Synthesis of orthogonally protected 1,2-diaminopropanoic acids by ring-opening of 3-unsubstituted N-activated aziridine 2-carboxylates with para-methoxybenzylamine: A study of the regioselectivity of the reaction

Article abstract of DOI:10.1016/j.tetlet.2013.09.116

Orthogonally protected 1,2-diaminopropanoic acids (DAPs) have been synthesised in good yields by the ring-opening of 3-unsubstituted N-activated aziridine 2-carboxylates with para-methoxybenzylamine. The choice of both the N-activating group and ester alk

Full text of DOI:10.1016/j.tetlet.2013.09.116

Tetrahedron Letters 54 (2013) 6627–6630
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of orthogonally protected 1,2-diaminopropanoic acids
by ring-opening of 3-unsubstituted N-activated aziridine
2-carboxylates with para-methoxybenzylamine: a study
of the regioselectivity of the reaction
⇑
Keith O’Brien, Fintan Kelleher
Molecular Design and Synthesis Group, Centre of Applied Science for Health, Institute of Technology Tallaght, Dublin 24, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 July 2013
Revised 2 September 2013
Accepted 25 September 2013
Available online 1 October 2013
Orthogonally protected 1,2-diaminopropanoic acids (DAPs) have been synthesised in good yields by the
ring-opening of 3-unsubstituted N-activated aziridine 2-carboxylates with para-methoxybenzylamine.
The choice of both the N-activating group and ester alkyl group had a significant influence on the ratio
of attack at the
a
or b positions of the aziridine. However, the regiochemical outcome is not predictable.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
1,2-Diaminopropanoic acids (DAPs)
Aziridine 2-carboxylates
Ring-opening
Regioselectivity
Interest in
a
,b-diaminopropanoic acids [1,2-diaminopropanoic
aziridines has been shown to be quite straightforward, mostly
involving the nucleophilic attack at the less hindered carbon atom,
with some exceptional cases comprising the nucleophilic attack at
the benzylic position’. An examination of the regioselectivity of
these reactions shows that the outcome is not always that predict-
able,6 particularly when the nucleophile is a primary amine, and
the 2-substituent is also electron-withdrawing (e.g., an ester
group). For example, Rich found that reaction of N-tosyl aziridine
2-carboxylic acid with methylamine gave a 1:1 ratio of products
acids (DAPs)] has expanded over the years, as evidenced by the in-
crease in publications in the area. A recent review,1 and its update,2
have highlighted methodologies for their synthesis, as well as their
possible applications and biological activities. Natural biologically
active molecules, which contain the
motif, are known and they are stereochemically pure as single
a,b-diaminopropanoic acid
enantiomers or diastereoisomers. Therefore methods from the
stereoselective synthesis of
important. In this context, the use of
the chiral pool, where one or more stereocentres are already pres-
ent, as precursors for ,b-diaminopropanoic acids, is a key method
a,b-diaminopropanoic acids are very
a
-amino acid derivatives from
resulting from
drance at the 2-position was increased, by using the 2-tert-butyl
ester derivative, the :b ratio was 1:6.3. The van Boom group found
a
- and b-attack.7 However, when the steric hin-
a
a
that has been employed. One of the main routes employed for the
synthesis of DAPs is the ring-opening of N-activated aziridine 2-
carboxylate esters with primary amines. Although there is a large
body of literature on the ring-opening of aziridines in general,3
as well as 2-acyl aziridines4 and N-activated aziridines,5 the
number of reports on the ring-opening of N-activated aziridine
2-carboxylate esters is fewer. In all such reactions the nucleophile
a 3.5:1 ratio, in favour of attack at the less hindered b-position, for
the reaction of benzylamine with N-ortho-nitrobenzenesulfonyl
(o-Ns) aziridine 2-t-butyl ester, which increased to a 15–17:1 ratio
when the primary amine was an
steric hindrance was increased significantly, by linking the ester
a
-amino acid ester.8 When the
group at the aziridine 2-position to a solid support, Olsen found
can attack at either the a- or b-position of the aziridine ring to give
regioisomeric products (Fig. 1).
H
H
CO2R
α
Nu: or
EWG
β
CO2R
In a recent review,3a De Kimpe and Ha stated that ‘the regiose-
lectivity in the ring-opening reactions of 2-substituted activated
CO2R
N-EWG
N
H
Nu
+
+
N
Nu
Nu-
H
EWG
α-attack
β-attack
⇑
Corresponding author. Address: Department of Science, Institute of Technology
Tallaght, Tallaght, Dublin 24, Ireland. Tel.: +353 1 404 2869; fax: +353 1 404 2700.
