(R)-4-phenyloxazolidin-2-thione: an efficient chiral auxiliary for [4+2] cycloaddition of 1-aminodiene and activated phosphonodienophiles

Article abstract of DOI:10.1016/j.tetlet.2009.01.036

A theoretical model for the facial selectivity of N-dienyl oxazolidin-2-(thi)one and thiazolidin-2-thione 2a-c is presented. Our analysis provides a clear understanding of factors controlling stereoselectivity in reaction of these dienes, and allows predi

Full text of DOI:10.1016/j.tetlet.2009.01.036

Tetrahedron Letters 50 (2009) 1314–1317
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
(R)-4-phenyloxazolidin-2-thione: an efficient chiral auxiliary for [4+2]
cycloaddition of 1-aminodiene and activated phosphonodienophiles
Jean-Christophe Monbaliu a, Raphaël Robiette a, Daniel Peeters b, Jacqueline Marchand-Brynaert a,
*
a Unité de Chimie organique et médicinale, Département de Chimie, Université catholique de Louvain, Bâtiment Lavoisier, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium
b Unité de Chimie structurale et des mécanismes réactionnels, Département de Chimie, Université catholique de Louvain, Bâtiment Lavoisier, Place Louis Pasteur 1,
B-1348 Louvain-la-Neuve, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
A theoretical model for the facial selectivity of N-dienyl oxazolidin-2-(thi)one and thiazolidin-2-thione
2a–c is presented. Our analysis provides a clear understanding of factors controlling stereoselectivity
in reaction of these dienes, and allows predictions of high diastereoselectivity in the case of oxazoli-
Received 18 November 2008
Revised 18 December 2008
Accepted 9 January 2009
Available online 14 January 2009
din-2-thionyl diene (2b). The application of this diene to the synthesis of b- and
c-aminophosphonic
derivatives is then investigated. Under classical conditions or under microwave activation, the D–A reac-
tion of diene 2b leads to aminophosphonic chirons with high regio- and stereoselectivities.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Diels–Alder cycloaddition
Aminodiene
Stereoselective synthesis
Microwave-assisted synthesis
The Diels–Alder (D–A) reaction occupies a central role in organ-
ic synthesis as it generates polyfunctionalized six-membered rings
with high regio- and stereo-control.1 Numerous examples of asym-
metric D–A reactions (chiral auxiliaries,2 chiral catalysts,3 organo-
catalysts,4 chiral media,5 etc.) have also shown that it is possible to
control the absolute configuration of cycloadducts. In this context,
we recently reported8b–e the successful development of a new chi-
ral N-protected aminodiene8f–k based on a simplified theoretical
model (model compounds 1a–b, R1 = H; see Fig. 1) and its use in
asymmetric D–A reactions.
The compatibility of the D–A reaction with a wide range of
chemical functions guarantees interest in many research areas,
from simple molecules to large and complex structures. Also, the
implementation of modern synthesis techniques6,7 (microwave
activation, high pressure reactors, micro-emulsions, etc.) over-
comes previous unsuccessful applications of the D–A process due
to low reactivity of the diene or/and the dienophile and instability
of both partners and/or cycloadducts under thermal or Lewis acid-
catalyzed conditions. This prompted us to envisage the use of diene
2 for the development of a general method for the asymmetric syn-
thesis of aminophosphonic derivatives.8a–e These compounds are
recognized as constituting an important class of pharmacologically
active molecules.9 Several strategies have been developed towards
In this Letter, we document further the theoretical model12,13
for the facial selectivity of dienes bearing thiazolidin-2-(thi)one
R1
R2
Y
N
X
R1
Y
N
X
TSsyn
R2
ΔEa
sel
TSanti
N*
R2
(S)
Ea
syn
N*
R2
Ea
anti
(R)
Ea
rot
ΔErot
dienesyn
Y
rel
dieneanti
Y
R1
X
H
N
1
R1
X
N
2
3
4
the
occurring
a-amino phosphonic compounds, related to the naturally
-aminoacids.10 Surprisingly, b-,
c- and d-aminophos-
X=Y=O, R1=H (1a) or Ph (2a);
a
X=S and Y=O, R1=H (1b) or Ph (2b);
X=Y=S, R1=H (1c) or Ph (2c)
phonic derivatives were obtained only via punctual methods.9a,11
Figure 1. Energy profiles for the cycloadditions of dienes 2a–c (R1 = Ph) and
* Corresponding author. Tel: +32 (0)10 472740; fax: +32 (0)10 474168.
