Synthesis of unusual amino acids: N-(tert-butoxycarbonyl)-L-vinyl glycine and N-(tert-butoxycarbonyl)-L-homophenylalanine

Article abstract of DOI:10.1016/S0957-4166(02)00120-9

The synthesis of the unusual amino acids N-(tert-butoxycarbonyl)-L-vinyl glycine and N-(tert-butoxycarbonyl)-L-homophenylalanine starting from commercially available D-xylose via an alkylative fragmentation method is described.

Full text of DOI:10.1016/S0957-4166(02)00120-9

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 423–428
Synthesis of unusual amino acids:
N-(tert-butoxycarbonyl)-
L
-vinyl glycine and
N-(tert-butoxycarbonyl)-
L
-homophenylalanine†
S. Chandrasekhar,* Abbas Raza and Mohamed Takhi
Indian Institute of Chemical Technology, Hyderabad 7, A.P, India
Received 18 February 2002; accepted 1 March 2002
Abstract—The synthesis of the unusual amino acids N-(tert-butoxycarbonyl)-
homophenylalanine starting from commercially available
Elsevier Science Ltd. All rights reserved.
L
-vinyl glycine and N-(tert-butoxycarbonyl)-
L-
D-xylose via an alkylative fragmentation method is described. © 2002
1. Introduction
Similiarly, L-homophenylalanine, which may be viewed
as a homologue of naturally occurring amino acid
phenylalanine, is a new agent of pharmacological inter-
est.7 However, the number of procedures available for
Unusual amino acids continue to be the targets of
much synthetic interest.1 Optically active, non-proteino-
genic amino acids deserve particular attention because
of their documented and potential biological activities.2
Obviously there is a demand for the synthesis of
uncommon amino acids in enantiomerically pure form
both for pure and applied organic and bioorganic
chemistry. In principle, the design of general strategies
for enantiopure amino acids is a challenge for the
synthetic organic chemist. Hence, there is considerable
interest in a-amino acids, both from pharmaceutical
and mechanistic viewpoints.3
the synthesis of
L
-homophenylalanine8 is limited.
Herein, we describe the full details of a common strat-
egy for the synthesis of these two unusual amino acids,
with the key reaction involving alkylative fragmenta-
tion of a sugar tosylhydrazone (vide infra).9
2. Results and discussion
Carbohydrates, which are abundant in nature, are con-
sidered as the main stream of nature’s ‘chiral pool’. For
Vinyl glycine, the simplest b,g-unsaturated-a-amino
acid, is a plant toxin,4 which has attracted considerable
interest both as an irreversible inhibitor of pyridoxal
phosphate-linked aspartate amino transferase as well as
a chiral building block for the elaboration of complex
natural products.5 Various methods have been reported
in the literature for the synthesis of vinyl glycine both
in racemic and enantiomerically pure form.6 However,
the utility of this amino acid is hampered by the
difficulties involved in its synthesis in enantiomerically
pure form, which are often complicated by rearrange-
ment of the double bond to the a,a-position or by an
increased tendency for racemisation.
the synthesis of N-(tert-butoxycarbonyl)-
I and N-(tert-butoxycarbonyl)- -homophenylalanine II,
we chose the inexpensive and readily available pentose
sugar, -xylose, as the chiral starting material. Accord-
ing to our retrosynthetic analysis (Scheme 1), the key
chiral allyl alcohols 6 and 10 could be obtained by
L-vinyl glycine
L
D
alkylative elimination of the tosylhydrazone of D-xylose
3. This unprecedented alkylative fragmentation of an
a-O-tosylhydrazone is studied in detail by our group.9
The hydroxyl group becomes a precursor for the intro-
duction of the amino group with inversion and the
1,2-O-isopropylidine would then be cleaved to the
–COOH group with the loss of one carbon atom.
