The case for the intermediacy of monomeric metaphosphate analogues during oxidation of H-phosphonothioate, H-phosphonodithioate, and H-phosphonoselenoate monoesters: Mechanistic and synthetic studies

Article abstract of DOI:10.1021/jo8006072

(Chemical Equation Presented) Studies on the reaction of H-phosphonothioate, H-phosphonodithioate, and H-phosphonoselenoate monoesters with iodine in the presence of a base led to identification of a unique oxidation pathway, which consists of the initial oxidation of the sulfur or selenium atom in these compounds, followed by oxidative elimination of hydrogen iodide to generate the corresponding metaphosphate analogues. The intermediacy of the latter species during oxidation of the investigated H-phosphonate monoester derivatives with iodine was supported by various diagnostic experiments. The scope and limitation of these oxidative transformations for the purpose of the synthesis of nucleoside phosphorothioate, nucleoside phosphorodithioate, and nucleoside phosphoroselenoate diesters was also investigated.

Full text of DOI:10.1021/jo8006072

The Case for the Intermediacy of Monomeric Metaphosphate
Analogues during Oxidation of H-Phosphonothioate,
H-Phosphonodithioate, and H-Phosphonoselenoate Monoesters:
Mechanistic and Synthetic Studies
Agnieszka Bartoszewicz,† Marcin Kalek,† and Jacek Stawinski*,†,‡
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm UniVersity, S-106 91 Stockholm,
Sweden, and Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14,
61-704 Poznan, Poland
js@organ.su.se
ReceiVed March 19, 2008
Studies on the reaction of H-phosphonothioate, H-phosphonodithioate, and H-phosphonoselenoate
monoesters with iodine in the presence of a base led to identification of a unique oxidation pathway,
which consists of the initial oxidation of the sulfur or selenium atom in these compounds, followed by
oxidative elimination of hydrogen iodide to generate the corresponding metaphosphate analogues. The
intermediacy of the latter species during oxidation of the investigated H-phosphonate monoester derivatives
with iodine was supported by various diagnostic experiments. The scope and limitation of these oxidative
transformations for the purpose of the synthesis of nucleoside phosphorothioate, nucleoside phospho-
rodithioate, and nucleoside phosphoroselenoate diesters was also investigated.
Introduction
require prior conversion into electrically neutral derivatives (e.g.,
silyl ethers) to react with electrophiles (oxidants).2,5
Oxidative transformation of P(III) compounds, e.g., tervalent
phosphite triesters (λ3σ3),1 or four-coordinated H-phosphonate
derivatives (λ5σ4),2,3 is a powerful synthetic method for the
preparation of biologically important phosphorus compounds and
their analogues, bearing single or multiple modifications at the
phosphorus center.2,4,5 In contrast to phosphorous acid triesters
(phosphites) and diesters (H-phosphonates), that undergo oxidation
under mild conditions, the corresponding monoesters (H-phospho-
nate monoesters) are more resistant to oxidation6 and usually
However, during studies on nucleoside H-phosphonate mo-
noester analogues it was observed that replacement of one of the
nonbridging oxygen atoms in H-phosphonate monoesters 1 (Chart
1) by sulfur to form H-phosphonothioate monoesters 2 made such
derivatives highly susceptible to oxidation by iodine.7,8 Since other
H-phosphonate monoester analogues, namely H-phosphonodithio-
ate 3 and H-phosphonoselenoate 4 monoesters, were found to also
undergo rapid oxidation with iodine, we decided to investigate this
phenomenon in more detail.
* Address correspondence to this author. Phone: +46-(0)8-16 24 85. Fax:
+46-(0)8-154908.
CHART 1
† Stockholm University.
‡ Polish Academy of Sciences.
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S. L.; Iyer, R. P. Tetrahedron 1993, 49, 6123–6194.
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Ed.; Marcel Dekker: New York, 1992; pp 377-434.
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(4) Beaucage, S. L.; Iyer, R. P. Tetrahedron 1993, 49, 10441–10488. Guga,
P.; Okruszek, A.; Stec, W. J. In New Aspects In Phosphorus Chemistry; Majoral,
J. P., Ed.; Springer: New York, 2002; Vol. 220, pp 169-200; Wozniak, L. A.;
Okruszek, A. Chem. Soc. ReV. 2003, 32, 158–169.
The observation that H-phosphonate monoester analogues
2-4 undergo easy oxidation with iodine without prior neutral-
(7) Brill, W. K.-D.; Yau, E. K.; Caruthers, M. H. Tetrahedron Lett. 1989,
30, 6621–6624.
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10.1021/jo8006072 CCC: $40.75
Published on Web 05/29/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 5029–5038 5029

