Hydroboration-Suzuki cross coupling of unsaturated amino acids; the synthesis of pyrimine derivatives

Article abstract of DOI:10.1016/S0040-4020(02)00541-0

Hydroboration of protected allylglycines with 9-BBN followed by Suzuki cross coupling of the resulting organoboranes proceeded smoothly giving a range of new bis-homophenylalanine and related derivatives in good yields (9 examples, 53-64%). One of the Suzuki coupling products has been elaborated to give the N-Cbz-protected natural product pyrimine. The hydroboration-Suzuki coupling of vinylglycine derivatives was also studied but was less efficient than with the allylglycine derivatives: the best results were obtained using disiamylborane·DMS as the hydroborating agent.

Full text of DOI:10.1016/S0040-4020(02)00541-0

TETRAHEDRON
Pergamon
Tetrahedron 58 12002) 6117±6125
Hydroboration±Suzuki cross coupling of unsaturated amino
acids; the synthesis of pyrimine derivatives
Philip N. Collier,a Andrew D. Campbell,a Ian Patelb and Richard J. K. Taylora,p
aDepartment of Chemistry, University of York, Heslington, York YO10 5DD, UK
bAstraZeneca, Avlon Works, Severn Road, Hallen, Bristol BS10 7ZE, UK
Received 11 March 2002; revised 14 May 2002; accepted 17 May 2002
AbstractÐHydroboration of protected allylglycines with 9-BBN followed by Suzuki cross coupling of the resulting organoboranes
proceeded smoothly giving a range of new bis-homophenylalanine and related derivatives in good yields 19 examples, 53±64%). One of
the Suzuki coupling products has been elaborated to give the N-Cbz-protected natural product pyrimine. The hydroboration±Suzuki coupling
of vinylglycine derivatives was also studied but was less ef®cient than with the allylglycine derivatives: the best results were obtained using
disiamylborane´DMS as the hydroborating agent. q 2002 Published by Elsevier Science Ltd.
Non-proteinogenic a-amino acids are of value in terms of
their biological importance and their utility as synthetic
building blocks.1,2 A number of preparative approaches to
enantiopure, non-proteinogenic a-amino acids have been
developed, many based on radical3 and cationic4 reagents
ultimately derived from the chiral pool. However, the most
popular approach in recent years has been to convert
proteinogenic amino acids into alanine, homoalanine and
bis-homoalanine anionic equivalents.5 In this area, the
organozinc approach of Jackson and co-workers is note-
worthy.6 They have prepared a family of organozinc
reagents 1 which can be coupled with aryl iodides and
related
electrophiles
under
palladium-catalysed
conditions to produce a range of phenylalanine, homo-
and bis-homophenylalanine derivatives in good yields
1Scheme 1).
Scheme 1.
Scheme 2.
Keywords: hydroboration±Suzuki cross coupling; unsaturated amino acids; pyrimine derivatives.
Corresponding author. Tel.: 144-1904-432606; fax: 144-1904-434523; e-mail: rjkt1@york.ac.uk
p
0040±4020/02/$ - see front matter q 2002 Published by Elsevier Science Ltd.
PII: S0040-4020102)00541-0

6118
P. N. Collier et al. / Tetrahedron 58 .2002) 6117±6125
Scheme 3.
Recently, we described the synthesis of novel organoborane
homoalanine anion equivalents 2, which were simply and
effectively transformed into a range of known and novel
non-proteinogenic a-amino acids under mild conditions
1Scheme 2).7
In extending the above methodology, we decided to investi-
gate whether organoborane reagents 3 could be prepared
directly by hydroboration of unsaturated amino acid deriva-
tives 1Scheme 3). Subsequent elaboration of the organo-
borane reagents 3 by Suzuki coupling would produce a
range of novel non-proteinogenic amino acids in protected
form. Such an approach, if successful, would avoid the need
for the oxidation step required in Scheme 2, thereby mini-
mising the risk of racemisation and extending the range of
compatible halide partners in the Suzuki coupling process.8
We felt that this methodology could complement the zinc
chemistry in terms of ease of reagent preparation, tolerance
in terms of reactions conditions and functional groups, etc.
Preliminary results in this area have been published;9 we
now wish to report our investigations in full.
Scheme 4.
acids some of which display interesting biological
activities.11 We ®rst carried out the hydroboration of
protected allylglycine 5 with 9-BBN 1Scheme 5); 2equiv.
of the borane were required for the reaction to proceed to
completion at room temperature. It seems likely that the
®rst equivalent of 9-BBN reacts with, or is complexed
by, the carbamate group. The triorganoborane adduct 6
was not isolated but was treated in situ with Suzuki coupling
conditions. The same Suzuki coupling conditions as
employed in our earlier studies7,8 were adopted, i.e.
PdCl21dppf)´CH2Cl2/aq. K3PO4 in a solvent mixture of
THF±DMF. The results of the couplings are summarised
in Table 1.