Accordingly,
D-xylose was converted to its ethylmer-
captal derivative10 using ethanethiol in concentrated
* Corresponding author. Tel.: +91-40-7193434; fax: +91-40-7170512;
e-mail: srivaric@iict.ap.nic.in
HCl, and was further converted to the di-O-isopropyl-
† IICT Communication No. 020212.
idine-D
-xylose-diethyl dithioacetal11 using standard
0957-4166/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00120-9

424
S. Chandrasekhar et al. / Tetrahedron: Asymmetry 13 (2002) 423–428
Scheme 1.
reported procedure (Scheme 2). Thioacetal was then
treated with HgCl2 and HgO in acetone–H2O12 to afford
the unstable aldehyde 2, which was immediately deriva-
tised with p-toluenesulfonyl hydrazine in methanol to
afford the corresponding tosylhydrazone 3.
the nitrogen and hydride from LiAlH4 adds to the olefinic
carbon with elimination of a nitrogen molecule, p-tolue-
nesulfonic acid and acetone (Scheme 3).
Alcohol 4 was then converted to azide 5 under Mitsunobu
conditions13 using TPP/DEAD (diethyl azodicarboxyl-
ate)/DPPA (diphenyl phosporyl azide) in THF. The azide
group was reduced to an amine (TPP, H2O in THF),14
which was further protected in a single pot transformation
as its tert-butyl carbamate derivative 6 by treatment with
1N aqueous NaOH and (Boc)2O (Scheme 4).
Exposure of the hydrazone 3 to sodium hexamethyldis-
ilazide (NaHMDS) and LiAlH4 in THF as solvent
furnished allyl alcohol 4 in 52% yield by an alkylative
fragmentation process,9 wherein the first equivalent of
non-nucleophilic base NaHMDS abstracts a proton from
Scheme 2.
Scheme 3.

S. Chandrasekhar et al. / Tetrahedron: Asymmetry 13 (2002) 423–428
425
Scheme 4.
Oxidation of the Boc-protected isopropylidine moiety 6
with Jones’ reagent15 provided the carboxylic acid 7,
again in one pot, the 1H NMR spectrum of which
showed resonances at 4.11 ppm as a doublet due to the
a-protons. The remaining protons resonated at their
respective positions. The specific rotation of this com-
pound was determined as [h]2D5=+2.6 (c 1.5, MeOH)
[lit.16 [h]2D5=+2.8 (c 4, MeOH)].
The alcohol was then converted to azide 8 under Mit-
sunobu conditions using TPP/DEAD (diethyl azodicar-
boxylate)/DPPA (diphenylphosporyl azide) in THF
(Scheme 6). The azide was reduced to an amine func-
tion by catalytic hydrogenation with palladium–carbon
under hydrogen gas, which on in situ treatment with
(Boc)2O afforded the NH-Boc isopropylidene 10 in one
pot.
As per our synthetic plan, we began the synthesis of
Oxidation of the Boc-protected isopropylidine 10 with
Jones’ reagent then provided carboxylic acid 11 again
in a single step. The specific rotation of this compound
was also obtained [h]2D5=+5.8 (c 0.9, EtOH) and other
spectral features were comparable [lit.17 value [h]D25=
+6.0 (c 1, EtOH)]. Finally, the structure of this unnatural
amino acid was confirmed by preparing its methyl ester
derivative 14 through treatment of the acid with an
ethereal solution of diazomethane at 0°C. The specific
L
-homophenylalanine from
plate. In an initial attempt,
D
-xylose as the chiral tem-
-xylose was converted to
D
the desired hydrazone precursor 3, in a series of four
transformations as depicted in Scheme 2. Exposure of
the hydrazone 3 to Grignard reaction with PhMgBr in
THF as solvent, furnished the allyl alcohol 8 in 56%
yield through an alkylative fragmentation process9
(Scheme 5).
Scheme 5.
Scheme 6.

