Synthesis of indolylalkylphosphonates and 3-(1-diphenylphosphinoalkyl) indoles by reaction of 3-(1-arylsulfonylalkyl) indoles with phosphorus derivatives

Article abstract of DOI:10.1016/j.tetlet.2008.07.077

Dialkyl phosphites as well as diphenylphosphine react with 3-(1-arylsulfonylalkyl) indoles under basic conditions leading to a formal substitution of the arylsulfonyl group through a reactive 3-alkylidene indole intermediate.

Full text of DOI:10.1016/j.tetlet.2008.07.077

Tetrahedron Letters 49 (2008) 5645–5648
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Tetrahedron Letters
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Synthesis of indolylalkylphosphonates and 3-(1-diphenylphosphinoalkyl)
indoles by reaction of 3-(1-arylsulfonylalkyl) indoles with phosphorus
derivatives
*
Marino Petrini , Rafik R. Shaikh
Dipartimento di Scienze Chimiche, Università di Camerino, via S. Agostino, 1, I-62032 Camerino, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Dialkyl phosphites as well as diphenylphosphine react with 3-(1-arylsulfonylalkyl) indoles under basic
conditions leading to a formal substitution of the arylsulfonyl group through a reactive 3-alkylidene
indole intermediate.
Received 19 June 2008
Revised 9 July 2008
Accepted 11 July 2008
Available online 17 July 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Conjugate addition
Imines
Indoles
Phosphines
Phosphites
Sulfones
The base-promoted hydrophosphonylation of electrophilic
compounds represents a viable procedure to introduce a phosphon-
yl group into a wide array of functionalized substrates. According
SO2Ar
R1
R1 CHO
ArSO2H
Base
to this synthetic strategy, a-hydroxy as well as a-aminophospho-
R
-ArSO2H
R
N
N
H
nates can be readily prepared using aldehydes and imino deriva-
tives as electrophiles.1 Although less explored, conjugate addition
of dialkyl phosphites to electron-poor olefins also represents a via-
ble procedure to obtain b-phosphonylated derivatives.2 In this con-
text, the utilization of vinylogous imino derivatives in the reaction
H
1
2
R1 = alkyl, aryl
Nu
R1
with dialkyl phosphites usually provides a-aminoalkenyl phospho-
nates through a regioselective 1,2-addition reaction.3 Phosphonic
acid derivatives can replace the more popular carboxylic group in
many pharmaceutical targets introducing a consistent modifica-
tion in their biological activity.4 Furthermore, arylphosphonic acids
have recently emerged as an important class of useful catalysts
amenable of promoting enantioselective as well as many other
synthetic processes.5 Recently, we have demonstrated that 3-(1-
arylsulfonylalkyl) indoles 2 readily obtained by a three-component
coupling from indoles 1 are effective precursors of vinylogous-type
imino derivatives 3 that regioselectively add nucleophilic reagent
at the exocyclic double bond leading to the corresponding substi-
tuted indoles 4 (Scheme 1).6
R1
Nu
R
R
N
N
H
3
4
Scheme 1.
Organometallic reagents as well as stabilized carbanions can be
efficiently used as nucleophiles in this process, thus providing a
straightforward synthesis of branched indole derivatives. The utili-
zation of dialkyl phosphites 7 as heteronucleophiles in the reaction
with sulfonyl indoles 6 is also successful in providing the corres-
ponding indolylalkylphosphonates (Scheme 2, Table 1).
Indoles bearing an alkylphosphonyl substituent at 3-position
are featured by
Furthermore, such derivatives are also pivotal intermediates in
a
consistent pharmacological activity.7
* Corresponding author. Tel.: +39 0737 402253; fax: +39 0737 402297.
