Synthesis of highly functionalized 2,5-disubstituted pyrrolidines via an aza-Morita-Baylis-Hillman-type reaction

Article abstract of DOI:10.1016/j.tet.2010.05.047

An aza-Morita-Baylis-Hillman-type reaction of Michael acceptors with 5-substituted cyclic N,O-acetals derived from pyrrolidines has been investigated. It has been found that the combination of Me₂S and TMSOTf work well with unhindered and reactive enals and enones whilst the use of quinuclidine and TMSOTf is superior for more hindered Michael acceptors. The reactions lead to 2,5-trans-disubstituted pyrrolidines with good to excellent diastereoselectivity. The origin of the selectivity is discussed.

Full text of DOI:10.1016/j.tet.2010.05.047

Tetrahedron 66 (2010) 6293e6299
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of highly functionalized 2,5-disubstituted pyrrolidines via
an aza-MoritaeBayliseHillman-type reaction
*
James E. Kitulagoda, Anders Palmelund, Varinder K. Aggarwal
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
An aza-MoritaeBayliseHillman-type reaction of Michael acceptors with 5-substituted cyclic N,O-acetals
derived from pyrrolidines has been investigated. It has been found that the combination of Me2S and
TMSOTf work well with unhindered and reactive enals and enones whilst the use of quinuclidine and
TMSOTf is superior for more hindered Michael acceptors. The reactions lead to 2,5-trans-disubstituted
pyrrolidines with good to excellent diastereoselectivity. The origin of the selectivity is discussed.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 8 March 2010
Received in revised form 5 May 2010
Accepted 13 May 2010
Available online 20 May 2010
This paper is dedicated with admiration to
Professor Steven Ley on the award of the
Tetrahedron Prize for creativity in organic
chemistry. Many congratulations!
1. Introduction
O
O
m
m
MeO
X
n
N
X
n
+
2,5-Disubstituted pyrrolidines are common motifs found in
many natural products, as well as molecules with potential phar-
maceutical applications. In addition they have been used as chiral
auxiliaries,1 chiral ligands2 and as organocatalysts for asymmetric
synthesis.3 A common strategy towards their synthesis involves
nucleophilic addition to 5-substituted N,O-acetals. A broad range of
nucleophiles have been reported, including organosilanes,4 allyl-
stannanes,5 organocuprates,6 cyanides7 and thiols,8 but the dia-
stereoselectivity of the addition reactions has been found to be
highly substrate- and nucleophile-dependent.
N
PG
PG
1
2
3
(i) TMSOTf, Me2S
(ii) NaHCO3 quench
(iii) DBU
TMS OMe
m
TMS
O
O
m
X
n
+
X
n
TfO
N
N
Recently, we reported a novel protocol for the synthesis of
PG
Me2S
PG
b
0-amido-
a,b-unsaturated carbonyl compounds 3 through a mecha-
Me2S
nism akin to the aza-MoritaeBayliseHillman reaction (aza-MBHR)
5
4
6
(Scheme 1).9 In this protocol, the combination of TMSOTf, Me2S and
PG = Ts, Boc or Cbz; m = 1 or 2, n = 0, 1 or 2; X = H, CH2, Me, SEt, OMe
Michael acceptor gave an intermediate b-sulfonium silyl enol ether 4,
which reacted with an iminium ion 5 to give the Mannich adduct 6.
Followingbasetreatmenttheaza-MBHadduct3wasgenerated. In this
paper we report an extension of this general protocol to 5-substituted
pyrrolidine-derived N,O-acetals to give highly functionalized 2,5-di-
substituted pyrrolidines with good-high trans-diastereoselectivity.
Scheme 1. Aza-MoritaeBayliseHillman-type reaction of N,O-acetals.
i)
ii)
RO
RO
RO
O
O
OMe
N
H
N
N
iii)
Cbz
Cbz
2. Results and discussion
O
O
O
7a
8a
9a R = Me (57%)
9b
R = Me
R = Me (76%)
R = Bn (71%)
7b R = Bn
8b
The required N,O-acetal substrates were prepared following the
R = Bn (67%)
general procedure shown in Scheme 2.10
i) LiHMDS, Cbz-Cl; ii) LiEt3BH; iii) p-TsOH, MeOH
Scheme 2. Synthesis of N,O-acetals.
* Corresponding author. Tel.: þ44 117 954 6315; fax: þ44 117 929 8611; e-mail
address: v.aggarwal@bristol.ac.uk (V.K. Aggarwal).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.05.047

6294
J.E. Kitulagoda et al. / Tetrahedron 66 (2010) 6293e6299
Application of our previously reported reaction conditions to
repeating the experiment in entry 7 at ꢀ50 ꢁC failed to afford any of
the desired product. Variable temperature NMR experiments of the
reaction of cyclohexenone, TBDMSOTf and Me2S performed by Lee
a series of Michael acceptors and N,O-acetals, but extending the
reaction time from 3 h to 36 h, gave the aza-MBH adducts in moder-
ate-good yield and with moderate-high trans-selectivity (Table 1).
and Iwasawa demonstrated that the b-sulfonium silyl enol ether was
Table 1
Aza-MoritaeBayliseHillman-type reactions of aminals using Me2S as Lewis base
Entry
1
N,O-Acetal
Michael Acceptor
Product
Yielda (%)
trans/cisb
9a
61
75
72
57
62
>95:5
2
3
4
5
6
9a
9a
9b
9b
9b
80:20
84:16c
>95:5
>95:5
71
25
70:30d
87:13
7
9a
a
Yield of isolated product.
b
c
Determined by 1H NMR of the crude product.
trans-Diastereoisomer isolated as a 1:4 (Z/E) mixture of diastereoisomers.
trans-Diastereoisomer isolated as a 1:3 (Z/E) mixture of diastereoisomer.
d
Acrolein gave very high trans-selectivity with both N,O-acetals (en-
tries 1 and 4) whilst MVK gave higher selectivity with the more hin-
dered N,O-acetal 9b (entries 2 and 5). Crotonaldehyde gave a mixture
of E- and Z-enals with moderate diastereoselectivity (entries 3 and 6).
