Amide-Assisted Hydrolysis of β-Carboxamido-Substituted Phosphinic Acid Esters

Article abstract of DOI:10.1021/jo962275w

Phosphinic acids are of interest due to their ability to inhibit metalloproteases. The hydrolysis of a phosphinic acid ester is typically one of the final steps in the synthesis of such inhibitors. We have found that the acid-catalyzed hydrolysis of a phosphinic acid ester containing a β-carboxamido group is facilitated by the presence of the amide. The promotion of the hydrolysis is dependent on the electron density of the amide suggesting the intermediacy of a cyclic imidate structure (C). The hydrolysis of phosphinic acid esters containing a β-carboxamido group is conveniently and quantitatively effected by treating the ester with 10:90 H₂O:TFA.

Full text of DOI:10.1021/jo962275w

2808
J . Org. Chem. 1997, 62, 2808-2812
Am id e-Assisted Hyd r olysis of â-Ca r boxa m id o-Su bstitu ted
P h osp h in ic Acid Ester s
Lawrence A. Reiter* and Brian P. J ones
Department of Medicinal Chemistry, Central Research Division, Pfizer Inc., Groton, Connecticut 06340
Received December 6, 1996X
Phosphinic acids are of interest due to their ability to inhibit metalloproteases. The hydrolysis of
a phosphinic acid ester is typically one of the final steps in the synthesis of such inhibitors. We
have found that the acid-catalyzed hydrolysis of a phosphinic acid ester containing a â-carboxamido
group is facilitated by the presence of the amide. The promotion of the hydrolysis is dependent on
the electron density of the amide suggesting the intermediacy of a cyclic imidate structure (C).
The hydrolysis of phosphinic acid esters containing a â-carboxamido group is conveniently and
quantitatively effected by treating the ester with 10:90 H2O:TFA.
Phosphinic acids are chelating agents for inorganic
Under these conditions, however, the phosphinic acid
ester was also cleaved.8 Our initial assumption that the
released carboxylic acid was assisting in the hydrolysis
of the phosphinic acid ester via A was, however, at
variance with the observation that the phosphinic acid
ester of 3 remained intact upon treatment with neat TFA,
yielding the monoacid 4.8 This suggested that rather
than the carboxylic acid, the carboxamide of 2 was
responsible for promoting the cleavage of the phosphinic
acid ester via intermediate B.
metal ions, and appropriately substituted phosphinic
acids have proven to be effective inhibitors of metallo-
proteases such as stromelysin (MMP-3),1 angiotensin
converting enzyme,2 endothelin converting enzyme,2a,3
neutral endopeptidase 24.11,2a endopeptidases 3.4.24.15
and 3.4.24.16,4 NAALADase,5 bacterial collagenase,6 and
thermolysin.7 The phosphinic acid moiety in an inhibitor
of this class ligates the metal ion at the enzyme's active
site and mimics the tetrahedral geometry attained by the
scissile amide bond of the enzyme’s substrate during
cleavage. We have been studying the potential of phos-
phinic acids as inhibitors of human fibroblast collagenase
(MMP-1) and sought to prepare a library of compounds
with general structure 1. To this end we prepared the
Interactions between â-carbonyl groups and phospho-
rus acid derivatives have been observed with â-carbox-
amido-substituted phosphonamidates,9 R-acylaminophos-
phinates,9,10 and R-acylaminophosphonates,11 and the
acid-catalyzed hydrolysis of phosphonate esters contain-
ing a 2-carboxamidophenyl group has been reported.12
Thus, assisted hydrolysis of a phosphinic acid ester by a
â-substituted carboxamide did not seem unlikely. Since
the hydrolysis of phosphinic acid esters with this par-
ticular motif has not been studied nor has such an acid-
catalyzed process been promoted as a method for phos-
phinic acid ester cleavage, we chose to study this reaction
in more detail with the aim of delineating an alternate
procedure for the cleavage of phosphinic acid esters of
this type. An alternate method for the cleavage of
phosphinic acid esters would be useful because the
hydrolysis of a phosphinic acid ester is typically one of
the final steps in the synthesis of phosphinic acid-based
inhibitors and current methods have limitations. For
example, hydrolyses with base (e.g. LiOH) at room
temperature are often slow and higher temperatures may
be too harsh for sensitive substrates. Likewise, hydroly-
phosphinic acid ester 2 and treated it with neat tri-
fluoroacetic acid in order to release the carboxylic acid
which we intended to broadly derivatize (Scheme 1).
X Abstract published in Advance ACS Abstracts, April 15, 1997.
(1) (a) Caldwell, C. G.; Sahoo, S. P.; Polo, S. A.; Eversole, R. R.;
Lanza, T. J .; Mills, S. G.; Niedzwiecki, L. M.; Izquierdo-Martin, M.;
Chang, B. C.; Harrison, R. K.; Kuo, D. W.; Lin, T.-Y.; Stein, R. L.;
Durette, P. L.; Hagmann, W. K. Bioorg. Med. Chem. Lett. 1996, 6, 323.
