Synthesis and comparative study on the reactivity of peptidyl-type phosphinic esters: Intramolecular effects the alkaline and acidic cleavage of methyl β-carboxyphosphinates

Article abstract of DOI:10.1021/jo0156363

Using the phosphinic analogue of Cbz-Phe-Gly-OEt 1a as a template for this study, several phosphinic esters (2a-g) were prepared, employing an efficient method for each case. The activity of these derivatives under conventional deprotection conditions was studied, and the results are listed comparatively. The effect of steric hindrance as well as the contribution of neighboring groups in the rate of hydrolysis of suitably selected β-carboxyphosphinates under acidic and deprotection conditions was examined. The results clearly demonstrate that a significant acceleration of phosphinate cleavage occurs due to the intermediacy of a five-membered, mixed anhydride-type species. This was supported by the observation that similar interactions were not observed in the case of hindered α-carboxyphosphinate homologous derivatives.

Full text of DOI:10.1021/jo0156363

6604
J . Org. Chem. 2001, 66, 6604-6610
Syn th esis a n d Com p a r a tive Stu d y on th e Rea ctivity of
P ep tid yl-Typ e P h osp h in ic Ester s: In tr a m olecu la r Effects in th e
Alk a lin e a n d Acid ic Clea va ge of Meth yl â-Ca r boxyp h osp h in a tes
Dimitris Georgiadis,† Vincent Dive,‡ and Athanasios Yiotakis*,†
Department of Chemistry, Laboratory of Organic Chemistry, University of Athens,
Panepistimiopolis Zografou, 15771, Athens, Greece, and CEA,
De´partement d’Inge´nerie et d’Etudes des Prote´ines, 91191 Gif/ Yvette Cedex, France
agiotak@cc.uoa.gr
Received March 19, 2001
Using the phosphinic analogue of Cbz-Phe-Gly-OEt 1a as a template for this study, several
phosphinic esters (2a -g) were prepared, employing an efficient method for each case. The reactivity
of these derivatives under conventional deprotection conditions was studied, and the results are
listed comparatively. The effect of steric hindrance as well as the contribution of neighboring groups
in the rate of hydrolysis of suitably selected â-carboxyphosphinates under acidic and alkaline
deprotection conditions was examined. The results clearly demonstrate that a significant acceleration
of phosphinate cleavage occurs due to the intermediacy of a five-membered, mixed anhydride-type
species. This was supported by the observation that similar interactions were not observed in the
case of hindered R-carboxyphosphinate homologous derivatives.
Sch em e 1. Gen er a l P r oced u r e for th e Syn th esis
of P h osp h in ic P ep tid es on a Solid P h a sea
In tr od u ction
Phosphinic peptides, an important class of protease
inhibitors, are peptidic isosters containing the chemically
stable phosphinic moiety -PO(OH)CH2- which mimics
the transition state tetrahedral geometry of a scissile
peptide bond during enzymatic hydrolysis.1 In the past
decade, several studies have demonstrated that the
synthesis of phosphinic peptides is a very effective
approach for the development of highly potent inhibitors
of zinc metalloproteases.2 In addition, the development
of solid-phase synthesis of phosphinic peptides, making
possible the use of either parallel or combinatorial
chemistry strategies, has been proven to be a fruitful
approach to the development of both potent and selective
inhibitors of zinc metalloproteases.3-5 Toward this end,
we have reported a strategy for the synthesis of phos-
phinic peptides by classical Fmoc solid-phase methodol-
a
Reagents and conditions: (i) CbzCl, NaOH; (ii) HMDS,
H2CdC(R2)COOEt and then EtOH; (iii) 1-AdBr, Ag2O; (iv) 0.4 M
NaOH/MeOH and then aq HCl; (v) HCOO-NH4+, 10% Pd/C; (vi)
FmocCl, Na2CO3.
* Corresponding author. Fax: 0030-1-7274498. Tel: 0030-1-7274498.
† University of Athens.
‡ CEA, Gif/Yvette.
(1) Morgan, P. B.; Scholtz, J . M.; Ballinger, M. D.; Zipkin, I. D.;
Bartlett, P. A. J . Am. Chem. Soc. 1991, 113, 297.
ogy, where it was envisaged that proper protection of the
hydroxyphosphinyl group is essential for the efficient
preparation of phosphinic peptides (Scheme 1).6
(2) (a) Karanewsky, D. S.; Badia, M. C.; Cushman, D. W.; DeForrest,
J . M.; Dejneka, T.; Loots, M. J .; Perri, M. G.; Petrillo, E. W.; Powell, J .
R. J . Med. Chem. 1988, 31, 204. (b) Chen, H.; Noble, F.; Coric, P.;
Fournie-Zaluski, M.-C.; Roques, B. P. Proc. Natl. Acad. Sci. U.S.A.
1998, 95, 12028. (c) Reiter, L. A.; Rizzi, J . P.; Pandit, J .; Lasut, M. J .;
McGahee, S. M.; Parikh, V. D.; Blake, J . F.; Danley, D. E.; Laird, E.
R.; Lopez-Anaya, A.; Lopresti-Morrow, L. L.; Mansour, M. N.; Marti-
nelli, G. J .; Mitchell, P. G.; Owens, B. S.; Pauly, T. A.; Reeves, L. M.;
Schulte, G. K.; Yocum, S. A. Bioorg. Med. Chem. Lett. 1999, 9, 127. (d)
Vassiliou, S.; Mucha, A.; Cuniasse, P.; Georgiadis, D.; Beau, F.;
Kannan, R.; Murphy. G.; Kna¨uper, V.; Rio, M.-C.; Basset, P.; Yiotakis,
A.; Dive, V. J . Med. Chem. 1999, 42, 2610. (e) Georgiadis, D.; Vazeux,
G.; Llorens-Cortes, C.; Yiotakis, A.; Dive, V. Biochemistry 2000, 39,
1152.
The protocol for the synthesis of phosphinic peptides
consists of two main parts: (i) preparation of the pseudo-
dipeptidic synthon; (ii) peptide elongation, using either
solid phase or solution methodologies. After a thorough
overview of the literature, numerous contradictory re-
ports concerning the protection of the hydroxyphosphinyl
function in both parts of the synthetic protocol mentioned
above were found. Some researchers claim that protection
of the hydroxyphosphinyl function is not necessary during
peptide coupling reactions, either on the solid phase or
(3) J iracek. J .; Yiotakis, A.; Vincent, B.; Checler, F.; Dive, V. J . Biol.
Chem. 1996, 271, 19606.
(4) Dive, V.; Cotton, J .; Yiotakis, A.; Michaud, A.; Vassiliou, S.;
J iracek, J .; Vazeux, G.; Chauvet, M.; Cuniasse, P.; Corvol, P. Proc. Natl.
Acad. Sci. U.S.A. 1999, 96, 4330.
(5) Buchardt, J .; Sciødt, C. B.; Krog-J ensen, C.; Delaisse´, J .-M.;
Foged, N. T.; Meldal, M. J . Comb. Chem. 2000, 2, 624.