However, cyclohexenone only gave the aza-MBH adduct in low yield
and the corresponding reactionwith cyclopentenone onlygave traces
of the desired product. The low yield obtained with cyclohexenone
and cyclopentenone prompted us to study this reaction further.
We reasoned that the b-sulfonium silyl enol ether of this more
hindered and less reactive substrate might require higher reaction
temperatures to undergo addition to the iminium ion. However,
Figure 1. Stability of sulfonium silyl enol ether triflate as a function of temperature.

J.E. Kitulagoda et al. / Tetrahedron 66 (2010) 6293e6299
6295
in a dynamic equilibrium with the starting materials.11 At low tem-
Table 2
Aza-MoritaeBayliseHillman-type reactions of aminals using amines as Lewis bases
perature (T <ꢀ50 ꢁC), the
b-sulfonium silyl enol ether was the
dominantspeciesbutathighertemperatures(T>ꢀ45 ꢁC)thestarting
materials were favoured (Fig. 1). We presumed therefore that at the
higher temperatures required for Mannich addition (i.e., T >ꢀ50 ꢁC)
the concentration of the b-sulfonium silyl enol ether was consider-
ably lower, which could account for the poor conversion (Fig. 1).
Minor changes in the silyl reagent/Lewis acid or reaction conditions
did not offer any significant improvements.12 We therefore considered
alternative Lewis bases, particularly as Kim had reported that b-am-
monium silyl enol ether triflates were stable even at room tempera-
ture.13 A range of amine bases were therefore tested inplace of Me2S. As
an illustration of the protocol, pyridine and TMSOTf were added to
cyclohexenone at ꢀ78 ꢁC and allowed to warm to room temperature.
The reaction mixture was re-cooled to ꢀ40 ꢁC and the N,O-acetal 9a
added. After 3 h at ꢀ40 ꢁC, the reaction was quenched with aqueous
NaHCO3, and subjected to DBU treatment, affording the product 16 in
a69%yield anda 25:75(cis/trans)diastereomeric ratio (Table 1,entry1).
Of the bases tested (Table 2), quinuclidine proved optimum giving the
aza-MBH adduct in high yield and high trans-selectivity (entry 7).
Extension of the amine-based protocol to other Michael accep-
tors and N,O-acetals was also effective (Table 3). Whilst
Entry
Tertiary amines
Yielda (%)
trans/cisb
1
2
3
4
5
6
7
Pyridine
69
85
90
<5
45
0
75:25
76:24
82:18
87:13
72:28
d
4-tert-Butyl pyridine
5,6,7,8-Tetrahydroquinoline
Quinoline
DMAP
DABCO
Quinuclidine
80
92:8
a
Yield of isolated product.
b
Determined by 1H NMR of the crude product.
Table 3
Aza-MoritaeBayliseHillman-type of N,O-acetals using quinuclidine as a Lewis base
Entry
1
N,O-Acetal
Michael acceptor
Product
Yielda (%)
trans/cisb
92:8
9a
80
2
3
4
9a
9a
9a
66
43
0
61:31
>95:5
d
5
9b
86
41
68:32
6
9c
<95:5
a
Yield of isolated product.
b
Determined by 1H NMR of the crude product.

6296
J.E. Kitulagoda et al. / Tetrahedron 66 (2010) 6293e6299
cyclopentenone gave lower diastereoselectivity (entries 2 and 5)
acrolein gave very high trans-selectivity albeit in lower yield than
what we had achieved with the sulfide protocol (entry 3). Un-
expectedly, MVK failed to give the aza-MBH adduct under these
conditions. Hemi-aminal 9c14 also performed well giving the adduct
in moderate yield but very high dr with cyclohexenone (entry 6).
Rationalisation of the stereochemical outcome of the reactions
involving pyroglutamic acid derivatives is challenging since these
systems are influenced by a balance between steric and electronic
factors.15 The carbamate moiety will force the C5-substituent into
an axial position to minimize A1,3 strain.4a,16 The C5-substituent will
then block one face and result in preferential formation of the
trans-isomer [Fig. 2 (Eq. 1)]. However, electronic effects are also
operative. It has been suggested that attractive interactions be-
tween the ester side chain and silyl-based nucleophiles can be in-
volved [Fig. 2 (Eq. 2)],4a which account for the cis-selectivity
observed in allyl silane4aec,17 or silyl enol ether additions4c to such
iminium ions. Representative results from Izawa are shown in
Scheme 3.
favoring the trans-isomer. The lower diastereoselectivity observed
in certain cases may result from either competing attractive in-
teractions between the silyl enol ether nucleophile and ester
moiety or the change in dihedral angle in the TS both of which
would favor the cis-isomer. Interestingly, replacement of the ester
substituent for a substituent that cannot provide attractive in-
t
teractions (CH2OSiPh2 Bu) led to complete trans-selectivity sug-
gesting the importance of the former electronic effect (Fig. 3).