(b) Goulet, J . L.; Kinneary, J . F.; Durette, P. L.; Stein, R. S.; Harrison,
R. K.; Izquierdo-Martin, M.; Kuo, D. W.; Lin, T.-Y.; Hagmann, W. K.
Bioorg. Med. Chem. Lett. 1994, 4, 1221.
(2) (a) McKittrick, B. A.; Stamford, A. W.; Weng, X.; Chackalaman-
nil, S.; Czarniecki, M.; Cleven, R. M.; Fawzi, A. B. Bioorg. Med. Chem.
Lett. 1996, 6, 1629. (b) Karanewsky, D. S.; Badia, M. C.; Cushman, D.
W.; DeForrest, J . M.; Dejneka, T.; Loots, M. J .; Perri, M. G.; Petrillo,
E. W., J r.; Powell, J . R. J . Med. Chem. 1988, 31, 204. Krapcho, J .; Turk,
C.; Cushman, D. W.; Powell, J . R.; DeForrest, J . M.; Spitzmiller, E.
R.; Karanewsky, D. S.; Duggan, M.; Rovnyak, G.; Schwartz, J .;
Natarajan, S.; Godfrey, J . D.; Ryono, D. E.; Neubeck, R.; Atwal, K. S.;
Petrillo, E. W., J r. J . Med. Chem. 1988, 31, 1148.
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Chacklamannil, S.; Chung, S.; Stamford, A. W.; McKittrick, B. A.;
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Chem. Lett. 1996, 6, 1257. Bertenshaw, S. R.; Rogers, R. S.; Stern, M.
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McDonald, J . F.; McMahon, E. G.; Palomo, M. A. J . Med. Chem. 1993,
36, 173.
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Chem. 1996, 271, 19606. Vincent, B.; Dive, V.; Yiotakis, A.; Smadja,
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E.; Donzanti. B. A.; Trainor, D. A. J . Med. Chem. 1996, 39, 619.
(6) Yiotakis, A.; Lecoq, A.; Vassiliou, S.; Raynal, I.; Cuniasse, P.;
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Bartlett, P. A. J . Am. Chem. Soc. 1991, 113, 297. Grobelny, D.; Goli,
U. B.; Galardy, R. E. Biochem. 1989, 28, 4948.
(8) Reiter, L. A.; Lasut, M. J . Unpublished observations.
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(10) J acobsen, N. E.; Bartlett, P. A. J . Am. Chem. Soc. 1983, 105,
1613.
(11) Rahil, J .; Pratt, R. F. J . Chem Soc. Perkin Trans. 2 1991, 947.
(12) Kluger, R.; Chan, J . L. W. J . Am. Chem. Soc. 1973, 95, 2362;
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S0022-3263(96)02275-X CCC: $14.00 © 1997 American Chemical Society

Amide-Assisted Hydrolysis of Phosphinates
J . Org. Chem., Vol. 62, No. 9, 1997 2809
Sch em e 1. P h osp h in a te Ester Hyd r olysissIn itia l
Sch em e 2. Syn th esis of P h osp h in a te Ester s 7a -7f;
Hyd r olysis of P h osp h in a te Ester s 7a -7ga
Obser va tion s
a
Reagents, reaction conditions, and yields: (i) 5 equiv of BTSP,
CH2Cl2, rt, 16 h (∼100%); (ii) 1.1 equiv of BnBr, 3 equiv of BSA,
CH2Cl2, reaction purged with N2, rt/reflux, 40 h (∼100%); (iii)
excess TMSCHN2, 4:1 toluene:MeOH, rt, 15 min (70% for 3 steps);
(iv) H2/5% Pd-BaSO4, 45 psi, MeOH, rt, 1 h (100%); (v) 1.5 equiv
of EDC•HCl, 1.5 equiv of HOSu, DMF, rt, 18 h (96%); (vi) 1.2 equiv
of HNR1R2, CH2Cl2, rt to reflux until conversion complete by TLC
or HPLC (29-88%); (vii) aqueous TFA, 0 °C to rt (100%); (viii) 1.1
equiv of EtI or BnBr, 2 equiv of Cs2CO3, DMF, rt, 2-3 d (28-
66%).
ses with trimethylsilyl bromide may be incompatible with
other functionalities. Herein we demonstrate that the
acid-catalyzed hydrolysis of â-carboxamido-substituted
phosphinic acid esters is an effective, rapid, and mild
method for the preparation of â-carboxamido-substituted
phosphinic acids.
the diastereomers.18 Reconversion of the salt into the
corresponding free acid 5S and repeating the steps
outlined in Scheme 2 for the preparation of 7f from 5 led
to 7g. HPLC analysis of the diastereomeric mixture 7g
revealed a 91:9 mixture of diastereomers. The ethyl and
benzyl phosphinates, 7h and 7i, respectively, were
prepared by O-alkylation of the corresponding phosphinic
acid 8, which was prepared as described below.