Figure 1. Regiochemistry of nucleophilic ring-opening of N-activated aziridine 2-
E-mail address: fintan.kelleher@ittdublin.ie (F. Kelleher).
carboxylates.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.09.116

6628
K. O’Brien, F. Kelleher / Tetrahedron Letters 54 (2013) 6627–6630
CO2All
H
H
H
PMB
CO2All
NHp-Nz
TsHN
MeO2C
CO2All
NHp-Nz
N
H
N
PMB
N
p-Nz
Figure 2. Synthesis of orthogonally protected azalanthionines (lanazanines).10
that this led to exclusive attack at the less hindered b-position,
with no evidence for any
-attack.9 The Franzyk group reported a
9:1 ratio, in favour of b-attack, when the ester group of an -amino
The required DAPs were prepared by the ring-opening of
3-unsubstituted N-activated aziridine 2-carboxylate esters with
benzylamines, or by the Mitsunobu reaction of serine derivatives.11
During the aziridine ring-opening reactions it was found that there
were issues with the regioselectivity of the reactions, depending on
the choice of the N-protecting group and ester group at the
aziridine 2-position. Since the initial results showed that the regio-
chemical outcome was not straightforward, an extended study was
undertaken to determine whether the regioselectivity was in any
way predictable. The required N-activated aziridine 2-carboxylates
were prepared as shown in Scheme 1.10,12
In each case the protecting groups chosen were those that have
been used previously in solid-phase peptide syntheses, particularly
in the preparation of lanthionine containing peptides and their
analogues.12 Thus the N-protecting groups used were Ts,
para-nitrobenzenesulfonyl (p-Ns), and para-nitrobenzyloxycarbon-
yl (p-Nz), while the methyl and allyl esters were chosen for the azi-
ridine 2-position. Preparation of a DAP with the b-amino group
also being protected was achieved by ring-opening of aziridine
2a with para-methoxybenzylamine (PMB-NH2) following the
method of Kim.13 This involved heating the reaction to 80 °C in
acetonitrile for 24 h with two molar equiv of PMB-NH2, which gave
the two regioisomeric products from nucleophilic attack at either
a
a
ester was linked to a solid support, and it was reacted with N–pNs
aziridine methyl 2-carboxylate.5h
As part of an ongoing programme of research towards the
synthesis of peptides incorporating unusual amino acid residues,
we are interested in the synthesis of b-monoalkyl substituted
1,2-diaminopropanoic acids (DAPs). In particular, the use of
orthogonally protected DAPs would allow their incorporation into
peptide structures and then their subsequent derivatisation. We
previously reported the synthesis, and subsequent use, of such
orthogonally protected DAPs for the preparation of orthogonally
protected azalanthionines (lanazanines), which are nitrogen-
linked analogues of the more common amino acid lanthionine
(Fig. 2).10
CO2R
CO2R
a
b
c
2a, R = Me
N
Trt
N
Ts
2b, R = Allyl
1a, R = Me
CO2R
CO2R
1b, R = Allyl
3a, R = Me
N
p-Ns
3b, R = Allyl
the
a- or b-positions of the aziridine. The PMB group was chosen
because it can be easily removed in solid-phase synthesis. The
yield of the product (Table 1) from attack at the less hindered b-po-
4a, R = Me
N
p-Nz
sition was 32%, while attack at the
(entry 1).
a-position gave a 41% yield
4b, R = Allyl
Conducting the reaction at room temperature for 24 h gave a
reversal in the observed selectivity, with the b-attack giving a yield
CO2Allyl
5b
Alloc
d
1b
N
of 70% and
higher temperatures was not unexpected, but the 23% yield of
the compound isolated from attack at the hindered -position, at
room temperature, was somewhat of surprise. Since the
a-attack of 23% (entry 2). The selectivity observed at
a
Scheme 1. Synthesis of N-activated aziridine 2-carboxylate esters.10,12 Reagents
and conditions: (a) (i) 50% TFA in CH2Cl2/MeOH (1:1), rt, 30 min, (ii) NaHCO3, H2O,
rt, (iii) p-TsCl, EtOAc, rt, 24 h, (2a 85%, 2b 83%); (b) (i) 50% TFA in CH2Cl2/MeOH
(1:1), rt, 30 min, (ii) NaHCO3, H2O, rt, (iii) p-nitrobenzenesulfonyl chloride, EtOAc,
rt, 24 h (3a 85%, 3b 80%); (c) (i) 50% TFA in CH2Cl2/MeOH (1:1), rt, 30 min, (ii)
NaHCO3, H2O, rt, (iii) p-nitrobenzyl chloroformate, EtOAc, rt, 24 h (4a 82%, 4b 82%);
(d) (i) 50% TFA in CH2Cl2/MeOH (1:1), rt, 30 min, (ii) NaHCO3, H2O, rt, (iii) allyl
chloroformate, EtOAc, rt, 24 h, 34%.