(6) Yiotakis, A.; Vassiliou, S.; J iracek, J .; Dive, V. J . Org. Chem.
1996, 61, 6601-6605.
10.1021/jo0156363 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/05/2001

Synthesis and Reactivity Study of Phosphinates
J . Org. Chem., Vol. 66, No. 20, 2001 6605
in solution,1,7 while others strongly emphasize the serious
limitations of coupling methods when phosphinopeptidic
units are not protected at the phosphorus center.8,9 It has
been previously reported that the 1-adamantyl (1-Ad)
group is a suitable protecting group for the hydroxyphos-
phinyl group during both the synthesis of phosphinic
building blocks and the development of the pseudopep-
tides on the solid phase.6,10 Nevertheless, 1-adamantyl
phosphinates exhibit increased sensitivity during hydro-
genation (Scheme 1), thus complicating the preparation
of the target compounds.6 Furthermore, as described in
the literature, the most commonly used methyl phosphi-
nates exhibit a structure-dependent behavior during
deprotection while, in many cases, the harsh conditions
needed for demethylation (e.g. TMSBr,8 I-/reflux11) limit
the flexibility of the synthetic plan.
Sch em e 2. P r ep a r a tion of P h osp h in a tes 2a -ga
The confusion caused by the lack of a reliable guide to
the behavior of phosphinic esters prompted us to inves-
tigate the protection and deprotection conditions of the
hydroxyphosphinyl moiety in a more systematic manner.
In this paper, the synthesis of a series of phosphinic
esters are described and their behavior under basic, acidic
and hydrogenation conditions is studied. Accordingly, the
effect of neighboring functional groups and steric hin-
drance in the deprotection rate is investigated. Finally,
the involvement of intramolecular carboxyl assistance in
the cleavage of methyl â-carboxyphosphinates is de-
scribed.
a
Reagents and conditions: 2a , CH2N2, Et2O, 10 min, 0 °C, 85%
or MeOH, (4-ClPh)3P, DIAD, Et3N, 2 h, rt, 72%; 2b, Cs2CO3,
i-PrBr, DMF, 48 h, 50 °C, 86%; 2c, Cs2CO3, BnBr, DMF, 4 h, rt,
87% or BnOH, (4-ClPh)3P, DIAD, Et3N, 2 h, rt, 67%; 2d , 9-diaz-
ofluorene, CH2Cl2, reflux, 24 h, 80%; 2e, HC(Ot-Bu)2N(Me)2,
benzene, reflux, 4 h, 76%; 2f, Ph2CdNNH2, PhI(OAc)2, CHCl3, 25
°C, 15 min, 90%; Ag2O, 1-AdBr, CHCl3, reflux, 3.5 h, 81%.
di-tert butyl acetal in benzene under neutral conditions.14
Attempts to protect the hydroxyphosphinyl moiety by
prior activation with coupling reagents (DCC/DMAP,
BOP/Et3N, pyBOP) gave only poor results while Mit-
sunobu esterification, as modified by Campbell et al.15
for analogous phosphonic derivatives, proceeded satis-
factorily for 2a ,c but sluggishly for the rest of the cases
(<20%).16 This low reactivity of phosphinic acids could
be attributed to the reduced basicity and presumably
nucleophilicity of phosphinate anions, as compared to
carboxylates,17 which would prevent the formation of the
phosphinyloxy-activated species.
With these esters in hand, their reactivity was exam-
ined using common deprotection conditions. Therefore,
phosphinates 2a -g were subjected to (i) saponification
conditions using 0.35 N NaOH/MeOH, (ii) acidolysis
conditions using TFA/CH2Cl2/H2O in various concentra-
tions, and (iii) hydrogenation conditions using 10% Pd/C
as a catalyst and either H2 or HCOONH4 as a hydrogen
donor. The progress of the reactions was being monitored
by TLC, and the conversion percentages were determined
after workup of the reaction mixtures and isolation of
the products. The results are summarized in Tables 1-3.
Cleavage of the phosphinic ester follows the saponifica-
tion of the ethyl carboxylate in all cases except from the
case of 2g (Table 1). Phosphinates 2a ,b were cleaved
simultaneously with the carboxylic ester while the benzyl
and fluorenyl groups were partially removed from the
corresponding phosphinates (2c,d ) under saponification
conditions, but complete cleavage took place during the
acidification step. 2e,f appeared to be stable to saponi-
fication conditions; however, rapid cleavage took place
during the acidification step (diphenylmethyl phosphi-
nate is cleaved even in pH ∼ 5). Upon treatment with
Resu lts a n d Discu ssion
The phosphinic pseudodipeptidic analogue of Cbz-Phe-
Gly-OEt 1a 6 was used as a template in this study. Seven
phosphinic esters of 1a with diverse protecting groups
covering a wide range of reactivity were prepared in high
yields (Scheme 2). Methyl (2a ), 9-fluorenyl (2d ), and
diphenylmethyl (2f) phosphinates were efficiently ob-
tained by the action of diazomethane, diazodiphenyl-
methane,12 and 9-diazofluorene, respectively, on 1a .
Introduction of isopropyl and benzyl groups was success-
fully achieved by reacting the cesium phosphinate of 1a
with isopropyl and benzyl bromide, respectively. At-
tempts to prepare adamantyl phosphinate 2g using the
cesium phosphinate of 1a and 1-bromoadamantane failed,
mainly due to the low rate of bromide ion displacement
caused by the angle strain of adamantane in the non-
planar transition state of this proccess.13 Thus, 2g was
prepared by conversion of 1a to silver phosphinate and
reaction of the latter with 1-bromoadamantane, as previ-
ously described.6 Finally, tert-butyl ester 2e was prepared
by refluxing 1a with an excess of N,N-dimethylformamide
(7) (a) Campagne, J .-M.; Coste, J .; Guillou, L.; J ouin, P. Tetrahedron
Lett. 1993, 34, 4181. (b) 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.
(8) Allen, M. C.; Fuhrer, W.; Tuck, B.; Wade, R.; Wood, J . M. J . Med.
Chem. 1989, 32, 1652.
(9) Miller, D. J .; Hammond, S. M.; Anderluzzi, D.; Bugg, T. D. H. J .
Chem. Soc., Perkin Trans. 1 1998, 131.
(10) Buchardt, J .; Ferreras, M.; Krog-jensen, C.; Delaisse´, J .-M.;
Foged, N. T.; Meldal, M. Chem. Eur. J . 1999, 5, 2877.
(11) Patel, D. V.; Gordon, E. M.; Schmidt R. J .; Weller, H. N.; Young,
M. G.; Zahler, R.; Barbacid, M.; Carboni, J . M.; Gullo-Brown, J . L.;
Hunihan, L.; Ricca, C.; Robinson, S.; Seizinger, B. R.; Tuomari, A. V.;
Manne, V. J . Med. Chem. 1995, 38, 435.
(12) Lloyd, J .; Schmidt, J . B.; Hunt, J . T.; Barrish, J . C.; Little, D.
K.; Tymiak, A. A. Biorg. Med. Chem. Lett. 1996, 6, 1323.