We began our study by confirming that a â-situated
carboxamide would facilitate the acid-catalyzed hydroly-
sis of a phosphinic acid ester and indeed, treatment of
7a with 10% aqueous TFA at room temperature led to
the rapid hydrolysis (<15 min) of the phosphinic acid
ester, quantitatively yielding 8a ,19 presumably via C. The
corresponding benzyl ester 6a was not affected by treat-
ment with 10% aqueous TFA at 0 °C; however, the
Resu lts a n d Discu ssion
A key intermediate in our study, phosphinate 6a , was
prepared through a Michael addition13 of bis(trimethyl-
silyl)phosphonite14 with benzyl R-isobutylacrylate15 fol-
lowed by an Arbuzov reaction16 with benzyl bromide
mediated by N,O-bis(trimethylsilyl)acetamide and sub-
sequent methylation with (trimethylsilyl)diazomethane17
(Scheme 2). Conversion of the benzyl ester into the
desired amides 7 was effected by hydrogenation and
coupling with N-hydroxysuccinimide followed by reaction
of the resulting active ester 6c with various amines.
Preparation of the enantiomerically enriched compound
7g began with the crystallization of the (S)-R-methyl-
benzylamine salt of 5 (Scheme 3).1a After two recrystal-
lizations, the salt was substantially enriched in one of
(18) 1H-NMR of the initially formed diastereomeric salt showed a
“shoulder” on the P-H resonance which diminished upon two recrys-
tallizations. Direct quantitative assessment of the diastereomeric
purity of the salt by 1H-NMR or 31P-NMR was not successful and
attempts to determine the e.e. of 5S by chiral HPLC, 1H-NMR, and
31P-NMR were likewise unsuccessful. On the basis of the literature
precedent (ref 1a), we assume the isolated salt is enriched in the
S-isomer of the phosphinic acid.
(13) Boyd, E. A.; Corless, M.; J ames, K.; Regan, A. C. Tetrahedron
Lett. 1990, 31, 2933.
(14) Boyd, E. A.; Regan, A. C.; J ames, K. Tetrahedron Lett. 1992,
33, 813.
(15) Stetter, H.; Kuhlmann, H. Synthesis 1979, 29.
(16) Thottathil, J . K.; Przybyla, C. A.; Moniot, J . L. Tetrahedron Lett.
1984, 25, 4737. Boyd, E. A.; Regan, A. C.; J ames, K. Tetrahedron Lett.
1994, 35, 4223.
(17) Hashimoto, N.; Aoyama, T.; Shioiri, T. Chem Pharm. Bull. 1981,
29, 1475.
(19) We did not observe any of the alternate cleavage which would
yield 6b or 9. This is consistent with the observations of Kluger and
Chan (ref 12).

2810 J . Org. Chem., Vol. 62, No. 9, 1997
Reiter and J ones
Sch em e 3. Syn th esis of P h osp h in a te Ester s 7g-7i;
Hyd r olysis of 7h -ia
carboxylic acid 6b was, like the amide 7a , rapidly
converted to the respective phosphinic acid (i.e. 9) under
these conditions. Although both a â-carboxamido or
â-carboxy group promote the hydrolysis of a phosphinic
acid ester, under more dilute conditions (1:9:90 H2O:TFA:
CH2Cl2 at 0 °C) the carboxamide-induced hydrolysis
proved to be more rapid with the hydrolysis being
complete in about 3 h and the hydrolysis of the acid being
only about 25% complete after 4 h (Figure 1).20 With the
effectiveness of the â-carboxamido-induced hydrolysis
confirmed, we next investigated the effect of altering the
amide substituents. Under the dilute conditions used
above, the dimethylamide 7b hydrolyzed even more
rapidly than 7a , demonstrating that increased electron
density of the amide promoted the hydrolysis and that
the presence of a proton on the amide nitrogen was
unnecessary.21 Hydrolysis of the anilide 7d was slower
than the benzyl amide and, as anticipated, the 4-meth-
oxyanilide 7c hydrolyzed more rapidly than 7d while the
4-chloroanilide 7e hydrolyzed more slowly (Figure 2). The
overall dependency of the hydrolysis on the electron
density of the carboxamide supports the intermediacy of
C in the hydrolysis process.
a
See footnote to Scheme 2 for reagents, reaction conditions, and
yields.
Altering the phosphinate ester functionality from
methyl to ethyl (7h ) or benzyl (7i) diminished the reaction
rate, presumably due to steric inhibition in forming
intermediate C (Figure 3). However, since the reaction
rate diminution was only slight, we expect that the
method will be effective in general, that is, not limited
to methyl phosphinates.
F igu r e 1. Phosphinate ester hydrolysis promoted by a car-
boxamide vs a carboxylic acid.
Since many of the phosphinic acids of interest as
protease inhibitors have chiral centers adjacent to the
participating carboxamido group or a chiral amino acid
as the carboxamido moiety, we examined whether the
integrity of these chiral centers was affected by the
hydrolysis procedure. To determine whether epimeriza-
tion at either center of concern occurred, we compared
the hydrolysis of 7f with that of 7g. Compound 7f is a
mixture of four diastereomers that upon hydrolysis yields
equal amounts of the two diastereomeric phosphinic
acids. Hydrolysis of compound 7g, which is also a
mixture of four diastereomers but which is enriched in
two diastereomers to the extent of 91:9, led to the same
two diastereomeric phosphinic acids as above and in the
same ratio as the initial mixture (i.e. 91:9). Had either
chiral center epimerized this ratio would have been
diminished. Thus, epimerization of either center adja-
cent to the carboxamide does not appear to occur to an
appreciable extent under the hydrolysis conditions.