Figure 3. Origin of trans-selectivity in addition of silyl enol ethers to pyroglutamic
acid-derived iminium ions.
3. Conclusions
Nu
(sterically favoured)
Nu
O
steric effects
In conclusion, we have developed two sets of reaction condi-
tions for effecting the aza-MBH-type reaction of 5-substituted cy-
clic aminals: a protocol employing Me2S and TMSOTf, which works
well with unhindered and reactive enals and enones; or the use of
quinuclidine and TMSOTf, which is better for more hindered Mi-
chael acceptors. The reactions lead to 2,5-trans-disubstituted pyr-
rolidines with good-high diastereoselectivity. The selectivity
appears to be dominated by steric effects where a bulky silyl enol
ether approaches the iminium ion from the face opposite to the
C5-substituent. In certain cases, the degree of trans-selectivity is
reduced, presumably because of attractive electronic interactions
between the ester substituent and the silyl enol ether.
(1)
(2)
N
trans-favoured
OR2
H
N
R1O2C
O
R1O2C
OR2
Nu
O
O
OR2
N
N
H
electronic effects
cis-favoured
OR2
Nu
CO2R1
Nu
R1O
SiMe3
O
Nu
O
O
Nu
OR2
N
disfavoured
O
favoured
N
(3)
OR2
4. Experimental
N
Nu
R1O2C
Nu
CO2R1
R1O
O
R2O
4.1. General procedure for the synthesis of aza-MBH adducts
using TMSOTf/Me2S protocol
Figure 2. Electronic and steric effects on the addition of nucleophiles to pyroglutamic
acid-derived iminium ions.
A
solution of N,O-acetals (0.34 mmol), olefin substrate
(0.68 mmol) and dimethyl sulfide (39 ml, 0.51 mmol) in CH2Cl2
(1.6 ml) was cooled to ꢀ78 ꢁC, to which TMSOTf (154
ml, 0.85 mmol)
was added drop-wise. The reaction mixture was allowed to warm
to ꢀ60 ꢁC and stirred overnight (ca. 16 h). The reaction was
quenched with saturated aqueous NaHCO3 (2.5 ml), allowed to
warm to room temperature. Water (20 ml) was added and the re-
action mixture extracted with CH2Cl2 (2ꢂ20 ml). The organic
phases were combined, dried (MgSO4) and the solvent removed.
The residue was re-dissolved in CH2Cl2 (1.6 ml) and DBU (76 ml,
0.51 mmol) added. The reaction mixture was allowed to stir at
room temperature for 30 min before being diluted with CH2Cl2
(20 ml) and saturated aqueous NH4Cl (20 ml) added. The layers
were separated and the organic layer further washed with satu-
rated aqueous NH4Cl (20 ml). The aqueous layers were combined
and extracted with CH2Cl2 (20 ml). The organic phases were com-
bined, dried (MgSO4) and the solvent removed. The resultant oil
was purified by flash column chromatography.
Scheme 3. Representative results by Izawa on addition of allyl silanes and silyl enol
ethers to pyroglutamic acid-derived iminium ions.
Furthermore, it has also been suggested that trans-addition of
a nucleophile pushes the N-carbamate group towards the C5-sub-
stituent (due to changes in the dihedral angle) during the TS
thereby incurring an energetic penalty. Conversely, cis-approach
pushes the N-carbamate group away from the ester substituent and
so will be favoured electronically (Fig. 2, Eq. 3).4a,18
4.1.1. (2S,5S)-1-Benzyl 2-methyl 5-(3-oxoprop-1-en-2-yl)pyrrolidine-
1,2-dicarboxylate (10). Purification by column chromatography (1:2
EtOAc/petrol) gave 10 (66 mg, 61%) as a colourless oil.
25
Rf (2:3 EtOAc/petrol) 0.19; [
a
]
ꢀ68.5 (c 1.3, CHCl3); nmax
D
In our case, the steric bulk of the silyl enol ether seems to
dominate the stereochemical outcome of the reactions thus
(CDCl3)/cmꢀ1 1691, 1745; 1H NMR dH (400 MHz, CDCl3, rotamers)
1.54e1.68 (1H, m), 1.84e1.95 (1H, m), 1.96e2.15 (1H, m), 2.17e2.35

J.E. Kitulagoda et al. / Tetrahedron 66 (2010) 6293e6299
6297
(1H, m), 3.48, 3.67 (3H, 2ꢂs), 4.42, 4.46 (1H, 2ꢂdd, J¼8.9, 1.1 Hz),
4.91e4.98 (2H, m), 5.08, 5.12 (1H, 2ꢂd, J¼12.2 Hz), 5.93, 6.01 (1H,
2ꢂbr s), 6.00, 6.08 (1H, 2ꢂd, J¼1.5 Hz), 7.13e7.29 (5H, m), 9.50, 9.55
(1H, 2ꢂs); 13C NMR dC (101 MHz, CDCl3, rotamers) 26.9, 28.1, 28.9,
29.8, 52.1, 52.3, 55.8, 56.6, 59.7, 59.8, 67.2, 127.6, 127.9, 127.9, 128.0,
128.3, 128.4, 132.6, 132.8, 136.2, 149.1, 150.1, 153.8, 154.5, 172.4,
172.7, 193.1, 193.3; HRMS (ESI) calcd for C17H20O5N [MþHþ]
318.1336, found 318.1344; calcd for C17H19O5NNa [MþNaþ]
340.1155, found 340.1163.
