Synthesis of boronic acid analogues of α-amino acids by introducing side chains as electrophiles

Article abstract of DOI:10.1021/jo015753y

A synthetic route has been developed which has allowed us to prepare novel α-aminoboronic acids as inhibitors of serine proteases. These compounds were prepared to study the roles of proteases in biological systems. This methodology affords α-aminoboronic

Full text of DOI:10.1021/jo015753y

J . Org. Chem. 2001, 66, 6375-6380
6375
Syn th esis of Bor on ic Acid An a logu es of r-Am in o Acid s by
In tr od u cin g Sid e Ch a in s a s Electr op h iles
Sharada J agannathan, Timothy P. Forsyth,† and Charles A. Kettner*
Chemical and Physical Sciences, Dupont Pharmaceuticals Company,
P.O. Box 80500, Wilmington, Delaware 19880-0500
Charles.a.kettner@dupontpharma.com
Received May 14, 2001
A synthetic route has been developed which has allowed us to prepare novel R-aminoboronic acids
as inhibitors of serine proteases. These compounds were prepared to study the roles of proteases
in biological systems. This methodology affords R-aminoboronic acids with the general formula
R′-NHCH(R)BO2-pinanediol, where R ) -CH2CHF2, -CH2CO2tBu, and -(CH2)2CO2Me and R′ )
either H or C(O)R′′. The latter two compounds are the boronic acid analogues of the natural amino
acids aspartic acid and glutamic acid with the side chain carboxylate protected as a tert-butyl or a
methyl ester, respectively. Following acylation of the amino group, the side chain tert-butyl ester
of boroaspartic acid was removed by treatment with TFA. Boroglutamic acid was obtained as
the free boronic acid by hydrolysis with HCl. Prior syntheses of R-aminoboronic acids involve the
initial addition of an organometallic reagent to a trialkyl borate ester. These conditions do not
allow the preparation of compounds with functionalities that are not stable to the strongly basic
reaction conditions. The methodology described here allows the preparation of R-aminoboronic
acids by introducing side chains as electrophiles. This is particularly advantageous for side chains
which are prone to elimination or unwanted enolate formation. Specifically, BrCH2CHF2, BrCH2-
COOtBu, and CH2dCHCOOMe were allowed to react with the stabilized anion of (phenylthio)-
methane boronate, PhSCH2BO2C6H12, to give the substituted boronate. The substituted (phenylthio)-
methane boronate was converted to the corresponding sulfonium ion by treatment with methyl
iodide and subsequently displaced with iodide. The R-iodo derivative was converted to the amine
by conventional methods.
In tr od u ction
cessing by the viral protease occurs almost exclusively
at a cysteine residue.8 On the basis of the observation
that -CHF2 is an isostere for a sulfhydryl group,9 we
have developed a synthesis for R-aminoboronic acids with
a difluoroethyl side chain 1. The synthesis of 1, together
with its incorporation into a peptide and its effectiveness
in the inhibition of hepatitis C protease, is reported. In
addition, the syntheses of new R-aminoboronic acids
boroaspartate 2 and boroglutamate 310 as their respective
esters are also described.
Serine proteases are a group of enzymes that have a
common catalytic mechanism. They are involved in the
mediation of a number of pathological conditions and,
therefore, are the targets for inhibition by therapeutic
agents.1 Serine proteases that are pharmacological tar-
gets include elastase2 involved in inflammation and em-
physema, the blood coagulation enzymes (such as throm-
bin, factor Xa, and factor VIIa),3 and, more recently, hep-
atitis C protease.4 The latter is required for viral replica-
tion,5 and consequently, the development of hepatitis C
protease inhibitors is an important therapeutic objective.
Highly effective inhibitors of serine proteases can be
obtained by preparing peptide analogues of substrates
where the scissile bond is replaced by an electrophilic
moiety such as trifluoromethyl ketones, aldehydes, R-keto
acids and amides, and R-aminoboronic acids.1 We have
targeted hepatitis C protease for inhibition by aminobo-
ronic acids due to our longstanding interest in these
analogues6,7 and their greater potencies determined from
earlier comparisons (Kettner, unpublished data). Pro-
The different synthetic approaches that have been used
in the synthesis of R-aminoboronic acids containing a
(6) Kettner, C. A.; Shenvi, A. B. J . Biol. Chem. 1984, 259, 15106.
(7) Kettner, C. A.; Mersinger, L. J .; Knabb, R. J . Biol. Chem. 1990,
265, 18289.
(8) Urbani, A.; Bianchi, E.; Narjes, F.; Tramontano, A.; De Francesco,
R.; Steinkuhler, C.; Pessi, A. J . Biol. Chem 1997, 272, 9204.
(9) Narjes, F.; Brunetti, M.; Colarusso, S.; Gerlach, B.; Koch, U.;
Biasol, G.; Fattori, D.; De Francesco, R.; Matassa, V.; Steinkuhler, C.
Biochemistry 2000, 39, 1849.
