. [c] Determined by GC for the starting 1-alkyn-
3-ols [(S) R1 =Me, ee>99%, (S) R1 =n-Pent, ee=96%, (R) R1 =n-Pent,
ee=96%, (S) R1 =CH2Ph, ee=86%] and HPLC for 5 using a chiral
stationary phase. [d] R enantiomer.
[+] Present address: AdPueriVitam, Laboratoire BioCIS
FacultØ de Pharmacie, UniversitØ Paris-Sud
5 rue J-B ClØment, 92296 Châtenay-Malabry (France)
++] Present address: Departamento de Química Orgµnica
Facultad de Ciencias, Universidad de Zaragoza
Pedro Cerbuna 12, 50009 Zaragoza (Spain)
[
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201509824.
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Scheme 1. Proposed mechanism for 5!1. pin=pinacol.
Scheme 3. Proposed mechanism for the formation of 9a. THF=tetra-
hydrofuran.
in which 1,3-diaxial interactions are minimized (intermediate
A) explains the E geometry of the new double bond.
Concerning the allylation step, its stereochemical course can
be rationalized by the favored transition-state B. The NCO
group adopts a pseudoaxial position because of the presence
of a polar a-substituent and the relatively bulky pinacol
moiety.[11] Finally, an intramolecular cyclization occurs after
substituent. Nucleophilic displacement of PhI, assisted by
the neighboring OAc substituent, led to the oxonium ion 11
which then afforded 9a. a-Acetoxycarbamates are prone to
undergoing Lewis acid catalyzed reactions with diverse
nucleophiles.[17] The compound 9a indeed reacted with
either allyltrimethylsilane or TMSCN in the presence of
Et2O.BF3 (Scheme 4). Surprisingly, instead of the expected
seven-membered ring, the pyrrolidines 12a and 13a, respec-
tively, were produced as single isomers.[8] These compounds
belong to the 3-hydroxypyrrolidine family, which is found in
a wide range of natural products and drug intermediates.[18]
À
cleavage of the O B bond (see the Supporting Information
for a detailed 1H NMR study). No erosion of the stereochem-
ical integrity of the starting alkynol was observed for 5a–h
(Table 1), thus demonstrating that the [3,3]-sigmatropic
rearrangement occurred with complete 1,3-chirality transfer.
In the case of 5i, partial loss of chiral information could result
from the participation of a cationic phenonium intermediate
which is trapped by SN2’ addition of the cyanate anion.[12–14]
Enecarbamates have been the center of intense interest
over the past few years[15,16] and led us to explore the reactivity
of 5 (Scheme 2). Hydrogenation with H2 in the presence of
Pd/C and methylation mediated by silver oxide afforded 6
(90%) and 7 (95%), respectively. Treatment with 1m HCl in
THF caused ring opening to provide the aldehyde 8, and
vicinal dioxygenation using PhI(OAc)2 occurred in high yield
to afford 9a as the major isomer.[8,16g]
This selectivity is consistent with the mechanism proposed
in Scheme 3. The N-acyliminium intermediate 10 underwent
addition of the acetate anion anti to the bulky a-iodo
Scheme 4. Synthesis of the pyrrolidines 12a and 13a from 9a.
TMS=trimethylsilyl.
We hypothesized that either the 1,3-oxazepan-2-one 14a
or 15a was initially produced following a classical reaction
pathway (Scheme 5), with the relative configuration at C4
directed by the formation of an oxonium intermediate 11
(Scheme 3). After the coordination of the oxygen atom of the
carbamate to boron, an intramolecular displacement by the
acetate group afforded either 16a or 17a, which, in turn, was
subjected to the attack of the internal nitrogen atom with
concomitant release of CO2.
A supplementary experiment confirmed the hypothesis of
a 1,3-oxazepan-2-one intermediate. Reduction of 9a with
NaBH3CN gave 18 in 88% yield with no pyrrolidine
formation. Further treatment with Et2O.BF3 led to ring
contraction to furnish 19 (Scheme 6).
To check the plausibility of this ring contraction, we
calculated the reaction profile associated with the 15!13
transformation. We used M06-2X[19] (including solvent
Scheme 2. Functionalization of 5.
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Scheme 5. Proposed mechanism for the formation of 12a and 13a.
Figure 1. Reaction profile and stationary points associated with the
15a,b!13a,b transformations. All the calculations were performed at
the M06-2X(PCM=CH2Cl2)/6–31+G*//B3LYP/6–31G* level of theory.
Relative standard free energies and bond distances are given in
kcalmolÀ1 and Š, respectively.