(1H, m), 4.45, 4.51 (1H, 2ꢂd, J¼7.6 Hz), 4.88e4.98 (3.5H, m),
5.05e5.15 (1.5H, m), 5.91, 5.99 (1H, 2ꢂs), 6.00, 6.06 (1H, 2ꢂd,
J¼1.2 Hz), 7.13e7.28 (10H, m), 9.49, 9.53 (1H, 2ꢂs); 13C NMR dC
(101 MHz, CDCl3, rotamers) 26.8, 28.1, 28.9, 29.8, 55.8, 56.7, 59.8,
60.0, 66.8, 66.9, 67.1, 67.1, 127.6, 127.9, 128.0, 128.1, 128.2, 128.3,
128.3, 128.4, 128.5, 132.6, 132.7, 135.3, 135.5, 136.1, 136.2, 149.1,
150.1,153.7,154.5, 171.7,172.0,193.1,193.2; MS (ESI) 394.17 [MþHþ],
416.15 [MþNaþ], 432.12 [MþKþ]; HRMS (ESI) calcd for
C23H23O5NNa [MþNaþ] 416.1468, found 416.1474.
4.1.2. (2S)-1-Benzyl 2-methyl 5-(3-oxobut-1-en-2-yl)pyrrolidine-1,2-
dicarboxylate (11). Purification by column chromatography (1:2
EtOAc/petrol) gave 11 (85 mg, 75%) as a 79:21 (trans/cis) mixture of
diastereoisomers, separable by column chromatography. Both di-
astereoisomers were afforded as colourless oils.
4.1.5. (2S,5S)-Dibenzyl
carboxylate (14). Purification by column chromatography (2:1
EtOAc/petrol) gave 14 (70 mg, 62%) as a colourless oil.
5-(3-oxobut-1-en-2-yl)pyrrolidine-1,2-di-
24
Rf (1:2 EtOAc/petrol) 0.19; [
a
]
ꢀ51.0 (c 0.95, CHCl3); nmax
D
(CDCl3)/cmꢀ1 1677, 1702, 1745; 1H NMR dH (400 MHz, DMSO-d6,
rotamers) 1.42e1.54 (1H, m, C9H2), 1.80e1.93 (1H, m), 2.02e2.19
(2H, m), 2.28, 2.32 (3H, 2ꢂs), 4.59 and 4.63 (1H, 2ꢂd, J¼7.8 Hz), 4.82
and 4.86 (1H, d, J¼8.0 Hz), 5.14e4.92 (4H, m), 5.76, 5.78 (1H, 2ꢂd,
J¼1.1 Hz), 6.23, 6.24 (1H, 2ꢂs), 7.19e7.40 (10H, m); 13C NMR dC
(101 MHz, DMSO-d6, rotamers) 26.3, 26.4, 26.4, 27.4, 29.0, 30.0,
56.5, 57.1, 59.2, 59.6, 66.1, 66.1, 66.3, 66.3, 124.9, 124.9, 127.1, 127.3,
127.8, 127.8, 127.9, 127.9, 128.1, 128.2, 128.3, 128.4, 128.5, 135.7,
135.9, 136.5, 136.7, 147.2, 148.1, 153.0, 153.7, 171.8, 172.1, 199.1; MS
(ESI) 408.18 [MþHþ], 430.16 [MþNaþ], 446.14 [MþKþ]; HRMS (ESI)
calcd for C24H26O5N [MþHþ] 408.1805, found 408.1803; calcd for
C24H25O5NNa [MþNaþ] 430.1625, found 430.1622.
cis-11: Rf (1:2 EtOAc/petrol) 0.18; [
a
]
25 ꢀ55.5 (c 2.0, CHCl3); nmax
D
(CDCl3)/cmꢀ1 1672, 1705, 1746; 1H NMR dH (400 MHz, CDCl3,
rotamers) 1.59e1.86 (2H, m), 2.06e2.27 (2H, m), 2.24, 2.31 (3H,
2ꢂs), 3.54, 3.72 (3H, 2ꢂs), 4.25e4.33 (1H, m), 4.85e4.87 (1H, m),
4.96e5.09 (2H, m), 6.11, 6.16 (1H, 2ꢂs), 6.61, 6.65 (1H, 2ꢂs),
7.15e7.29 (5H, m); 13C NMR dC (101 MHz, CDCl3, rotamers) 26.1,
26.2, 28.0, 28.9, 31.4, 32.3, 52.0, 52.3, 57.6, 58.5, 60.3, 60.6, 67.1, 67.4,
126.3, 126.6, 127.6, 127.8, 128.0, 128.4, 136.3, 136.4, 147.4, 148.4,
154.0, 154.9, 173.2, 173.4, 199.2; MS (ESI) 332.15 [MþHþ], 354.13
[MþNaþ], 370.11 [MþKþ]; HRMS (ESI) calcd for C18H21O5NNa
[MþNaþ] 354.1312, found 354.1312.