(10) The prefix “boro” is used to designate the boronic acid analogue
of the corresponding amino acid where the -COOH group is replaced
by B(OH)2. The suffixes -C10H16 and -C6H12 indicates the boronate
pinanediol and pinacol ester, respectively. In the peptide Pz-CO-Val-
Val-Hyp(Bzl)-OH, Pz is pyrazine and Hyp(Bzl) is 4-hydroxyproline with
the hydroxyl group protected as a benzyl ether.
* To whom correspondence should be addressed.
† Current address: Albany Molecular Research, Inc., 21 Corporate
Circle, P.O. Box 15098, Albany, NY 12212-5098.
(1) Powers, J . C.; Harper, J . Inhibitors of Serine Proteases; Elsevier
Science Publishing Co.: New York, 1986; Vol. 12.
(2) Edwards, P. D.; Bernstein, P. R. Med. Res. Rev. 1994, 14, 127.
(3) Lorand, L.; Mann, K. G. Methods Enzymol. 1993, 222, 1-10.
(4) Love, R. A.; Parge, H. E.; Wickersham, J . A.; Hostomsky, Z.;
Habuka, N.; Moomaw, E. W.; Adachi, T.; Hostomska, Z. Cell 1996, 87,
331.
(5) Grakoui, A.; McCourt, D. W.; Wychowski, C.; Feinstone, S. M.;
Rice, C. M. J . Virol. 1993, 67, 2832.
10.1021/jo015753y CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/22/2001

6376 J . Org. Chem., Vol. 66, No. 19, 2001
J agannathan et al.
Sch em e 1
Sch em e 2
acids, but limitations exist for these approaches. Either
a stable nucleophile, Grignard reagent, or olefin must be
available for generation of the boronate ester 5. Here,
we describe a procedure for the synthesis of novel
R-aminoboronic acids where the R-side-chain substituent
is derived from the reaction of an electrophile with the
stabilized PhSCH2BO2-pinacol methide anion. In addition
to the synthesis of an R-aminoboronic acid with a 2,2-
difluoroethyl side-chain 1, we have also prepared com-
pounds with a carboxymethyl and a carboxyethyl side
chain (boroaspartate 2 and boroglutamate 3, respec-
tively). Analogues of aspartic acid and glutamic acid
where the side-chain carboxylates were replaced with a
boronic acid have been synthesized;15-17 however, boro-
aspartate and boroglutamate, the analogues of natural
amino acids, have not been previously reported. They are
expected to be applicable to the inhibition of other serine
proteases of general interest. It should be noted that we
have drawn from the earlier chemistry developed by
Matteson et al.18(See Discussion).
variety of side chains are outlined (Scheme 1). In the first
approach, a Grignard reagent or other suitable nucleo-
phile is added to trialkyl borate to give a substituted
dialkyl boronate 4 (Scheme 1a). Transesterification with
a suitable diol protecting group like pinanediol gives the
boronate ester 5. The R-chloroalkyl intermediate 6 is
obtained by the stereospecific addition of the anion of
methylene chloride to the boronic pinanediol ester.11
Nucleophilic displacement of the chloride of 6 by a
nitrogen nucleophile such as lithium bis(trimethylsilyl)-
amide gives the bissilane-protected amine 7.12 Subse-
quent treatment of 7 with anhydrous HCl gives the
amine as the hydrochloride salt 8. The alkyl side chain
can also be introduced as an olefin13 wherein hydrobo-
ration with catecholborane and transesterification with
pinanediol gives the alkyl boronate 5 (Scheme 1b). The
preparation of R,R-dichloromethyl ester 10 also allows
the introduction of the side chain as a nucleophile to give
6 (Scheme 1c).14
Resu lts
The procedures outlined in Scheme 1 have been suc-
cessfully used to synthesize a number of R-aminoboronic
Scheme 2 shows the synthetic scheme for the prepara-
tion of an R-aminoboronic acid with a difluoroethyl side
(11) Matteson, D. S.; Majumdar, D. Organometallics 1983, 2, 1529.
(12) Matteson, D. S.; Sadhu, K. M. Organometallics 1984, 3, 614.
(13) Matteson, D. S.; J esthi, P. K.; Sadhu, K. M. Organometallics
1984, 3, 1284.
(14) Kinder, D. H.; Katzenellenbogen, J . A. J . Med. Chem. 1985,
28, 1917.
(15) Denniel, V.; Bauchat, P.; Danion, D.; Danion-Bougot, R. Tet-
rahedron Lett. 1996, 37, 5111.
(16) Kinder, D. H.; Ames, M. M. J . Org. Chem. 1987, 52, 2452.
(17) Hangauer, D. G.; Hsiao, G. K. Synthesis 1998, 1043.
(18) Matteson, D. S.; Arne, K. Organometallics 1982, 1, 280.