E-mail address: jacqueline.marchand@uclouvain.be (J. Marchand-Brynaert).
ethylene (R2 = H) or propene (R2 = Me).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.01.036

J.-C. Monbaliu et al. / Tetrahedron Letters 50 (2009) 1314–1317
1315
Table 1
Energetic discrimination between the two anti and syn conformers (DErreolt) of dienes 2a–c and selectivities (DEasel) calculated for the D–A reaction between 2a-c (R1 = Ph) and
ethylene (R2 = H) or propene (R2 = Me) (values in brackets correspond to calculations in solution)
Diene
2a (X = Y = O)
2b (X = S, Y = O)
2c (X = Y = S)
rot
rel
DE
(kcal.molꢀ1
)
anti
syn
anti
syn
anti
syn
0
1.5 (1.7)
Propenea
1.7 (1.1)
0
3.6 (3.7)
Propenea
4.5 (3.9)
0
4.6 (4.4)
Propenea
5.9 (5.2)
D
Easel (kcal.molꢀ1
)
Ethylene
1.6 (1.8)
Ethylene
3.5 (3.1)
Ethylene
4.8 (4.5)
a
Illustrated for the endo ortho cycloadducts.
or oxazolidin-2-thione chiral auxiliaries, using realistic systems
and including now solvent effects. The application of aminodienes
over the syn one because of (1) the higher stabilization by elec-
tronic delocalization involved in the anti orientation, (2) the unfa-
vourable orientation of respective dipoles of the dienyl and
heterocyclic moieties in the syn conformer and despite (3) the
emergence of a stabilizing electrostatic interaction between oxy-
gen and the nearest hydrogen in the syn isomer (Table 1).
2 for the asymmetric synthesis of b- and
c-aminophosphonic
derivatives is then investigated.
The factors controlling the facial selectivity for chiral N-dienyl
oxazolidin-2-(thi)ones 2a–b and N-dienyl thiazolidin-2-thione 2c
(R1 = Ph, Fig. 1) have been identified: the absolute configuration
of the cycloadducts can be accounted for by the approach of the
Our calculations12a on realistic models and taking solvents ef-
fects into account predict that structural modification by succes-
sive oxygen–sulfur replacement (compounds 2a–c) leads to an
increase of the energetic discrimination between the syn and the
anti conformers by further (relative) destabilization of the former
(Table 1). The most important effect is observed when going from
oxazolidin-2-one 2a to oxazolidin-2-thione 2b, most probably due
to a weaker stabilizing effect by the hydrogen bond in the syn con-
former.8e,15 Indeed, similar trends observed in solution phase sug-
gest that the dipole–dipole interaction between butadienyl and
heterocyclic moieties is not a determining factor for the anti–syn
energetic discrimination. Also, the fact that the rotation barrier
dienophile from the less hindered face (a
-face)14 of the s-cis diene
presenting an anti conformation around the C(1)–N bond (see
Fig. 1).8c–e Indeed, this conformation was found to be preferred
was found to be constant16 Eraot is about 9.5 kcal molꢀ1 for dienes
(D
2a–c) indicates that oxygen–sulfur replacement does not affect sig-
nificantly electronic delocalization.
Our calculations on TSs confirm the increase of facial selectivity
upon progressive replacement of oxygen by sulfur (Table 1). The
main effect is observed for the oxazolidin-2-one (2a) to oxazoli-
din-2-thione (2b) shift. A structural analysis of the TSs shows a sys-
tematic concerted synchronous addition (Fig. 2). Activation barriers
for both regioisomers (ortho and meta) for propene cyclization indi-
2a TSanti
2a TSsyn
φ
φ
φ
C(=X)-N-C(1)-C(2) = 176.0°
φ
φ
φ
C(=X)-N-C(1)-C(2) = 16.7°
24.15 kcal.mol-1
25.85 kcal.mol-1
cated a slight preference for the ortho-pathway (by 0.5 kcal molꢀ1
)
with no particular effect comparing the three dienes 2a–c. This low
regioselectivity arises from the low activating effect of the methyl
group of propene, and should be stronger when considering more
activated dienophiles. Generally, the results collected indicate that
the endo approach is favoured over the exo. Interestingly, the cyclo-
addition of the sulfur-containing dienes (compounds 2b–c) with
2b TSanti
2b TSsyn
C(=X)-N-C(1)-C(2) = 17.5°
28.30 kcal.mol-1
O
C(=X)-N-C(1)-C(2) = 170.6°
23.80 kcal.mol-1
S
N
O
S
ref. 8e
a
N
2b
O
O
4
S
N
2c TSanti
2c TSsyn
C(=X)-N-C(1)-C(2) = 168.0°
23.70 kcal.mol-1
C(=X)-N-C(1)-C(2) = 21.4°
29.60 kcal.mol-1
OTBDMS
2d
Figure 2. Structure and energy of two relevant transition states for the cycload-
dition of propene and dienes 2a–c (Energy of both transition states is relative to the
sum of the reactants energies considering the diene in its anti conformation).