25
trans-11: Rf (1:2 EtOAc/petrol) 0.14; [
a
]
ꢀ6.6 (c 0.3, CHCl3);
D
nmax (CDCl3)/cmꢀ1 1673, 1705, 1744; 1H NMR dH (400 MHz, DMSO-
d6, rotamers) 1.43e1.55 (1H, m), 1.81e1.93 (1H, m), 2.02e2.17 (2H,
m), 2.28, 2.33 (3H, 2ꢂs), 3.55, 3.65 (3H, 2ꢂs), 4.53, 4.57 (1H, 2ꢂd,
J¼8.0 Hz), 4.81, 4.86 (1H, 2ꢂd, J¼8.1 Hz), 4.91e5.10 (2H, m), 5.77,
5.79 (1H, 2ꢂd, J¼1.2 Hz), 6.23, 6.24 (1H, 2ꢂs), 7.18e7.23 (1H, m),
7.23e7.41 (4H, m); 13C NMR dC (101 MHz, DMSO-d6, rotamers) 26.3,
26.4, 27.4, 29.0, 30.0, 52.0, 56.5, 57.1, 59.0, 59.4, 66.0, 66.2, 124.8,
124.8, 127.1, 127.3, 127.7, 127.8, 128.3, 128.3, 136.5, 136.7, 147.2, 148.1,
153.0, 153.5, 172.4, 172.7, 198.9, 199.0; MS (ESI) 332.15 [MþHþ],
354.13 [MþNaþ], 370.11 [MþKþ]; HRMS (ESI) calcd for
C18H21O5NNa [MþNaþ] 354.1312, found 354.1301.
4.1.6. (2S)-Dibenzyl
5-(1-oxobut-2-en-2-yl)pyrrolidine-1,2-di-
carboxylate (15). Purification by column chromatography (1:2
EtOAc/petrol), gave the product (95 mg, 69%) as a 70:30 (trans/cis)
mixture of diastereoisomers. The diastereoisomers could be par-
tially separated, with the cis-product obtained as a single isomer.
cis-15 isolated as a >95:5 (Z/E) mixture of double bond isomers: Rf
(1:2 EtOAc/petrol) 0.31; [
a
]
D
25 ꢀ17.1 (c 0.23, CHCl3); nmax (film)/cmꢀ1
1669, 1702, 1743; 1H NMR dH (400 MHz, CDCl3, rotamers) 1.57e1.68
(1H, m), 1.72e1.86 (1H, m), 1.90, 1.99 (3H, 2ꢂd, J¼7.3 Hz), 2.06e2.21
(2H, m), 4.30, 4.36 (1H, 2ꢂt, J¼7.8 Hz), 4.79 (1H, br s), 4.92e5.13 (4H,
m), 7.14e7.28 (11H, m), 10.0, 10.1 (1H, 2ꢂs); 13C NMR dC (101 MHz,
CDCl3, rotamers) 12.6,12.7, 28.1, 29.0, 31.4, 32.2, 57.0, 57.9, 60.5, 60.8,
67.0, 67.3, 127.7, 127.9, 128.1, 128.2, 128.2, 128.4, 128.6, 135.2, 135.5,
136.2, 136.4, 139.0, 139.8, 144.5, 145.0, 154.0, 154.9, 172.4, 172.7, 190,
190.1; MS (ESI) 408.18 [MþHþ], 430.16 [MþNaþ], 446.16 [MþKþ]; MS
(ESI) calcd for C24H25O5NNa [MþNa] 430.1625, found 430.1630.
trans-15 isolated as a 25:75 (Z/E) mixture of double bond iso-
mers: Rf (1:2 EtOAc/petrol) 0.20; nmax (film)/cmꢀ1 1702, 1743; 1H
NMR dH (400 MHz, CDCl3, rotamers) 1.56e1.65 (0.5H, m), 1.69e1.79
(1.5H, m), 1.88e2.20 (3H, m), 2.22e2.37 (1H, m), 2.45e2.67 (1H, m),
4.42, 4.47 (0.23H, 2ꢂd, J¼8.8 Hz), 4.54 (0.77H, app. dt, J¼7.9,
2.7 Hz), 4.89e5.31 (5H, m), 6.34 (0.12H, q, J¼7.6 Hz), 6.38e6.46
(0.43H, m), 6.64 (0.45H, q, J¼7.2 Hz), 7.18e7.40 (10H, m), 9.23, 9.33
(0.77H, 2ꢂs), 10.12, 10.17 (0.23H, 2ꢂs); 13C NMR dC (101 MHz, CDCl3,
rotamers) 12.6, 12.8, 14.2, 14.6, 27.0, 28.2, 28.8, 29.6, 29.8, 30.4, 30.9,
53.8, 54.5, 56.8, 57.8, 66.7, 66.8, 66.8, 66.9, 67.1, 67.3, 127.7, 127.9,
128.1, 128.2, 128.3, 128.3, 128.4, 128.4, 128.4, 128.5, 128.5, 128.6,
135.4, 135.5, 135.6, 135.7, 136.3, 136.4, 136.5, 139.4, 140.2, 142.1,
142.2, 143.2, 143.9, 151.2, 152.2, 153.8, 153.8, 154.4, 154.6, 171.9,
172.2, 172.6, 172.9, 189.7, 189.8, 193.8, 194.0; MS (ESI) 408.18
[MþHþ], 430.16 [MþNaþ], 446.16 [MþKþ]; MS (ESI) calcd for
C24H25O5NNa [MþNa] 430.1625, found 430.1632.
4.1.3. (2S,5S)-1-Benzyl 2-methyl 5-(1-oxobut-2-en-2-yl)pyrrolidine-
1,2-dicarboxylate (12). Purification by column chromatography (1:2
EtOAc/petrol), gave 12 (81 mg, 72%) as a 84:16 (trans/cis) mixture of
diastereoisomers. The diastereoisomers could be partially sepa-
rated, with the trans-product (colourless oil) obtained as a 20:80
(Z/E) mixture of double bond isomers.