Boronic Acid Analogues of R-Amino Acids
J . Org. Chem., Vol. 66, No. 19, 2001 6377
Sch em e 3
Sch em e 4
chain 1, its incorporation into a peptide, and resolution
of the peptide diastereomers. Pinacol chloromethyl bor-
onate19 11 was allowed to react with 1 equiv of ben-
zenethiol in the presence of diisopropylethylamine to give
the boronate 12. (Phenylthio)methane boronate18 12 was
added to 1 equiv of LDA at 0 °C, and the precipitated
anion was quenched with 2-bromo-1,1-difluoroethane to
provide the substituted boronate 13 as a mixture of
enantiomers (56% yield). The boronate 13 was alkylated
with an excess of methyl iodide to give the sulfonium salt
14 which, in the presence of sodium iodide, was converted
to the iodoboronic ester 15 (49% yield) in situ. The iodide
15 was treated with LHMDS at -78 °C to form the
disilazane boronate, which was readily hydrolyzed with
3 equiv of anhydrous HCl in dioxane to give 1 as the HCl
salt (52% yield). The amine 1 was coupled to Pz-CO-Val-
Val-Hyp(Bzl)-OH10 using the mixed anhydride proce-
dure20 to provide the peptide boronic ester as a mixture
of diastereomers. The pinacol ester was converted to the
pinanediol ester by transesterification, due to its greater
stability, and the diastereomers were separated by silica
gel chromatography (67% yield).
The utility and versatility of the chemistry outlined is
further demonstrated in the preparation of derivatives
of boroaspartate 2 (Scheme 3) and boroglutamate 3
(Scheme 4). For the preparation of 2, the anion of 12 was
treated with an excess of tert-butyl bromoacetate at 0 °C
to give the substituted boronic ester 16 (35% yield).
Following the steps in Scheme 2, the iodide 18 was
obtained (31% yield). Treatment of 18 with sodium azide
under phase transfer catalysis conditions21 followed by
transesterification to the pinanediol ester provided the
R-azido derivative 19 (59% yield).22 Catalytic hydrogena-
tion in the presence of 1 equiv of anhydrous HCl gave
the amine 2 as the HCl salt (79% yield). 2 was further
characterized as its benzoyl derivative 2a . It was treated
with benzoyl chloride, and the tert-butyl ester in the side
chain was removed with TFA in methylene chloride to
give the free acid (43%).
H-boroGlu(OMe)-C10H16 3 was also prepared by a
similar series of reactions (Scheme 4). After generation
of the anion of 12, a Michael acceptor (in this case methyl
acrylate) was added to the anion at 0 °C to provide 21
(21% yield). Following the steps in Scheme 2, the iodide
23 was obtained (38% yield). Treatment of 23 with
sodium azide in DMF at 60 °C followed by transesteri-
fication to the pinanediol ester provided the R-azido
derivative 24 (26% yield). Catalytic hydrogenation in the
presence of 1 equiv of anhydrous HCl afforded the amine
3 as the HCl salt (55% yield). Amine 3 was refluxed in 6
N HCl to give 3a as the free boronic acid (90%).
Discu ssion
In the conventional preparation of R-aminoboronic
acids, side chains are introduced as nucleophiles24 or they
are introduced by hydroboration of the appropriately
functionalized alkene13 (Scheme 1). In the current syn-
thetic approach, the side chain is introduced as an
(22) The phase transfer catalysis conditions used in the preparation
of 19 employed dichloromethane as one solvent component. Diazi-
domethane is a potential, hazardous byproduct of this reaction. It is
recommended that alternative solvent systems described by Singh and
Matteson23 be used.
(23) Singh, R. P.; Matteson, D. S. J . Org. Chem. 2000, 65, 6650.
(24) Matteson, D. S. Chem. Rev. 1989, 89, 1535.
(19) Matteson, D. S.; Majumdar, D. J . Organomet. Chem. 1979, 170,
259.
(20) Anderson, G. W.; Zimmerman, J .; Callahan, F. J . Am. Chem.
Soc. 1967, 89, 5012.
(21) Matteson, D. S.; Sadhu, K. M.; Peterson, M. L. J . Am. Chem.
Soc. 1986, 108, 810.

6378 J . Org. Chem., Vol. 66, No. 19, 2001
J agannathan et al.
electrophile following the preparation of the (phenylthio)-
methide boronate anion (Schemes 2-4). This anion was
first reported by Matteson et al.18 and was shown to react
with simple, activated electrophiles. The sulfonium salt
of PhSCH(R)BO2C6H12 was prepared by alkylation with
methyl iodide, and it was converted to the R-iodoboronic
ester by treatment with sodium iodide using an approach
described by Corey and J autelat25 for non-boronic acid
derivatives. However, the usefulness of this chemistry
was not fully realized due to the difficulty of converting
the substituted (phenylthio)methane boronate to the
R-iodoboronic ester. This reaction is reported to be slow
at room temperature (3 days) and can be hampered by
competing dehydrohalogenation. We have found the
conversion of PhSCH(R)BO2C6H12 (13 for example) to the
R-iodoboronic ester could be achieved in good yields by
refluxing in anhydrous acetonitrile for 3-6 h. This key
improvement allows easy access to a wide variety of
R-iodoboronic esters. We recognized that these R-iodobo-
ronic esters could be easily converted into the corre-
sponding R-aminoboronic esters. Interestingly, the for-
mation of the sulfonium ion and the conversion to the
R-iodoboronate does not take place when a pinanediol
ester is used in place of a pinacol ester, presumably due
to steric hindrance. It should be noted that modifying
existing reaction pathways for the preparation of R-ami-
noboronic acids previously had received little consider-
ation due to the versatility of the homologation reaction
and displacement of the R-chloro group with a nucleo-
phile.11,23 We found that the (phenylthio)methide anion
chemistry and its reaction with electrophiles could allow
us access R-aminoboronic esters with functionalities that
would otherwise not survive the harsh basic conditions.