1. Hydroboration±Suzuki coupling studies of protected
allylglycines
Table 1. Suzuki coupling of organoborane 6 with aryl and vinyl halides
1l)-Allylglycine 4 is commercially available or can readily
be prepared by literature methods.10 N-Boc protection of
amino acid 4 using 1Boc)2O/NaOH was followed by treat-
ment with 1trimethylsilyl)diazomethane giving N-Boc 1l)-
allylglycine methyl ester 5 {[a]Dˆ118.8 1c 1.0, CHCl3);
lit.10c 119.3 1c 1.5, CHCl3)} in 91% overall yield as the key
substrate for hydroboration±Suzuki coupling studies
1Scheme 4).
Entry
RX
Product
Yield 1%)
i
ii
PhI
2-NO2±C6H4I
4-MeO±C6H4I
4-MeO2C±C6H4Br
2-Br±py
2-Br±py N-oxide´HBr
1Z)-IHCvCHCO2Et
CH2vCBrCH3
7
8
9
10
11
12
13a
14
62
53
60
64
62
56
60
58
iii
iv
v
vi
vii
viii
Danion et al. have recently described the hydroboration of
protected allyl- and vinylglycine derivatives and other
unsaturated amino acids as a route to v-borono-a-amino
a
Estimated NMR yield 113 contaminated by an inseparable boron
impurity).
Scheme 5.

P. N. Collier et al. / Tetrahedron 58 .2002) 6117±6125
6119
The ®rst electrophile investigated in the Suzuki coupling
process was iodobenzene 1Table 1, entry i). We were
delighted to observe ef®cient formation of the bis-homo-
phenylalanine derivative 7 {[a]Dˆ121.3 1c 0.2, CHCl3)}
in 62% yield after stirring the reaction overnight at room
temperature. Both electron withdrawing and electron
donating aromatic bromides and iodides were found to be
viable coupling partners successfully giving adducts 8±10
1Table 1, entries ii±iv). 2-Bromopyridine also underwent
ef®cient transformation and the product 11 displayed
spectroscopic data consistent with those previously reported
for the racemic material 1Table 1, entry v).12 Likewise,
Suzuki coupling of 2-bromopyridine N-oxide hydrobromide
was accomplished to give 12 1Table 1, entry vi). Vinyl
iodides and bromides also coupled smoothly. Ethyl Z-iodo-
acrylate gave adduct 13 with the Z-alkenyl stereochemistry
present in the coupling partner being retained in the product
13 1Table 1, entry vii, Jˆ11.6 Hz). 2-Bromopropene also
coupled successfully giving 14 in 58% yield 1Table 1, entry
viii); in this example, 3 equiv. of 2-bromopropene were
used due to its high volatility.
Polarimetry measurements con®rmed that stereo-
chemical integrity was conserved during the hydro-
boration±Suzuki coupling sequence. Hydrolysis of 9
with 6 M HCl at 708C gave the known13 amino acid
product 15 which had an optical rotation consistent
with that previously reported {[a]Dˆ131.21 c 0.3,
5 M HCl±DMF, 1:1); lit.13 1ent-15) 231.8 1c 2.0, 5 M
HCl±DMF, 1:1)} 1Scheme 6).
Other N-protected allylglycine derivatives were investi-
gated to establish their compatibility with the hydro-
boration±Suzuki coupling sequence. N,N-DiBoc- and
N-trityl-allylglycine methyl ester were prepared from
commercially available allylglycine and subjected to
9-BBN hydroboration followed by Suzuki coupling with
2-nitro-iodobenzene. In both cases coupling proceeded to
Scheme 6.
Scheme 7.

6120
P. N. Collier et al. / Tetrahedron 58 .2002) 6117±6125
give the N-protected analogue corresponding to adduct 8.
However, the overall yields were lower than with N-Boc
protection 1N-Boc, 53%; N,N-DiBoc, 45%; N-trityl 44%)
and so the choice of the N-Boc derivative 5 for the
hydroboration-coupling sequence seems appropriate.
3. Application to the synthesis of pyrimine derivatives
Finally, the utility of the bis-homophenylalanine equivalent
6 was demonstrated in the natural product arena by prepar-
ing derivatives of pyrimine 18, an Fe1II)-binding agent
isolated from a Pseudomonas species found in soil by
Shiman et al. 1Scheme 8).15,16
2. Hydroboration±Suzuki coupling studies of protected
vinylglycines
The availability of the N-oxide Suzuki adduct 12 led us to
investigate the possibility of using the Boekelheide reac-
tion17,18 for oxidation at the benzylic position as required
in the natural product. However, treatment of N-oxide 12
with tri¯uoroacetic acid anhydride 1TFAA) under the
improved conditions developed for the Boekelheide reac-
tion by Balavoine et al.18 resulted in decomposition of the
starting material 12 and none of the expected product 19
was obtained 1Scheme 8).
Having achieved encouraging results for the preparation of
novel bis-homophenylalanine derivatives via the hydro-
boration±Suzuki coupling of protected allylglycine 5, we
next turned our attention to vinylglycine. In view of the
commercial availability of N-Cbz vinylglycine methyl
ester 16, attention was focussed on its elaboration 1Scheme
7 and Table 2). Initial attempts involved using iodobenzene
and the same 9-BBN-Suzuki coupling conditions which
were successful in the allylglycine studies. However, the
starting alkene 16 required 5 molar equiv. of 9-BBN for
complete consumption and none of the desired phenyl
adduct 17 was obtained 1entry i).14 We therefore investi-
gated the use of other dialkylboranes which are known8a
to provide trialkylboranes suitable for Suzuki coupling
1entries ii±v). The best result, a 32% yield for the formation
of adduct 17, was obtained using disiamylborane´SMe2 as
the hydroborating agent; the dimethyl sul®de 1DMS) was
removed in vacuo prior to Suzuki coupling otherwise cata-
lyst poisoning occurred. The product 17 {[a]Dˆ114.4 1c
0.5, CH2Cl2); lit.6c [a]Dˆ114.4 1c 0.5, CH2Cl2)} was
obtained in enantiomerically pure form.