These results demonstrate that the acid-catalyzed
hydrolysis of phosphinic acid esters containing a â-car-
boxamido group is an effective method for converting
such esters into the corresponding acids. The carbox-
amido group may be secondary or tertiary and chiral
centers adjacent to the carboxamido group are unaffected.
The ready hydrolysis of methyl, ethyl, and benzyl phos-
phinic acid esters suggests that phosphinic acid esters
of this general motif should all be readily cleaved, and
thus, esters especially sensitive to acid hydrolysis such
as diphenylmethyl3a or adamantyl22 need not be utilized
for this purpose. This method compliments existing
methods for phosphinic acid ester hydrolysis and should
prove useful in the preparation of phosphinic acid-
containing inhibitors of metalloproteases; we have suc-
cessfully applied the method, using the “standard” con-
ditions, to the preparation of a wide variety of phosphinic
acid-based inhibitors of MMP-1.
(20) Continued reaction at room temperature for 20 h led to complete
conversion.
(21) The hydrolysis of the dimethyl amide 7b was complete within
10 min vs 180 min for 7a .
(22) Yiotakis, A.; Vassiliou, S.; J iracek, J .; Dive, V. J . Org. Chem.
1996, 61, 6601.

Amide-Assisted Hydrolysis of Phosphinates
J . Org. Chem., Vol. 62, No. 9, 1997 2811
acid (quantitative yield) as an orange oil: 1H-NMR (CDCl3) δ
0.82/0.87 (6H, 2d), 1.2-1.7 (4H, m), 2.04 (1H, m), 2.82 (1H,
m), 2.96 (2H, d), 5.12 (2H, dd), 7.15-7.45 (10H, m), 8.34 (1H,
br s).
Crude benzyl[2-(benzyloxycarbonyl)-4-methylpentyl]phos-
phinic acid from above (23.4 mmol) was taken up in toluene/
methanol (160 mL/40 mL) and treated with excess TMSCHN2
(2M solution in hexanes) for 30 min. The excess TMSCHN2
was quenched with acetic acid and the reaction mixture
concentrated in vacuo to an oil. This was chromatographed
(ethyl acetate) to give 6.39 g (70.5% for three steps) of 6a as a
+
yellow oil: mass spectrum (CI) M+ + NH4 406 (22.0), M+
+
H
+ 389 (100); 1H-NMR (CDCl3) δ 0.83/0.90 (6H, 2d), 1.29 (1H,
m), 1.46 (1H, m), 1.58 (1H, m), 1.71 (1H m), 2.13 (1H, m), 2.80
(1H, m), 3.10/3.13 (2H, dd/d), 3.50/3.53 (3H, 2d), 5.12 (2H, dd),
7.2-7.3 (5H, m), 7.3-7.4 (5H, m); 31P-NMR (CDCl3) δ 52.09,
F igu r e 2. Phosphinate ester hydrolysis promoted by carbox-
amides of differing electron density.
52.50 (2m); HPLC (30/90) 17.9 min. Anal. Calcd for C22H29
PO4 (388.45): C, 68.03; H, 7.52. Found: C, 67.63; H, 7.74.
-
Ben zyl[2-(b en zyloxyca r b on yl)-4-m et h ylp en t yl]p h os-
p h in ic Acid N-Hyd r oxysu ccin im id e Ester (6c). The ben-
zyl ester 6a (1.94 g, 5.0 mmol) was hydrogenated in methanol
(60 mL) at 45 psi over 5% palladium on barium sulfate (1.0 g)
for 1.5 h. The catalyst was filtered off and washed with
methanol, and the filtrate was concentrated in vacuo to give
6b (quantitative yield) as an oil: 1H-NMR (CDCl3) δ 0.8-0.95
(6H, m), 1.25 (1H, m), 1.55 (3H, m), 2.18 (1H, m), 2.68/2.85
(1H, 2m), 3.25/3.28 (2H, d/dd), 3.65/3.70 (3H, 2d), 7.28 (5H,
m); HPLC (20/80) 13.1/13.4 min.
The crude acid 6b (5.0 mmol) was dissolved in dry DMF
(50 mL), and 1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide
hydrochloride (1.44 g, 7.5 mmol) and N-hydroxysuccinimide
(863 mg, 7.5 mmol) were added. The resulting solution was
stirred at room temperature for 19 h and was then diluted
with ethyl acetate (100 mL) and washed with 1 N HCl (2 ×
100 mL), NaHCO3 (2 × 100 mL) and brine (100 mL). The
organic layer was dried over MgSO4, filtered, and concentrated
in vacuo to give 1.89 g (95.5%) of 6c as an oil: 1H-NMR (CDCl3)
δ 0.85-1.0 (6H, m), 1.48 (1H, m), 1.75 (3H, m), 2.13 (1H, m),
2.85 (4H, s), 2.98/3.1-3.3 (3H, m), 3.62/3.68 (3H, 2d), 7.30 (5H,
m).