E-mail address: marino.petrini@unicam.it (M. Petrini).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.07.077

5646
M. Petrini, R. R. Shaikh / Tetrahedron Letters 49 (2008) 5645–5648
Table 1 (continued)
O
OR3
OR3
R2
O
PH
Sulfone Phosphonate Productsa
8
Yieldb (%)
(Method)c
P
SO2 Tol
R2
Entry
R3O
R3O
R
R
6
7
7
n
-C5H11
R1
R1
N
basic promoter
(see Table 1)
N
O
P
H
H
OEt
7
6e
7a
88 (B)
84 (B)
76 (B)
80 (B)d
66 (A)
6
8
OEt
N
Ph
R = H; R1= H; R2= PhCH2CH2
6b R = H; R1= Me; R2= PhCH2CH2
-C6 H11
-C5H11
-C5H1 1
7a R3 = Et
7b
H
6a
8g
8h
R3 = Me
6c R = H; R1= Me; R2=
6d R = H; R1= H; R2=
6e R = H; R1= Ph; R2=
6f R = H; R1= Ph; R2= Et
6g R = MeO; R1= H; R2= Et
6h R = H; R1= CO2Et; R2= Et
c
Et
n
O
P
OEt
n
8
6f
7a
7b
7a
7b
OEt
N
H
Ph
Et
Scheme 2.
O
P
OMe
9
6f
OMe
N
H
Ph
Table 1
Synthesis of indolylalkylphosphonates 8 by reaction of 3-(1-arylsulfonylalkyl) indoles
6 with dialkyl phosphites 7
8i
Entry
Sulfone
Phosphonate Productsa
8
Yieldb (%)
(Method)c
Et
6
7
MeO
O
P
OEt
OEt
Ph
10
6g
O
N
H
P
OEt
OEt
1
6a
6a
6b
6c
6c
6d
7a
65 (A)
66 (A)
68 (A)
70 (A)
75 (B)
77 (B)
8j
N
H
Et
8a
8b
O
P
OMe
OMe
CO2Et
Ph
11
6h
O
N
H
P
OMe
OMe
2
3
4
5
6
7b
7a
7a
7b
7a
8k
N
H
a
All products were identified on the basis of their IR and NMR spectra.
Yields of pure products isolated by column chromatography.
Method A: dialkyl phosphite (1.5 mmol), NaH (2 mmol) in THF (10 mL), and
b
c
sulfonyl indole (1 mmol) 2 h at rt. Method B: dialkyl phosphite (1.5 mmol), KF/basic
alumina (2 g) in THF (5 mL), and sulfonyl indole (1 mmol) 15 h at rt.
Ph
O
P
d
Reaction time 3 h.
OEt
OEt
N
H
the synthesis of biologically active compounds.8 Potassium fluoride
on basic alumina at room temperature acts as an efficient basic
promoter for the addition of dialkyl phosphites 7 to sulfonyl
indoles 6 (method B). The utilization of KF/basic alumina as
promoter introduces a considerable simplification in the work-up
procedures. Indeed, after removal of the solvent, the resulting solid
mixture can be directly applied on the head of a chromatographic
column for the final purification. However, we observed that for
some substrates this heterogeneous basic system is not completely
satisfactory since the starting material is still present in the reac-
tion mixture even after prolonged reaction time. For these runs,
sodium hydride in tetrahydrofuran is more effective than the
former promoter (method A).
Good to excellent results are obtained with sulfonyl indoles 6
tested for this process (Table 1).9 Steric crowding in close proxim-
ity to the reaction center does not usually affect the efficiency of
the addition as evidenced for the utilization of sulfonyl indole 6c
bearing a cyclohexyl group at 10 position (Table 1, entries 4 and
5). Similarly, the presence of a phenyl group at 2-position of the
indole ring in compounds 6e,f looks even beneficial in providing
good yields of the phosphonylated products 8g–i (Table 1, entries
7–9). Of particular interest is the reaction of sulfonyl indole 6h
8c
c-C6H11
O
P
OEt
OEt
N
H
8d
c
-C6H11
O
P
OMe
OMe
N
H
8e
8f
n
-C5H11
O
P
OEt
OEt
N
H

M. Petrini, R. R. Shaikh / Tetrahedron Letters 49 (2008) 5645–5648
5647
and amides usually requires the utilization of the corresponding
lithium salt.13
SO2Tol
PPh2
R2
R
R
R2
R1
In conclusion, 3-alkylidene indoles, generated under basic con-
ditions from sulfonyl indoles 6, react with dialkyl phosphites and
diphenylphosphine leading to the corresponding adducts. Sodium
hydride or potassium fluoride on basic alumina can be used as
basic promoters showing different levels of efficiency depending
on the heteronucleophile employed. The obtained phosphonates
8 may be considered as synthetic analogues of carboxylic acids
or esters with potential biological activity.