On the assumption that N-Boc protection is incompatible
with the modi®ed Boeklelheide reaction conditions, we
followed the same approach but with the more robust
N-Cbz protecting group 1Scheme 9). Thus, treatment of
the known allylglycine derivative 2019 with 9-BBN
followed by Suzuki coupling with 2-bromopyridine
N-oxide hydrobromide salt gave adduct 21, as expected.
Exposure of N-oxide 21 to TFAA gave the hoped-for
Boekelheide reaction producing alcohol 22 as a mixture of
diastereomers 1approximately 1:1) in 62% yield after
saponi®cation of the intermediate tri¯uoroacetates.
Alcohol 22 was oxidised with manganese dioxide and then
saponi®cation using aqueous lithium hydroxide in THF
gave N-Cbz-protected pyrimine 23, which was fully char-
acterised. Attempted synthesis of pyrimine 18 from N-Cbz
pyrimine 23 by hydrogenolysis gave the unexpected amino
acid product 24 1ca. 1:1 mixture of diastereoisomers)
presumably arising from cyclisation of the intermediate
amino ketone 18 and then non-selective imine reduction.20
Table 2. Hydroboration±Suzuki coupling of alkene 16
Entry
R2BH
Yield of 17 1%)
i
9-BBN 15 equiv.)
Cy2BH´THF
Cy2BH´DMS
Sia2BH´THF
Sia2BH´DMS
0
ii
iii
iv
v
10
11
22
32
In conclusion, a facile procedure for the synthesis of a range
of new bis-homophenylalanine derivatives has been
developed by the hydroboration±Suzuki coupling of
protected allylglycines. Protected vinylglycine has also
been employed in a similar sequence, although less success-
fully. We are currently developing a polymer-supported
version of this methodology so as to allow a combinatorial
approach to the synthesis of non-proteinogenic amino acids
for use in medicinal chemistry.
In view of these disappointing results, no further studies
were carried out in this area. It should be noted that homo-
phenylalanine derivatives are available using the related
chemistry shown in Scheme 2,7 and this is our preferred
route.
Scheme 8.

P. N. Collier et al. / Tetrahedron 58 .2002) 6117±6125
6121
Scheme 9.
4. Experimental
vacuo. The residue was puri®ed by column chromatography
1light petroleum±EtOAc, 2:1) to yield the title compound 5
11.54 g, 98%) as a colourless oil, Rf 0.39 1light petroleum±
EtOAc, 3:1); [a]Dˆ118.8 1c 1.0, CHCl3); lit.10c
4.1. General
1
[a]Dˆ119.3 1c 1.5, CHCl3); H NMR 1270 MHz, CDCl3)
See Ref. 7c.
d 1.47 19H, s), 2.42±2.61 12H, m), 3.74 13H, s), 4.34±4.44
11H, m), 5.06 11H, br s), 5.1212H, m), 5.7211H, m). The
spectroscopic data were consistent with those reported.10c
4.1.1. Methyl ,2S)-2-[,tert-butoxycarbonyl)amino]-4-
pentenoate ,5). 1i) A solution of l-2-amino-4-pentenoic
acid 4 11 g, 8.69 mmol) in 1 M NaOH 120 mL) and
1,4-dioxane 110 mL) at 08C was treated with di-tert-butyl
dicarbonate 12.28 g, 10.5 mmol) and the mixture allowed to
warm to rt and the pH re-adjusted to 9 when required. The
reaction was complete in 2days by TLC. The dioxane was
removed in vacuo and the mixture washed with Et2O
120 mL) to remove any di-tert-butyl dicarbonate. The
aqueous layer was acidi®ed to pH 2with 2M H 2SO4 and
extracted with EtOAc 14£50 mL) saturating the aqueous
layer with NaCl each time. The combined organic layers
were dried, ®ltered and the solvent removed in vacuo to
furnish the N-Boc protected amino acid 11.74 g, 93%) as a
colourless oil which was used without further puri®cation.
4.2. General procedure for hydroboration±Suzuki
coupling
To the alkene 5, 16 or 20 11 equiv.) in THF 11 mL per
0.2mmol alkene) at 0 8C under nitrogen was added 9-BBN
10.5 M in THF, 2equiv.). The mixture was warmed to rt and
stirred for 2h. The ¯ask was covered with foil and K 3PO4
13 M in H2O, 2equiv.) was added carefully 1H 2 evolution)
followed quickly by addition of the organic halide 1ca.