Scheme 1. Synthesis of dienes 2b and 2d. Reagent ands conditions: (a) TBDMSCl,
NaI., Et3N, rt, overnight (2b, quant.).

1316
J.-C. Monbaliu et al. / Tetrahedron Letters 50 (2009) 1314–1317
O
EWG
Ph
5'
4'
O
O
PO(OMe)2
Ph
X
X
Ph
X
N
N
N
EWG
3a
(EtO)2OP
3b-e
MeCN, Δ
EWG
1
4
EWG
2
PO(OMe)2
3
MeCN, Δ
R3
R3
R3
PO(OEt)2
major stereoisomer
5b-d unique stereoisomer
2b
2d
Scheme 2. Cycloaddition of dienes 2b and 2d with the selected dienophiles 3a–e.
Table 2
Table 3
Cycloadditions of dienes 2b with the selected dienophiles (3a–e)
Microwave activation assays illustrated for the reaction of 2b and 3c
Entry
Dienophile
X
Yielda
Regioselect.b
endo/exo
dec
Solvent
Power (W)
Temp. (°C)
Time (h)
Conv. (%)
1
2
O
S
80
>98
100
100
84:16
86:14
80
>99
Toluene
(CH2Cl)2
MeCN
300
500
750
500
60
75
80
145
24
24
24
4
0
15
22
MeO2C
PO(OMe)2
3a
DMF
100
3
4
O
S
69
73
100
100
76:24
100:0
82
>99
CO2Me
tivity19 and the reactivity order are compatible with the orienta-
tion and the strength of the polarization among the system of
(EtO)2OP
p
3b
the dienophiles 3a–e. Surprisingly, dienophile 3e remains unreac-
tive even under prolonged heating. Thermal instability of the cyc-
loadducts prevented, in most cases, the use of GC for the
determination of selectivities, but 500 MHz NMR analysis of the
crude mixtures offered a convenient tool for structural assign-
ments.19 In each case, the facial discrimination induced by the chi-
ral 4-phenyloxazolidin-2-thione auxiliary is total. Endo-selectivity
is also total for dienophiles 3b–d; in the case of the gem-substi-
tuted phosphonodienophile 3a, endo-selectivity drops to 86/14
(Table 2, entry 1), mainly because of steric hindrance. What is very
encouraging is the observed enhancement of the facial selectivity
as compared to the oxazolidin-2-one series (X = O).8e–k This struc-
tural modification does not affect the intrinsic reactivity of the
diene as indicated by similar yields and reaction times.
5
6
O
S
63
70
100
100
92:8
100:0
95
>99
COMe
3c
(EtO)2OP
7
8
O
S
59
54
100
100
85:15
100:0
>85
>99
CN
(EtO)2OP
(EtO)2OP
3d
3e
9
10
O
S
0
0
/
/
/
/
/
/
CF3
Despite the excellent regio- and stereoselectivities offered by
the chiral aminodiene 2b, the application field suffers from the lack
of reactivity of the phosphonodienophiles. Microwave (MW) acti-
vation could provide a useful way to overcome this inherent low
reactivity. Using toluene or dichloromethane as solvent (totally
permeable to MW), no, or very low, conversion was observed after
prolonged heating (Table 3). However, in acetonitrile, conversion
occurred to the same extent as under classical heating conditions
and in DMF, at 145 °C, complete conversion was observed after
only 4 h. These observations highlight the lack of specific MW acti-
vation effect as a consequence of an isopolar mechanism20 (in good
agreement with the predicted concerted synchronous character of
these cycloadditions, see above). There is no modification of the
observed selectivities under microwave activation. Dienophile 3e
remains unreactive towards diene 2b, and no D–A reaction of diene
2d was observed. This MW activation method allows an increased
efficiency to reach the corresponding cycloadducts 5a–d. Complete
structural analysis of these cycloadducts has been published
elsewhere.8b,d
a
After purification by column chromatography.
b
c
Determined by 1H NMR (500 MHz) analysis on the crude mixtures.