426
S. Chandrasekhar et al. / Tetrahedron: Asymmetry 13 (2002) 423–428
rotation of the compound was determined to be [h]2D5=
−13.1 (c 1, MeOH) and other spectral features were
also comparable [lit.18 [h]2D5=−14.7 (c 1.2, MeOH)].
water, brine, dried (Na2SO4) and solvent was evapo-
rated under reduced pressure. The product was isolated
by column chromatography (1:10 ethyl acetate–hexane
as eluent) to furnish 4 as a colourless liquid (0.237 g,
52%). 1H NMR (CDCl3): l 1.31 and 1.42 (2s, 6H),
2.20–2.32 (bs, 1H), 3.68–3.80 (m, 1H), 3.90–4.02 (m,
3H), 5.15–5.41 (dd, 2H, J=11.2, 17.9 Hz), 5.67–5.85
(m, 1H); IR (CHCl3): 3500 cm−1, [h]D25: +6.2 (c 1,
CHCl3); HRMS: calcd for C8H14O3 158.1968. Found
158.1937.
3. Conclusion
In conclusion, we have developed a novel procedure for
the preparation of N-(tert-butoxycarbonyl)-
glycine and N-(tert-butoxycarbonyl)- -homophenylala-
nine from -xylose via fragmentation of sugar hydra-
L-vinyl
L
D
zones, which is convenient and flexible and can
conveniently be used in the synthesis of molecules of
biological and pharmaceutical interests.
4.1.3. (3S)-Azido-(4R)-5-isopropylidene-1-pentene 5. To
a solution of PPh3 (2.72 g, 10.4 mmol) in dry THF (30
mL) under a nitrogen atmosphere at −20°C was added
diethyl azodicarboxylate (1.45 mL, 10.4 mmol) drop-
wise. After stirring for 10 min, a solution of alcohol 4
(1.5 g, 9.5 mmol) in THF (10 mL) was added slowly.
The reaction mixture was stirred for 30 min at the same
temperature and then cooled to 0°C and diphenyl phos-
phoryl azide (2.45 mL, 11.3 mmol) was added dropwise
and the reaction mixture was stirred for 6 h at room
temperature, after which it was quenched with water
and extracted with ethyl acetate (50 mL, 2×25 mL). The
combined extracts were washed with brine, dried
(Na2SO4), concentrated and the residue was purified by
silica gel column chromatography to afford azide 5 as a
colourless viscous liquid (1.21 g, 70%). 1H NMR
(CDCl3): l 1.35 and 1.44 (2s, 6H), 3.75–4.15 (series of
m, 4H), 5.25–5.40 (2H, dd, J=10.1, 18.5 Hz), 5.90–6.01
(m, 1H); IR (neat): 2105, 1600 cm−1; HRMS: calcd for
C18H13N3O2 183.2098. Found 183.1995.
4. Experimental
4.1. General
All solvents and reagents were purified by standard
techniques. Column chromatography was performed on
SiO2 (60–120 mesh). IR spectra were recorded on
Perkin–Elmer 683 spectrophotometer. Optical rotations
were obtained on Jasco Dip 360 digital polarimeter.
Melting points (uncorrected) were obtained using a
Buchi 535 melting point apparatus. NMR spectra were
recorded on a Varian Gemini 200 or Varian Unity 400
using TMS as the internal standard. Chemical shifts (l)
are reported in ppm, coupling constants (J) are
reported in Hz. Mass spectra were obtained on a
Finnegan-MAT 1020B (70 eV).
4.1.4.
(3S)-[(tert-Butoxycarbonyl)amino]-(4R)-5-iso-
4.1.1.
(methylphenyl)sulfonyl]hydrazone 3. 2,3,4,5-Di-O-iso-
propylidene-
-xylose aldehyde12 2 (1.7 g, 7.39 mmol)
D
-Xylose-(2R,3R,4R)-5-di-O-isopropylidene-[4-
propylidene-1-pentene 6. A solution of 5 (0.95 g, 5.19
mmol) and TPP (1.35 g, 5.19 mmol) in THF (15 mL)
was heated under reflux for 2 h. After cooling, water (1
mL) was added and the mixture was stirred overnight
at room temperature. To the reaction mixture, 10% aq.