Ph2PH 9
R1
KF/Basic Al2O3
THF, 4 h, rt
N
N
H
H
6
10
6a R = H; R1= H; R2= PhCH2CH2
6c R = H; R1= Me; R2=
6e R = H; R1= Ph; R2=
c
-C6H11
n
-C5H11
6f R = H; R1= Ph; R2= Et
Acknowledgments
Scheme 3.
Financial support from University of Camerino (National Project
‘Sintesi organiche ecosostenibili mediate da nuovi sistemi cataliti-
ci’) is gratefully acknowledged.
obtained from ethyl 2-indolecarboxylate with phosphonate 7b,
which affords indole 8k bearing two ester functions of different
acidic systems. Such derivatives are important structural
analogues of glutamic and glutaric acids, which can be used in
biological assays.10
The widespread interest in the preparation of trisubstituted
phosphines prompted us to exploit the feasibility of a related pro-
cess involving the utilization of diphenylphosphine as nucleophile
in the reaction with sulfonyl indoles 6.11 Thus, diphenylphosphine
9 efficiently adds to sulfonyl indoles 6 in the presence of KF/basic
alumina leading to the corresponding trisubstituted phosphines
10 in excellent yields (Scheme 3, Table 2).12 The reaction condi-
tions adopted for the addition of phosphine 9 are surprisingly
mild, considering that the conjugate addition of 9 to enoates
References and notes
1. Reviews: (a) Merino, P.; Marqués-López, E.; Herrera, R. P. Adv Synth Catal. 2008,
350, 1195–1208. For some recent papers see: (b) Nakamura, S.; Nakashima, H.;
Yamamura, A.; Shibata, N.; Toru, T. Adv. Synth. Catal. 2008, 350, 1209–1212; (c)
Hosseini-Sarvari, M. Tetrahedron 2008, 64, 5459–5466; (d) Ambica Kumar, S.;
Taneja, S. C.; Hundal, M. S.; Kapoor, K. K. Tetrahedron Lett. 2008, 49, 2208–2212;
(e) Tajbakhsh, M.; Heydari, A.; Alinezhad, H.; Ghanei, M.; Hhaksar, S. Synthesis
2008, 352–354; (f) Bhagat, S.; Chakraborti, A. K. J. Org. Chem. 2007, 72, 1263–
1270; (g) Saito, B.; Egami, H.; Katsuki, T. J. Am. Chem. Soc. 2007, 129, 1978–
1986.
2. (a) Yao, Q. Tetrahedron Lett. 2007, 48, 2749–2753; (b) Yeom, C.-E.; Kim, M. J.;
Kim, B. M. Tetrahedron 2007, 63, 904–909; (c) Carta, P.; Puljic, N.; Robert, C.;
Dhimane, A.-L.; Fensterbank, L.; Lacôte, E.; Malacria, M. Org. Lett. 2007, 9,
1061–1063; (d) Fonvielle, M.; Mariano, S.; Therisod, M. Bioorg. Med. Chem. Lett.
2005, 15, 2906–2909.
3. (a) Dieltiens, N.; Moonen, K.; Stevens, C. V. Chem. Eur. J. 2007, 13, 203–214.
Double phosphonylation is observed using P(OR)2OTMS as reagent: (b)
Moonen, K.; Van Meenen, E.; Verwée, A.; Stevens, C. V. Angew. Chem., Int. Ed.
2005, 44, 7404–7411.
4. Moonen, K.; Laureyn, I.; Stevens, C. V. Chem. Rev. 2004, 104, 6177–6215.
5. (a) Zhang, Y.; Desai, A.; Li, Z.; Hu, G.; Ding, Z.; Wulff, W. Chem. Eur. J. 2008, 14,
3785–3803; (b) Lou, S.; Shaus,S. E. J. Am. Chem. Soc. 2008, 130, 6922–6923; (c)
Liang, Y.; Rowland, E. B.; Rowland, G. B.; Perman, J. A.; Antilla, J. C. Chem.
Commun. 2007, 4477–4479.
Table 2
Synthesis of 3-(1-diphenylphosphinoalkyl) indoles 10 by reaction of 3-(1-aryl-
sulfonylalkyl) indoles 6 with diphenyl phosphine 9a
Entry
Sulfone 6
Productsb 10
Yieldc (%)
6. (a) Ballini, R.; Palmieri, A.; Petrini, M.; Torregiani, E. Org. Lett. 2006, 8, 4093–
4096; (b) Palmieri, A.; Petrini, M. J. Org. Chem. 2007, 72, 1863–1866; (c)
Palmieri, A.; Petrini, M.; Torregiani, E. Tetrahedron Lett. 2007, 48, 5653–5656;
(d) Ballini, R.; Palmieri, A.; Petrini, M.; Shaikh, R. R. Adv. Synth. Catal. 2008, 350,
129–134.