One example involved the attempted preparation of
1-amino-3,3-difluoropropyl boronic ester 1. In attempts
to prepare 1, low-temperature transmetalation of 2-bromo-
1,1-difluoroethane with tert-butyllithium26 followed by
treatment with triisopropyl borate and isolation as its
pinanediol or pinacol ester did not give the desired
product. Presumably this was due to the formation of the
volatile â-elimination product, difluoroethylene. An at-
tempt was made to prepare the Grignard reagent derived
from 2-bromo-1,1-difluoroethane and to treat it with
dichloromethyl boronate 10 (Scheme 1c). This reaction
also failed probably due to a similar â-elimination.
However, the possibility exists that the (difluoroethyl)-
lithium anion and the Grignard reagent had formed and
were not sufficiently reactive to provide the desired
product. Other conventional methods of preparing R-ami-
noboronic acids also failed to give compound 1. For
example, hydroboration of 1,1′-difluoroethene gave an
intractable mixture from which the desired product,
difluoroethyl boronate, (compound 5 in Scheme 1b where
R is -CH2CHF2) was present at levels not exceeding 4%.
Another approach to prepare difluoro compounds involves
treatment of an aldehyde with (diethylamino)sulfur
trifluoride (DAST).27 Attempts to prepare the key inter-
mediate (the protected aldehyde side chain derived from
2-bromomethyl-1,3-dioxolane) in a manner analogous to
that described by Mantri et al.28 resulted in elimination
of the acetal to the vinyl glycol ether.29 Utilizing our
present approach, we were able to prepare 1-amino-3,3-
difluoropropyl boronate 1 in good yields and to incorpo-
rate it into a peptide to give Pz-CO-Val-Val-Hyp(Bzl)-
NH-CH(CH2CHF2)-BO2C10H16. The two diastereomers
were separated to give enantiomerically pure isomers,
25A,B. In general, serine proteases have a strong prefer-
ences for substrates containing L-amino acids and R-ami-
noboronic acids6 in the R configuration in this position.
25A was the most effective inhibitor of hepatitis C
protease binding with a Ki of 12 nM. This compound is
most likely in the R configuration.
In addition to the R-aminoboronic acid containing a
difluoroethyl side chain, we were interested in R-ami-
noboronic acids with alkyl carboxylate side chains (either
as an ester or a free carboxylate). Attempts to prepare
the boronic acid analogue of aspartic acid using the
chemistry outlined in Scheme 1 were unsuccessful. We
felt that the methodology developed above would be
useful in preparing these R-aminoboronic acids.
For example, in efforts to prepare boroaspartate 2,
treatment of both 9 and 10 (Scheme 1c) with tert-butyl
lithioacetate with and without a Lewis acid catalyst did
not provide the corresponding R-chloroboronic ester 6 (R
is tert-butoxycarbonylethyl). The enolate was not nucleo-
philic enough to displace the chloride. In contrast, the
preparation of boroaspartate (Scheme 3) was readily
achieved with the current methodology. The anion of 12
was treated with an excess of tert-butyl bromoacetate at
0 °C to give the substituted boronic ester 16. The key
intermediate, R-iodoboronic ester 18, was obtained and
treated with sodium azide under phase transfer catalysis
conditions21,22 followed by transesterification to the pi-
nanediol ester to provide the R-azido derivative 19.
Catalytic hydrogenation in the presence of 1 equiv of
anhydrous HCl provided amine 2 as the hydrochloride
salt. This compound was incorporated into peptides, and
the tert-butyl ester was readily removed with TFA to
generate the free acid.
We have prepared the boronic acid analogue of glutam-
ic acid by introducing the side-chain carboxyethyl group
as an electrophile (Scheme 4). In contrast to the difluo-
roethyl compound and boroaspartic acid, boroglutamic
acid can probably be prepared by the traditional routes
(Scheme 1). Hiscox and Matteson30 have prepared the
1-chloro-tert-butoxycarbonylpropionyl boronate by react-
ing the anion of methylene chloride with tert-butoxycar-
bonylethyl boronate. This reagent was used as an inter-
mediate in the preparation of the J apanese beetle
pheromone, [R-(Z)]-5-(1-decenyl)dihydro-2(3H)-furanone.
In our attempts to prepare the boroglutamate 3, the thiol
ether 21 (Scheme 4) was not obtained when the anion of
12 (Scheme 2) was allowed to react with 1 equiv of methyl
3-bromopropionate. The boronate 12 was recovered along
with methyl acrylate, the latter arising from enolate
formation followed by bromide elimination. The desired
product 21 was obtained when the anion of 12 was
allowed to react with an excess of methyl acrylate. This
led to the successful synthesis of 3, H-boroGlu(OMe)-
C10H16. The amine 3 was incorporated into peptides, and
the side-chain methyl ester was cleaved by treatment
with potassium trimethylsilanolate.31 This reaction gave
(25) Corey, E. J .; J autelet, M. Tetrahedron Lett. 1968, 55, 5787.