NaOH solution (5 mL) was added and the flask was
cooled to 0°C. (Boc)2O (1.35 g, 6.2 mmol) in THF (5
mL) was added dropwise and the same temperature was
maintained for 4 h. The reaction mixture was extracted
with ethyl acetate (3×10 mL) and the combined extracts
were washed with water, brine and dried (Na2SO4).
Concentration and purification by column chromatog-
raphy provided 6 as a viscous oil (0.97 g, 73% yield). 1H
NMR (CDCl3): l 1.35 and 1.40 (2s, 6H), 1.52 (s, 9H),
3.71–4.51 (series of m, 4H), 5.15 (bd, 1H), 5.55–5.60 (m,
3H); IR (CHCl3): 1679, 3283 cm−1; [h]D25: −35.3 (C1,
CHCl3); HRMS: calcd for C13H23NO4 257.329. Found
257. 298.
D
was dissolved in methanol (20 mL) and to this solution
was added p-toluenesulfonylhydrazine (1.38 g, 7.39
mmol) and stirred for 6 h at room temperature under
nitrogen. Methanol was evaporated under reduced
pressure and the residue was triturated with petroleum
ether (15 mL). The resulting white solid was then
filtered and dried to yield tosyl hydrazone 3 (2.49 g,
1
72%). H NMR (CDCl3): l 1.24–1.38 (3s, 12H), 2.40 (s,
3H), 3.25–4.0 (series of m, 5H), 7.10 (d, J=6.5 Hz, 1H),
7.30 (d, J=7.8 Hz, 2H, aromatic protons), 7.70 (d,
J=7.8 Hz, 2H, aromatic protons), 11.20 (s, 1H); mp:
153–155°C; IR (CHCl3): 1630 cm−1; [h]D25: +48 (c 0.6,
CHCl3); HRMS: calcd for C18H26N2O6 398.4732.
Found. 398.4725.
4.1.2. (3R)-Hydroxy-(4R)-5-isopropylidine-1-pentene 4.
The tosyl hydrazone of 2,3,4,5-di-O-isopropylidene-
D
-
4.1.5. (2S)-[N-(tert-Butoxycarbonyl)amino]-3-pentenoic
acid 7. To a solution of 6 (0.5 g, 1.57 mmol) in alcohol-
free acetone (10 mL) at 0°C, freshly prepared Jones
reagent (1 mL) was added dropwise. After stirring for 2
h at room temperature, excess of Jones reagent was
quenched with isopropanol. Acetone was removed and
extracted with ethyl acetate (4×10 mL). The extracts
were dried (Na2SO4), concentrated and purified by
column chromatography to afford 7 as a highly viscous
liquid (0.226 g, 58%). 1H NMR (CDCl3): l 1.49 (s, 9H),
4.75 (bd, 1H), 5.32–6.10 (m, 3H); IR (CHCl3): 1637,
xylose 3 (1.152 g, 1 mmol) was placed in an oven-dried,
nitrogen flushed 100 mL flask fitted with a septum inlet
and a magnetic stirrer bar. The flask was charged with
dry THF (30 mL) and cooled to 0°C. NaHMDS (1 M
solution, 1 mL, 1 mmol) was added dropwise over 3
min, followed by addition of LiAlH4 (0.225 g, 2 mmol)
at once under a stream of N2 at the same temperature
and allowed to stir at room temperature for 8 h.
Saturated aqueous Na2SO4 solution was added and the
reaction mixture was extracted with ether, washed with

S. Chandrasekhar et al. / Tetrahedron: Asymmetry 13 (2002) 423–428
427
2995 cm−1; [h]2D5: +2.6 (c 1.5, MeOH); HRMS: calcd for
4.1.9.
