Transformation of glutamic acid into (S)-benzyl 2-(dibenzylamino)-6- (dimethoxyphosphoryl)-5-oxohexanoate for a convenient access to 5-substituted prolines

Article abstract of DOI:10.1055/s-0029-1217340

L-Glutamic acid was transformed into δ-keto-phosphonate in two steps. This compound was employed in the stereocontrolled synthesis of cis-5-substituted prolines through a Woodward-Horner-Emmons (WHE) reaction with different aldehydes followed by hydrogeno

Full text of DOI:10.1055/s-0029-1217340

1562
LETTER
Transformation of Glutamic Acid into (S)-Benzyl 2-(dibenzylamino)-
6-(dimethoxyphosphoryl)-5-oxohexanoate for a Convenient Access to
5-Substituted Prolines
A
ccess to 5-Sub
l
stitute
d
e
Prolines na Cini,a Gianluca Giorgi,b Manuela Rodriquez,c Maurizio Taddei*a
a
Dipartimento Farmaco Chimico Tecnologico, Università degli Studi di Siena, Via A. Moro, 53100 Siena, Italy
Fax +39(0577)234333; E-mail: taddei.m@unisi.it
b
c
Dipartimento di Chimica, Università degli Studi di Siena, Via A. Moro, 53100 Siena, Italy
Dipartimento di Scienze Farmaceutiche, Università degli Studi di Salerno, Via Ponte don Melillo, 84084 Fisciano, Salerno, Italy
Received 02 September 2008
organometallic reagents followed by intramolecular cycli-
sation.9 Other approaches include cyclisation of sulfon-
amides,10 sulfinimines,11 acetylenic-derived amino acids12
or stereoselective hydrogenation of pyrrolines,13 and five-
Abstract: L-Glutamic acid was transformed into b-keto-phospho-
nate in two steps. This compound was employed in the stereocon-
trolled synthesis of cis-5-substituted prolines through a Woodward–
Horner–Emmons (WHE) reaction with different aldehydes fol-
lowed by hydrogenolysis/hydrogenation-mediated ring closure. We membered iminium ions.14 Phosphonates structurally re-
found also that a one-pot sequential hydroformylation–WHE reac-
tion was possible with cis-5-substituted prolines. In addition to dif-
ferently functionalised prolines, an indolizidine amino acid with a
lated to 3 have been prepared by Lubell and co-workers in
3–5 steps starting from glutamic or pyroglutamic acid.15,16
structure related to constrained peptidomimetics was obtained.
O
Key words: amimo acids, hydrogenation, peptidomimetics, micro-
waves
P
R
O
O
Bn
O
Bn
Bn
O
Bn
ref. 7, 8
BnO
base
N
N
L-Glu
L-Glu
H
BnO
n
R
n
The importance of proline derivatives in peptide and nat-
ural product chemistry and in organic synthesis is well ev-
idenced by the high number of reviews and scientific
literature on the subject.1 Substituted prolines are consid-
ered as conformationally constrained amino acids2 and
they have been introduced in peptidomimetics and peptide
analogues with successful results.3 Many natural products
containing heterocyclic rings related to proline have been
isolated and some of them show very interesting proper-
ties.4 Consequently, proline has been considered as a priv-
ileged structure in the search for new hits in drug
discovery.5 Moreover, proline and substituted prolines
have found a broad application as organocatalysts in
asymmetric synthesis.6
O
1 n = 1
2 n = 2
1) RCHO, base
2) H2, catalyst
Bn
O
Bn
N
R
BnO
O
HOOC
N
P
H
O
3
O
O
Scheme 1
We considered a simpler sequence starting from tetraben-
zyl glutamic acid 4 and using the hindrance of the benzyl
protection at the a-nitrogen to force the phosphonometh-
ylene anion to discriminate between the two benzyl ester
groups. Different variables such as the solvent, the nature,
and the concentration of the base employed to generate
the anion, the temperature and the order of the addition of
the reagents were addressed in order to obtain a higher ra-
tio in favour of the desired (S)-benzyl 2-(dibenzylamino)-
6-(dimethoxyphosphoryl)-5-oxohexanoate (5) with re-
spect to the other possible products (6 and 7) formed in the
reaction (Table 1). First attempts carried out with stoichi-
ometric amounts of (MeO)2POMe and BuLi in THF gave
a mixture of the two regioisomers 5 and 6 in 50:50 ratio in
42% yields together with 40% of unreacted starting mate-