(26) Bailey, W. F.; Patricia, J . J . J . Organomet. Chem. 1988, 352, 1.
(27) Middleton, W. J . J . Org. Chem. 1975, 40, 574.
(28) Mantri, P.; Duffy, D. E.; Kettner, C. A. J . Org. Chem. 1996, 61,
5690.
(29) Feugeas, C. Bull. Soc. Chim. Fr. 1963, 2568.
(30) Hiscox, W. C.; Matteson, D. S. J . Organomet. Chem. 2000, 614-
615, 314.
(31) Laganis, E. D.; Chenard, B. Tetrahedron Lett. 1984, 25, 5831.

Boronic Acid Analogues of R-Amino Acids
J . Org. Chem., Vol. 66, No. 19, 2001 6379
Hz), -116.77 (t, 1F, J ) 16.8 Hz); HRMS calcd for C15H21
-
the desired product in low yields. The major products
were the components resulting from the cleavage of the
carbon-boron bond. However, treatment of 3 with 6 N
HCl simultaneously hydrolyzed the side-chain methyl
ester and the pinanediol ester to give the boronic acid
3a in good yields.
In another case, tert-butyl acrylate was used in place
of methyl acrylate in an attempt to prepare the tert-butyl
ester analogue of 3. The tert-butyl analogue of 21 was
obtained, but further conversion to the corresponding
R-iodo analogue failed due to cleavage of the tert-butyl
ester. We speculate that the boronic ester moiety is acting
as an intramolecular Lewis acid in the cleavage of the
tert-butyl ester.
The sequence of reactions outlined in Schemes 2-4 has
made it possible to prepare many structurally diverse
R-aminoboronic acids. In addition to the specific com-
pounds we have prepared, higher order acrylates or alkyl
halides can be used to give more complex side chains.
This is particularly valuable for the preparation of com-
pounds with side chains that contain sensitive function-
alities such as ketones, phosphonates, and sulfonamides.
Clearly, there is significant value in the present syn-
thetic approach for preparing R-aminoboronic acids with
versatile side chains containing sensitive functionalities.
Similarly, the modification of the reaction (Scheme 2)
allowing the introduction of side chains as Michael
addition products is novel (Scheme 4). In conclusion, we
have developed a method for the synthesis of R-aminobo-
ronic acids with diverse substituents under conditions
amenable to the introduction of side chains as electro-
philes.
BF2O2S (M+) 314.1323, found 314.1328.
P in a col 1-Iod o-3,3-d iflu or op r op a n e-1-bor on a te (15).
Boronate 13 (6.00 g, 19.1 mmol) was dissolved in anhydrous
acetonitrile (60 mL). Anhydrous methyl iodide (24.0 mL, 380
mmol) and sodium iodide (5.76 g, 38.2 mmol) were added. The
reaction mixture was vigorously refluxed for 5 h. The solvent
was evaporated in vacuo. The residue was partitioned between
water (40 mL) and ether (40 mL). The phases were separated,
and the aqueous phase was washed with an equal volume of
ether. The combined organic phases were dried over Na2SO4
and evaporated to give a brown oil. This material was distilled
to give 3.1 g (48.8%) of the R-iodo boronate 15: bp 63-65 °C,
1
0.4 mmHg; H NMR (CDCl3) δ 6.18-5.64 [tt, 1H, J ) 4.4 Hz
(inner triplet), J ) 56.4 Hz (outer triplet)], 3.21 (t, 1H, J )
8.4 Hz), 2.43-2.36 (m, 2H), 1.25 (s, 12H); 13C NMR (CDCl3) δ
120.49-114.13 (t, J ) 238.4 Hz), 84.14, 39.38-38.79 (t, J )
21.8 Hz), 24.32, 24.16; HRMS calcd for C9H16BF2IO2 (M + H)
333.0334, found 333.0326.
P in a col 1-Am in o-3,3-d iflu or op r op a n e-1-bor on a te-Hy-
d r och lor id e (1). Iodide 15 (2.7 g, 8.1 mmol) was dissolved in
anhydrous THF (10 mL) and was added dropwise to a cool
(-78 °C) solution consisting of lithium bis(trimethylsilyl)amide
(9.68 mL, 9.68 mmol, 1.0 M in THF) and THF (10 mL). The
reaction mixture was allowed to warm to room temperature
and stirred for 12 h. It was concentrated in vacuo, and hexane
was added. The reaction mixture was cooled to -78 °C, and 4
N anhydrous hydrogen chloride in dioxane (6.05 mL, 24.2
mmol) was added dropwise. The mixture was warmed to room
temperature and stirred for 5 h. Solvent was removed by
evaporation, and chloroform was added. Insoluble material was
removed by filtration. The filtrate was evaporated almost to
dryness, and hexanes were added. The desired product crys-
tallized. It was isolated and washed with cold hexane to yield
1.1 g (52%) of 1 as the amine hydrochloride: mp 138-141 °C;
1H NMR (CDCl3) δ 7.86 (br, 3H), 6.22-6.01 [(tt, 1H, J ) 4.0
Hz (inner triplet), J ) 55.7 (outer triplet)], 3.42 (m, 1H), 2.76-
2.51 (m, 2H), 1.32 (s, 12H); 13C NMR (CDCl3) δ 119.21-116.10
(t, J ) 240.3 Hz), 86.28, 33.56-32.97 (t, J ) 22.1 Hz), 24.92,
24.82; 19F NMR -115.33 (t, 1F, J ) 16.9 Hz), -115.51 (t, 1F,
J ) 16.9 Hz); HRMS calcd for C9H18BO2F2N (M+) 222.1399,
found 222.1415.