(2S)-[(tert-Butoxycarbonyl)amino]-4-phenylbu-
C9H15NO4 201.2218. Found 201.2025.
tanoic acid 11. To a solution of 10 (0.5 g, 1.5 mmol) in
alcohol-free acetone (10 mL) at 0°C freshly prepared
Jones’ reagent (1 mL) was added dropwise. After stir-
ring for 24 h at room temperature, excess Jones’
reagent was quenched with isopropanol. Acetone was
removed on a rotavapor and to the residue, water was
added and extracted with ethyl acetate (4×10 mL). The
extracts were dried (Na2SO4), concentrated and purified
by column chromatography to afford 11 as a highly
4.1.6. 1-Phenyl-(3R)-hydroxy-(4R)-5-isopropylidene-1-
pentene 8. To a solution of PhMgBr [freshly prepared
from PhBr (9.45 mL, 0.09 mmol) and magnesium (2.33
g, 0.096 mmol)] in ether (200 mL) at 0°C was added
dropwise a solution of 3 (11.94 g, 0.03 mmol) in a
mixed solvent of THF (100 mL) and ether (50 mL) for
30 min with constant stirring under a nitrogen atmo-
sphere. The reaction mixture was slowly warmed up to
room temperature and stirred for an additional 4 h.
The reaction mixture was quenched with saturated aq.
solution of NH4Cl and the organic layer was separated
and washed with water, brine and dried (Na2SO4).
After concentration and column chromatography, allyl
alcohol 8 was obtained as a colourless syrupy liquid
1
viscous liquid (0.208 g, 50%). H NMR (CDCl3): l 1.51
(S, 9H), 1.93–2.33 (m, 2H), 2.65 (t, 2H, J=4.76 Hz),
4.26–4.42 (m, 1H), 5.12 (bd, 1H), 7.1–7.35 (m, 5H,
aromatic); 13C NMR (CDCl3): 176.5, 157, 140.6, 143.3,
126.0, 80, 54, 34.2, 31.5, 28.1, IR (neat): 1720, 2975,
3400 cm−1; [h]2D5=+5.8 (c 1, EtOH); HRMS: calcd for
C15H19NO4 277.2194. Found. 277.2085.
1
(3.93 g, 56%). H NMR (CDCl3): l 1.35 and 1.41 (2S,
6H), 2.45 (m, 1H, -OH), 3.65–4.12 (series of m, 3H),
4.32–4.48 (m, 1H), 5.62 (dd, J=9.2, 13.3 Hz, 1H), 6.68
(d, J=13.3, 1H), 7.20–7.38 (m, 5H, aromatic); IR
(neat): 3500 cm−1; [h]D25:+21.2 (c 0.5, CHCl3), HRMS:
calcd for C14H18O3 234.2944. Found 238.2543.
4.1.10. (2S)-[N-(tert-Butoxycarbonyl)amino]-4-phenyl-
methyl butanoate 12. A 25 mL round-bottomed flask
was charged with ether (6 mL) and 50% aq. KOH
solution (6 mL) and chilled to −5°C using coolant. To
this well stirred cooled solution, NMO (82 mg, 0.8
mmol) was added portionwise. After stirring for 5 min,
the yellow ether layer was separated and dried over
KOH. The yellow ethereal solution of diazomethane
was added to the cooled solution of 11 (0.05 g, 0.16
mmol) in ether (5 mL) and the mixture was allowed to
stir for 30 min. Ether was removed and the residue was
purified by silica gel column chromatography to afford
methyl ester 12 as a syrupy liquid (0.05 g, 96% yield).
1H NMR (CDCl3): l 1.48 (S, 9H), 2.08–2.21 (m, 2H),
2.62 (t, 2H, J=5 Hz), 3.79 (s, 3H), 4.20–4.40 (m, 1H),
5.05 (bd, 1H), 7.15–7.3 (m, 5H, aromatic), IR (neat):
1635, 1732 cm−1; [h]D25=−13.1 (c 1, MeOH), HRMS:
calcd for C16H21NO4 291.3462. Found. 291. 3123.
4.1.7. 1-Phenyl-(3S)-azido-(4R)-5-isopropylidene-1-pen-
tene 9. To a solution of TPP (4.32 g, 16.35 mmol) in
dry THF (40 mL) under a nitrogen atmosphere, −20°C
was added diethyl azodicarboxylate (2.56 mL, 16.5
mmol) dropwise. After stirring for 10 min, alcohol 8
(3.51 g, 15 mmol) in THF (20 mL) was added slowly.