rial. Keeping the reaction mixture to –78 °C and quench-
ing at this temperature improved the 5/6 ratio but
decreased the yields. Then, the nature of the base was
changed trying LDA, LiN(SiMe3)2, NaN(SiMe3)2,
KN(SiMe3)2, without any improvement in the amount of
5 isolated (entries 4–8 in Table 1). Changes in the solvent
from THF to DME or toluene did not improve yields or se-
In the search of new simple approaches to biologically rel-
evant natural and non-natural amino acids, we described
recently the synthesis of aldehydes 1 and 2 starting from
L-glutamic acid.7,8 These aldehydes, after reaction with
phosphonium ylides7 or phosphonates,8 could provide an
useful entry to lipophilic or long-chain-substituted
enantiomerically pure a-amino acids. We envisaged the
possibility to invert this reactivity sequence, installing the
phosphonate on the glutamic moiety (3 in Scheme 1) and
permitting it to react with different aldehydes to produce,
after hydrogenation, different 5-substituted prolines
(Scheme 1). This class of amino acids has been previously
prepared by reaction of protected pyroglutamic acid with
SYNLETT 2009, No. 10, pp 1562–1566
x
x
.x
x
.2
0
0
9
Advanced online publication: 02.06.2009
DOI: 10.1055/s-0029-1217340; Art ID: D31208ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Access to 5-Substituted Prolines
1563
Compound 5 was then reacted with different aldehydes
under standard neutral Horner–Wadsworth–Emmons
conditions to give compounds 8–17 in good yields, main-
Table 1 Optimisation of the Procedure for the Selective Formation
of the b-Keto Phosphonate 5
Bn
Bn
MePO(OMe)2, base
N
1
L-Glu
ly as the E-isomer as revealed by H NMR analysis
BnO
OBn
(3JCH=CH >15 Hz). It is noteworthy that the aldehydes em-
ployed for preparing compounds 12–14 were obtained by
microwave-assisted hydroformylation18 of the corre-
sponding alkenes, in a sort of tandem hydroformylation–
Horner–Wadsworth–Emmons reaction.19
4
O
O
Bn
Bn
Bn
O
Bn
N
N
O
BnO
O
OBn
O
P
P
O
O
O
O
O
O
O
5
6
Compounds 8–17 were then submitted to hydrogenolysis
in a Parr bottle at 4.8 bar in MeOH and in the presence of
Pd(OH)2/C at room temperature for 12–48 hours depend-
ing on the substrate, giving prolines 18–28 in good to ac-
ceptable yields (see Table 2).20 Boc2O was added to the
reaction mixture, in order to isolate N-Boc prolines as the
reaction products. The presence of the Boc group facili-
tates the isolation and purification of the product especial-
ly with smaller substituents in position 5. However,
without Boc2O, unprotected prolines could be isolated, as
in the case of compounds 18, 20, 22, and 28 whose lipo-
philicity allowed purification by simple column chroma-
tography on silica gel. Unfortunately, the hydrogenolysis
conditions led to a complete reduction of the phenyl ring
in product 26; although, by monitoring the conversion of
the reaction every hour, it was possible to isolate com-
pound 27 in acceptable yields after six hours at room tem-
perature. In the case of compound 14, the bromine atom
was removed during cyclisation, giving the simple alkyl
proline 25. The hydrogenolysis/hydrogenation cyclisation
of 8–17 can be carried out also in less than one hour under
microwave dielectric heating at 80 °C and under 4.8 bar of
H2 in the presence of Pd(OH)2.21 However, the yields of
the products obtained were lower than working at room
temperature for a longer time. All compounds 18–28 were
obtained in diastereomeric ratio higher than 90% (1H
NMR analysis, 400 MHz). NOE experiments showed a
significant effect between the proton in position 2 and
protons in position 3 and 4, and analogously an effect was
observed on these protons when CH(5) was irradiated. No
effects were observed between CH(2) and CH(5), sug-
gesting a 2,5-trans relationship. However, X-ray crystal-
structure analysis of compound 2022 revealed a cis rela-
tionship between the substituents of the pyrrolidinium
Bn
Bn
N
O
O
P
P
O
O
O
O
O
7
Entry Base
Solvent, temperature,
reaction conditionsa
Ratio of
5/6/7
Yield
(%)b
1
2
BuLi
BuLI
LDA
THF, –78 °C to r.t., A.