a
conditions required to remove the tosyl group are very harsh for
solid-phase peptide synthesis the alternative N-activating,
peptide-friendly, p-Ns and p-Nz groups were chosen, for
comparison with the N-tosyl aziridine 2a. (Note: the N-Fmoc
protected aziridine was not chosen because it was felt that the
Table 1
Regioselectivity of the ring-opening of aziridines with p-methoxybenzylamine
H
H
CO2R
α
β
EWG
CO2R
PMB
PMB
CO2R
CH3CN
rt, 24 h
H2N
N
H
N
H
+
+
N
HN
NHEWG
6
OMe
EWG
7
β-attack
α-attack
Entry
Aziridine
% b-attacka
% a
-attacka
1
2
3
4
5
6
7
8
2a (R = Me, EWG = Ts)
2a (R = Me, EWG = Ts)
32 (6a)b
70 (6a)
63 (6b)
56 (6c)
—
41 (7a)b
23 (7a)
21 (7b)
—
—
—
—
—
3a (R = Me, EWG = p-Ns)
4a (R = Me, EWG = p-Nz)
2b (R = Allyl, EWG = Ts)
3b (R = Allyl, EWG = p-Ns)
4b (R = Allyl, EWG = p-Nz)
5b (R = Allyl, EWG = Alloc)
—
66 (6d)
—
a
Product number is in parentheses.
Reaction conducted at 80 °C.
b

K. O’Brien, F. Kelleher / Tetrahedron Letters 54 (2013) 6627–6630
6629
secondary amino DAP product might lead to a loss of the Fmoc
group.) As can be seen, from Table 1, the p-Ns protected aziridine
methyl ester 3a (entry 3) gave a similar result to aziridine 2a
opening of 3-methyl N-activated aziridine 2-carboxylate esters,
en route to the aza analogues of the b-methyllanthionines found
in the ring structures of many important lantibiotics.15 The results
of all of these studies will be reported in due course.
A typical procedure is exemplified by the synthesis of 6a and 7a.
To a solution of the relevant aziridine (2 mmol) in CH3CN (5 ml)
was added p-methoxybenzylamine (0.52 ml, 4 mmol) and the
solution was stirred for 24 h at room temperature. The solvent
was removed in vacuo, and then the residue was re-dissolved in
EtOAc (20 ml), washed with brine (2 ꢀ 20 ml), dried over
anhydrous MgSO4 and concentrated in vacuo. The crude product
was purified by flash column chromatography on silica gel in
petroleum ether/EtOAc (2:1).
(63% b and 21%
was no product obtained from attack at the hindered
a
). However, when the p-Nz group was used there
-position,
a
with the DAP product 6c, from attack at the b-position, being the
sole product isolated in a yield of 56% (entry 4). This result is
comparable to the outcome of Harada’s study of the reaction of a
2-carboxamido N-Cbz aziridine with m-methylbenzylamine in
toluene at 80 °C, where the product from attack at the b-position
was the sole product isolated, in a 79% yield.14 It therefore appears
that incorporation of a sulfonamide group on the aziridine nitrogen
leads to an increase in the amount of nucleophilic attack at the
hindered
a
-position. It was thought that perhaps the increased
(L)-Methyl 3-(4-methoxybenzylamino)-2-(4-methylphenylsulfo-
namido) propanoate (6a). Colourless oil (0.54 g, 70%); Rf: 0.12
electron-withdrawing properties of the sulfonamide group were
responsible for this.
petroleum ether/EtOAc (1:1); ½a D20
ꢁ
+13.35 (c 1.0 in CHCl3); 1H
Therefore, in order to extend this study, it was thought that
changing the ester methyl group, again to the more peptide-
friendly allyl ester group, would be advantageous. However, this
small change led to some surprising results (Table 1). To keep
the results comparable PMB-NH2 was again used as the nucleo-
phile. In this case aziridines 2b and 3b did not give any reaction
products (entries 5 and 6), at either room temperature or at
80 °C in acetonitrile, while the p-Nz aziridine 4b again gave only
b-attack, giving DAP 6d, in an isolated yield of 66% (entry 7). The
N-alloc protected aziridine allyl ester 5b also did not give any
reaction at either room temperature or 80 °C (entry 8), which
was surprising when compared to Harada’s study with a Cbz-pro-
tected aziridine (vide supra).