The reaction mixture was stirred for 30 min at the same
temperature and then allowed to come to 0°C and
diphenyl phosphoryl azide (3.9 mL, 18 mmol) was
added dropwise. After stirring for 6 h at room temper-
ature, the reaction mixture was quenched with water
and extracted with ethyl acetate (100 mL, 2×50 mL).
The combined extracts were washed with brine, dried
(Na2SO4), concentrated and the residue was purified by
silica gel column chromatography to afford azide 9 as a
colourless viscous liquid (2.86 g, 74%). 1H NMR
(CDCl3): l 1.35 and 1.44 (2S, 6H), 3.78–4.28 (series of
m, 4H), 6.13 (dd, J=8.1, 13.7 Hz, 1H), 6.66 (dd,
J=5.5, 13.7 Hz), 7.22 –7.45 (m, 5H, aromatic); IR
(neat): 2105, 1600 cm−1; [h]D25: −13.5 (c 1.0, CHCl3),
HRMS: calcd for C14H17N3O2 259.3072. Found 259.
2975.
Acknowledgements
A.R. thanks INSA, New Delhi for financial assistance.
References
4.1.8.
1-Phenyl-(3S)-[N-(tert-butoxycarbonyl)amino]-
1. Williams, R. M. In Synthesis of Optically Active h-Amino
Acids, Vol. of Organic Chemistry Series; Baldwin, J. E.;
Magnus, P. D., Eds.; Pergamon Press: Oxford, 1989.
2. (a) Mass, G.; Poralla, K.; Zahner, H. Naturwissenschaften
1971, 58, 603; (b) Trowitzsch, W.; Sahm, H. J. Natur-
forschung, Teil C 1977, 32, 78.
3. (a) Chemistry and Biochemistry of the Amino Acids;
Barett, G. C., Ed.; Chapman and Hall: London, 1985; (b)
Jones, J. H. (Sen. Reported), Amino Acids and Peptides,
Specialist Periodical Report; The Royal Society of Chem-
istry, 1992; Vol. 23.
4. Dardene, G.; Casimir, J.; Marlier, M.; Larsen, P. O.
Phytochemistry 1974, 13, 1897.
5. Shaw, K. J.; Luly, J. R.; Rapport, U. J. J. Org. Chem.
1985, 50, 3306.
6. (a) Hanessian, S.; Sahoo, S. P. Tetrahedron Lett. 1984,
25, 1425; (b) Weber, T.; Aeschimann, R.; Maetzke, T.;
(4R)-5-isopropylidene-1-pentene 10. A solution of azide
9 (1 g, 3.86 mmol) in absolute ethanol (30 mL) was
added and hydrogenated over 10% Pd/C (10 mg) at
room temperature at 1 atm pressure for 24 h. The
reaction was analysed to check for completion by TLC.
A solution of (Boc)2O (0.925 g, 4.24 mmol) in EtOH (5
mL) was added in one portion and stirring was contin-
ued for another 6 h, then the catalyst was removed
by filtration through Celite. The filtrate was concentra-
ted to furnish amine 10 as a syrup after column chroma-
tography (0.838 g, 65%). 1H NMR (CDCl3): l 1.35
and 1.45 (2S, 6H), 1.47 (s, 9H), 1.80–1.92 (m, 2H),
2.60–2.82 (m, 2H), 3.61–4.15 (series of m, 3H), 4.65
(bd, 1H), 7.15–7.42 (m, 5H, aromatic); IR (neat):
1638 cm−1; HRMS: calcd for C19H27NO4 333.4276.
Found 333.4156.

428
S. Chandrasekhar et al. / Tetrahedron: Asymmetry 13 (2002) 423–428
Moloney, M. G.; North, M. Tetrahedron 1989, 45,
Seebach, D. Helv. Chim. Acta 1986, 69, 1365; (c) Schol-
6309.
lkopf, U.; Nozulak, J.; Growth, U. Tetrahedron 1984,
40, 1409; (d) Seebach, D.; Burger, H. M; Schekli, C. P.