F igu r e 3. Carboxamide-promoted hydrolysis of differing
phosphinate esters.
Exp er im en ta l Section
Ca u tion : bis(trimethylsilyl)phosphonite (BTSP), used in the
preparation of 5, is highly pyrophoric; appropriate precaution
should be taken to avoid contacting this reagent with air.
Solvents and reagents were used as obtained from com-
mercial sources. Sure/Seal® quality solvents were purchased
from Aldrich Chemical Co., Inc., and used when dry solvents
were required. HPLC was performed using a Waters NovaPak
C18 column (3.9 mm × 15 cm) with a CH3CN (0.1% TFA):H2O
(0.1% TFA) gradient (CH3CN concentrations in the gradient
are listed for each compound) and a flow rate of 1.0 mL/min.
All preparative chromatography was performed on 40 µM silica
gel.
Meth yl Ben zyl[(2-(ben zyloxyca r bon yl)-4-m eth ylp en -
tyl]p h osp h in a te (6a ). In a flame-dried flask under N2,
hexamethyldisilazane (24.6 mL, 116.8 mmol) was added to
NH4H2PO2 (9.7 g, 117 mmol) and the mixture heated at 105
°C with stirring for 2 h. The mixture was cooled to 0 °C in an
ice bath and benzyl R-isobutylacrylate (5.1 g, 23.4 mmol) in
dry CH2Cl2 (60 mL) was added by syringe. The reaction
mixture was stirred at room temperature for 16 h and then
carefully quenched (still under N2) with 1 N HCl (40 mL). After
30 min, the organic layer was separated and washed with 1
N HCl (2 × 50 mL) and brine and dried over MgSO4. Filtration
and concentration in vacuo gave 5 (quantitative yield) as an
oil: 1H-NMR (CDCl3) δ 0.90/0.92 (6H, 2d), 1.41 (1H, m), 1.5-
1.75 (2H, m), 1.86 (1H, m), 2.15 (1H, m), 2.92 (1H, m), 5.13
(2H, dd), 7.12 (1H, d, J ) 559 Hz), 7.37 (5H, m), 10.76 (1H, s);
31P-NMR (CDCl3) δ 35.05 (dm, J ) 559 Hz).
Meth yl Ben zyl[2-(N-ben zylca r ba m oyl)-4-m eth ylp en -
tyl]p h osp h in a te (7a ). A portion of the above crude N-
hydroxysuccinimide ester 6c (593 mg, 1.5 mmol) in dry CH2Cl2
(15 mL) was treated with benzylamine (0.33 mL, 3.0 mmol).
After 5 days at room temperature the reaction was diluted with
additional CH2Cl2 (30 mL), washed with 1 N HCl (2 × 50 mL),
NaHCO3 (2 × 50 mL), and brine (50 mL), dried over MgSO4,
filtered, and concentrated in vacuo to an oil. This was
chromatographed (ethyl acetate) to give 511 mg (87.9%) of 7a
1
as an oil: mass spectrum (CI) M+ + H+ 388 (100); H-NMR
(CDCl3) δ 0.80-0.95 (6H, m), 1.05-1.80 (4H, m), 2.10 (1H, m),
2.57 (1H, m), 3.07 (2H, m), 3.47/3.58 (3H, 2d), 4.40 (2H, m),
6.42/6.50 (1H, 2m), 7.30 (10H, m); 31P-NMR (CDCl3) δ 53.23,
53.65 (2m); HPLC (30/90) 11.4/12.0 min. Exact Mass Calcd
for C22H30NO3P: 388.2042. Found: 388.2056.
Amides 7b-g were prepared analogously.
Meth yl ben zyl[2-(N,N-d im eth ylca r ba m oyl)-4-m eth yl-
p en tyl]p h osp h in a te (7b): yield 76.7%; mass spectrum (CI)
1
M+ + H+ 326 (100), M+ - N(Me)2 281 (86); H-NMR (CDCl3)
δ 0.80-0.90 (6H, m), 1.16-1.75 (4H, m), 2.27 (1H, m), 2.98/
3.02 (3H, 2s), 3.05-3.2 (6H, m), 3.55/3.59 (3H, 2d), 7.30 (5H,
m); HPLC (10/70) 18.6/19.4 min.
Meth yl ben zyl[2-(N-(4-m eth oxyp h en yl)ca r ba m oyl)-4-
m eth ylp en tyl]p h osp h in a te (7c): yield 66.3%; mass spec-
+
1
trum (CI) M+ + NH4 421 (4), M+ + H+ 404 (100); H-NMR
(CDCl3) δ 0.80-0.95 (6H, m), 1.00-1.90 (4H, m), 2.12 (1H, m),
2.76 (1H, m), 3.11 (2H, m), 3.50/3.65 (3H, 2d), 3.77/3.78 (3H,
2s), 6.85 (2H, m), 7.15-7.35 (5H, m), 7.48 (2H, m), 8.40/8.48
(1H, 2s); HPLC (30/90) 11.9/12.2 min.