THF, –78 °C, A
50:50:0
70:30:0
60:40:0
55:40:5
42
22
35
43
3
THF, –78 °C to r.t., A
THF, –78 °C to r.t., A
4
NaHDMS
NaHDMS
NaHDMS
LiHDMS
KHDMS
KOt-Bu
i-PrMgBr
t-BuLi
5
toluene, –78 °C to r.t., A
DME, –78 °C to r.t., A
THF, –78 °C to r.t., A
THF, –78 °C to r.t., A
THF, –78 °C to r.t., A
THF, 0 °C, A
30:55:15 40
6
65:35:0
65:35:0
50
33
7
8
60:30:10 45
c
9
–
–
–
–
–
–
c
10
11
12
13
d
Et2O, –78 °C to r.t., A
THF, –78 °C to r.t., B
THF, –78 °C, C
BuLi
60:20:20 76
75:25:0 82
BuLi
a Conditions A: MePO(OMe)2 (1 equiv), base (1.3 equiv). Conditions
B: MePO(OMe)2 (3 equiv), base (3 equiv). Conditions C: 0.1 M soln
MePO(OMe)2 (3 equiv), base (3 equiv).
b Yields of isolated 4, 5, and 6.
c Decomposition of 4 observed.
d Starting material recovered.
lectivity. Surprisingly, in toluene the phosphonate 6 was
the major isomer formed. Finally, an increase of the
amount of the phosphonomethylene anion gave an im-
provement of the yields, although with concomitant in-
crease of the diphosphonate 7.
Better yields and selectivity were observed by slow addi-
tion of a solution of 4 in THF to the lithium anion. The
best result was achieved by slow addition of a diluted so-
lution of 4 in THF to the lithium phosphonate in THF at
–78 °C and carrying out the aqueous workup at this tem-
perature after four hours of stirring. However, 5 could be
obtained in pure form in 65% yield exclusively after flash
chromatography separation from 6.17
Figure 1 X-ray crystal structure of compound 20; ellipsoids for
nonhydrogen atoms enclose 50% probability
Synlett 2009, No. 10, 1562–1566 © Thieme Stuttgart · New York

1564
E. Cini et al.
LETTER
ring and an R-configuration for the stereocentre at C(5) As with other analogues,23 20 crystallises as 5-isopen-
(Figure 1).
tylpyrrolidinium-2-carboxylate with the N(1)–C(2) and
N(1)–C(5) distances equal to 1.503(2) and 1.522(2) Å, re-
Table 2 Synthesis of Different 5-Substituted Prolines
R1
Bn
Bn
R1CHO, LiCl, DIPEA
MeCN, r.t., 24–48 h
N
HOOC
H2 (70 psi), Pd(OH)2/C
r.t., MeOH, (Boc2O)
R1
Bn
O
Bn
N
R2
N
BnO
BnO
O
R2 = H or Boc ; 18–29
P
O
O
8–17
O
O
O
5
Entry
1
Aldehyde
MeCHO
Benzyl 2-dibenzylamino-
5-oxa-enoic acid (R1)
Proline
Yields (%)a
75
Me
8 (77%)
HOOC
N
H
18
2
MeCH2
75
68
CHO
9 (76%)a
HOOC
N
Boc
19
3
4
Me2CH
10 (69%)
HOOC
N
CHO
H
20
Me(CH2)12
11 (85%)
75
60
12CHO
HOOC
N
14
21 R2 = Boc
22 R2 = H
5
6
BocNH(CH2)3
56
78
70
67
56
66
NHBoc
12 (75%)a
BocHN
EtOOC
3CHO
CHO
HOOC
N
5
Boc
23
EtOOC(CH2)4
COOEt
4
13 (70%)a
HOOC
N
6
5
Boc
24
7
Br(CH2)4
14 (81%)a
Br
4 CHO
HOOC
N
Boc
25
8
Ph
15 (85%)a
HOOC
N
CHO
Boc
26
9
4-MeOC6H4
MeO
16 (81%)a
HOOC
N
OMe
CHO
Boc
27
10
Boc
Boc
Boc
N
N
O
N
HOOC
N
O
H
CHO
17 (76%)a
O
28
a Isolated yields.