Liebigs Ann. Chem. 1991, 669; (e) Muzler, J.; Anger-
mann, A.; Schubert, B.; Seilz, C. J. Org. Chem. 1986,
51, 5294; (f) Itaya, T.; Shimizu, S.; Nakagawa, S.;
Morisue, M. Chem. Pharm. Bull. 1994, 42, 1927; (g)
Griesbeck, A. G.; Hirt, J. Liebigs Ann. Chem. 1995,
1957; (h) Ardakani, A.; Rapport, H. J. Org. Chem.
1980, 45, 4817; (i) Hannesian, S.; Sahoo, P. Tetra-
hedron. Lett. 1984, 25, 1425; (j) Pelliciari, R.; Natalini,
B.; Marinozzi, M. Synth. Commun. 1988, 18, 1715; (k)
Badorrey, R.; Cativiela, C.; Villegas, M. D. D.; Glvez,
J. A. Synthesis 1997, 747.
9. (a) Chandrasekhar, S.; Takhi, M.; yadav, J. S. Tetra-
hedron Lett. 1995, 36, 5071; (b) Chandrasekhar, S.;
Mohapatra, S.; Takhi, M. Synlett 1996, 759; (c) Chan-
drasekhar, S.; Reddy, M. V.; Takhi, M. Tetrahedron
Lett. 1998, 39, 6535; (d) Chandrasekhar, S.; Reddy, M.
V.; Reddy, K. S.; Rmarao, C. S. Tetrahedron Lett.
2000, 41, 2667.
10. Wolform, M. L.; Newlin, M. R.; Stanley, E. E. J. Am.
Chem. Soc. 1931, 53, 4379.
11. Wolform, M. L.; Hanessian, S. J. Org. Chem. 1962,
188.
12. Corey, E. J.; Enckson, B. W. J. Org. Chem. 1971, 36,
3553.
7. Patchett, A. A.; Harris, E.; Tristram, E. W.; Wyvratt,
M. J.; Wu, M. T.; Taub, D.; Peterson, E. R.; Ikeler, T.
J.; Broeke, J.; Payne, L. G.; Ondeyka, D. L.; Thorgett,
E. D.; Greenlee, W. J.; Lohr, N. S.; Hoffsommar, R.
D.; Joghua, H.; Ruyle, W. V.; Rothrock, J. W.; Aster,
J. D.; Maycock, A. L.; Robinson, F. M.; Hirshmann,
R.; Sweet, C. S.; Ulm, E. H.; Gross, D. M.; Vassil, T.
C.; Stonne, C. A. Nature (London) 1980, 288, 280.
8. (a) Jackson, K. I.; Wishart, W.; Wood, A.; James, K.;
Wythes, M. J. J. Org. Chem. 1992, 57, 3397; (b) Regi-
nato, G.; Mordini, A.; Caraccido, M. J. Org. Chem.
1997, 62, 6187; (c) Fraser, J. L.; Jackson, R. F. W.;
Porter, B. Synlett 1994, 379; (d) Baldwin, J. E.;
13. Lal, B.; Pramanik, B.; Manhas, M. S.; Bose, A. K.
Tetrahedron Lett. 1977, 23, 1977.
14. Cha, J. S.; Kim, J. E.; Bose, A. K. Tetrahedron Lett.
1985, 26, 6453.
15. Evans, P. A.; Roseman, J. D.; Garber, L. T. Synth.
Commun. 1996, 26, 4685.
16. (a) Mirzani, D. M.; Gottilieb, H. E.; Marles, V.; Nudel-
man, A. J. Org. Chem. 1996, 61, 8402; (b) Ardakini, A.;
Rapport, H. J. Org. Chem. 1980, 45, 4817.
17. Weller, H. W.; Gordon, E. M. J. Org. Chem. 1982, 47,
4160.
18. Bajrowicz, J. A.; El Hallaoui, A.; Jacquier, R.; Pigiere,
C.; Villefont, P. Tetrahedron 1985, 41, 1833.