(13) Schleyer, P. R.; Nicolas, R. D. J . Am. Chem. Soc. 1961, 83, 2700.
(14) Georgiadis, D.; Matziari, M.; Vassiliou, S.; Dive, V.; Yiotakis,
A. Tetrahedron 1999, 55, 14635.
(15) Campbell, D. A.; Bermak, J . C. J . Org. Chem. 1994, 59, 658.
(16) An example of phosphinate preparation via Mitsunobu reaction
has been recently reported: Hum, G.; Wooler, K.; Lee, J .; Taylor, S.
D. Can. J . Chem. 2000, 78, 642.
(17) Chaudhry, A.; Harger, M. J . P.; Shuff, P.; Thompson, A. J .
Chem. Soc., Perkin Trans. 1 1999, 1347.

6606 J . Org. Chem., Vol. 66, No. 20, 2001
Georgiadis et al.
Ta ble 1. Rea ctivity of 2a -g u n d er Ba se-Ca ta lyzed
Ta ble 4. Ba se-Ca ta lyzed Hyd r olysis of 2g-i a n d 1a -c
Hyd r olysis Con d ition sa
(0.35 N Na OH/MeOH)
reactant
product
R
X
T1/2 (min)
ester
product
time
conversion (%)
2g
2h
2i
1a
1b
1c
4a
4b
4c
3a
3b
3c
H
Ad
Ad
Ad
H
H
H
12
72
450
4.1
27
2a
2b
2c
2d
2e
2f
3a
3a
3a
3a
3a
3a
4a
40 min
60 min
45 min
60 min
60 min
45 min
24 h
98
100
91
Me
i-Bu
H
Me
i-Bu
95
100
100
89
220
2h
Sch em e 3. Dem eth yla tion Con d ition s of 6, 8, a n d
10a
a
Conditions used: 0.35 N NaOH/MeOH.
Ta ble 2. Rea ctivity of 2a -g u n d er Acid -Ca ta lyzed
Hyd r olysis Con d ition s
ester
TFA/CH2Cl2/H2O
time
24 h
24 h
24 h
24 h
24 h
4 h
1 h
5 min
5 min
5 min
3 h
conversion (%)
2a
2b
2b
2b
2c
2c
2c
2d
2e
2f
99.5/0/0.5
10/89.5/0.5
50/49.5/0.5
99.5/0/0.5
10/89.5/0.5
50/49.5/0.5
99.5/0/0.5
5/94.5/0.5
5/94.5/0.5
5/94.5/0.5
10/89.5/0.5
50/49.5/0.5
<5
4
35
81
66
100
100
100
100
100
95
a
Reagents and conditions: (i) 0.35 N NaOH/MeOH and then
aq HCl; (ii) TMSBr, CH2Cl2, 2 h, rt, 93%; (iii) PhSH/Et3N/dioxane
(2/2/1), 5 h, rt, 89%
2g
2g
30 min
100
mantyl phosphinates 2g-I, where the bulkiness of the
side chain in the R-position of the carboxylate function
was increased. Comparison of the half-life values in these
processes indicates that the reaction is significantly
slower with bulkier side chains, since nucleophilic attack
of OH- is prevented by steric encumbrance (Table 4).
Interestingly, when phosphinates 1a -c were subjected
to the same conditions, a considerable acceleration of the
ethyl carboxylate hydrolysis was observed (Table 4). We
assumed that the free hydroxyphosphinyl group of 1a -c
is participating to the ethyl carboxylate cleavage while
in the case of 2g-i any hydroxyphosphinyl contribution
to the saponification rate is suppressed. This effect is
dependent on the size of the side chain R, reaching a
maximum in the case of R ) H (∼3-fold acceleration
based on the half-lives). This acceleration was attributed
to an intramolecular effect of the phosphinic anion
assisting in the hydrolysis of the ethyl carboxylate,
probably via a five-membered intermediate.
Ta ble 3. Rea ctivity of 2a -g u n d er Ca ta lytic
Hyd r ogen a tion Con d ition s
ester
product
X
conversion (%)a
2a
2b
2c
2d
2e
2f
5a
5b
5c
5c
5d
5c
5e
Me
i-Pr
H
H
t-Bu
H
91 (89)
84 (87)
96 (100)
90 (100)
93 (82)
91 (100)
95 (90)
2g
1-Ad
a
Conversion yields given outside and within parentheses cor-
respond to H2 and HCOONH4 hydrogen donors, respectively.
TFA, 2a is the only compound that remained unaffected
(Table 2). Phosphinates 2d -f were immediately cleaved
using 5% TFA/CH2Cl2/H2O, whereas 2c,g were suf-
ficiently cleaved only in more concentrated TFA solutions.
Isopropyl phosphinate (2b) was less resistant than 2a
since it was significantly cleaved using concentrated TFA
solutions. Finally, under catalytic hydrogenation, benzyl,
diphenylmethyl, and fluorenyl groups were removed
simultaneously with the Cbz group while methyl, iso-
propyl, tert-butyl, and adamantyl groups were stable
under these conditions (Table 3).
To explore further this observation we initially exam-
ined whether the aforementioned intermediate can be
formed by the attack of a carboxylic anion to the vicinal
phosphorus center. When the alkaline hydrolysis rates
of 2a (Table 1) and 6 were compared, it was observed
that conversion of 6 to 7 is 78% after 3 h while 2a is
converted quantitatively to 3a within 40 min (Scheme
3). In the case of 6, where the carboxylic moiety remains
protected throughout the reaction, assisting effects are
not operable and hydrolysis occurs only due to the direct
action of OH-. On the contrary, in the case of 2a the
initially liberated carboxylate anion significantly acceler-
ates methyl phosphinate cleavage.
Our next objective was to investigate the contribution
of steric and neighboring effects to the base-catalyzed
hydrolysis of â-carboxyphosphinates. At the outset, we
examined the rate of carboxylate saponification in ada-
The assumption of the cyclic intermediate was con-
firmed by the following experiment: methyl phosphinates

Synthesis and Reactivity Study of Phosphinates
J . Org. Chem., Vol. 66, No. 20, 2001 6607
Sch em e 4. Syn th esis of 8 a n d 10a
Sch em e 5. Ch em ica l Beh a vior of 6 a n d 11 u p on
Tr ea tm en t w ith TF Aa
a
Reagents and conditions: (i) SOCl2, CH2Cl2, rt, 2 h; (ii)
BnCH2COOEt, LDA, THF, -78 to -10 °C, 45 min; (iii) TFA, rt, 3
h.
8 and 10, bearing free carboxylic acid and tert-butyl
carboxylate functions in their R-position respectively,
were synthesized (Scheme 4) and subjected to saponifica-
tion conditions. We observed that cleavage of methyl
phosphinate 8 proceeded extremely slowly (58%, 36 h)
while phosphinate 10 was completely stable even after
several days (Scheme 3). This observation is in perfect
accordance with our previous remarks since an analogous
four-membered cyclic intermediate of an R-carboxyl-
substituted phosphinate would not be thermodynamically
favorable. In the case of such hindered structures as
methyl R-carboxyphosphinate 8, cleavage by nucleophilic
displacement using Me3SiBr18 or PhS-Et3NH+19 proved
to be more efficient for demethylation.