Rf (1:2 EtOAc/petrol) 0.18; nmax (CDCl3)/cmꢀ11701, 1744; 1H NMR
dH (400 MHz, CDCl3, rotamers) 1.59e1.66 (0.2H), 1.72e1.80 (1.6H),
1.93e2.19 (3.4H), 2.27e2.67 (1.8H), 3.53, 3.73, 3.75 (3H, 3ꢂs), 4.46,
4.49 (0.2H, 2ꢂd), 4.55, 4.57 (0.8H, 2ꢂt, J¼2.5 Hz), 4.91e5.22 (3H, m),
6.32 (0.1H, q, J¼7.5 Hz), 6.38e6.45 (0.4H, m), 6.64 (0.5H, m),
7.21e7.35 (5H, m), 9.22, 9.32 (0.8H, 2ꢂs),10.11,10.17 (0.2H, 2ꢂs); 13C
NMR dC (101 MHz, CDCl3, rotamers) 12.5, 12.7, 14.2, 14.5, 26.9, 28.1,
28.7, 29.5, 29.7, 29.7, 30.3, 30.8, 51.9, 52.0, 52.15, 52.2, 53.6, 54.3, 56.6,
57.7, 59.8, 59.9, 60.4, 60.8, 66.8, 66.9, 67.1, 67.2, 127.6, 127.7, 127.8,
127.8,127.9,128.0,128.0,128.1,128.3,128.3,128.3,128.3,136.2,136.3,
136.3, 136.5, 139.3, 140.1, 142.0, 142.1, 143.1, 143.8, 151.2, 152.1, 153.7,
154.3,172.5,172.8,173.2,173.5,189.6,189.8,193.7,193.9; HRMS (ESI)
332.15 [MþHþ], 354.13 [MþNaþ], 371.13 [MþKþ]; MS (ESI) calcd for
C18H21O5NNa [MþNaþ] 354.1312, found 354.1319.
4.1.4. (2S,5S)-Dibenzyl 5-(3-oxoprop-1-en-2-yl)pyrrolidine-1,2-di-
carboxylate (13). Purification by column chromatography (2:1
4.2. General procedure for the synthesis of aza-MBH adducts
using TMSOTf/quinuclidine protocol
EtOAc/petrol) gave 13 (61 mg, 57%) as a colourless oil.
25
Rf (1:2 EtOAc/petrol) 0.34; [
a
]
ꢀ62.5 (c 1.2, CHCl3); nmax
A solution of olefin substrate (0.68 mmol) and quinuclidine
(76 mg, 0.68 mmol) in CH2Cl2 (1 ml) was cooled to ꢀ78 ꢁC, to which
D
(CDCl3)/cmꢀ1 1692, 1745; 1H NMR dH (400 MHz, CDCl3, rotamers)
1.56e1.62 (1H, m), 1.85e1.92 (1H, m), 1.96e2.13 (1H, m), 2.18e2.30
TMSOTf (185 ml, 1.02 mmol) was added drop-wise. The cooling bath

6298
J.E. Kitulagoda et al. / Tetrahedron 66 (2010) 6293e6299
was removed and the reaction mixture allowed to warm to room
temperature. After 1 h, the reaction mixture was re-cooled to
ꢀ40 ꢁC and a solution of N,O-acetal (0.34 mmol) in CH2Cl2 (0.6 ml)
added drop-wise. The reaction mixture was stirred at ꢀ40 ꢁC for 3 h
before being quenched with saturated aqueous NaHCO3 (2.5 ml).
Water (20 ml) was added and the reaction mixture extracted with
CH2Cl2 (2ꢂ20 ml). The organic phases were combined, dried
(MgSO4) and the solvent removed. The residue was re-dissolved in
J¼8.8 Hz), 4.87 (0.5H, d, J¼8.8 Hz), 4.90 (0.5H, d, J¼7.6 Hz), 4.97 (1H,
app. t, J¼12.7 Hz), 5.19, 5.25 (1H, 2ꢂd, J¼12.2 Hz), 7.07 (0.5H, br s),
7.22e7.37 (5.5H, m); 13C NMR dC (101 MHz, CDCl3, rotamers) 26.1,
26.2, 27.5, 28.4, 28.6, 29.4, 35.3, 35.5, 52.0, 52.3, 54.4, 55.3, 59.5,
59.6, 66.9, 67.1, 127.8, 127.9, 128.0, 128.3, 128.4, 136.3, 136.5, 145.8,
146.7, 153.9, 154.5, 157.6, 158.1, 172.5, 172.9, 207.7, 207.9; MS (ESI)
344.15 [MþHþ], 366.13 [MþNaþ]; HRMS (ESI) calcd for C19H22O5N
[MþHþ] 344.1492, found 344.1506.
CH2Cl2 (1.6 ml) and DBU (76 ml, 0.51 mmol) added. The reaction
mixture was allowed to stir at room temperature for 30 min before
being diluted with CH2Cl2 (20 ml) and 1 M HCl(aqueous) (20 ml)
added. The layers were separated and the organic layer further
washed with 1 M HCl(aqueous) (20 ml). The aqueous layers were
combined and extracted with CH2Cl2 (20 ml). The organic phases
were combined, dried (MgSO4) and the solvent removed. The re-
sultant oil was purified by flash column chromatography.