Scheme 6. Synthesis of 19 from 9a. Ts=4-toluenesulfonyl.
TS1 and TS2 have a significant cationic character. The
computed NBO charges for CipsoH in TS1a and TS2a are
+ 0.45 e and + 0.44 e, respectively. Accordingly, a p-electron
withdrawing group at the 4-position of the aryl group (R2)
should result in larger activation energies. In effect, the
transition structures TS1b and TS2b are significantly desta-
bilized with respect to their respective bromo congeners and
the corresponding activation energies are 4.7 kcalmolÀ1 and
5.4 kcalmolÀ1 higher than those associated with TS1a and
TS2a, respectively. In terms of relative rates, the theoretical
kinetic ratio at 298 K is rate(NO2):rate(Br) ꢀ 10À6. This much
lower reactivity was confirmed experimentally, as no pyrro-
lidine formation was observed from 9b (see Table 2).
To better define the scope of the ring contraction process,
several additional 4,5-diacetoxy-1,3-oxazepanones (9) were
reacted with TMSCN in the presence of Et2O.BF3 (Table 2).
Depending upon the R2 substituent, and, in agreement with
the previous theoretical study, the pyrrolidines 13c, 13d, and
13j were obtained in good yields while seven-membered rings
were only produced from 9b and 9e.
effects[20]) and B3LYP[21] hybrid DFT functionals for single-
point calculations and geometry optimizations, respectively.
The reported results correspond to the M06-2X(PCM =
CH2Cl2)/6-31 + G*//B3LYP/6–31G* level of theory as imple-
mented in the Gaussian 09 suite of programs.[22] According to
these results (Figure 1), the intermediate 15a coordinates to
BF3, thus generating a more polar species which gives the
zwitterionic intermediate 17a (6-exo-tet process). The NH
moiety then acts as a nucleophile to form the pyrrolidine 13a
by a second intramolecular 5-exo-tet cyclization, with con-
comitant loss of OAc, release of CO2, and recovery of BF3.
This latter event provides a favorable entropic balance, which
is compatible with the exergonocity computed for the 15a!
13a transformation.
Two additional factors contribute to the efficient con-
version of 15a into 13a: first, the benzylic character of the
carbon atom on which the two consecutive nucleophilic
substitutions take place; second, the stereogenic centers
bearing the CN, Me, and aryl groups in TS1a and TS2a
possess an adequate configuration to occupy equatorial
positions, and thus results in less congested saddle points
(Figure 1). Formally, both consecutive nucleophilic substitu-
tions can be envisaged as SN2 reactions via the saddle points
TS1a and TS2a. However, in both transition structures, the
bond orders between the Cipso group and the nucleophiles and
the leaving groups are quite low. Thus the CipsoH groups in
In conclusion, we have developed an efficient [3,3]-
sigmatropic
rearrangement/allylboration/cyclization
sequence which gives access to cyclic enecarbamates. These
versatile intermediates can be further engaged in a series of
functional-group modifications. An unprecedented ring con-
traction reaction was demonstrated to be a promising strategy
for the enantiocontrolled synthesis of tetrasubstituted pyrro-
lidines. The scope and limitations of this process were
determined and computational evidence fully supports the
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Table 2: Reactivity of 4,5-diacetoxy-1,3-oxazepan-2-ones 9.
Entry
9
R2
Yield [%][a]
13
71
–
15
–
1
2
3
9a
9b
9c
70
–
59
4
9d
87
–
5
6
9e
9j
–
71
–
73
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Acknowledgements
This work was supported by the University of Rennes 1 and
the Centre National de la Recherche Scientifique (CNRS). P.
A. thanks the Spanish MINECO (Grant CTQ2010-17436) for
a fellowship. F. P. C. thanks the Gobierno Vasco/Eusko
Jaurlariza (Grant IT-324-07) and the Spanish MINECO
(Grant CTQ2013-45415-P) for financial support, and the
SGI/IZO-SGIker of the UPV/EHU for generous allocation of
computational resources. The authors thank C. Lalli and T.
VivØs (UMR 6226, Rennes) for the determination of the
ee values by chiral-phase HPLC and G. Masson (ICSN, Gif/
Yvette) for helpful discussions on mechanistic questions.
Keywords: allylic compounds · asymmetric synthesis ·
heterocycles · reaction mechanisms · rearrangements
How to cite: Angew. Chem. Int. Ed. 2016, 55, 1025–1029
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Received: October 20, 2015
Published online: December 7, 2015
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