Similar intramolecular solvolysis effects have been
previously reported for the cases of â-carboxyphosphonic
and â-carboxyphosphonamidic acids20 as well as for
various structurally diverse phosphates and phospho-
nates.21 However, in the case of phosphinates the effect
is weaker probably due to reduced electrophilicity of the
phosphorus atom. Attempts to differentiate between the
significantly more stable methyl phosphinate and the
ethyl carboxylate moieties of 2a using 1 equiv of base
afforded only poor results.22 It has been previously
described that differentiation can be achieved by enzy-
matic cleavage of the carboxylic ester but, even in these
cases, cleavage of methyl phosphinate due to intramo-
lecular catalysis cannot be completely suppressed.1,22a
As we wished to examine whether a similar assisting
effect to the one mentioned above could participate in the
acid-catalyzed cleavage of phosphinates, we compared the
stability of methyl phosphinates 2a and 6 upon treatment
with dilute TFA (50/49.5/0.5 TFA/CH2Cl2/H2O). 2a was
stable even when 99.5/0.5 TFA/H2O is used for prolonged
periods (Table 2). Surprisingly, in the case of 6, the
methyl group was removed quantitatively within 14 h
(Scheme 5). This unexpected observation implies that the
released carboxylic acid can promote methyl phosphinate
hydrolysis via a five-membered intermediate even under
mild acidic conditions. Furthermore, in the case of the
R-carboxy-substituted methyl phosphinate 11, unmask-
ing of the hydroxyphosphinyl group did not occur even
after treatment with 99.5/0.5 TFA/H2O for 15 days. In
fact, this methyl phosphinate is resistant even on treat-
a
Reagents and conditions: (i) TFA/CH2Cl2/H2O (50/49.5/0.5),
14 h, quant; (ii) TFA/H2O (99.5/0.5), 15 d.
ment with 33% HBr/AcOH for a long period of time.
Obviously, as noted above for 8, in the case of 11 the
formation of a four-membered intermediate that would
promote phosphinate cleavage is unfavorable due to steric
hindrance as well as thermodynamic constraint.
Some authors claim that the methyl group of methyl
carboxyphosphinates can be removed in acidic conditions
(TFA,23 HCl/dioxane24) while others report that it is
completely stable upon treatment with TFA25,26 or even
refluxing HBr/H2O for 6 h.8 Furthermore, Reiter and
J ones mention that cleavage of a phosphinic acid methyl
ester with TFA can be promoted by a â-situated carboxa-
mide due to the formation of a five-membered reactive
intermediate that involves the carboxamidic function.27
In the structural motifs described herein, a five-mem-
bered intermediate involving the vicinal carbamate can
be safely ruled out, since if the carbamate were involved
in the cleavage, no differentiation between the R and
â-carboxyphosphinates examined would be observed. All
these observations perfectly supplement the study of
Reiter and J ones and reveal the strong dependence of
methyl phosphinates’ behavior on the type and position-
ing of carbonyl-containing functional groups within the
molecule at hand.
In Scheme 6, a general pathway for the mechanism of
the alkaline and acidic cleavage of methyl â-carboxy-
phosphinates is proposed. In the case of basic hydrolysis,
an intramolecular nucleophilic attack can lead to a
phosphorane-type anionic TS intermediate (Ia ) which
is readily cleaved to give the deprotected derivative via
the five-membered cyclic mixed-anhydride II.28 In the
case of acidic cleavage, the mechanism involves proto-
nation of the phosphinate which could lead to two open
protonated TS intermediates being in equilibrium with
two cyclic phosphorane-type cationic TS intermediates.28
When protonation of the methanolic oxygen occurs (Ib),
equilibrium is rapidly displaced to the five-membered
intermediate II, via its protonated form in equilibrium
in the acidic medium, affording the fully deprotected
compound after hydrolysis. The breakdown of the TS
(18) Abdel-Meguid, S. S.; Zhao, B.; Murthy, K. H. M.; Winborne,
E.; Choi, J .-K.; DesJ arlais, R. L.; Minnich, M. D.; Culp, J . S.; Debouck,
C.; Tomaszeck, T. A., J r.; Meek, T. D.; Dreyer, G. B. Biochemistry 1993,
32, 7972.
(19) Campbell, D. A.; Bermak, J . C.; Burkoth T. S.; Patel, D. V. J .
Am. Chem. Soc. 1994, 117, 5381.
(23) Goulet, J . G.; Kinneary, J . F.; Durette, P. L.; Stein, R. L.;
Harrison, R. K.; Izquierdo-Martin, M.; Kuo, D. W.; Lin, T.-Y.; Hag-
mann, W. K. Bioorg. Med. Chem. Lett. 1994, 4, 1221.
(24) McKittrick, B. A.; Stamford, A. W.; Weng, X.; Ma, K.; Chack-
alamannil, S.; Czarniecki, M.; Cleven, R. M.; Fawzi, A. B. Bioorg. Med.
Chem. Lett. 1996, 6, 1629.
(20) J acobsen, N. E.; Bartlett, P. A. J . Am. Chem. Soc. 1983, 105,
1613; 1983, 105, 1619.
(21) Kirby, A. J . Adv. Phys. Org. Chem. 1980, 17, 183.
(22) Our observation is in accordance with that of the following: (a)
Ross, F. C.; Botting, N. P. Bioorg. Med. Chem. Lett. 1996, 6, 2643. And
it is contrary to that of Patel et al.11
(25) Chackalamannil, S.; Chung, S.; Stamford, A. W.; McKittrick,
B. A.; Wang, Y.; Tsai, H.; Cleven, R.; Fawzi, A.; Czarniecki, M. Bioorg.
Med. Chem. Lett. 1996, 6, 1257.
(26) Bartlett, P. A.; Hanson, J . E.; Giannousis, P. P. J . Org. Chem.
1990, 55, 6268.
(27) Reiter, L. A.; J ones, B. P. J . Org. Chem. 1997, 62, 2808.

6608 J . Org. Chem., Vol. 66, No. 20, 2001
Georgiadis et al.
Sch em e 6. P u ta tive Mech a n istic P a th w a ys for th e Ba se- a n d Acid -Ca ta lyzed Clea va ge of Meth yl
â-Ca r boxyp h osp h in a tes
have been observed for the various stereoisomers of these
compounds. Column chromatography was carried out on silica
gel (70-230 mesh). Melting points are uncorrected. Assign-
ment of the 1H NMR signals was achieved using COSY and,
in some cases only, HOM2DJ experiments. 13C partial assign-
ment is based on DEPT experiments. 1H, 13C, and 31P NMR
spectra were recorded on a 200 MHz Mercury Varian spec-
trometer. The presence of asymmetric centers in the described
compounds complicates the interpretation of the spectra,
especially when the hydroxyphosphinyl function is protected
by the adamantyl group. Numbers I and II are used for the
assignment of NMR signals that correspond to resonances of
different diastereoisomers. 13C and 31P NMR spectra are fully
proton decoupled. 31P chemical shifts are reported on the δ
scale (in ppm) downfield from 85% H3PO4 (external standard).