1.1 equiv.) in dry degassed DMF 11 mL per 0.2mmol
alkene) and PdCl21dppf)´CH2Cl2 10.05 equiv.) under
nitrogen. The reaction was stirred overnight and the solvent
was removed in vacuo. The residue was taken up in Et2O
125 mL per 0.2 mmol alkene) and saturated aq. NaHCO3
110 mL per 0.2mmol alkene). The aqueous layer was
re-extracted with Et2O 125 mL per 0.2 mmol alkene) and
the combined organic layers were dried, ®ltered and concen-
trated in vacuo to give the crude product which was puri®ed
by ¯ash column chromatography eluting with light
1ii) N-1tert-Butoxycarbonyl)-l-2-amino-4-pentenoic acid
11.47 g, 6.83 mmol) was dissolved in MeOH 113 mL) and
toluene 113 mL) and 1trimethylsilyl)diazomethane 12M
solution in hexanes, 6.84 mL, 13.7 mmol) added until
there was a permanent change in colour 1colourless to
straw coloured). The reaction was left to stir for 30 min
after which all volatile components were removed in

6122
P. N. Collier et al. / Tetrahedron 58 .2002) 6117±6125
1
petroleum±EtOAc mixtures to afford the Suzuki coupling
product.
1610, 1520, 1365 cm21; H NMR 1270 MHz, CDCl3) d
1.43 19H, s), 1.56±1.98 14H, m), 2.51±2.62 12H, m), 3.71
13H, s), 3.77 13H, s), 4.27±4.40 11H, m), 5.03 11H, br d,
Jˆ8.5 Hz), 6.81 12H, d, Jˆ8.5 Hz), 7.06 12H, d, Jˆ8.5 Hz);
13C NMR 167.9 MHz, CDCl3) d 27.9, 28.9, 32.8, 35.0, 52.8,
53.9, 55.9, 80.5, 114.4, 129.9, 134.4, 156.0, 158.4, 174.0;
CIMS m/z 355 1MNH41, 7), 338 1MH1, 8), 238 1100);
HRCIMS Found: MH1, 338.1962. C18H27NO5 requires
MH, 338.1967.
In some cases, the product was accompanied by a 9-BBN
derived impurity which could be removed by the following
procedure: the crude reaction product was dissolved in THF
13 mL per 0.2mmol alkene) and aq. NaOH 11 M, 1 mL per
0.2mmol alkene) was added followed subsequently by aq.
H2O2 160% w/v, 0.2mL per 0.2mmol alkene) and the
mixture was stirred vigorously for 10 min at 08C. The reac-
tion was diluted with Et2O 120 mL per 0.2 mmol alkene) and
saturated aq. NaHCO3 110 mL per 0.2mmol alkene). The
aqueous layer was re-extracted with Et2O 125 mL per
0.2mmol alkene) and the combined organic layers were
dried, ®ltered and concentrated in vacuo prior to chromato-
graphy as above.
4.2.4. Methyl 4-{,4S)-4-[,tert-butoxycarbonyl)amino]-5-
methoxy-5-oxopentyl}benzoate ,10). The title compound
10 was prepared according to the general hydroboration±
Suzuki coupling procedure outlined above by reaction with
methyl 4-bromobenzoate 142mg, 0.2mmol). Puri®cation
by column chromatography eluting with light petroleum±
EtOAc 13:1) yielded 10 142mg, 64%) as a colourless oil, Rf
0.27 1light petroleum±EtOAc, 3:1); [a]Dˆ116.21 c 1.5,
CHCl3); IR 1®lm) 3363, 2953, 2864, 1720, 1610, 1513,
4.2.1. Methyl ,2S)-2-[,tert-butoxycarbonyl)amino]-5-
phenylpentanoate ,7). The title compound 7 was prepared
according to the general hydroboration±Suzuki coupling
procedure outlined above by reaction with iodobenzene
1124 mg, 0.61 mmol, 1.05 equiv.). Puri®cation by column
chromatography eluting with light petroleum±EtOAc 14:1)
yielded 7 1110 mg, 62%) as a colourless oil, Rf 0.35 1light
petroleum±EtOAc, 3:1); [a]Dˆ121.3 1c 0.2, CHCl3); IR
1®lm) 3369, 3025, 2977, 2864, 1745, 1715, 1603,
1
1437, 1366, 1281 cm21; H NMR 1270 MHz, CDCl3) d
1.42 19H, s), 1.55±1.91 14H, m), 2.57±2.78 12H, m), 3.71
13H, s), 3.89 13H, s), 4.25±4.41 11H, m), 5.03 11H, br d,
Jˆ8.0 Hz), 7.22 12H, d, Jˆ8.0 Hz), 7.94 12H, d, Jˆ8.0 Hz);
13C NMR 167.9 MHz, CDCl3) d 26.7, 28.3, 32.2, 35.2, 51.9,
52.2, 53.0, 79.9, 127.9, 128.4, 129.7, 147.1, 155.3, 167.0,
173.2; CIMS m/z 383 1MNH41, 4), 366 1MH1, 1), 266
1100); HRCIMS Found: MNH41, 383.2189. C19H27NO6
requires MNH4, 383.2182.