(3R) isomer versus (3S) isomer, for the major endo-stereoisomer.
propene is characterized by a higher endo/exo selectivity as com-
pared to the oxazolidin-2-one diene 2a.
Since the most important effects were predicted for diene 2b,
we selected the latter for the development of an asymmetric syn-
thesis of b- and
fact that the regioselectivity of the D–A reaction should allow the
selective synthesis of b- and -aminophosphonic derivatives by
reaction of diene 2b with unsymmetrical gem- and vic-activated
phosphonodienes, respectively.17 Diene 2b was synthesized from
intermediate 4 according to the reported procedure (Scheme 1).8e
The corresponding 3-siloxy-activated diene 2d18 was also obtained
quantitatively from the vinylogous amide 4, in view to enlarge the
application field of the 4-phenyloxazolidin-2-thione chiral
auxiliary.
Dienes 2b and 2d were engaged under classical conditions
(refluxing MeCN) with the selected dienophiles (Scheme 2 and Ta-
ble 2).
Degradation of the diene 2d was observed in all cases, whereas
diene 2b reacts with phosphonodienophiles 3a–e to give the corre-
sponding amino-phosphonic derivatives with good yields. Low
reactivity of phosphonodienophiles 3a–e is, however, responsible
for long reaction times (1–15 days). Nevertheless, the regioselec-
c-aminophosphonates. Our strategy is based on the
c
In summary, we have further illustrated the D–A cycloadditions
of chiral N-oxazolidin-2-thione diene 2b with a representative set
of gem- and vic- activated phosphonodienophiles (3a–d). Under
both thermal conditions and MW activation, these reactions pro-
ceed with total regio- and stereoselectivities, leading to b- and c-
aminophosphonic chirons.21 Our computational investigation, on
realistic model and including solvent effects, of the factors govern-
ing the stereoselectivity provides a complete rational for the asym-
metric D–A reaction of N-protected dienes bearing a oxazolidin-2-
(thi)one or thiazolidin-2-thione moiety. This study shows that fa-

J.-C. Monbaliu et al. / Tetrahedron Letters 50 (2009) 1314–1317
11. Palacios, F.; Alonso, C.; de los Santos, J. M. Chem. Rev. 2005, 105, 899.
1317
cial selectivity is controlled by the energetic discrimination be-
tween the two reactive conformers of the diene (anti and syn con-
formers). This energetic preference for the anti conformer increases
with progressive oxygen–sulfur replacement in the chiral auxiliary.
Calculations taking solvent effects into account indicate that the
main factor responsible for the observed increase in facial selectiv-
ity upon going from oxazolidin-2-one to oxazolidin-2-thione chiral
auxiliary is the lower stabilization of the syn conformer by hydro-
gen bond in the case of sulfur atom.
12. (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A. Jr.; Stratmann, R. E.;
Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M.
C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.;
Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.;
Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko,
A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham; M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill,
P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A., Gas Phase Calculations were Done Using
the GAUSSIAN Suite of Programs, Revision A.7, Gaussian, Inc., Pittsburgh, PA,
1998.; (b) For solution phase calculations (THF), given energies are obtained
Acknowledgements
after corresponding fully analytical single point calculations (B3LYP/6-31G**
)
using the fine DFT grid and the polarizable continuum-Poisson method as
incorporated in Jaguar program, version 6.5, Schrödinger, LLC, New York, NY,
2005.
This work was supported by FRIA (fellowship to J.-C.M.), and
F.R.S.-FNRS by its support to access computational facilities (FRFC
Project No. 2.4556.99 ‘Simulations numériques et traitement des
données’). R.R. is a Chargé de recherches F.R.S.-FNRS. J.M.-B. is a se-
nior research associate of F.R.S.-FNRS.
13. Unconstrained optimizations were performed at the modified B3LYP/6-31G
level (with supplementary
d orbitals for sulfur atoms). SP energies were
calculated at the B3LYP/6-31G** level.
14. The approach of the dienophile from the b-face of the diene was found to be
disfavoured by about 5 kcal molꢀ1 (B3LYP/6-31G).
15. Due to the intrinsic geometrical features of the oxazolidin-2-one, oxazolidin-2-
thione and thiazolidin-2-thione, we have observed an increased XꢁꢁꢁH–C(2)
distance for compounds 2a–c (2.28–2.61 Å).
References and notes
16.