Before microanalyses, samples were dried under high vacuum
at 40 °C for 24 h in a dry pistol. Electron impact mass spectra
(EIMS) were obtained on a mass spectrometer with positive
ionization mode.
intermediates (Ia ,b) to the intermediate cyclic phosphi-
nate (II) is irreversible in both pathways. In addition,
two main structural features limit the rate enhancement
effect, as compared to electron-richer phosphates, phos-
phonates, and phosphonamidates: first, the reduced
electrophilicity of the phosphorus atom, which favors the
acyclic charged intermediates (Scheme 6), and, second,
the presence of an endocyclic P-C bond, in lieu of a P-O
or a P-N bond, in the intermediate (II) which could
relieve the ring from the large strain described in the
case of the more reactive phosphonate and phosphate
(and less in phosphonamidate) intermediates.17,29
Con clu sion
Several diverse phosphinates have been synthesized,
and results concerning the behavior of these compounds
under typical deprotection conditions have been obtained.
Upon examination of the dependency of steric factors and
intramolecular interactions on acid- and base-catalyzed
hydrolysis, using methyl â-carboxyphosphinates as tem-
plates, we concluded that a five-membered cyclic inter-
mediate (II) is responsible for promoting cleavage in both
processes. Absence of this assisting effect in methyl
R-carboxyphosphinate hydrolysis strongly supports this
proposal. The results of this study aim to improve the
understanding of the reactivity of phosphinates, thus
offering a consistent guide to the protection/deprotection
conditions during the synthesis of this important class
of bioactive molecules.
(R,R,S,S)-3-((1′-(N-Ben zyloxyca r b on yla m in o)-2′-p h e-
n yleth yl)m eth yloxyp h osp h in yl)p r op a n oic Acid , Eth yl
Ester (2a ). To an ice-cooled mixture of 40% KOH (3 mL) and
Et2O (20 mL) was added N-nitrosomethylurea (1.03 g, 10.0
mmol) under vigorous stirring. After the mixture was stirring
for 15 min, the organic layer was added to a suspension of
compound 1a (0.84 g, 2.0 mmol) in Et2O (20 mL). In the
resulting yellow solution, some drops of AcOH were added. The
colorless solution obtained was washed with 5% NaHCO3 (2
× 10 mL) and H2O (10 mL). The organic layer was dried with
Na2SO4 and concentrated to dryness. The crude product was
purified by column chromatography using CHCl3/i-PrOH (9.7/
1
0.3) as eluent: yield 0.74 g (85%); H NMR (CDCl3) δ 1.25 (t,
3J HH ) 7.2 Hz, 3H, CH2CH3), 2.01-2.29 (m, 2H, PCH2), 2.50-
2.74 (m, 2H, CH2CO), 2.80-3.06 (m, 1H, PhCHH), 3.14-3.37
(m, 1H, PhCHH), 3.66 (s, 3H/2, POCH3, I), 3.70 (s, 3H/2,
Exp er im en ta l Section
3
POCH3, II), 4.14 (q, J HH ) 7.2 Hz, 2H, CH2CH3), 4.28-4.50
3
(m, 1H, PCH), 5.00 (s, 2H, OCH2Ph), 6.47 (d, J HH ) 9.7 Hz,
All of the compounds, for which analytical and spectroscopic
data are quoted, were homogeneous by TLC (silica gel 60
F-254). In most solvent systems close, but different, Rf values
1H, NH), 7.11-7.35 (m, 10H, aryl); 31P NMR (CDCl3) δ 53.54,
54.24; ESMS (m/z) calcd for C22H29NO6P (M + H)+ 434.2, found
434.1. Anal. Calcd for C22H28NO6P: C, 60.96; H, 6.51; N, 3.23.
Found: C, 61.03; H, 6.78; N, 3.21.
(28) It is emphasized that the structures in Scheme 6 are only
indicative and nothing is implied for the geometry and the stereo-
chemistry of the TS phosphorane-type intermediates. According to the
rules established by Westheimer (Westheimer, F. H. Acc. Chem. Res.
1968, 1, 70), (i) attack of a nucleophile in tetrahedral P leads to trigonal
bipyramidal (TBP) intermediates which are rather transitional state
forms and (ii) nucleophile enters and leaving group leaves from apical
positions. We should also note that these forms are presumably in
constant polytopic isomerisation. All the above have been demonstrated
for a large number of phosphorous analogues of tetrahedral ground-
state geometry (Thatcher, G. R. J .; Kluger, R. Adv. Phys. Org. Chem.
1989, 25, 99). Nevertheless, the case of such phosphinates has never
been studied.
(R,R,S,S)-3-((1′-(N-Ben zyloxyca r b on yla m in o)-2′-p h e-
n yleth yl)isop r op yloxy p h osp h in yl)p r op a n oic Acid , Eth -
yl Ester (2b). Compound 1a (0.84 g, 2.0 mmol) and Cs2CO3
(0.33 g, 1.0 mmol) were dissolved in 50% EtOH-H2O (20 mL).
The resulting solution was concentrated to dryness, and the
residue was well dried over P2O5. To a solution of the resulting
cesium salt in dry DMF (10 mL) was added i-PrBr (9.1 mL,
96.0 mmol). The mixture was stirred for 24 h at 50 °C, and
then the solvents were evaporated. The residue was dissolved
in H2O (10 mL) and Et2O (25 mL). The organic phase was
washed with 5% NaHCO3 (2 × 10 mL) and H2O (10 mL), dried
(29) Kluger, R.; Taylor, S. D. J . Am. Chem. Soc. 1990, 112, 6669.

Synthesis and Reactivity Study of Phosphinates
J . Org. Chem., Vol. 66, No. 20, 2001 6609
with Na2SO4, and concentrated to dryness. The residue was
treated with Et2O/hexane (1/1). After cooling of the mixture
at 0 °C for 24 h, the white crystalline precipitate was filtered
CH3), 4.10-4.39 (m, 1H, PCH), 4.95 (s, 2H, OCH2Ph), 6.44 (d,
3J HH ) 10.3 Hz, 1H, NH), 6.99-7.37 (m, 10H, aryl); 31P NMR
(CDCl3) δ 52.32, 52.66; ESMS (m/z) calcd for C25H35NO6P (M
+ H)+ 476.2, found 476.1. Anal. Calcd for C25H34NO6P: C,
63.15; H, 7.21; N, 2.95. Found: C, 63.42; H, 6.93; N, 2.86.