It was clear that the observed regioselectivity of ring-opening
was not predictable, with small changes in the choice of activating
groups leading to quite different outcomes. Since the primary
amine employed in these studies was kept constant we next
examined the aziridine partner, in order to try to understand the
observed regioselectivity. Perhaps an analysis of the 1H NMR
NMR (CDCl3, d ppm) 7.72 (d, 2H, J = 8.2 Hz), 7.28 (d, 2H,
J = 8.8 Hz), 7.16 (d, 2H, J = 8.8 Hz), 6.85 (d, 2H, J = 8.8 Hz), 4.03 (t,
1H, J = 4.8 Hz), 3.79 (s, 3H), 3.65 (d, 1H, J = 12.9 Hz), 3.61 (d, 1H,
J = 12.9 Hz), 3.52 (s, 3H), 2.88 (d, 2H, J = 5.2 Hz), 2.40 (s, 3H); 13C
NMR (CDCl3, d ppm) 171.2, 158.7, 143.7, 136.6, 131.3, 129.6,
129.3, 127.2, 113.8, 60.4, 55.3, 52.6, 52.3, 50.0, 21.5; IR (thin film,
NaCl, cmꢂ1) 3347, 3089, 2981, 1744, 1188, 1150; MS (ES+) for C19-
H25N2O5S, expected [M+H] 393.1479, observed [M+H] 393.1477.
(D)-Methyl 2-(4-methoxybenzylamino)-3-(4-methoxyphenylsulfo-
namido) propanoate (7a). Colourless oil (0.17 g, 23%); Rf: 0.24
petroleum ether/EtOAc (1:1); ½a D20
ꢁ
+12.41 (c 1.0 in CHCl3); 1H
d ppm) 7.70 (d, 2H, J = 8.6 Hz), 7.28 (d, 2H,
NMR (CDCl3,
J = 8.0 Hz), 7.15 (d, 2H, J = 8.6 Hz), 6.85 (d, 2H, J = 8.6 Hz), 5.15 (br
s, 1H) 3.80 (s, 3H), 3.68 (s, 3H), 3.65 (m, 2H), 3.51 (d, 1H,
J = 12.9 Hz), 3.30 (m, 1H), 3.26 (dd, 1H, J = 8.6, 8.6 Hz), 2.95 (dd,
1H, J = 7.5, 7.4 Hz), 2.41 (s, 3H), 1.88 (br s, 1H); 13C NMR (CDCl3,
d ppm) 172.9, 158.9, 143.5, 136.6, 131.0, 129.7, 129.4, 127.0,
113.9, 59.0, 55.3, 52.3, 51.2, 44.2, 21.5; IR (thin film, NaCl, cmꢂ1
3359, 3082, 2991, 1743, 1224, 1124.
)
chemicals shifts of the
a- and b-protons of the aziridine rings,
along with their corresponding carbon chemical shifts in 13C
NMR spectra, would help the understanding of the observed re-
sults (Table 2).
Acknowledgements
We are grateful to Strand I of the Irish Government’s National
Development Plan Technological Sector Research Program for
funding (Grant PP07/TA07) and the Higher Education Authority’s
Programme for Research in Third-Level Institutions IV for funding
for K.O’B.
However, it can be seen that there is very little difference
between the analogous 1H and 13C NMR spectral chemical shifts
of the aziridines 2–5. It is therefore difficult to make a definitive
prediction of the regioselectivity for a particular ring-opening
reaction of 3-unsubstituted N-activated aziridine 2-carboxylate
esters with a primary amine.