Synlett 2009, No. 10, 1562–1566 © Thieme Stuttgart · New York

LETTER
Access to 5-Substituted Prolines
1565
spectively. The pyrrolidinium ring shows an envelope
conformation with N(1), C(2), C(4), C(5) defining the
best least-squares plane while C(3) is out-of-plane with a
deviation of 0.59 Å from it. The two substituents, that is,
the carboxylate and the isopentyl chain are in axial and bi-
sectional position, respectively (Figure 1).
Acknowledgment
This work was financially supported by MIUR (Rome, Progetto
PRIN 2006) and the University of Siena (Progetto PAR 2005).
Reference and Notes
(1) For example, search for ‘proline’ in ISI Web of
KnowledgeTM of Thomson Reuters) gives more than 200
reviews in ‘chemistry’ between 2000 and 2008.
(2) Koskinen, A. M. P.; Helaja, J.; Kumpulainen, E. T. T.;
Koivisto, J.; Mansikkamaeki, H.; Rissanen, K. J. Org.
Chem. 2005, 70, 6447; and references therein.
The crystal structure is stabilised by strong intermolecular
hydrogen bonds involving the pyrrolidinium NH2 and
both the oxygen atoms of the carboxylate group.
Starting from compound 28 we designed a convenient
synthetic scheme to prepare the indolizidine amino acid
33, having a structure related to other indolizidines that
have applications as peptidomimetics in medicinal chem-
istry.24 Proline 28 was first protected as the Cbz derivative
and then methylated at the carboxylic group. O-Methyl-
isourea 30, prepared through reaction of commercially
available PS-supported microporous DCC in dry metha-
nol under microwave heating,25 was used for methylation
of the COOH group. Addition of 29 to resin 30 at 60 °C in
dichloromethane gave compound 31 in 75% overall yield
(Scheme 2). The oxazolidine acetonide was then removed
using PPTS in hot ethanol26 to give the proline derivative
32. The terminal OH was transformed into the tosylate
and the product submitted to microwave-assisted transfer
hydrogenation.27 The Cbz group was removed and the free
proline NH underwent intramolecular nucleophilic substi-
tution at the carbon carrying the tosylate to give the
indolizidine amino acid 33 as a single diasteromer as re-
vealed by 1H NMR (400 MHz) and HPLC analyses.
(3) Cheng, X.-C.; Wang, Q.; Fang, H.; Xu, W.-F. Curr. Med.
Chem. 2008, 15, 374.
(4) See, for example: Schardl, C. L.; Grossman, R. B.;
Nagabhyru, P.; Faulkner, J. R.; Mallik, U. P. Phytochemistry
2007, 68, 980; and references therein.
(5) Fournie-Zaluski, M.-C.; Coric, P.; Thery, V.; Gonzales, W.;
Mendal, H.; Turcaud, S.; Michel, J.-B.; Roques, B. P.
J. Med. Chem. 1996, 39, 2594.
(6) Doyle, A. G.; Jacobsen, E. N. Chem. Rev. 2007, 107, 5713.
(7) Rodriquez, M.; Taddei, M. Synthesis 2005, 493.
(8) Rodriquez, M.; Bruno, I.; Cini, E.; Marchetti, M.; Taddei,
M.; Gomez-Paloma, L. J. Org. Chem. 2006, 71, 103.
(9) Mota, A. J.; Langlois, N. Tetrahedron Lett. 2003, 44, 1141.
(10) Knight, D. W.; Redfern, A. L.; Gilmore, J. J. Chem. Soc.,
Perkin Trans. 1 2002, 622.
(11) Davis, F. A.; Zhang, H.; Lee, S. H. Org. Lett. 2001, 3, 759.
(12) van Esseveldt, B. C. J.; van Delft, F. L.; Smits, J. M. M.;
de Gelder, R.; Rutjes, F. P. J. T. Synlett 2003, 2354.
(13) Li, H.; Sakamoto, T.; Kikugawa, Y. Tetrahedron Lett. 1997,
38, 6677.
(14) Rudolph, A. C.; Machauer, R.; Martin, S. F. Tetrahedron
Lett. 2004, 45, 4895.
(15) Dietrich, E.; Lubell, W. D. J. Org. Chem. 2003, 68, 6988.