(R,R,S,S)-3-((1′-(N-Ben zyloxyca r b on yla m in o)-2′-p h e-
n yleth yl)diph en ylm eth yloxyph osph in yl)pr opan oic Acid,
Eth yl Ester (2f). To a solution of compound 1a (0.63 g, 1.5
mmol) in CHCl3 (8 mL) were added benzophenone hydrazone
(0.35 g, 1.8 mmol) and diacetoxy(phenyl)iodine (0.58 g, 1.8
mmol) portionwise and alternately. After 30 min the solvent
was evaporated and the residue was dissolved in AcOEt (40
mL). The solution was rinsed with 0.1 N HCl (2 × 15 mL), 5%
NaHCO3 (2 × 15 mL), and H2O (15 mL) and then dried with
Na2SO4. The solvent was removed in vacuo, and the residue
was treated with Et2O/hexane (3/7). The white crystalline
precipitate was filtered out and dried: yield 0.78 g (89%); mp
1
out and dried: yield 0.79 g (86%); mp 107-109 °C; H NMR
(CDCl3) δ 1.25 (t, 3J HH ) 7.1 Hz, 3H, CH2CH3), 1.23 (d, 3J HH
)
6.4 Hz, 3H, CH(CH3)2, I), 1.29 (d, 3J HH ) 6.4 Hz, 3H, CH(CH3)2,
II), 1.98-2.20 (m, 2H, PCH2), 2.49-2.73 (m, 2H, CH2CO),
2.75-2.93 (m, 1H, PhCHH), 3.20-3.40 (m, 1H, PhCHH), 4.16
(q, 3J HH ) 7.1 Hz, 2H, CH2CH3), 4.23-4.42 (m, 1H, PCH), 4.71
3
3
(q, J HH ) 6.4 Hz, 1H/2, CH(CH3)2, I), 4.75 (q, J HH ) 6.4 Hz,
1H/2, CH(CH3)2, II), 4.99 (s, 2H, OCH2Ph), 5.41 (d, 3J HH ) 10.3
Hz, 1H, NH), 7.01-7.39 (m, 10H, aryl); 31P NMR (CDCl3) δ
51.37, 52.43; ESMS (m/z) calcd for C24H33NO6P (M + H)+ 462.2,
found 462.1. Anal. Calcd for C24H32NO6P: C, 62.46; H, 6.99;
N, 3.04. Found: C, 62.18; H, 6.99; N, 3.27.
(R,R,S,S)-3-((1′-(N-Ben zyloxyca r b on yla m in o)-2′-p h e-
n yleth yl)ben zyloxyp h osp h in yl)p r op a n oic Acid , Eth yl
Ester (2c). Compound 1a (0.84 g, 2.0 mmol) was converted to
its cesium salt as described above. To a solution of this salt in
dry DMF (10 mL) was added BnBr (0.24 mL, 2.3 mmol). The
reaction mixture was stirred for 4 h at room temperature, and
then the solvents were evaporated. The residue was treated
with H2O (10 mL) and Et2O (30 mL). The organic phase was
washed with 5% NaHCO3 (2 × 10 mL) and H2O (10 mL), dried
with Na2SO4, and concentrated to dryness. The oily residue
was treated with Et2O/hexane (1/4). After cooling of the
mixture at 0 °C for 48 h, the white crystalline precipitate was
filtered out and dried: yield 0.89 g (87%); mp 111-112 °C;1H
1
3
115-117 °C; H NMR (CDCl3) δ 1.20 (t, J HH ) 7.1 Hz, 3H,
CH2CH3), 1.97-2.18 (m, 2H, PCH2), 2.29-2.63 (m, 2H, CH2-
CO), 2.74-3.04 (m, 1H, PhCHH), 3.15-3.33 (m, 1H, PhCHH),
4.05 (q, 3J HH ) 7.1 Hz, 2H, CH2CH3), 4.23-4.47 (m, 1H, PCH),
3
4.97 (s, 2H, OCH2Ph), 6.40 (d, J HH ) 6.7 Hz, 1H, NH), 6.63
(s, 1H/2, POCHPh2, I), 6.67 (s, 1H/2, POCHPh2, II), 7.00-7.48
(m, 20H, aryl); 31P NMR (CDCl3) δ 54.23, 54.32; ESMS (m/z)
calcd for C34H37NO6P (M + H)+ 586.2, found 586.1. Anal. Calcd
for C34H36NO6P: C, 69.73; H, 6.20; N, 2.39. Found: C, 69.64;
H, 6.55; N, 2.25.
(R,R,S,S)-3-((1′-(N-Ben zyloxyca r b on yla m in o)-2′-p h e-
n yleth yl)a d a m a n tyloxyp h osp h in yl)p r op a n oic Acid , Eth -
yl Ester (2g). To a refluxing solution of compound 1a (0.42 g,
1.0 mmol) and 1-adamantyl bromide (0.26 g, 1.2 mmol) in
CHCl3 (10 mL) was added silver(I) oxide (0.28 g, 1.2 mmol)
portionwise over 1 h. After the solution was refluxed for 3 h,
the solvent was removed in vacuo and the residue was treated
with Et2O (5 mL). The resulting mixture was filtered through
Celite, and the filtrates were evaporated. The residue was
purified by column chromatography, using CHCl3/i-PrOH (9.8:
0.2) as eluent. The product was treated with dry Et2O, and
the white solid, which was precipitated after cooling for 24 h,
was filtered out and dried: Yield 0.45 g (81%); mp 138-140
3
NMR (CDCl3) δ 1.22 (t, J HH ) 7.1 Hz, 3H, CH2CH3), 2.04-
2.31 (m, 2H, PCH2), 2.48-2.69 (m, 2H, CH2CO), 2.83-3.08 (m,
3
1H, PhCHH), 3.12-3.38 (m, 1H, PhCHH), 4.09 (q, J HH ) 7.1
Hz, 2H, CH2CH3), 4.33-4.55 (m, 1H, PCH), 4.90-5.20 (m, 4H,
3
POCH2Ph, OCOCH2Ph), 6.80 (d, J HH ) 10.1 Hz, 1H, NH),
7.15-7.45 (m, 15H, aryl); 31P NMR (CDCl3) δ 53.47, 53.95;
ESMS (m/z) calcd for C28H33NO6P (M + H)+ 510.2, found 510.1.
Anal. Calcd for C28H32NO6P: C, 66.00; H, 6.33; N, 2.75.
Found: C, 65.65; H, 6.67; N, 2.60.
(R,R,S,S)-3-((1′-(N-Ben zyloxyca r b on yla m in o)-2′-p h e-
n yleth yl)-9-flu or en yloxyph osph in yl)pr opan oic Acid, Eth -
yl Ester (2d ). A solution of compound 1a (0.84 g, 2.0 mmol)
and 9-diazofluorene (0.38 g, 2.0 mmol) in CH2Cl2 (15 mL) was
refluxed for 4 h. Then the reaction mixture was stirred for 24
h in room temperature. The mixture was concentrated, and
the residue was taken up to Et2O (20 mL). The organic phase
was washed with 5% NaHCO3 (10 mL) and H2O (10 mL), dried
with Na2SO4, and concentrated to dryness. The residue was
treated with Et2O/hexane (1/1) After cooling of the mixture at
0 °C for 24 h, the white crystalline precipitate was filtered
1
3
°C; H NMR (CDCl3) δ 1.23 (t, J HH ) 7.0 Hz, 3H, CH2CH3),
1.52-1.66 (m, 6H, CHCH2CH of Ad group), 2.00-2.19 (m, 11H,
CCH2 of Ad group, CH of Ad group, PCH2), 2.20-2.42 (m, 2H,
CH2CO), 2.77-3.00 (m, 1H, PhCHH), 3.13-3.36 (m, 1H,
3
PhCHH), 4.11 (q, J HH ) 7.0 Hz, 2H, CH2CH3), 4.06-4.32 (m,
3
1H, PCH), 4.95 (s, 2H, OCH2Ph), 6.41 (d, J HH ) 10.3 Hz, 1H,
NH), 7.09-7.36 (m, 10H, aryl); 31P NMR (CDCl3) δ 48.51, 48.76;
ESMS (m/z) calcd for C31H41NO6P (M + H)+ 554.2, found 554.2.