4.2.3. (2S)-Dibenzyl 5-(5-oxocyclopent-1-en-1-yl)pyrrolidine-1,2-di-
23
carboxylate (18). cis-18: Rf (1:2 EtOAc/petrol) 0.22; [
a
]
þ29.2 (c
D
0.48, CHCl3); nmax (film)/cmꢀ1 1636, 1693, 1743; 1H NMR dH
(400 MHz, CDCl3, rotamers) 1.70e1.89 (2H, m), 2.02e2.58 (6H, m),
4.31, 4.38 (1H, 2ꢂt, J¼7.6 Hz), 4.66, 4.71 (1H, 2ꢂd, J¼7.2 Hz),
4.92e5.14 (4H, m), 7.13e7.30 (10H, m), 7.74 (1H, br s); 13C NMR dC
(101 MHz, CDCl3, rotamers) 23.9, 25.8, 26.7, 27.5, 28.6, 32.7, 32.9,
52.2, 53.3, 57.5, 57.8, 64.4, 64.8, 125.2, 125.3, 125.4, 125.5, 125.6,
125.7, 125.8, 125.8, 125.9, 125.9, 126.0, 132.7, 132.9, 133.7, 133.8,
142.9, 143.8, 151.4, 152.0, 157.4, 157.6, 169.9, 170.1, 205.9, 206.0; MS
(ESI) 420.18 [MþHþ], 442.16 [MþNaþ], 458.14 [MþKþ]; MS (ESI)
4.2.1. (2S)-1-Benzyl 2-methyl 5-(6-oxocyclohex-1-enyl)pyrrolidine-
1,2-dicarboxylate (16). Purification by column chromatography
(20%/33% EtOAc/petrol) gave 16 as a colourless oil (97 mg, 80%) in
a 92:8 (trans/cis) mixture of diastereoisomers, separable by column
chromatography.
calcd for C25H26O5N [MþHþ] 420.1805, found 420.1817.
23
trans-18: Rf (1:2 EtOAc/petrol) 0.15; [
a
]
D
ꢀ73.6 (c 1.21, CHCl3);
cis-16: Rf (1:2 EtOAc/petrol) 0.20; [
a
]
25 þ18.3 (c 0.6, CHCl3); nmax
nmax (film)/cmꢀ1 1635, 1693, 1742; 1H NMR dH (400 MHz, CDCl3,
rotamers) 1.74e1.81 (1H, m), 1.94e1.99 (1H, m), 2.09e2.66 (6H, m),
4.52 (0.5H, d, J¼8.1 Hz), 4.58 (0.5H, d, J¼8.6 Hz), 4.86 (0.5H, d,
J¼7.3 Hz), 4.91 (0.5H, d, J¼8.6 Hz), 4.94e5.05 (2.5H, m), 5.14e5.28
(1.5H, m), 7.08 (0.5H, br s), 7.21e7.38 (10.5H, m); 13C NMR dC
(101 MHz, CDCl3, rotamers) 26.1, 26.2, 27.4, 28.4, 28.6, 29.4, 35.3,
35.5, 54.4, 55.3, 59.7, 59.8, 66.8, 66.9, 67.0, 127.8, 127.8, 127.9, 127.9,
128.0, 128.1, 128.2, 128.3, 128.3, 128.5, 135.3, 135.5, 136.2, 136.4,
145.8, 146.7, 153.9, 154.5, 157.6, 158.1, 171.9, 172.2, 207.7, 207.9; MS
(ESI) 420.18 [MþHþ], 442.16 [MþNaþ]; MS (ESI) calcd for
C25H26O5N [MþHþ] 420.1805, found 420.1818.
D
(CDCl3)/cmꢀ1 1670, 1705, 1744; 1H NMR dH (400 MHz, CDCl3, rotam-
ers) 1.64e2.54 (10H, m), 3.61, 3.78 (3H, 2ꢂs), 4.31, 4.37 (1H, 2ꢂt,
J¼7.3 Hz), 4.91 (1H, br t, J¼10.3 Hz), 4.99, 5.04, 5.12, 5.18 (2H, 4ꢂd,
J¼12.5 Hz), 7.23e7.35 (5H, m), 7.50 (1H, app. br q, J¼3.6 Hz); 13C NMR
dC (101 MHz, CDCl3, rotamers) 22.8, 25.7, 28.2, 28.9, 31.4, 32.3, 38.5,
38.5, 52.0, 52.2, 56.7, 57.8, 60.2, 60.4, 67.0, 67.3, 127.6, 127.8, 127.9,
128.0,128.4,136.3,136.5,138.1,139.0,145.5,145.9,154.0,154.9,173.3,
173.5, 198.9; MS (ESI) 358.17 [MþHþ], 380.15 [MþNaþ]; HRMS (ESI)
calcd for C20H23O5NNa [MþNaþ] 380.1468, found 380.1475.
25
trans-16: Rf (1:2 EtOAc/petrol) 0.14; [
a]
ꢀ79.1 (c 0.37, CHCl3);
D
nmax (CDCl3)/cmꢀ1 1672, 1700, 1741; 1H NMR dH (400 MHz, CDCl3,
rotamers) 1.59e1.67 (1H, m), 1.84e2.49 (9H, m), 3.54 and 3.74 (3H,
2ꢂs), 4.45, 4.50 (1H, 2ꢂdd, J¼8.9, 1.1 Hz), 4.92, 4.98, 5.19, 5.27 (2H,
4ꢂd, J¼12.4 Hz), 5.00e5.04 (1H, m), 6.46, 6.58 (1H, 2ꢂdt, J¼4.2,
1.1 Hz), 7.22e7.37 (5H, m); 13C NMR dC (101 MHz, CDCl3, rotamers)
22.6, 22.6, 25.4, 25.5, 26.9, 28.1, 29.5, 30.3, 38.4, 38.5, 52.0, 52.2,
56.3, 57.4, 59.8, 59.9, 66.7, 67.0,127.7,127.8,127.9,127.9,128.3,128.3,
136.3, 136.6, 138.3, 139.2, 143.2, 14.3,153.7, 154.5, 172.5, 172.8, 198.4;
MS (ESI) 358.17 [MþHþ], 380.15 [MþNaþ], 396.12 [MþKþ]; HRMS
(ESI) calcd for C20H23O5NNa [MþNaþ] 380.1468, found 380.1475.