Crude 5 (23.4 mmol) from above was taken up in dry CH2Cl2
(120 mL), and benzyl bromide (3.06 mL, 25.7 mmol) and BSA
(14.5 mL, 70.1 mmol) were added. The solution was purged
for ∼30 min with a slow stream of N2. The mixture was then
stirred at room temperature for 20 h, refluxed for 8 h, and
stirred at room temperature for 18 h. The reaction was
quenched with 1 N HCl (40 mL) and the separated organic
layer washed with 1 N HCl (2 × 50 mL) and brine (50 mL)
and dried over MgSO4. Filtration and concentration in vacuo
gave benzyl[2-(benzyloxycarbonyl)-4-methylpentyl]phosphinic
Meth yl ben zyl[2-(N-p h en ylca r ba m oyl)-4-m eth ylp en -
tyl]p h osp h in a te (7d ): yield 39.0%; mass spectrum (CI) M+
+ NH4+ 391 (4), M+ + H+ 374 (100); 1H-NMR (CDCl3) δ 0.80-
0.95 (6H, m), 1.15 (1H, m), 1.5-1.9 (3H, m), 2.14 (1H, m), 2.80
(1H, m), 3.15 (2H, m), 3.48/3.65 (3H, 2d), 7.09 (1H, m), 7.15-
7.35 (7H, m), 7.57 (2H, m), 8.58/8.67 (1H, 2s); HPLC (30/90)
13.7 min.

2812 J . Org. Chem., Vol. 62, No. 9, 1997
Reiter and J ones
Met h yl b en zyl[2-(N-(4-ch lor op h en yl)ca r b a m oyl)-4-
m eth ylp en tyl]p h osp h in a te (7e): yield 29.4%; mass spec-
Ben zyl[2-(N-((S)-1-(m et h oxyca r b a m oyl)-1-et h yl)ca r -
ba m oyl)-4(R,S)- m eth ylp en tyl]p h osp h in ic a cid (8f): mass
+
1
1
trum (CI) M+ + NH4 425 (5), M+ + H+ 408 (100); H-NMR
(CDCl3) δ 0.80-1.0 (6H, m), 1.0-1.9 (4H, m), 2.20 (1H, m),
2.85-3.25 (3H, m), 3.41/3.56 (3H, 2d), 7.15-7.45 (7H, m), 7.52
(2H, m), 9.66/9.74 (1H, 2s); HPLC (30/90) 17.0 min.
spectrum (CI) M+ + H+ 370 (100), 338 (6); H-NMR (CD3OD)
δ 0.80-1.0 (6H, m), 1.20-1.81 (4H, m), 1.39 (3H, d), 2.08 (1H,
m), 2.80 (1H, m), 3.22 (2H, m) 3.68 (3H, d), 4.43 (1H, m), 7.30
(5H, m); HPLC (10/70) 17.2/17.6 min (50/50 R/S). Exact Mass
Calcd for C18H28NO5P: 370.1783. Found: 370.1811.
Meth yl ben zyl[2-(N-((S)-1-(m eth oxyca r ba m oyl)-1-eth -
yl)car bam oyl)-4(R,S)-m eth ylpen tyl]ph osph in ate (7f): yield
80.9%; mass spectrum (CI) M+ + H+ 384 (100); 1H-NMR
(CDCl3) δ 0.80-0.95 (6H, m), 1.05-1.80 (4H, m), 1.40 (3H, m),
2.12 (1H, m), 2.60 (1H, m), 3.15 (2H, m), 3.61 (3H, m), 3.74
(3H, s), 4.55 (1H, m), 6.48/6.57/6.86/6.93 (1H, 4d), 7.31 (5H,
m); HPLC (10/70) 16.1/16.8 (two isomers)/17.9 min (50/50 R/S).
(S)-r-Meth ylben zyla m in e sa lt of 5S. (S)-R-Methylben-
zylamine (1.47 mL, 11.4 mmol) was added to crude 5 (6.50 g,
22.9 mmol) in dry CH2Cl2 (50 mL). The solvent was removed
in vacuo and 1:1 hexane/ether (40 mL) added to the resulting
oil. After standing at 0 °C, white crystals formed. These
crystals were collected, washed with hexane, and recrystallized
from ethyl acetate (2×) to give 2.82 g (60.9% with respect to
(S)-R-methylbenzylamine) of the (S)-R-methylbenzylamine salt
Ben zyl[2-(N-((S)-1-(m et h oxyca r b a m oyl)-1-et h yl)ca r -
ba m oyl)-4(S)-m eth ylp en tyl]p h osp h in ic a cid (8g): mass
1
spectrum (CI) M+ + H+ 370 (100), 338 (6); H-NMR (CD3OD)
δ 0.90/0.96 (6H, 2d), 1.28 (1H, m), 1.40 (3H, d), 1.60 (3H, m),
2.03 (1H, m), 2.79 (1H, m), 3.16 (2H, m), 3.69 (3H, s), 4.43
(1H, m), 7.30 (5H, m); HPLC (10/70) 17.1 min (91% S-isomer)/
17.5 min (9% R-isomer). Exact Mass Calcd for C18H28NO5P:
370.1783. Found: 370.1788.
Ben zyl[2-ca r b oxy-4-m et h ylp en t yl]p h osp h in ic
a cid
(9): mass spectrum, (TS) M+ - H+ 283 (100); 1H-NMR (CDCl3)
δ 0.80/0.87 (6H 2d), 1.1-1.8 (4H, m), 2.05 (1H, m), 2.72 (1H,
m), 3.12 (2H, d), 7.25 (5H, m); HPLC (20/80) 10.6 min.