D
Earot was found constant as a consequence of the conformational adaptability
1. Fringuelli, F.; Taticchi, A. The Diels–Alder Reaction; John Wiley & Sons, Ltd, 2002.
2. (a) Liu, X.; Snyder, J. K. J. Org. Chem. 2008, 73, 2935–2938; (b) Songis, O.; Geant,
P. Y.; Sautrey, G.; Martinez, J.; Calmes, M. Eur. J. Org. Chem. 2008, 308–318; (c)
Henderson, J. R.; Parvez, M.; Keay, B. A. Org. Lett. 2007, 9, 5167–5170; (d)
Calmes, M.; Escale, F.; Didierjean, C.; Cazals, G.; Martinez, J. Tetrahedron:
Asymmetry 2007, 18, 2491–2496.
of the nitrogen atom.
17. Dienophile 3a was commercially available. Dienophiles 3b–e were synthesized
from the described protocol in the literature. (a) McClure, C.; Hansen, K.
Tetrahedron Lett. 1996, 37, 2149–2152; (b) Afarinkia, K.; Evans, M.; Graham, J.;
Jimenez-Bueno, G. Tetrahedron Lett. 1998, 39, 433–434; (c) Nickson, T. J. Org.
Chem. 1988, 53, 3870–3872.
3. Carmona, D.; Lamata, M. P.; Viguri, F.; Rodriguez, R.; Lahoz, F. J.; Dobrinovitch, I.
T.; Oro, L. A. Dalton Trans. 2008, 3328–3338.
18.
A
solution of (R)-3-(30-oxobut-10-enyl)-4-phenyloxazolidin-2-thione
4
(2.54 mmol, 0.63 g), TBDMSCl (3.2 mmol, 0.48 g) and triethylamine
(3.2 mmol, 0.44 cm3) in anhydrous MeCN (5 mL) was treated dropwise with
an anhydrous solution of NaI (3.2 mmol, 0.48 g) in MeCN (2 mL) at room
temperature over 10 min. After 15 h, the solution was concentrated under
reduced pressure. The oily residue is diluted in petroleum ether (5 cm3) and
filtrated over a slight celite pad. Petroleum ether was removed under reduced
pressure (temperature bath less 30 °C) to give pure 2d as a yellow oil (0.91 g,
>99%); 1H NMR (500 MHz, CDCl3) d: 7.67 (d, J1,2 = 14.4 Hz, C(1)–H, 1H), 7.2–
4. Akalay, D.; Duerner, G.; Goebel, M. W. Eur. J. Org. Chem. 2008, 2365–2368.
5. (a) Winkel, A.; Reddy, P. V. G.; Wilhelm, R. Synthesis 2008, 999–1016; (b)
Headley, A. D.; Ni, B. Aldrichim. Acta 2007, 40, 107–117; (c) Van Buu, O. N.; Vo-
Thanh, G. Lett. Org. Chem. 2007, 4, 158–167; (d) Pegot, B.; Van Buu, O. N.; Gori,
D.; Vo-Thanh, G. B. J. Org. Chem. 2006, 2.
6. (a) Lidström, P.; Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57,
9225; (b) Kappe, C. O. Angew. Chem., Int. Ed. 2004, 43, 6250–6284; (c) Perreux,
L.; Loupy, A. Tetrahedron 2001, 57, 9199–9223.
0
0
7.38 (m, 5H), 5.31 (d, J2,1 = 14.4 Hz, C(2)–H, 1H), 5.21 (dd, J4 ,5a = 9.0 Hz and
7. Jenner, G. Tetrahedron 2002, 58, 5185–5202.
J4 ,5b = 4.4 Hz, C(40)–H, 1H), 4.86 (t, J5a ,5b = 9.0 Hz, C(50)–Ha, 1H), 4.36 (dd,
0
0
0
0
8. (a) Monbaliu, J.-C.; Marchand-Brynaert, J. Tetrahedron Lett. 2008, 49, 1839–
1842; (b) Monbaliu, J.-C.; Tinant, B.; Marchand-Brynaert, J. J. Mol. Struct. 2008,
879, 113–118; (c) Robiette, R.; Defacqz, N.; Peeters, D.; Marchand-Brynaert, J.