4.2.4. (2S)-Benzyl 2-(((tert-butyldiphenylsilyl)oxy)methyl)-5-(6-oxo-
cyclohex-1-en-1-yl)pyrrolidine-1-carboxylate (19). Purification by
column chromatography (1:4 EtOAc/petrol) gave 19 (79 mg, 41%) as
a colourless oil.
25
Rf (1:4 EtOAc/petrol) 0.29; [
a
]
ꢀ65.6 (c 0.9, CHCl3); nmax
D
(film)/cmꢀ1 1697, 1671; 1H NMR dH (400 MHz, DMSO-d6, rotamers)
0.98, 1.00 (9H, 2ꢂs), 1.37 (1H, app. ddd, J¼12.7, 12.7, 5.1 Hz),
1.66e1.97 (4H, m), 2.04e2.37 (5H, m), 3.60 (0.4H, dd, J¼9.8, 7.1 Hz),
3.72e3.81 (1.6H, m), 4.01e4.10 (1H, m), 4.69, 4.75 (1H, 2ꢂd,
J¼8.3 Hz), 4.78 (0.6H, d, J¼12.6 Hz), 4.89 (0.8H, m), 5.16 (0.6H, d,
J¼12.6 Hz), 6.45, 6.48 (1H, 2ꢂt, J¼4.0 Hz), 7.07e7.12 (0.8H, m),
7.18e7.35 (4.2H, m), 7.37e7.50 (6H, m), 7.54e7.65 (4H, m); 13C
NMR dC (101 MHz, DMSO-d6, rotamers) 18.7, 18.8, 22.3, 22.4, 24.4,
24.9, 25.3, 26.5, 28.8, 30.3, 38.0, 37.9, 55.7, 56.2, 58.4, 59.0, 63.1,
63.9, 65.5, 65.9, 127.4, 127.6, 127.6, 127.7, 127.8, 1278, 127.9, 128.2,
128.2, 129.8, 132.8, 132.9, 132.9, 135.0, 136.6, 137.0, 138.0, 139.2,
142.5, 142.9, 152.7, 153.2, 197.8, 198.0; MS (ESI) 568.29 [MþHþ],
590.27 [MþNaþ]; HRMS (ESI) calcd for C35H42O4NSi [MþHþ]
568.2878, found 568.2903; calcd for C35H41O4NNaSi [MþNaþ]
590.2697, found 590.2722.
4.2.2. (2S)-1-Benzyl 2-methyl 5-(5-oxocyclopent-1-en-1-yl)pyrroli-
dine-1,2-dicarboxylate (17). A portion of the crude material (116 mg
of 130 mg) was purified by column chromatography (1:2 EtOAc/
petrol), affording the product as a 69:31 (trans/cis) mixture of di-
astereoisomers, partially separable by column chromatography.
Both diastereoisomers were afforded as colourless oils (71 mg, 66%
scaled yield).
cis-17: Rf (1:2 EtOAc/petrol) 0.11; [
a
]
24 þ27.3 (c 0.22, CHCl3); nmax
D
(film)/cmꢀ1 1636, 1696, 1747; 1H NMR dH (400 MHz, CDCl3, rotam-
ers) 1.81e1.94 (2H, m), 2.20e2.27 (2H, m), 2.31e2.70 (4H, m), 3.62,
3.79 (3H, 2ꢂs), 4.34, 4.39 (1H, 2ꢂt, J¼7.5 Hz), 4.74, 4.80 (1H, 2ꢂt,
J¼7.3 Hz), 5.05, 5.14, 5.15 (2H, 3ꢂd, J¼12.5 Hz), 7.23e7.36 (5H, m),
7.94 (1H, br s), 13C NMR dC (101 MHz, CDCl3, rotamers) 26.5, 28.4,
29.2, 30.1, 31.2, 35.3, 35.5, 52.1, 52.3, 54.8, 55.8, 59.9, 60.1, 67.1, 67.4,
127.8, 128.0, 128.4, 136.3, 136.4, 145.6, 146.5, 153.9, 154.6, 159.9,
160.1, 173.3, 173.4, 208.6, MS (ESI) 344.15 [MþHþ], 366.13 [MþNaþ],
382.11 [MþKþ]; HRMS (ESI) calcd for C19H22O5N [MþHþ] 344.1492,
Acknowledgements
We would like to thank the EPSRC for support of this work. We
are grateful to Dr. Craig Butts for assistance with NMR analysis.
Supplementary data
found 344.1503.
24
trans-17: Rf (1:2 EtOAc/petrol) 0.07; [
a
]
þ25 (c 0.36, CHCl3);
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2010.05.047. This data in-
clude MOL file and InChIKey of the most important compounds
described in this article.
D
nmax (film)/cmꢀ1 1636, 1696, 1747; 1H NMR dH (400 MHz, CDCl3,
rotamers) 1.76e1.84 (1H, m), 1.93e2.02 (1H, m), 2.09e2.66 (6H, m),
3.55, 3.74 (3H, 2ꢂs), 4.48 (0.5H, d, J¼7.8 Hz), 4.52 (0.5H, d,

J.E. Kitulagoda et al. / Tetrahedron 66 (2010) 6293e6299
6299
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