1
1518 cm21; H NMR 1270 MHz, CDCl3) d 1.44 19H, s),
1.54±1.91 14H, m), 2.55±2.70 12H, m), 3.72 13H, s),
4.28±4.42 11H, m), 4.99 11H, d, Jˆ8.0 Hz) and 7.13±7.34
15H, m); 13C NMR 167.9 MHz, CDCl3) d 27.7, 28.9, 32.9,
35.9, 52.9, 53.9, 80.5, 126.5, 129.0 1£2), 142.3, 156.0,
174.0; CIMS m/z 325 1MNH41, 1), 308 1MH1, 7), 208
1100); HRCIMS Found: MH1, 308.1857. C17H25NO4
requires MH, 308.1862.
4.2.5. Methyl ,2S)-2-[,tert-butoxycarbonyl)amino]-5-,2-
pyridinyl)pentanoate ,11). The title compound 11 was
prepared according to the general hydroboration±Suzuki
coupling procedure outlined above by reaction with
2-bromopyridine 134 mg, 0.22 mmol). Puri®cation by
column chromatography eluting with light petroleum±
EtOAc 11:2) yielded 11 138 mg, 62%) as a colourless oil,
Rf 0.35 1EtOAc); [a]Dˆ115.0 1c 0.8, CHCl3); IR 1®lm)
4.2.2. Methyl ,2S)-2-[,tert-butoxycarbonyl)amino]-5-,2-
nitrophenyl)pentanoate ,8). The title compound 8 was
prepared according to the general hydroboration±Suzuki
coupling procedure outlined above by reaction with
1-iodo-2-nitrobenzene 1140 mg, 0.56 mmol). Puri®cation
by column chromatography eluting with hexane±EtOAc
13:1) yielded 8 195 mg, 53%) as a light yellow oil, Rf 0.24
1light petroleum±EtOAc, 2:1); [a]Dˆ115.0 1c 0.8, CHCl3);
IR 1®lm) 3376, 2977, 2939, 2871, 1745, 1713, 1610, 1577,
1525, 1456, 1365 cm21; 1H NMR 1270 MHz, CDCl3) d 1.43
19H, s), 1.62±1.98 14H, m), 2.89 12H, app t, Jˆ8.0 Hz), 3.73
13H, s), 4.27±4.42 11H, m), 5.05 11H, d, Jˆ7.5 Hz), 7.31±
7.36 12H, m), 7.47±7.54 11H, m), 7.88 11H, d, Jˆ7.0 Hz);
13C NMR 167.9 MHz, CDCl3) d 27.0, 28.9, 33.1, 33.2, 53.0,
53.7, 80.6, 125.4, 127.8, 132.5, 133.6, 137.3, 149.9, 156.0,
173.8; CIMS m/z 370 1MNH41, 13), 353 1MH1, 3) and 253
1100); HRCIMS Found: MNH41, 370.1979. C17H24N2O6
requires MNH4, 370.1978.
3201, 2886, 2726, 1740, 1704, 1538, 1455, 1377 cm21; H
1
NMR 1270 MHz, CDCl3) d 1.41 19H, s), 1.58±1.9214H, m),
2.79 12H, td, Jˆ7.0, 2.5 Hz), 3.70 13H, s), 4.21±4.40 11H,
m), 5.17 11H, br s, Jˆ7.5 Hz), 7.06±7.13 12H, m), 7.57 11H,
td, Jˆ7.5, 1.5 Hz), 7.50 11H, d, Jˆ4.5 Hz); 13C NMR
167.9 MHz, CDCl3) d 25.3, 28.3, 32.1, 37.5, 52.2, 53.3,
79.7, 121.1, 122.8, 136.3, 149.2, 155.7, 161.2, 173.2;
CIMS m/z 309 1MH1, 100), 209 146); HRCIMS Found:
MH1, 309.1810. C16H24N2O4 requires MH, 309.1814.
The spectroscopic data were consistent with those reported
for the racemate.12
4.2.6. Methyl ,2S)-2-[,tert-butoxycarbonyl)amino]-5-,1-
oxido-2-pyridinyl)pentanoate ,12). The title compound
12 was prepared according to the general hydroboration±
Suzuki coupling procedure by reaction with 2-bromo-
pyridine-N-oxide hydrobromide 156 mg, 0.22 mmol,
1.2equiv.) in the presence of K 3PO4 13 M in H2O, 0.2mL,
0.59 mmol, 3.2equiv.). In addition to the standard pro-
cedure the reaction was heated to 658C overnight, then
cooled and the solvent was removed in vacuo and puri®ed
directly by column chromatography eluting with EtOAc±
EtOH 15:1) to afford 12 133 mg, 56%) as a colourless oil, Rf
0.10 1EtOAc±EtOH, 10:1); [a]Dˆ115.4 1c 0.2, CHCl3); IR
1®lm) 3363, 2976, 2930, 1741, 1707, 1526, 1492, 1439,
4.2.3. Methyl ,2S)-2-[,tert-butoxycarbonyl)amino]-5-,4-
methoxyphenyl)pentanoate ,9). The title compound 9
was prepared according to the general hydroboration±
Suzuki coupling procedure outlined above by reaction
with 4-methoxy-iodobenzene 177 mg, 0.33 mmol). Puri®ca-
tion by column chromatography eluting with light
petroleum±EtOAc 14:1) yielded 9 161 mg, 60%) as a colour-
less oil, Rf 0.35 1light petroleum±EtOAc, 3:1); [a]Dˆ117.5
1c 0.8, CHCl3); IR 1®lm) 3362, 2977, 2936, 1744, 1716,

P. N. Collier et al. / Tetrahedron 58 .2002) 6117±6125
6123
1366 cm21; 1H NMR 1270 MHz, MeOH-d4) d 1.43 19H, s),
1.60±1.93 14H, m), 2.96 12H, app t, Jˆ6.0 Hz), 3.71 13H, s),
4.15±4.20 11H, m), 4.86±4.90 11H, m), 7.37±7.47 11H, m),
7.50±7.60 12H, m), 8.33 11H, d, Jˆ6.5 Hz); 13C NMR
167.9 MHz, CDCl3) d 22.1, 28.2, 30.0, 32.2, 52.3, 53.1,
79.8, 123.6, 125.5, 126.1, 139.7, 151.8, 155.4, 173.1;
CIMS m/z 325 1MH1, 65), 309 1MH2O, 100); HRCIMS
Found: MH1 325.1763. C16H24N2O5 requires MH,
325.1763.