Ph
PPh2
1
6a
88
7. Greco, M. N.; Hawkins, M. J.; Powell, E. T.; Almond, H. R., JrJr.; de Garavilla, L.;
Hall, J.; Minor, L. K.; Wang, Y.; Corcoran, T. W.; Di Cera, E.; Cantwell, A. M.;
Savvides, S. V.; Damiano, B. P.; Maryanoff, B. E. J. Med. Chem. 2007, 50, 1727–
1730.
N
H
10a
c
-C6H11
PPh2
8. Mohanakrishnan, A. K.; Srinivasan, P. C. J. Org. Chem. 1995, 60, 1939–1946.
9. General procedure for the preparation of indolylalkylphosphonates 8. Method A: To
a stirred suspension of NaH (2.0 mmol) in dry THF (10 mL), dialkyl phosphite 7
(1.5 mmol) was added at room temperature. After stirring at the same
temperature for 20 min, sulfonyl indole 6 dissolved in dry THF (5 mL) was
added dropwise. After stirring at room temperature for 2 h, the reaction
mixture was quenched with satd. NH4Cl (4 mL) and the resulting solution was
extracted with CH2Cl2 (3 ꢀ 10 mL). The organic phase was then dried over
MgSO4 and after removal of the solvent at reduced pressure, the crude
indolylphosphonate was purified by column chromatography (chloroform–
2
3
6c
6e
6f
90
86
95
N
H
10b
n-C5H11
methanol, 9:1). Method B: To
a
stirred solution of dialkyl phosphite
7
PPh2
Ph
(1.5 mmol) and sulfonyl indole
6 (1 mmol) in THF (5 mL), KF on basic
alumina [2 g, prepared from basic alumina (Baker, grade I) following the
Bergbreiter’s procedure]14 was added at room temperature. After stirring for
the appropriate time, silica gel (0.8 g) was added and solvent was removed at
reduced pressure. The solid mixture thus obtained was directly charged on a
chromatographic column and eluted to afford pure indolylphosphonates.
Selected data of compounds prepared: compound 8a: Oil. IR (cmꢁ1, neat):
3648, 1280, 1170. 1H NMR (400 MHz, CDCl3) d: 0.96 (t, 3H, J = 6.8 Hz), 1.25 (t,
3H, J = 6.8 Hz), 2.25–2.39 (m, 1H), 2.40–2.53 (m, 2H), 2.62–2.73 (m, 1H), 3.34
(ddd, 1H, J = 3.8, 11.1, 22.2), 3.52–3.62 (m, 1H), 3.78–3.85 (m, 1H), 3.96–4.21
(m, 2H), 7.02–7.25 (m, 8H), 7.36 (d, 1H, J = 8.1 Hz), 7.58 (d, 1H, J = 7.7 Hz), 9.22
(s, 1H). 13C NMR (100 MHz, CDCl3) d: 16.3 (d, 3J(CP) = 5.3 Hz, CH3CH2O), 16.5 (d,
3J(CP) = 5.3 Hz, CH3CH2O), 31.8, 33.6, 34.3 (d, 1J(CP) = 141.9 Hz, CHP), 61.8 (d,
2J(CP) = 7.6 Hz, CH3CH2O), 62.6 (d, 2J(CP) = 7.6 Hz, CH3CH2O), 109.4, 109.6,
111.6, 119.2, 119.4, 121.9, 123.8, 126.0, 127.7, 128.4, 128.6, 128.7, 136.1, 141.5.
Compound 8g: Oil. IR (cmꢁ1, neat): 3639, 1273, 1176. 1H NMR (400 MHz,
CDCl3) d: 0.67 (t, 3H, J = 6.9 Hz), 0.95–1.15 (m, 9H), 1.24 (t, 3H, J = 7.3 Hz), 1.91–
2.06 (m, 1H), 2.21–2.40 (m, 1H), 3.49 (ddd, 1H, J = 3.8, 11.1, 23.5 Hz), 3.75–3.84
N
H
10c
Et
PPh2
Ph
4
N
H
10d
a
Diphenylphosphine (1.1 mmol), KF/basic alumina (2 g) in THF (5 mL), and sul-
fonyl indole (1 mmol) 4 h at rt.
b
All products were identified on the basis of their IR and NMR spectra.