Hyd r olysis of P h osp h in a te/Am id ess“Dilu te” Con d i-
tion s. All hydrolysis experiments plotted in Figures 1-3 were
conducted at 0.10 M in 1:9:90 H2O:TFA:CH2Cl2 at 0 °C.
Aliquots (50 µL) were taken at 5, 10, 15, 30, 60, 120, 180, and
240 min time points and quenched in 1.0 mL of 50:50 H2O:
CH3CN. Quenched samples were analyzed by HPLC (ap-
propriate gradient of CH3CN:H2O based on HPLC retention
of the starting ester and product acid). After the 240 min
aliquot, the reactions were allowed to come to room temper-
ature and stir overnight. The solvent was then removed in
vacuo, CH2Cl2 added, the mixture re-concentrated (3×), and
the product analyzed by HPLC and shown to be the expected
1
of 5S as a white solid: mp 130-134 °C; H-NMR (CDCl3) δ
0.82/0.88 (6H, 2d), 1.22-2.30 (5H, m), 1.57 (3H, d), 2.72 (1H,
m), 4.19 (1H, q), 5.07 (2H, dd), 6.83 (1H, d, J ) 513 Hz), 7.2-
7.45 (10H, m). Anal. Calcd for C22H32NO4P (405.48) C, 65.17;
H, 7.95; N, 3.45. Found: C, 65.10; H, 8.21; N, 3.54.
The above salt of 5S (1.22 g, 3.00 mmol) was converted to
5S by dissolution in EtOAc (20 mL) and washing with 1 N
HCl (2 × 20 mL). The organic layer was dried with MgSO4,
filtered, and concentrated in vacuo to give 5S as an oil that
was used directly in the next step.
Meth yl ben zyl[2-(N-((S)-1-(m eth oxyca r ba m oyl)-1-eth -
yl)ca r ba m oyl)-4(S)-m eth ylp en tyl]p h osp h in a te (7g): yield
74.2%; mass spectrum (CI) M+ + H+ 384 (100); 1H-NMR
(CDCl3) δ 0.80-0.95 (6H, m), 1.06-1.80 (4H, m), 1.42 (3H, d),
2.12 (1H, m), 2.58 (1H, m), 3.14 (2H, m), 3.60/3.64 (3H, 2d),
3.75 (3H, s), 4.56 (1H, m), 6.48/6.56 (1H, 2d), 7.31 (5H, m);
HPLC (10/70) 17.6/18.1 (two isomers)/18.9 min (one minor
isomer, 4.5% of total) (91/9 S/R).
1
phosphinic acid by H-NMR and MS analysis.
Ben zyl[2-(N,N-d im et h ylca r b a m oyl)-4-m et h ylp en t yl]-
p h osp h in ic a cid (8b): mass spectrum (CI) M+ + H+ 312
(100), M+- N(Me)2 267 (11); 1H-NMR (CDCl3) δ 0.86/0.91 (6H,
2d), 1.40 (3H, m), 1.81 (1H, m), 2.18 (1H, m), 2.97/3.05 (6H,
2s), 3.16 (3H, m/d), 7.17 (5H, m); HPLC (10/70) 17.2 min.
Exact Mass Calcd for C16H26NO3P: 312.1728. Found: 312.1714.
Ben zyl[2-(N-(4-m eth oxyp h en yl)ca r ba m oyl)-4-m eth yl-
p en tyl]p h osp h in ic a cid (8c): mass spectrum (CI) M+ + H+
390 (100), 267 (16); 1H-NMR (CD3OD) δ 0.85/0.88 (6H, 2d),
1.22 (1H, m), 1.4-1.8 (3H, m), 2.15 (1H, m), 2.70 (1H, m), 3.04
(2H, d), 3.74 (3H, s), 6.77 (2H, d), 7.15-7.35 (7H, m), 8.34 (1H,
s); HPLC (30/90) 9.7 min. Exact Mass Calcd for C21H28NO4P:
390.1834. Found: 390.1818.
Ben zyl[2-(N-p h en ylca r ba m oyl)-4-m eth ylp en tyl]p h os-
p h in ic a cid (8d ): mass spectrum (CI) M+ + H+ 360 (100),
267 (15); 1H-NMR (CD3OD) δ 0.87/0.90 (6H, 2d), 1.22 (1H, m),
1.45-1.8 (3H, m), 2.15 (1H, m), 2.71 (1H, m), 3.02 (2H, d),
7.05-7.35 (8H, m), 7.40 (2H, d), 8.45 (1H, s); HPLC (30/90)
10.3 min. Exact Mass Calcd for C20H26NO3P: 360.1729.