Curr. Org. Synth. 2005, 2, 453; (d) Tinant, B.; Defacqz, N.; Robiette, R.; Touillaux,
R.; Marchand-Brynaert, J. Phosphorus, Sulfur, Silicon 2004, 179, 389; (e) Robiette,
R.; Cheboub-Benchaba, K.; Peeters, D.; Marchand-Brynaert, J. J. Org. Chem. 2003,
68, 9809. and references cited therein; For references on other chiral
aminodienes, see: (f) Kozmin, S.; Rawal, V. H. J. Am. Chem. Soc. 1997, 119,
7165–7166; (g) Kozmin, S.; Rawal, V. H. J. Am. Chem. Soc. 1999, 121, 9562–
9573; (h) Janey, J.; Iwama, T.; Kozmin, S.; Rawal, V. H. J. Org. Chem. 2000, 65,
9059–9068; (i) Murphy, J. P.; Nieuwenhuyzen, M.; Reynolds, K.; Sarma, P. K. S.;
Stevenson, P. J. Tetrahedron Lett. 1995, 36, 9533–9536; (j) McAlonan, H.;
Murphy, M.; Nieuwenhuyzen, J. P.; Reynolds, K.; Sarma, P. K. S.; Stevenson, P. J.;
Thompson, N. J. Chem. Soc., Perkin Trans. 1 2002, 1, 69–79; (k) Huebner, S.; Jiao,
H.; Michalik, D.; Neumann, H.; Klaus, S.; Struebing, D.; Spannenberg, A.; Beller,
M. Chem. Asian J. 2007, 2, 720–733.
9. (a) Kukhar, V.; Hudson, H. Aminophosphonic and Aminophosphinic Acids:
Chemistry and Biological Activity; John Wiley & Sons, Ltd, 2000; (b) Collinsová,
M.; Jirácek, J. Curr. Med. Chem. 2000, 7, 629; (c) Kafarski, P.; Lejcak, B. Curr. Med.
Chem. 2001, 1, 301.
10. (a) Mandal, T.; Samanta, S.; Zhao, C.-G. Org. Lett. 2007, 9, 943; (b) Romanenko,
V. D.; Kukhar, V. P. Chem. Rev. 2006, 106, 3868–3935; (c) Kobayashi, S.;
Kiyohara, H.; Nakamura, Y.; Matsubara, R. J. Am. Chem. Soc. 2004, 126, 6558.
J5b ,5a = 9.0 Hz, C(50)–Hb 1H), 4.18 (s, C(4a)–H, 1H), 4.04 (s, C(4b)–H, 1H), 0.98
(s, 9H), 0.16 (s, 3H), 0.15 (s, 3H); 13C NMR (125 MHz, CDCl3) d: 186.18 (s, C@S),
153.20 (s, C(3)), 137.23 (s, Cq u), 129.46 (2 s, u), 129.01 (s, u), 126.01 (s, C(1)),
125.62 (s, u), 114.29 (s, C(4)), 95.10 (s, C(2)), 74.81 (s, C(50)), 62.04 (s, C(40)),
0
0
25.70 (s, t-Bu), 18.06 (s, Cq t-Bu), -4.73 (s, Me), ꢀ4.87 (s, Me); IR (NaCl,
m,
cmꢀ1): 2955, 1689, 1957, 1472, 1377, 1259, 1175; ESI MS m/z for C19H27NO2SSi
[M+H+]: 362.25 (18), 746.16 (100).
19. For
a detailed structural analysis (by NMR and X-ray diffraction) of
cycloadducts 5a–d, see Refs. 8b and 8d.
20. (a) Loupy, A.; Maurel, F.; Sabatie-Gogova, A. Tetrahedron 2004, 60, 1683–1691;
(b) Diaz-Ortiz, A.; Carrillo, J.; Cossio, F.; Gomez-Escalonilla, M.; de la Hoz, A.;
Moreno, A.; Prieto, P. Tetrahedron 2000, 56, 1569–1577; (c) Langa, F.; de la Cruz,
P.; de la Hoz, A.; Espildora, E.; Cossio, F.; Lecea, B. J. Org. Chem. 2000, 65, 2499–
2507.
21. We have shown in a previous paper (see Monbaliu, J.-C.; Tinant, B.; Marchand-
Brynaert, J. Heterocycles 2008, 75, 2459–2475) that oxazolidin-2-thione could
be (quantitatively) transformed into their oxazolidin-2-one analogues for
which several deprotection methods (to lead to corresponding primary amine)
have been reported. See: (a) Turconi, J.; Lebeau, L.; Paris, J.-M.; Mioskowski, C.
Tetrahedron 2006, 62, 8109–8114; (b) Fisher, J.; Duningan, J.; Hatfield, L.;
Hoying, R.; Ray, J.; Thomas, K. Tetrahedron Lett. 1993, 34, 4755–4758; (c) Katz,
S.; Bergmeier, S. Tetrahedron Lett. 2002, 43, 557–559.