Exp er im en ta l Section
Ma ter ia ls a n d Meth od s. All reactions were carried out
under anhydrous conditions under a positive pressure of dry
nitrogen unless otherwise stated. Sure-seal solvents and
reagents were used as purchased from Aldrich and Lancaster
P z -C O -Va l-Va l-H y p (B z l)-N H -C H (C H 2 C H F 2 )-B O 2 -
C
10H16 (25A).10 Pz-CO-Val-Val-Hyp(Bzl)-OH (1.2 g, 2.28 mmol)
was dissolved in THF (10 mL), and N-methylmorpholine (0.25
mL, 2.28 mmol) was added. The solution was cooled to -20
°C, and isobutyl chloroformate (0.30 mL, 2.28 mmol) was
added. After 5 min, a cold (-20 °C) solution of 1 dissolved in
chloroform (10 mL) was added followed by the addition of
triethylamine (0.32 mL, 2.28 mmol). The reaction was allowed
to warm to room temperature and stirred overnight. The
mixture was filtered, and the filtrate was concentrated in
vacuo. After the oily residue was dissolved in ethyl acetate
(30 mL), it was washed with 0.2 N HCl, 5% NaHCO3, and
saturated aqueous NaCl. The organic layer was dried over Na2-
SO4 and concentrated. The material was redissolved in metha-
nol (10 mL) and transesterified with (+)-pinanediol (0.38 g,
2.28 mmol). After 2 h, the methanol was evaporated and the
crude reaction mixture was purified on a 4 × 90 cm Sephadex
LH-20 column using methanol as a solvent. The desired
product was obtained as an amorphous solid (1.19 g, 67%). TLC
in 100% ethyl acetate indicated the two diastereomers with
Rf of 0.31 and 0.25, respectively. The diastereomer with Rf of
0.31 was isolated by silica gel chromatography. The diaster-
eomeric mixture (0.05 g) was loaded onto the column, and it
was eluted with a stepwise gradient of ethyl acetate: hexane
from a ratio of 20:80 to a ratio of 80:20. Fractions containing
the product were concentrated in vacuo to give 0.019 g (36%)
of the diastereomer 25A: 1H NMR (CDCl3) δ 9.40 (d, 1H), 8.76
(d, 1H), 8.57 (m, 1H), 8.43 (d, 1H), 7.78 (m, 1H), 7.34 (m, 5H),
6.10-5.91 [(tt, 1H, J ) 4.0 Hz (inner triplet), J ) 56.6 Hz
(outer triplet)], 4.78 (m, 2H), 4.61 (t, 1H), 4.42 (q, 2H), 4.22
(d, 1H), 4.08 (d, 1H), 3.62 (m, 1H), 3.18 (m, 1H), 2.31-1.85
(m, 10H), 1.32 (s, 3H), 1.27 (s, 3H), 0.98-0.81 (m, 15H); 13C
NMR (CDCl3) δ 173.19, 171.84, 170.66, 163.20, 147.46, 144.49,
1
Chemical Co. H and 13C NMR spectra were recorded at 300,
500, and 600 MHz, and 19F NMR spectra were recorded at 300
MHZ. Pz-CO-Val-Val-Hyp(Bzl)-OH was synthesized using the
mixed anhydride coupling conditions20 employing isobutyl
chloroformate, triethylamine, and N-methylmorpholine.
P in acol 1-(P h en ylth io)-3,3-diflu or opr opan e-1-bor on ate
(13). Butyllithium (50.6 mL, 126 mmol, 2.5 M in hexanes) was
added dropwise to a solution of diisopropylamine (18.4 mL,
133 mmol) dissolved in THF (40 mL) at 0 °C in a 500 mL
round-bottom flask. A solution of pinacol (phenylthio)methane
boronate18 (31.6 g, 126 mmol) in THF (40 mL) was added
dropwise over a period of 10 min to yield a white precipitate.