reported.21 The melting point of this compound has not
been reported before.
4.2.10. Methyl ,2S)-2-{[,benzyloxy)carbonyl]amino}-4-
phenylbutanoate ,17). 1a) By hydroboration±Suzuki
coupling of 16 with disiamylborane´DMS. To alkene 16
124 mg, 96 mmol) in THF 10.6 mL) at 08C under nitrogen
was added disiamylborane freshly prepared from
BH3´DMS22 11.2M solution in THF, 0.16 mL, 0.19 mmol)
and the reaction was warmed to rt and stirred for 1.5 h until
TLC showed consumption of starting material. The solvent
and DMS were removed in vacuo and the residue re-
dissolved in THF 10.4 mL) and DMF 10.4 mL). K3PO4
13 M in H2O, 0.06 mL, 0.18 mmol) was added followed
by quick addition of iodobenzene 121 mg, 0.10 mmol) and
®nally PdCl21dppf)´CH2Cl2 13 mg, 4 m mol) under nitrogen.
The reaction was heated to 708C for 5 h and then cooled and
the solvent was removed in vacuo. The residue was taken up
in Et2O 110 mL) and saturated aq. NaHCO3 110 mL). The
aqueous layer was re-extracted with Et2O 12£10 mL) and
the combined organic layers were dried, ®ltered and concen-
trated in vacuo to give the crude product as a brown oil
which was puri®ed by column chromatography 1light
petroleum±EtOAc, 3:1) to afford the title compound 17
110 mg, 32%) as a colourless oil, [a]Dˆ114.4 1c 0.5,
CH2Cl2); lit.6c [a]Dˆ114.4 1c 0.5, CH2Cl2); 1H NMR
1270 MHz, CDCl3) d 1.95±2.07 11H, m), 2.14±2.27 11H,
m), 2.69 12H, app t, Jˆ8.0 Hz), 3.7213H, s), 4.40±4.5211H,
m), 5.15 12H, s), 5.48 11H, d, Jˆ8.0 Hz), 7.12±7.50 110H,
m).
4.2.7. Methyl ,2S,6Z)-2-[,tert-butoxycarbonyl)amino]-8-
ethoxy-8-oxo-6-octenoate ,13). The title compound 13 was
prepared according to the general hydroboration±Suzuki
coupling procedure outlined above by reaction with 1Z)-3-
iodoethylacrylate 140 mg, 0.18 mmol). Puri®cation by
column chromatography eluting with light petroleum±
EtOAc 14:1) yielded 13 156 mg, contaminated by a 9-BBN
derived impurity, approx. yield 60%) as a colourless oil, Rf
1
0.28 1light petroleum±EtOAc, 3:1); H NMR 1270 MHz,
CDCl3) d 1.27 13H, t, Jˆ7.5 Hz), 1.43 19H, s), 1.50±1.90
14H, m), 2.66 12H, app q, Jˆ7.5 Hz), 3.73 13H, s), 4.15 12H,
q, Jˆ7.5 Hz), 4.34±4.43 11H, m), 5.00±5.21 11H, app d,
Jˆ7.5 Hz), 5.77 11H, dt, Jˆ11.5, 2.0 Hz), 6.15 11H, dt,
Jˆ11.5, 7.5 Hz); 13C NMR 167.9 MHz, CDCl3) d 14.2,
24.7, 28.2, 28.3, 32.3, 52.2, 53.3, 59.8, 79.8, 120.3, 149.0,
155.3, 166.2, 173.3; CIMS m/z 347 1MNH41, 2), 330 1MH1,
9), 230 1100); HRCIMS Found: MH1, 330.1923.
C16H27NO6 requires MH, 330.1917.