Found 360.1695.
Eth yl Ben zyl[2-(N-ben zylca r ba m oyl)-4-m eth ylp en tyl]-
p h osp h in a te (7h ). In a flame-dried flask under nitrogen was
added 8a (140 mg, 0.375 mmol), cesium carbonate (0.25 g, 0.75
mmol), dry DMF (4.0 mL), and ethyl iodide (0.04 mL, 0.41
mmol). After stirring at room temperature for 2 days, the
reaction mixture was diluted with ethyl acetate (20 mL),
washed with 1 N HCl (3 × 20 mL) and brine (20 mL), dried
over MgSO4, and concentrated in vacuo to an oil. This was
chromatographed (ethyl acetate) to give 99 mg (65.6%) of 7h
as an oil: mass spectrum (CI) M+ + H+ 402 (100), M+
-
PhCH2NH 295 (23); 1H-NMR (CDCl3) δ 0.80-0.90 (6H, m) 1.1-
1.35 (4H, m), 1.3-1.8 (3H, m), 2.10 (1H, m), 2.50/2.61 (1H,
2m), 3.09 (2H, m), 3.91 (2H, m), 4.41 (2H, m), 6.46/6.60 (1H,
2m), 7.28 (10H, m); HPLC (30/90) 13.2/13.7 min.
Ben zyl[2-(N-(4-ch lor op h en yl)ca r ba m oyl)-4-m eth ylp en -
tyl]p h osp h in ic Acid (8e): mass spectrum (CI) M + H+ 394
Ben zyl Ben zyl[2-(N-b en zylca r b a m oyl)-4-m et h ylp en -
tyl]p h osp h in a te (7i): By the same method as above, 8a (140
mg, 0.375 mmol), cesium carbonate (0.25 g, 0.75 mmol), dry
DMF (4.0 mL), and benzyl bromide (0.050 mL, 0.41 mmol) gave
after chromatography (EtOAc) 49 mg (28%) of 7i as an oil:
1
(100), 267 (30); H-NMR (CD3OD) δ 0.87/0.90 (6H, 2d), 1.23
(1H, m), 1.4-1.8 (3H, m), 2.15 (1H, m), 2.67 (1H, m), 3.05 (2H,
d), 7.10-7.50 (9H, m), 8.45 (1H, s); HPLC (30/90) 13.9 min.
Exact Mass Calcd for C20H25ClNO3P: 394.1339. Found
394.1314.
1
mass spectrum (CI) M+ + H+ 464 (100); H-NMR (CDCl3) δ
0.75-0.90 (6H, m), 1.10-1.35 (1H, m), 1.35-1.6 (1H, m), 1.70
(2H, m), 2.13 (1H, m), 2.59 (1H, m), 3.05 (2H, m), 4.40 (2H,
m), 4.90 (2H, m), 6.23/6.50 (1H, 2m), 7.15/7.45 (15H, m); HPLC
(30/90) 17.4/17.8 min.
Hyd r olysis of P h osp h in a te/Am id ess“Sta n d a r d ” Con -
d ition s: The phosphinate ester substrate, 0.1 M in 10:90 H2O:
TFA, was stirred at room temperature for 2 h. The solvent
was removed in vacuo, CH2Cl2 added, the mixture re-
concentrated (3×), toluene added, and the mixture re-
concentrated (3×) to give a quantitative yield of the phosphinic
acid.
Ben zyl[2-(N-ben zylca r b a m oyl)-4-m et h ylp en t yl]p h os-
p h in ic a cid (8a ): mass spectrum, (CI) M+ + H+ 374 (100);
1H-NMR (CD3OD) δ 0.87/0.93 (6H 2d), 1.30 (1H m), 1.4-1.75
(3H, m), 2.05 (1H, m), 2.72 (1H, m), 3.08 (2H, d), 4.37 (2H,
dd), 7.26 (10H, m); 31P-NMR (CDCl3) δ 52.15 (m); HPLC (30/
90) 9.4 min. Anal. Calcd for C21H28NO3P (373.43): C, 67.54;
H, 7.56; N, 3.75. Found: C, 67.17; H, 7.70; N, 3.79.
Ack n ow led gm en t. We thank Drs. Ralph P. Robin-
son and Kim F. McClure for their review of the
manuscript and the Pfizer Scientific Advisory Commit-
tee for support of B.P.J . as an undergraduate research
fellow.
Su p p or tin g In for m a tion Ava ila ble: 1H-NMR spectra of
5, 6a -c, 7a -i, 8a -g, 9, benzyl[2-(benzyloxycarbonyl)-4-
methylpentyl]phosphinic acid and the (S)-R-methylbenzyl-
amine salt of 5, HPLC’s of 6a -b, 7a -i, 8a -g, 9 and of a mix
of 7f/7g, and tables of the data points for Figures 1-3 (45
pages). This material is contained in libraries on microfiche,
immediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
J O962275W