After the mixture was stirring for 1 h at 0 °C, 2-bromo-1,1-
difluoroethane (51 mL, 630 mmol) was added dropwise. The
precipitate slowly dissolved. The solution was allowed to warm
to room temperature and stirred for 16 h. Excess cold 10%
phosphoric acid was added, and the mixture was stirred for 5
min. Ether (100 mL) was added, and the phases were
separated. The organic layer was dried over sodium sulfate
and filtered. The filtrate was concentrated in vacuo and
distilled (bp 119-122 °C, 0.4 mmHg) to give 22 g (56%) of 13
as a clear oil: 1H NMR (CDCl3) δ 7.43-7.19 (m, 5H), 6.16-
5.78 [tt, 1H, J ) 4.6 Hz (inner triplet), J ) 56.8 Hz (outer
triplet)], 2.82 (m, 1H), 2.38-2.19 (m, 2H), 1.23 (s, 12H); 13C
NMR (CDCl3) δ 134.95, 130.70, 128.94, 126.84, 119.39-113.04
(t, J ) 238.9 Hz), 84.39, 36.15-35.58 (t, J ) 21.4 Hz), 24.63,
24.57, 23.98 (br);32 19F NMR (CDCl3) δ -116.97 (t, 1F, J ) 16.8
(32) The carbon R to boron is broad and not always visible due to
coupling to the boron atom.

6380 J . Org. Chem., Vol. 66, No. 19, 2001
J agannathan et al.
144.17, 142.75, 137.57, 128.43, 127.82, 127.68, 118.70-117.12
(t, J ) 238.7 Hz), 85.26, 71.15, 57.81, 57.44, 55.84, 52.75, 51.70,
39.73, 38.19, 35.89, 35.36, 34.39, 31.93, 31.67, 28.75, 27.19,
40.22, 38.34, 36.72, 29.27, 27.36, 26.82, 24.23; HRMS calcd
for C20H26BNO5 (M+) 371.1904, found 371.1888.
P in a col 1-(P h en ylth io)-3-(m eth oxyca r bon yl)p r op a n e-
1-bor on a te (21). Boronate 21 was prepared by the procedure
described for the preparation of 13 (21%): 1H NMR (CDCl3) δ
7.37-7.12 (m, 5H), 3.60 (s, 3H), 2.74 (t, 1H), 2.42 (m, 2H), 1.98
(m, 2H), 1.18 (s, 12H); 13C NMR (CDCl3) δ 173.51, 135.96,
130.32, 128.71, 126.27, 84.08, 51.50, 32.54, 29.12 (br), 26.07,
24.71, 24.63; HRMS calcd for C17H25BO4S (M+) 336.1567, found
336.1569.
26.35, 24.08, 19.41, 19.33, 18.07, 17.58; HRMS calcd for C40H55
-
BF2N6O7 (M + H) 781.4271, found 781.4275.
P z -C O -Va l-Va l-H y p (B z l)-N H -C H (C H 2 C H F 2 )-B O 2 -
C10H16 (25B). The peptide was synthesized from Pz-CO-Val-
Val-Hyp(Bzl)-OH and R-aminoboronic acid 1 using the proce-
dure described for peptide 25A. The diastereomeric mixture
(0.05 g) was loaded onto a silica gel column, and it was eluted
with a stepwise gradient of ethyl acetate: hexane from a ratio
of 50:50 to 90:10. TLC in 100% ethyl acetate indicated the
product at Rf of 0.25. Fractions containing the product were
concentrated in vacuo to give 0.016 g (32%) of the desired
product 25B: 1H NMR (CDCl3) δ 9.35 (d, 1H), 8.76 (d, 1H),
8.56 (m, 1H), 8.33 (d, 1H), 7.60 (m, 1H), 7.35 (m, 5H), 6.91 (d,
1H), 6.01-5.78 [(tt, 1H, J ) 4.0 Hz (inner triplet), J ) 56.8
Hz (outer triplet)], 4.65 (m, 1H), 4.54 (m, 3H), 4.42 (m, 3H),
4.28 (m, 3H), 3.98 (m, 1H), 3.58 (m, 1H), 2.95 (m, 1H), 2.44
(m, 1H), 2.36-1.80 (m, 10H), 1.33 (s, 3H), 1.27 (s, 3H), 1.05-
0.97 (m, 12H), 0.82 (s, 3H); 13C NMR (CDCl3) δ 172.63, 171.43,
171.15, 163.44, 147.49, 144.35, 144.03, 142.76, 137.49, 128.52,
127.94, 127.52, 118.92-117.01 (t, J ) 238.5 Hz), 85.16, 71.71,
58.59, 57.51, 57.02, 52.31, 51.69, 39.74, 38.15, 35.96, 34.91 (br),
34.19, 30.65, 30.28, 28.74, 27.18, 26.34, 24.06, 19.53, 19.48,
18.26, 18.04; HRMS calcd for C40H55BF2N6O7 (M + H) 781.4271,
found 781.4260.
P in a col 1-Iod o-3-(m et h oxyca r b on yl)p r op a n e-1-b or -
on a te (23). Iodide 23 was prepared by the procedure described
for the preparation of 15 (38%): 1H NMR (CDCl3) δ 3.63 (s,
3H), 3.30 (t, 1H), 2.40 (m, 2H), 2.15 (m, 2H), 1.24 (s, 12H), 13
C
NMR (CDCl3) δ 172.95, 84.10, 51.63, 35.34, 29.65, 24.39, 24.23;
HRMS calcd for C11H20BIO4 (M+) 354.0499, found 354.0512.