4.2.8. Methyl ,2S)-2-[,tert-butoxycarbonyl)amino]-6-
methyl-6-heptenoate ,14). The title compound 14 was
prepared according to the general hydroboration±Suzuki
coupling procedure outlined above by reaction with
2-bromopropene 172 mg, 0.59 mmol, 3 equiv.). Puri®cation
by column chromatography eluting with light petroleum±
EtOAc 13:1) yielded 14 131 mg, 58%) as a colourless oil, Rf
0.31 1light petroleum±EtOAc, 4:1); [a]Dˆ113.1 1c 0.5,
CHCl3); IR 1®lm) 3447, 3357, 2967, 2931, 2863, 1747,
The spectroscopic data were consistent with those repor-
ted.6c,7c
1b) By hydroboration±Suzuki coupling of 16 with dicyclo-
hexylborane´DMS. The title compound 17 was prepared
using the same procedure as in 1a) hydroboration of alkene
16 120 mg, 80 mmol) was carried out with a 0.07 M solution
of dicyclohexylborane´DMS.22 The Suzuki coupling
reaction was complete in 2h at 50 8C. The product 13 mg,
11%) displayed spectroscopic properties identical to those
of compound 17.
1
1715, 1699, 1525, 1453, 1169 cm21; H NMR 1270 MHz,
CDCl3) d 1.44 19H, s), 1.40±1.90 14H, m), 1.69 13H, s), 2.02
12H, t, Jˆ7.5 Hz), 3.74 13H, s), 4.27±4.42 11H, m), 4.66
11H, br s), 4.71 11H, br s), 5.0211H, d, Jˆ9.0 Hz); 13C NMR
167.9 MHz, CDCl3) d 22.2, 23.1, 28.3, 32.2, 37.1, 52.2,
53.3, 79.9, 110.4, 145.0, 155.3, 173.4; CIMS m/z 289
1MNH41, 11), 272 1MH1, 45), 1721100); HRCIMS
Found: MH1, 272.1859. C14H25NO4 requires MH,
272.1862.
1c) By hydroboration±Suzuki coupling of 16 with disiamyl-
borane´THF. The title compound 17 was prepared using the
same procedure as in 1a) hydroboration of alkene 16 114 mg,
56 mmol) was carried out with a 0.83 M solution of
disiamylborane´THF.22 The Suzuki coupling reaction was
complete in 2h at 60 8C. The product 14 mg, 22%) displayed
spectroscopic properties identical to those of compound 17.
4.2.9. ,2S)-2-Amino-5-,4-methoxyphenyl)pentanoic acid
hydrochloride ,15). A stirred solution of protected amino
acid 9 147 mg, 0.14 mmol) and anisole 131 mg, 0.29 mmol)
in 6 M HCl was heated at 708C for 6 h. The reaction was
cooled and then diluted with H2O 110 mL) and washed with
EtOAc 12£10 mL) and the aqueous layer was concentrated
in vacuo and azeotroped with toluene 13£5 mL) to give the
title compound 15 133 mg, 91%) as a colourless solid, mp
191±1938C; [a]Dˆ131.21 c 0.3, 5 M HCl±DMF, 1:1); lit.
1ent)13 [a]Dˆ231.8 1c 2.0, 5 M HCl±DMF, 1:1); 1H NMR
1270 MHz, CF3CO2D) d 2.11±2.33 12H, m), 2.40±2.58 12H,
m), 3.00±3.1212H, m), 4.3213H, s), 4.71 11H, t, Jˆ6.5 Hz),
7.33 12H, d, Jˆ8.0 Hz), 7.50 12H, d, Jˆ8.0 Hz).
1d) By hydroboration±Suzuki coupling of 16 with dicyclo-
hexylborane´THF. The title compound 17 was prepared
using the same procedure as in 1a) hydroboration of alkene
16 132mg, 0.13 mmol) was carried out with a 1 M solution
of dicyclohexylborane´THF.22 The Suzuki coupling reaction
was complete in 2h at 50 8C. The product 14 mg, 10%)
displayed spectroscopic properties identical to those of
compound 17.
4.2.11. Methyl ,2S)-2-{[,benzyloxy)carbonyl]amino}-5-
,1-oxido-2-pyridinyl)pentanoate ,21). Using alkene 20,
the title compound 21 was prepared according to the general
The spectroscopic data were consistent with those

6124
P. N. Collier et al. / Tetrahedron 58 .2002) 6117±6125
hydroboration±Suzuki coupling procedure by reaction with
2-bromopyridine N-oxide hydrobromide 1184 mg,
mixture extracted with EtOAc 13£5 mL). The organic
layer was dried, ®ltered and concentrated in vacuo to give
the crude product which was puri®ed by column chromato-
graphy eluting with EtOAc to afford 23 122 mg, 82%) as a
colourless oil, Rf 0.14 1EtOH±EtOAc, 1:9); [a]Dˆ25.1 1c
0.9, CHCl3); IR 1®lm) 3422, 2944, 1731, 1699, 1682, 1538,
0.72mmol) in the presence of K 3PO4 13 M in H2O,
0.7 mL, 2.1 mmol, 3.2 equiv.). In addition to the standard
procedure the reaction was heated to 658C overnight, then
cooled and the solvent was removed in vacuo and puri®ed
directly by column chromatography eluting with EtOAc±
EtOH 11:1) to afford 21 1145 mg, 62%) as a colourless oil, Rf
0.27 1EtOAc±EtOH, 1:1); [a]Dˆ112.5 1c 0.2, CHCl3); IR
1
1404, 1345, 1216 cm21; H NMR 1270 MHz, CDCl3) d
2.18±2.43 12H, m), 3.22±3.51 12H, m), 4.50±4.63 11H,
m), 5.07 12H, s), 5.95 11H, d, Jˆ8.0 Hz), 7.20±7.41 15H,
m), 7.46±7.51 11H, m), 7.83 11H, t, Jˆ7.5 Hz), 8.0211H, d,
Jˆ7.5 Hz), 8.67 11H, d, Jˆ4.0 Hz), 11.0 11H, br s); 13C
NMR 167.9 MHz, CDCl3) d 26.4, 33.9, 53.4, 67.1, 122.5,
127.7, 128.2, 128.5, 128.6, 136.3, 137.8, 148.7, 152.5,
156.3, 174.9, 200.0; CIMS m/z 343 1MH1, 15), 325 1100);
HRCIMS Found: MH1 343.1304. C18H18N2O5 requires MH,
343.1294.