Anal. Calcd for C31H40NO6P: C, 67.25; H, 7.28; N, 2.53.
Found: C, 67.65; H, 7.01; N, 2.25.
1
out and dried: yield 0.93 g (80%); mp 117-119 °C; H NMR
3
(CDCl3) δ 1.29 (t, J HH ) 7.1 Hz, 3H, CH2CH3), 2.20-2.42 (m,
2H, PCH2), 2.59-2.92 (m, 3H, CH2CO, PhCHH), 3.38-3.55
P r oced u r e for th e P r ep a r a tion of 2a ,c by Mitsu n obu
Ester ifica tion . In a solution of 1a (0.2 g, 0.5 mmol) in THF
(3 mL) were added tris(4-chlorophenyl)phosphine (0.13 g, 0.5
mmol), DIAD (101 mg, 0.5 mmol), and Et3N (0.2 g, 2 mmol).
Then the alcohol (0.75 mmol) was added and the reaction was
stirred for 2 h in room temperature. After the end of the
reaction, the solvents were evaporated and the residue was
dissolved in Et2O. The solution was washed with 0.1 N HCl
(2 × 10 mL), 5% NaHCO3 (2 × 10 mL), and H2O (15 mL). After
evaporation, a crude oil was obtained which was purified by
column chromatography using CHCl3/i-PrOH (9.7/0.3) as elu-
ent. The products 2a ,c were obtained in 72 and 67% yield,
respectively.
Gen er a l P r oced u r e for th e Ba se-Ca ta lyzed Hyd r olysis
Exp er im en ts. To a stirred solution of phosphinate (50 mg)
in MeOH (2 mL) was added a 4 N aqueous solution of NaOH
(0.2 mL) dropwise. The progress of the reaction was being
monitored by TLC before and after treatment of small aliquots
with dilute HCl and AcOEt.
3
(m, 1H, PhCHH), 4.18 (q, J HH ) 7.1 Hz, 2H, CH2CH3), 4.43-
4.63 (m, 1H, PCH), 4.98 (s, 2H, OCH2Ph), 5.20 (d, 3J HH ) 10.3
Hz, 1H, NH), 6.31 (s, 1H/2, POCHC12H8, I), 6.34 (s, 1H/2,
POCHC12H8, II), 7.07-7.89 (m, 18H, aryl); 31P NMR (CDCl3)
δ 54.58, 54.79; ESMS (m/z) calcd for C34H35NO6P (M + H)+
584.2, found 584.1. Anal. Calcd for C34H34NO6P: C, 69.97; H,
5.87; N, 2.40. Found: C, 69.65; H, 6.01; N, 2.25.
(R,R,S,S)-3-((1′-(N-Ben zyloxyca r b on yla m in o)-2′-p h e-
n yleth yl)-ter t-bu tyloxy p h osp h in yl)p r op a n oic Acid , Eth -
yl Ester (2e). A suspension of compound 1a (0.83 g, 2.0 mmol)
in dry benzene (4 mL) was heated to reflux. N,N-Dimethyl-
formamide di-tert-butyl acetal (1.92 mL, 8.0 mmol) was added
dropwise to the refluxing mixture over a period of 15 min. The
mixture was refluxed for an additional 2 h and then was
concentrated in vacuo. The oily residue was treated with Et2O
(10 mL) and 5% NaHCO3 (10 mL). The organic layer was
separated, and two more extractions with Et2O (2 × 10 mL)
were performed. The combined organic layers were dried with
Na2SO4 and concentrated in vacuo. The residue was purified
by column chromatography using CHCl3/i-PrOH (9.7/0.3):
For the determination of the data reported in Table 1, the
reactions were quenched after 40-60 min (in the case of 2g,
the reaction was interrupted after 24 h) by addition of 0.3 N
HCl (to pH ∼ 3) at 0 °C. The resulting suspensions were
extracted with AcOEt (2 × 10 mL), and the organic layers were
dried over Na2SO4 and concentrated to give the crude products.
1
3
yield 0.72 g (76%); H NMR (CDCl3) δ 1.21 (t, J HH ) 7.1 Hz,
3H, CH2CH3), 1.51 (bs, 9H, C(CH3)3), 1.64-1.93 (m, 2H, CH2-
CO), 2.22-2.55 (m, 2H, PCH2), 2.70-2.98 (m, 1H, PhCHH),
3
3.12-3.35 (m, 1H, PhCHH), 4.13 (q, J HH ) 7.1 Hz, 2H, CH2-

6610 J . Org. Chem., Vol. 66, No. 20, 2001
Georgiadis et al.
The conversion ratios were determined by 31P NMR. The
products were purified, when necessary, by silica gel column
chromatography using appropriate eluent systems and char-
acterized.
3.7 mmol) under an Ar atmosphere. The mixture was stirred
for 2 h at room temperature and then concentrated to dryness.
CH2Cl2 (30 mL) was added to the residue, and the solution
was concentrated to dryness. This procedure was repeated
twice with CH2Cl2 and twice with THF. A colorless thick oil
was obtained. In a solution of diisopropylamine (1.43 mL, 10.2
mmol) in THF (20 mL) was added n-BuLi (6.75 mL of a
solution 1.51 M in hexane, 10.2 mmol) under Ar at -20 °C.
The resulting mixture was cooled to -78 °C, and 3-phenyl-
propionic acid, tert-butyl ester (2.1 g, 10.2 mmol), was added
dropwise during 10 min. After 15 min, a solution of the
phosphinochloridate in THF (13 mL) was added during 5 min.
The mixture was stirred for 30 min at -10 °C, and then the
reaction was quenched by the addition of 10% HCl (30 mL).