(+)-P in a n ed iol 1-Azid o-3-(m eth oxyca r bon yl)p r op a n e-
1-bor on a te (24). The azide 24 was prepared by treating 23
with sodium azide in DMF followed by transesterification with
(+)-pinanediol (26%): 1H NMR (CDCl3) δ 4.37 (d, 1H), 3.68
(s, 1H), 3.21(m, 1H), 2.51-1.82 (m, 9H), 1.43 (s, 3H), 1.29(s,
3H), 1.10 (d, 1H), 0.84 (s, 3H); 13C NMR (CDCl3) δ 173.36,
86.99, 78.59, 51.65, 51.07. 39.44, 38.11, 35.22, 31.47, 28.49,
26.99, 26.49, 26.03, 25.95, 23.94; IR (film) 2094, 1738 cm-1
.
1-Am in o-3-(h ydr oxycar bon yl)pr opan e-1-bor on ate-Hy-
d r och lor id e (3a ). The azide 24 was converted to the amine
3 by catalytic hydrogenation. The amine was characterized as
the free boronic acid 3a following hydrolysis with aqueous HCl
(90%): 1H NMR (CD3OD) δ 2.88 (d, 1H), 2.40 (m, 2H), 1.90
(m, 2H); 13C NMR (CD3OD) δ 174.61, 40.43 (br), 31.37, 25.44;
HRMS calcd for C14H24BNO4 (pinanediol ester adduct, M +
H) 282.1876, found 282.1868.
P in a col 1-(P h en ylth io)-2-(ter t-bu toxyca r bon yl)eth a n e-
1-bor on a te (16). Compounds 16 was prepared by the proce-
dure described for the preparation of 13 (35%): 1H NMR
(CDCl3) δ 7.43-7.19 (m, 5H), 2.96 (t, 1H), 2.66 (d, 2H), 1.42
(s, 9H), 1.24 (d, 12H); 13C NMR (CDCl3) δ 171.82, 135.62,
130.74, 128.72, 126.44, 84.01, 80.69, 37.57, 28.04, 24.05 (br),
24.71, 24.51; HRMS calcd for C19H29BO4S (M+) 364.1880, found
364.1883.
Ack n ow led gm en t. We thank Lawrence Mersinger
for performing the enzyme assays. We also thank Carl
Decicco and Scott Priestley for their suggestions and
encouragement during the course of these studies.
Special acknowledgment is given to Xiaojun Zhang, who
suggested that we introduce the carboxyethyl side chain
as a Michael acceptor in the preparation of H-boroGlu-
(OMe)-C10H16. We thank Don Matteson, Department of
Chemistry, Washington State University, Pullman, WA,
for participating in the review of this manuscript and
for his useful suggestions. Greg Nemeth, Thomas Scholz,
and Laurie Galya in the NMR group and Michael Haas
and Wayne Danekar in the mass spectrometry group
provided assistance which is appreciated.
(+)-P in a n ed iol 1-Azid o-2-(ter t-bu toxyca r bon yl)eth a n e-
1-bor on a te (19). Boronate 16 was converted to the iodide 18
according to the procedure described for the preparation of 15.
Iodide 18 was in turn converted to the azide boronate under
phase transfer catalysis conditions.21,22 The azide was trans-
esterified to the (+)-pinanediol ester derivative 19 and isolated
(59%): 1H NMR (CDCl3) δ 4.32 (t, 1H), 3.32 (m, 1H), 2.58 (m,
2H), 2.28-1.85 (m, 6H), 1.39 (s, 9H), 1.35(d, 3H), 1.22 (s, 3H),
0.77 (s, 3H); 13C NMR (CDCl3) δ 170.97, 86.94, 81.36, 78.56,
51.19, 44.20 (br), 39.41, 38.16, 35.21, 28.41, 28.02, 27.01, 26.29,
23.99; IR (film) 2092, 1724 cm-1
.
(+)-P in a n ed iol 1-(Ben zoyla m id o)-2-(ter t-bu toxyca r bo-
n yl)eth a n e-1-bor on a te (2a ). 1-Amino-2-(tert-butoxycarbon-
yl)ethane-1-boronate (2) was obtained by catalytic hydroge-
nation of the azide 19 (79%). The amine was treated with
benzoyl chloride in the presence of triethylamine. Subsequent
deprotection with TFA followed by HPLC purification yielded
the benzoyl derivative 2a (43%): 1H NMR (CDCl3) δ 8.62 (s,
1H), 7.82 (d, 2H), 7.55 (t, 1H), 7.39 (t, 2H), 4.32 (d, 1H), 3.21
(t, 1H), 2.78 (m, 2H), 2.42-1.81 (m, 5H), 1.51 (d, 1H), 1.45 (s,
3H), 1.27 (s, 3H), 0.87 (s, 3H); 13C NMR (CDCl3) δ 174.88,
172.16, 134.16, 128.90, 128.27, 128.24, 84.23, 83.85, 52.38,
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and characterization data for compounds 2a , 3a , 16,
19, 21, 23, and 24 are provided along with 1H and 13C NMR
spectra of compounds 1, 2a , 3a , 13, 15, 16, 19, 21, 23, 24, and
25A,B. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O015753Y