References and notes
In conclusion, we have shown that the ring-opening of N-acti-
vated aziridine 2-carboxylate esters with p-methoxybenzylamine
is far from predictable. We are currently undertaking in-depth
computational studies on these reactions, including an examina-
tion of possible transition states and their energies, in order to
try to understand the experimental results observed. The
chemistry is also being extended to the synthesis of the
corresponding orthogonally protected b-methyl DAPs, by ring-
1. Viso, A.; Fernández de la Pradilla, R.; García, A.; Flores, A. Chem. Rev. 2005, 105,
3167–3196.
2. Viso, A.; Fernández de la Pradilla, R.; Tortosa, M.; García, A.; Flores, A. Chem.
Rev. 2011, 111, PR1–PR42.
3. (a) Stankovic´, S.; D’hooghe, M.; Catak, S.; Eum, H.; Waroquier, M.; Van
Speybroeck, V.; De Kimpe, N.; Ha, H.-J. Chem. Soc. Rev. 2012, 41, 643–665; (b)
Lu, P. Tetrahedron 2010, 66, 2549–2560; (c) Schneider, C. Angew Chem., Int. Ed.
2009, 48, 2082–2084; (d) Pineschi, M. Eur. J. Org. Chem. 2006, 4979–4988; (e)
Wu, J.; Sun, X.; Sun, W. Org. Biomol. Chem. 2006, 4, 4231–4235; (f) Hu, X. E.
Tetrahedron 2004, 60, 2701–2743; (g) Cardillo, G.; Gentilucci, L.; Tolomelli, A.
Aldrichim. Acta 2003, 36, 39–50.
4. (a) Lee, K. M.; Kim, J. C.; Kang, P.; Lee, W. K.; Eum, H.; Ha, H.-J. Tetrahedron 2012,
68, 883–893; (b) Yook, C.-M.; Eum, H.; Ha, H.-J.; Kang, K. Y.; Lee, W. K. Bull.
Korean Chem. Soc. 2011, 32, 2879–2880; (c) Park, J.-H.; Ha, H.-J.; Lee, W. K.;
Généreux-Vincent, T.; Kazlauskas, R. J. ChemBioChem 2009, 10, 2213–2222; (d)
Gnecco, D.; Orea, L.; Galando, F. A.; Enríquez, R. G.; Toscano, R. A.; Reynolds, W.
F. Molecules 2000, 5, 998–1003.
5. A non-exhaustive list of recent examples includes: (a) Bera, M.; Pratihar, S.;
Roy, S. J. Org. Chem. 2011, 76, 1475–1478; (b) Samanta, K.; Panda, G. Chem.
Asian J. 2011, 6, 189–197; (c) Ottesen, L. K.; Jaroszewski, J. W.; Franzyk, H. J. Org.
Chem. 2010, 75, 4983–4991; (d) Kelley, B. T.; Joullie, M. Org. Lett. 2012, 12,
4244–4247; (e) Ghoral, M. K.; Kumar, A.; Tiwari, D. P. J. Org. Chem. 2010, 75,
137–151; (f) Ottesen, L. K.; Olsen, C. A.; Witt, M.; Jaroszewski, J. W.; Franzyk, H.
Chem. Eur. J. 2009, 15, 2966–2978; (g) Moss, T. A.; Fenwick, D. R.; Dixon, D. J. J.
Am. Chem. Soc. 2008, 130, 10076–10077; (h) Crestey, F.; Witt, M.; Frydenvang,
K.; Staerk, D.; Jaroszewski, J. W.; Franzyk, H. J. Org. Chem. 2008, 73, 3566–3569;
Table 2
1H (300 MHz) and 13C (75 MHz) NMR chemical shifts (in ppm) of the
a
- and b-protons
and carbons of aziridines 2–5, run in CDCl3
Aziridine
a
-H
b-H
a-C
b-C
2a
3a
4a
2b
3b
4b
5b
3.33
3.38
3.09
3.36
3.47
3.22
3.13
2.76 and 2.59
2.83 and 2.60
2.57 and 2.45
2.76 and 2.58
2.91 and 2.68
2.65 and 2.57
2.61 and 2.50
35.6
36.1
35.0
35.8
36.3
34.9
31.3
31.9
32.5
31.4
32.0
32.5
31.4
29.7

6630
K. O’Brien, F. Kelleher / Tetrahedron Letters 54 (2013) 6627–6630
(i) Paixao, M. W.; Nielsen, M.; Jacobsen, C. B.; Jorgensen, K. A. Org. Biomol. Chem.
2008, 6, 3467–3470; (j) Bisai, A.; Bhanu Prasad, B. A.; Singh, V. K. ARKIVOC 2007,
v, 20–37.