1
1®lm) 3343, 3227, 3027, 2952, 1715, 1543, 1440 cm21; H
NMR 1270 MHz, MeOH-d4) d 1.70±1.98 14H, m), 2.89±
3.00 12H, m), 3.75 13H, s), 4.40±4.49 11H, m), 5.11 12H, s),
5.48 11H, d, Jˆ8.0 Hz), 7.14±7.35 18H, m), 8.30 11H, d,
Jˆ6.0 Hz); 13C NMR 167.9 MHz, CDCl3) d 22.1, 29.8,
31.7, 51.2, 53.6, 66.6, 123.5, 125.4, 126.1, 127.8, 128.0,
128.2, 136.1, 139.4, 151.4, 156.0, 172.7; CIMS m/z 359
1MH1, 4) and 343 1MH2O, 100); HRCIMS Found: MH1
359.1607. C19H22N2O5 requires MH, 359.1607.
4.2.14. ,2S)-5-,2-Pyridinyl)-2-pyrrolidinecarboxylic acid
,24). Carbamate 23 115 mg, 44 mmol) in EtOH 11 mL) was
exposed to an atmosphere of hydrogen gas 1introduced by
balloon after evacuation of a nitrogen atmosphere) in the
presence of Pd±C 110%, 2mg) at rt overnight. The reaction
mixture was ®ltered through Celite and concentrated in
vacuo to give the title compound 24 18 mg, 95%, ca. 1:1
mixture of diastereomers) as a white solid, mp 170±1728C;
4.2.12. Methyl ,2S)-2-{[,benzyloxy)carbonyl]amino}-5-
hydroxy-5-,2-pyridinyl)pentanoate ,22). To N-oxide 21
195 mg, 0.27 mmol) in CH2Cl2 12.5 mL) at rt under nitrogen
was added tri¯uoroacetic anhydride 1695 mg, 3.32mmol)
over 5 min. The solution was stirred overnight at rt and
then concentrated in vacuo. The residue was dissolved in
CH2Cl2 10.7 mL) and saponi®ed with aq. Na2CO3 12M,
2mL) for 3 h. The organic layer was separated and the
aqueous layer washed with CH2Cl2 110 mL) and the
combined organic layers were dried, ®ltered and concen-
trated in vacuo to give the crude product which was puri®ed
by column chromatography eluting with EtOAc to give the
title compound 22 159 mg, 62%, approx. 1:1 mixture of
diastereomers) as a colourless oil, Rf 0.25 1EtOAc); IR
1®lm) 3333, 3033, 2953, 2864, 1746, 1731, 1716, 1595,
IR 1®lm) 3484, 2971, 2926, 2858, 1716, 1699, 1455 cm21
;
1H NMR 1500 MHz, D2O) d 1.85±2.05 14H, m), 3.90±3.98
11H, m), 5.04±5.11 11H, m), 7.79±7.89 12H, m), 8.38±8.46
11H, m), 8.51±8.58 11H, m); 13C NMR 1125 MHz, D2O) d
²
25.7, 31.8, 31.9, 52.6, 52.7, 68.6, 124.5, 126.1, 140.5,
147.2, 156.9, 171.9; CIMS m/z 193 1MH1, 100), 106 134);
HRCIMS Found: MH1 193.0978. C10H12N2O2 requires MH,
193.0977.
1538, 1455, 1436, 1215 cm21
;
1H NMR 1270 MHz,
²
CDCl3) d 1.60±2.14 14H, m), 3.70 13H, s), 3.7213H, s),
3.90±4.3211H, m), 4.33±4.50 11H, m), 4.71±4.83 11H, m),
5.00±5.17 12H, m), 5.53 11H, d, Jˆ8.0 Hz), 5.64 11H, d,
Jˆ8.0 Hz), 7.15±7.28 12H, m), 7.29±7.40 15H, m), 7.66
11H, td, Jˆ8.0, 2.0 Hz), 8.51 11H, d, Jˆ4.5 Hz); 13C NMR
167.9 MHz, CDCl3) d 28.4, 33.9, 52.3, 53.5, 53.8, 66.9,
71.5, 71.9, 120.2, 122.4, 128.0, 128.1, 128.4, 136.2, 136.9,
148.0, 148.1, 156.0, 161.2, 172.8; CIMS m/z 359 1MH1,
15), 91 1100); HRCIMS Found: MH1 359.1607.
C19H22N2O5 requires MH, 359.1607.
Acknowledgements
We thank AstraZeneca and the University of York for ®nan-
cial assistance 1P. N. C.).
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