(16) Beausoleil, E.; L’Archeveque, B.; Belec, L.; Atfani, M.;
Lubell, W. D. J. Org. Chem. 1996, 61, 9447.
(17) (S)-2-Dibenzylamino-6-(dimethoxyphosphoryl)-5-
oxohexanoic Acid Benzyl Ester (5)
30, THF, MW
CbzCl, Et3N
CH2Cl2
Boc
N
80 °C, 2 min
HOOC
N
28
90%
84%
Cbz
29
Boc
O
PPTS, EtOH,
To a soln of dimethyl methylphosphonate (0.191 g, 1.54
mmol) in dry THF (17 mL) cooled at –78 °C, BuLi (0.62 mL
of a 2.5 M soln in hexane) was added dropwise. After 40
min, a soln of 4 (0.390 g, 7.7 mmol) in dry THF (14 mL) was
added. After stirring for 4 h at –78 °C, a sat. soln of NH4Cl
(6 mL) was added followed by EtOAc (30 mL). After
warming to r.t., the organic phase was separated. The
aqueous phase was extracted with EtOAc, all the organic
fractions were collected and dried over Na2SO4. The solvent
was removed in vacuo, and the crude material was purified
by column chromatography (PE–EtOAc, 1:3) to give
compound 5 as colourless oil (0.26 g, 65% yield). 1H NMR
(400 MHz, CDCl3): d = 1.95–2.02 (m, 2 H, H-3), 2.41–2.78
(m, 2 H, H-4), 2.79–2.97 (m, 2 H, H-6), 3.29 (t, J = 6.8 Hz,
1 H, H-2), 3.67 (AB system, 4 H, NCH2Ph), 3.68 (d, J = 2.8
Hz, 3 H, OCH3), 3.71 (d, J = 2.8 Hz, 3 H, OCH3), 5.19 (AB
system, 2 H, OCH2Ph), 7.19–7.36 (m, 15 H, ArH). 13C NMR
(50 MHz, CDCl3): d = 22.6, 40.0, 40.4, 42.5, 52.8, 53.0,
54.4, 59.8, 66.1, 127.1 54.5, 59.8, 66.2, 127,1, 128.2, 128.3,
128.5, 128.6, 128.9, 135.9 (2 C), 139.21, 172.13, 200.7,
200.8. HRMS (ES): m/z calcd for C29H35NO6P [M + H]+:
524.2202; found: 524.2180.
MeOOC
60 °C, 12 h
N
N
71%
Cbz
31
O
1) TsCl, pyridine, r.t.
2) HCOONH4, Pd/C
i-PrOH, MW, 10 min
MeOOC
N
N
NHBoc
NHBoc
54%
Cbz
32
MeOOC
33
HO
Scheme 2 Preparation of indolizidine amino acid 31
Compound 33 has the two functional groups orthogonally
protected and it is in a suitable form to be inserted into a
peptide sequence.
In conclusion we have developed a convenient approach
to enantiomerically pure cis-5-substituted prolines that
can be applied to differently functionalised substrates
(apart alkenes or alkyl halides). The resulting prolines can
be used as modified amino acids, potential organocata-
lysts soluble in low polarity organic solvents or as starting
material for the preparation of indolizidine alkaloids or
peptidomimetics.
(18) Petricci, E.; Mann, A.; Rota, A.; Schoenfelder, A.; Taddei,
M. Org. Lett. 2006, 8, 3725.
(19) For the domino hydroformylation–Wittig reaction, see:
(a) Breit, B.; Zhan, S. K. Angew. Chem. Int. Ed. 1999, 38,
969. (b) Breit, B.; Zhan, S. K. Tetrahedron 2005, 61, 6171.
Synlett 2009, No. 10, 1562–1566 © Thieme Stuttgart · New York

1566
E. Cini et al.
LETTER
pyrophoric) and washed several times with MeOH. The
(20) (S,E) 2-Dibenzylamino-5-oxoicos-6-enoic Acid Benzyl
Ester (11) – General Procedure
solvent was removed under vacuum and the crude mixture
was purified by column chromatography (CHCl3–MeOH,
98:2) to give compound 21 as white gel (51 mg, 75% yield);
[a]D25 –20.7 (c 0.1, CDCl3). 1H NMR (400 MHz, CHCl3):
d = 0.84 (t, J = 7.2 Hz, 3 H), 1.09–1.43 (m, 26 H), 1.47 (s, 9
H), 1.59–1.67 (m, 2 H), 1.82–1.94 (m, 2 H), 1.95–2.28 (m, 2
H), 3.71–3.83 (m, 1 H), 4.18–4.26 (m, 1 H), 9.91 (br s, 1 H).