15. A non-exhaustive recent list includes: (a) Arnison, P. G.; Bibb, M. J.; Bierbaum,
G.; Bowers, A. A.; Bugni, T. S.; Bulaj, G.; Camarero, J. A.; Campopiano, D. J.;
Challis, G. L.; Clardy, J.; Cotter, P. D.; Craik, D. J.; Dawson, M.; Dittmann, E.;
Donadio, S.; Dorrestein, P. C.; Entian, K.-D.; Fischbach, M. A.; Garavelli, J. S.;
Göransson, U.; Gruber, C. W.; Haft, D. H.; Hemscheidt, T. K.; Hertweck, C.; Hill,
C.; Horswill, A. R.; Jaspars, M.; Kelly, W. L.; Klinman, J. P.; Kuipers, O. P.; Link, A.
J.; Liu, W.; Marahiel, M. A.; Mitchell, D. A.; Moll, G. N.; Moore, B. S.; Müller, R.;
Nair, S. K.; Nes, I. F.; Norris, G. E.; Olivera, B. M.; Onaka, H.; Patchett, M. L.; Piel,
J.; Reaney, M. J. T.; Rebuffat, S.; Ross, R. P.; Sahl, H.-G.; Schmidt, E. W.; Selsted,
M. E.; Severinov, K.; Shen, B.; Sivonen, K.; Smith, L.; Stein, T.; Süssmuth, R. D.;
Tagg, J. R.; Tang, G.-L.; Truman, A. W.; Vederas, J. C.; Walsh, C. T.; Walton, J. D.;
Wenzel, S. C.; Willey, J. M.; Van der Donk, W. Nat. Prod. Rep. 2013, 30, 108–160;
(b) Knerr, P. J.; Van der Donk, W. A. Annu. Rev. Biochem. 2012, 81, 479–505; (c)
Montalbán-López, M.; Zhou, L.; Buivydas, A.; van Heel, A. J.; Kuipers, O. P.
Expert Opin. Drug Discov. 2012, 7, 695–709; (d) Al-Mahrous, M. M.; Upton, M.
Expert Opin. Drug Discov. 2011, 6, 155–170; (e) Lee, H.; Kim, H.-Y. J. Microbiol.
Biotechnol. 2011, 21, 229–235; (f) Kuipers, A.; Rink, R.; Moll, G. N. Prokaryotic
Antimicrobial Peptides; Springer, 2011. pp 147–169; (g) Asaduzzaman, S. M.;
Sonomoto, K. J. Biosci. Bioeng. 2009, 107, 475–487; (h) Piper, C.; Cotter, P. D.;
Ross, R. P.; Hill, C. Curr. Drug Discov. Technol. 2009, 6, 1–18.
6. For example, see: Tinotta, I.; Fifer, N. L.; Eakins, J.; Hutton, C. A. Tetrahedron Lett.
2013, 54, 618–620.
7. Solomon, M. E.; Lynch, C. L.; Rich, D. H. Tetrahedron Lett. 1995, 36, 4955–4958.
8. Turner, J. J.; Sikkema, F. D.; Filippov, D. V.; van der Marel, G. A.; van Boom, J. H.
Synlett 2001, 1727–1730.
9. Olsen, C. A.; Christensen, C.; Nielsen, B.; Mohamed, F. M.; Witt, M.; Clausen, R.
P.; Kristensen, J. L.; Franzyk, H.; Jaroszewski, J. W. Org. Lett. 2006, 8, 3371–3374.
10. O’Brien, K.; Ó Proinsias, K.; Kelleher, F. Tetrahedron Lett. 2013, 54, 2395–2397.
11. Kelleher, F.; Ó Proinsias, K. Tetrahedron Lett. 2007, 48, 4879–4882.
12. (a) Ross, A. C.; McKinnie, S. M. K.; Vederas, J. C. J. Am. Chem. Soc. 2012, 134,
2008–2011; (b) Liu, H.; Pattabiraman, V. R.; Vederas, J. C. Org. Lett. 2009, 11,
5574–5577; (c) Liu, H.; Pattabiraman, V.; Vederas, J. Org. Lett. 2007, 9, 4211–
4214.
13. Kim, B. M.; So, S. M. Tetrahedron Lett. 1999, 40, 7687–7690.
14. Harada, H.; Morie, T.; Suzuki, T.; Yoshida, T.; Kato, S. Tetrahedron 1998, 54,
10671–10676.