Yields of pure products isolated by column chromatography.
c

5648
M. Petrini, R. R. Shaikh / Tetrahedron Letters 49 (2008) 5645–5648
(m, 1H), 3.87–4.03 (m, 2H), 4.09–4.20 (m, 1H), 7.08 (t, 1H, J = 7.3 Hz), 7.16 (t,
Díaz, A.; Guillin, J. J.; Blanco, O.; Ruiz, M.; Ojea, V. J. Org. Chem. 2006, 71, 6958–
6974; (c) Han, L.; Hiratake, J.; Tachi, N.; Suzuki, H.; Kumagai, H.; Sakata, K.
Bioorg. Med. Chem. 2006, 14, 6043–6054.
1H, J = 7.3 Hz), 7.35–7.50 (m, 4H), 7.65–7.69 (m, 2H), 8.01 (d, 1H, J = 8.1 Hz),
8.70 (s, 1H). 13C NMR (100 MHz, CDCl3) d: 14.1, 16.4 (d, 3J(CP) = 5.3 Hz,
CH3CH2O), 16.6 (d, 3J(CP) = 5.3 Hz, CH3CH2O), 22.5, 27.3, 27.5, 28.2, 31.3, 36.1
(d, 1J(CP) = 142.7 Hz, CHP), 61.7 (d, 2J(CP) = 7.1 Hz, CH3CH2O), 61.8 (d,
2J(CP) = 7.1 Hz, CH3CH2O), 61.9, 62.0, 62.3, 62.4, 110.9, 119.6, 122.12, 128.1,
128.3, 128.7, 128.9, 129.1, 133.1, 136.3, 137.3, 137.5. Compound 8h: Oil. IR
(cmꢁ1, neat): 3645, 1277, 1179. 1H NMR (400 MHz, CDCl3) d: 0.70 (t, 3H,
J = 7.3 Hz), 1.07 (t, 3H, J = 7.3 Hz), 1.23 (t, 3H, J = 6.9 Hz), 2.05–2.18 (m, 1H),
2.21–2.38 (m, 1H), 3.40 (ddd, 1H, J = 4.3, 11.6, 23.5 Hz), 3.71–3.80 (m, 1H),
3.82–4.04 (m, 2H), 4.07–4.20 (m, 1H), 7.09 (t, 1H, J = 6.8 Hz), 7.16 (t, 1H,
J = 6.8 Hz), 7.38–7.51 (m, 4H), 7.63–7.66 (m, 2H), 8.02 (d, 1H, J = 8.1 Hz), 8.73 (s,
1H). 13C NMR (100 MHz, CDCl3) d: 12.7, 12.9, 16.5 (d, 3J(CP) = 5.3 Hz, CH3CH2O),
16.6 (d, 3J(CP) = 5.3 Hz, CH3CH2O), 21.8, 38.1 (d, 1J(CP) = 142.7 Hz, CHP), 61.7 (d,
2J(CP) = 7.6 Hz, CH3CH2O), 62.3 (d, 2J(CP) = 7.6 Hz, CH3CH2O), 111.0, 119.5,
122.1, 128.1, 128.9, 129.2, 133.0, 136.3, 136.4. Compound 8k: mp 144–146. IR
(cmꢁ1, KBr): 3648, 1715, 1270, 1178. 1H NMR (400 MHz, CDCl3) d: (mixture of
rotamers) 0.82 (t, 1.5H, J = 7.2 Hz), 0.83 (t, 1.5H, J = 7.3 Hz), 1.30 (t, 1.5H,
J = 7.3 Hz), 1.39 (t, 1.5H, J = 7.3 Hz), 2.20–2.38 (m, 1H), 2.40–2.56 (m, 0.5H),
2.57–2.65 (m, 0.5H), 3.53 (d, 1.5H, J = 10.7 Hz), 3.74 (d, 1.5H, J = 10.7 Hz), 3.77
(d, 1.5H, J = 11.3 Hz), 3.80 (d, 1.5H, J = 11.3 Hz), 4.11–4.22 (m, 2H), 4.26–4.40
(m, 2H), 4.57–4.67 (m, 0.5H), 5.60 (dd, 0.5H, J = 4.2, 11.5 Hz), 7.08–7.15 (m, 1H),
7.24–7.45 (m, 2H), 8.05–8.09 (m, 1H), 9.08 (s, 0.5H) 9.14 (s, 0.5H). 13C NMR
(100 MHz, CDCl3) d: 12.1, 12.9, 13.1, 14.5, 14.6, 20.4, 21.8, 22.4, 22.5, 36.6 (d,
1J(CP) = 140.4 Hz, CHP), 52.8 (d, 2J(CP) = 7.6 Hz, CH3O), 53.1 (d, 2J(CP) = 7.6 Hz,
CH3O), 61.1, 61.2, 65.3, 112.1, 112.3, 114.0, 120.5, 121.3, 123.8, 124.1, 125.8,
125.9, 128.9, 129.2, 135.7, 136.1, 136.3, 144.2, 161.2, 162.2.