13C NMR (100 MHz, CDCl3): d = 14.0, 22.7, 26.6, 28.3 (3
C), 29.3 (2 C), 29.7 (10 C), 31.9, 34.3, 58.9, 60.0, 67.0,
173.7, 179.4. MS (ES): m/z = 424 [M – H]–.
To a soln of 5 (0.900 g, 1.7 mmol) in dry MeCN (18 mL),
dry LiCl (72.0 mg, 1.7 mmol) was added followed by
DIPEA (180 mg, 239 mL, 1.4 mmol). After stirring for 2 h at
r.t., tetradecanal (0.276 g, 1.3 mmol) in MeCN was added
and the mixture stirred at r.t. for 72 h. Brine was added
and the organic layer separated. The aqueous phase was
extracted with EtOAc, all the organic fractions were
collected and dried over Na2SO4. The solvent was removed
under vacuum and the crude mixture was purified by column
chromatography (PE–EtOAc, 5:1) to give compound 11 as a
colourless oil (0.673 g, 85% yield); [a]D25 –55.5 (c 0.1,
CHCl3). 1H NMR (400 MHz, CDCl3): d = 0.91 (t, J = 6.6 Hz,
3 H), 1.24–1.47 (m, 20 H), 1.45 (m, 2 H), 2.07–2.19 (m, 4
H), 2.41–2.69 (m, 2 H), 3.39 (dd, J = 8.4, 6.2 Hz, 1 H), 3.73
(AB system, 4 H), 5.23 (AB system, 2 H), 6.00 (d, J = 15.6
Hz, 1 H), 6.71 (dt, J = 15.6, 7.0 Hz, 1 H), 7.22–7.43 (m, 15
H). 13C NMR (100 MHz, CDCl3): d = 14.0, 20.8, 22.6, 23.3,
28.0, 29.1, 29.2, 29.3 (3 C), 29.4, 29.5, 31.8 (2 C), 32.3, 36.3,
54.4, 60.1, 65.9, 126.9 (3 C), 128.1 (4 C), 128.2 (2 C), 128.4
(4 C), 128.7 (2 C), 130.1, 135.9, 139.2 (2 C), 147.2, 172.2,
199.3. MS (ES): m/z = 632 [M + Na]+.
(21) Piras, L.; Genesio, E.; Ghiron, C.; Taddei, M. Synlett 2008,
1125.
(22) Crystallographic data (excluding structure factors) for X-ray
crystal structure of 18 have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC 710555. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK [fax: +44 (1223)336033 or
e-mail: deposit@ccdc.cam.ac.uk].
(23) Hanessian, S.; Shao, Z.; Warrier, J. S. Org. Lett. 2006, 8,
4787.
(24) Manzoni, L.; Bassanini, M.; Belvisi, L.; Motto, A.;
Scolastico, C.; Castorina, M.; Pisano, C. Eur. J. Org. Chem.
2007, 1309; and references therein.
(2S,5R)-5-Pentadecylpyrrolidine-1,2-dicarboxylic Acid
1-tert-Butyl Ester (21) – General Procedure
To a soln of 11 (100 mg, 0.16 mmol) in CH2Cl2–MeOH (1:9,
5 mL) placed in the bottle connected with a Parr apparatus,
Boc2O (54 mg, 0.25 mmol) and Pd(OH)2/C (20%, 9 mg,
0.012 mmol) were added. The bottle was filled with H2 at 4.8
bar and shaken at r.t. for 8 h. The bottle was degassed, the
catalyst filtered (attention: the residue Pd may be
(25) Crosignani, S.; White, P. D.; Linclau, B. Org. Lett. 2002, 4,
1035.
(26) D’Aniello, F.; Mann, A.; Schoenfelder, A.; Taddei, M.
Tetrahedron 1997, 53, 1447.
(27) Daga, M. C.; Taddei, M.; Varchi, G. Tetrahedron Lett. 2001,
42, 5191.
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