Extractions with Et2O (2 × 20 mL) followed. The organic phase
was washed with 5% NaHCO3 (10 mL), H2O (10 mL), and brine
(10 mL), dried with Na2SO4, and concentrated to dryness. The
residue was purified by column chromatography using CHCl3/
i-PrOH (9.8/0.2) as eluent: yield 1.1 g (60%); 1H NMR (CDCl3)
δ 1.31 (s, 9H, C(CH3)3), 2.89-3.17 (m, 2H, PhCH2CHNH),
3.10-3.46 (m, 3H, PhCH2CHCO, PhCH2CHCO), 3.74 (d, 3H/
2, POCH3, I, II), 3.85 (d, 3H/2, POCH3, III, IV), 4.51-4.75 (m,
1H, PhCH2CHNH), 5.03 (s, 2H, OCH2Ph), 6.64 (d, 3J HH ) 10.0
Hz, 1H, NH), 7.06-7.40 (m, 15H, aryl); 31P NMR (CDCl3) δ
46.55, 46.64, 46.94, 48.31; ESMS (m/z) calcd for C30H37NO6P
(M + H)+ 538.2, found 538.1. Anal. Calcd for C30H36NO6P‚
0.5H2O: C, 65.92; H, 6.82; N, 2.56. Found: C, 65.99; H, 7.08;
N, 2.67.
For the determination of the data reported in Table 4, small
aliquots of the reaction mixtures were collected in certain time
periods (see Supporting Information) and treated with 0.3 N
HCl and AcOEt at 0 °C. The organic layers were dried with
Na2SO4 and concentrated in vacuo. The conversion percentages
were determined in every case by 31P NMR spectroscopy.
CDCl3 was used as NMR solvent, but when the samples had
low solubility, D2O/Na2CO3 was used. After the end of the
reactions, the products were purified by silica gel column
chromatography using CHCl3/MeOH/AcOH (9.5/0.5/0.01) for
4a -c while, in the case of 3a -c, the products were obtained
pure after crystallization of the crude product by AcOEt.
Progress curves were generated from these data. Half-lives
were calculated from the progress curves and correspond to
the time required for 50% conversion.
Gen er a l P r oced u r e for th e Acid -Ca ta lyzed Hyd r olysis
Exp er im en ts. The phosphinate (20 mg) was dissolved in an
ice-cooled solution of TFA/CH2Cl2 and 10 µL of H2O (total
volume: 2 mL), and the resulting mixture was stirred at room
temperature. After a reaction period, judged by TLC, the
mixture was concentrated to dryness. The solid residue was
dissolved in Et2O (10 mL), and the product was extracted with
5% NaHCO3 (2 × 4 mL). The aqueous phase was acidified with
0.5 N HCl to pH 1, and two extractions with AcOEt (2 × 10
mL) followed. The organic phase was dried with Na2SO4 and
concentrated in vacuo affording the crude products.
Dem eth yla tion of P h osp h in a te 8. Meth od A. A solution
of 8 (50 mg, 0.10 mmol) and (TMS)Br (23 mg, 0.15 mmol)
in dry CH2Cl2 (1 mL) was stirred for 2 h at room tempera-
ture. Then the solvent was evaporated and the residue was
treated with AcOEt (2 mL) and MeOH (2 mL). The solution
was stirred for 30 min at room temperature and evaporated
in vacuo. The crude product was purified by column chroma-
tography using CHCl3/MeOH/AcOH (7:0/4/0.4) as eluent.
Compound 9 was obtained in a yield of 45 mg (93%): mp 142-
145 °C; 1H NMR (DMSO) δ 2.69-2.97 (m, 2H, PhCH2CHNH),
3.04-3.36 (m, 3H, PhCH2CHCO, PhCH2CHCO), 4.48-4.68 (m,
1H, PhCH2CHNH), 5.01 (s, 2H, OCH2Ph), 7.08-7.36 (m, 15H,
The determination of the conversion percentages reported
in Table 2 was based on the mass of the isolated products,
which in all cases corresponded exclusively to 1a as deter-
mined by the 31P NMR spectra of the products.
Gen er a l P r oced u r e for Ca ta lytic Hyd r ogen a tion Us-
in g H 2, P d /C. The phosphinate (50 mg) was dissolved in
absolute ethanol (2 mL). In this solution, 10% Pd/C (15 mg)
was carefully added. Then H2 was introduced in a pressure of
1 atm. After 3 h, the catalyst was removed by filtration through
Celite and the filtrates were concentrated to dryness affording
the final products. Yields are listed in Table 3. Compounds
5a ,b were isolated as hydrochloric salts after treatment with
2 N HCl/dioxane in dry THF. Compounds 5d ,e were not stable
to prolonged storage because of their sensitivity to moisture
and to their tendency to form of pseudodiketopiperazines.6
Gen er a l P r oced u r e for Ca ta lytic Hyd r ogen a tion Us-
in g HCOONH4, P d /C. To a solution of the phosphinate (50
mg) in MeOH (0.5 mL) were added ammonium formate (20
mg, mmol) and 10% Pd/C (20 mg). After 12 min of vigorous
stirring at room temperature, the catalyst was removed by
filtration through Celite, and the filtrates were evaporated to
dryness. CH2Cl2 was added to the residue, and the solution
was evaporated to dryness. This procedure was repeated twice.
In the cases of 5a ,b, CHCl3 (5 mL) and H2O (5 mL) were added
to the mixture. The aqueous phase was separated, and the
organic phase was washed with H2O (2 × 5 mL). The organic
layer was concentrated, and the residue was treated with 2 N
HCl/dioxane in dry THF. The precipitate was isolated by
filtration affording compounds 5a ,b as hydrochloric salts.
Yields are listed in Table 3.
3
aryl), 7.67 (d, J HH ) 11.3 Hz, 1H, NH); 31P NMR (DMSO) δ
32.79 (broad); ESMS (m/z) calcd for C25H27NO6P (M + H)+
468.2, found 468.1. Anal. Calcd for C25H26NO6P·0.5H2O: C,
63.02; H, 5.71; N, 2.94. Found: C, 62.83; H, 5.78; N, 3.18.
Meth od B. A solution of 8 (50 mg, 0.10 mmol) in a mixture
of 2/2/1 PhSH/Et3N/dioxane (1 mL) was stirred for 5 h at room
temperature. Then the reaction was quenched by the addition
of 1 N HCl (2 mL) and Et2O (5 mL) was added. The organic
phase was separated and evaporated. The residue was dis-
solved in
a small quantity of Et2O. A white solid was
precipitated after the addition of hexane (20 mL), which was
filtered out and washed with hexane to afford 9 in pure form:
yield 43 mg (89%).
Ack n ow led gm en t. This work was supported by
funds from the University of Athens and from BIO-
TECH 2 (Contract No. ERBBIO4CT96-0464).
Su p p or tin g In for m a tion Ava ila ble: Full spectroscopic
and analytical data for compounds 1a , 3a -c, 4a -c, 5a -c,
6-8, and 12 a n d 13C NMR and IR data for all the compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(R,R,R,S,S,S)-3-P h en yl-2-((1′-(ben zyloxycar bon yl)am in o-
2′-p h en ylet h yl)m et h yloxyp h osp h in yl)p r op a n oic Acid ,
ter t-Bu tyl Ester (10). In a solution of 2-phenyl-1-(N-benzy-
loxycarbonylamino)aminophosphonic acid, monoethy lester
(1.19 g, 3.4 mmol) in CH2Cl2 (8 mL) was added SOCl2 (2.7 mL,
J O0156363