11. (a) Yanagisawa, A.; Arai, T. Chem. Commun. 2008, 1165–1172; (b) Caminade, A.
M.; Servin, P.; Laurent, R.; Majoral, J. P. Chem. Soc. Rev. 2008, 37, 56–67; (c)
Gluek, D. S. Synlett 2007, 2627–2634.
12. General procedure for the preparation of 3-(1-diphenylphosphinoalkyl) indoles 10.
Following the procedure reported in method B, diphenylphosphine
9
(1.1 mmol) and sulfonyl indole 6 (1 mmol) were made to react with KF on
basic alumina (2 g) in THF (5 mL) for 4 h at room temperature. Selected data of
compounds prepared: compound 10a: Oil. IR (cmꢁ1, neat): 3642, 1423. 1H NMR
(400 MHz, CDCl3) d: 2.01–2.11 (m, 1H), 2.20–2.31 (m, 1H), 2.40–2.48 (m, 1H),
2.71–2.79 (m, 1H), 3.76–3.81 (m, 1H), 6.91–7.60 (m, 18H), 7.73–7.76 (m, 2H),
9.53 (s, 1H). 13C NMR (100 MHz, CDCl3) d: 31.7, 33.4 (d, 3J(CP) = 13.0 Hz,
CH2Ph), 35.4 (d, 1J(CP) = 71.0 Hz, CHP), 109.0, 111.6, 118.5, 119.2, 121.6, 124.8,
124.9, 126.2, 128.0, 128.1, 128.2, 128.4, 128.8, 128.9, 129.1, 130.8, 130.9, 131.2,
131.3, 131.5, 132.1, 132.6, 136.0, 141.2. Compound 10c: mp 242–244. IR (cmꢁ1
,
KBr): 3648, 1428. 1H NMR (400 MHz, CDCl3) d: 0.69 (t, 3H, J = 6.8 Hz), 1.02–
1.19 (m, 5H), 1.20–1.33 (m, 1H), 1.82–1.94 (m, 1H), 2.51–2.65 (m, 1H), 3.87 (dt,
1H, J = 3.0, 12.4 Hz), 7.00–7.42 (m, 12H), 7.45–7.59 (m, 4H), 7.73–7.82 (m, 2H),
8.23 (d, 1H, J = 7.7 Hz), 8.43 (s, 1H). 13C NMR (75 MHz, CDCl3) d: 14.1, 22.5,
27.7, 28.1, 28.3, 31.6, 39.1 (d, 1J(CP) = 106.4 Hz, CHP), 110.7, 120.0, 122.4, 123.4,
127.8, 128.0, 128.2, 128.7, 128.9, 129.1, 131.0, 131.1, 131.3, 131.7, 131.8, 133.1,
133.4, 134.3, 136.3, 137.3, 137.5.
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J.-C. Tetrahedron Lett. 2002, 43, 9237–9240; (b) Minami, T.; Okada, Y.; Otaguro,
T.; Tawaraya, S.; Furuichi, T.; Okauchi, T. Tetrahedron: Asymmetry 1995, 6,
2469–2474.
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MacDonald, T. L. Bioorg. Med. Chem. 2007, 15, 663–677; (b) Fernández, M. C.;
14. Bergbreiter, D. E.; Lalonde, J. J. J. Org. Chem. 1987, 52, 1601–1603.