Bartoszewicz et al.
SCHEME 1
SCHEME 2
counterparts 1,11 and thus oxidation of H-phosphonothioates 2
could proceed via path A as shown in Scheme 2. In this scenario,
the initially formed phosphorothioite dianion 5 reacts with iodine
to form phosphorothioiodidate 6 which, via elimination of a
good leaving group (iodide), collapses to metathiophosphate 8.
Another possibility, although less likely, is that phosphorothio-
iodidate 6 might undergo direct bimolecular displacement of
iodide by a nucleophile to form the final product of the reaction,
the corresponding phosphorothioate monoester 9 (Scheme 2,
path C).
However, one can also envisage another mechanism that
avoids, probably unfavorable, formation of phosphite dianion
5. Since P(V) compounds with the P-S-X functions (X ) Br,
Cl)12 are known, and intermediacy of species containing the
P-S-I bond system is assumed in various iodine-promoted
desulfurizations of phosphorothioate diesters,13,14 a mechanism
involving initial oxidation of sulfur, to form an intermediate of
type 7 containing the S-I bond, appears as a viable alternative
(Scheme 2, path B). Since the P-H bond in 7 is likely to be
significantly more acidic than that in the starting H-phospho-
nothioate 2, elimination of hydrogen iodide should easily occur
to produce the same reactive intermediate as that formed via
path A, metathiophosphate 8.
In this paper we present our mechanistic and synthetic studies
on oxidation of H-phosphonate monoester analogues of type
2-4 with iodine. Several different techniques, including com-
petition experiments, stereochemical studies, and trapping
experiments, were used to provide evidence for the intermediacy
of the corresponding metathiophosphates, metadithiophosphates,
and metaselenophosphates in these reactions. Also, additional
experiments were carried out to elucidate the most probable
way in which these intermediates are formed (path A vs path B
in Scheme 2). Finally, a number of preparative examples of
oxidative couplings of nucleoside H-phosphonothioate and its
dithio and seleno analogues with hydroxylic compounds are also
described.
ization of negative charges was significant on three counts. First,
it may suggest that, in contradistinction to H-phosphonate
monoester 1, for the thio and seleno analogues 2-4, a new,
low-energy pathway for the reaction with electrophiles exists.
Second, since on theoretical grounds oxidation of 2-4 might
occur with intermediacy of the corresponding metaphosphate
analogues, it was tempting to experimentally verify this as-
sumption. If true, this may provide a convenient way to generate
otherwise difficultly accessible metathio-, metadithio-, and
metaselenophosphates for mechanistic studies, under exceed-
ingly mild reaction conditions. Third, the thio-, dithio-, and
selenometaphosphate analogues were expected to be powerful
phosphorylating species, and thus a direct oxidation of 2-4 with
iodine could be developed into a one-pot preparative oxidative
phosphorylation of alcohols, for the synthesis of phosphorothio-
ate, phosphorodithioate, and phosphoroselenoate diesters.
For oxidation of H-phosphonate diesters, iodine is commonly
used9 and a mechanism of this reaction was studied in detail
(Scheme 1a).10 In the first step of this reaction, the phosphorus-
bound hydrogen is abstracted by a base and the tervalent
phosphite anion formed reacts with iodine to produce the
corresponding phosphoroiodidate intermediate, which may
undergo various SN2(P)-type reactions. As was mentioned
above, in the instance of H-phosphonate monoesters (1, Scheme
1b) the reaction with iodine does not occur, because abstraction
of the proton from the P-H bond, which would lead to a
phosphite dianion, is apparently much more difficult to perform
due to the presence of negative charge. This problem is usually
overcame by a transient conversion of H-phosphonate mo-
noesters into electrically neutral species by silylation2,5 prior
to oxidation. In this context, it may come somewhat as a surprise
that H-phosphonothioate monoesters (2, Scheme 1c) are readily
oxidized by iodine, without invoking a presilylation step.
On mechanistic grounds, higher susceptibility of H-phospho-
nothioate monoesters 2 vs H-phosphonate monoesters 1 to
oxidation with iodine could be due to the higher acidity of the
P-H bond in the thio analogues 2 relative to their oxygen
Results and Discussion
Since the first reports on the possibility of a dissociative
mechanism involvement in phosphate monoester hydrolysis,15
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M.; Lopusinski, A. Angew. Chem., Int. Ed. 1982, 21, 135–136. Lopusinski, A.;
Potrzebowski, M. Phosphorus Sulfur Relat. Elem. 1987, 32, 55–64. Michalski,
J.; Mikolajczak, J.; Skowronska, A. J. Am. Chem. Soc. 1978, 100, 5386–5390.
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L. Biochemistry 1984, 23, 3443–3453. Wyrzykiewicz, T. W.; Cole, D. L. Nucleic
Acids Res. 1994, 22, 2667–2669. Lowe, G.; Tansley, G.; Cullis, P. M. J. Chem.
Soc., Chem. Commun. 1982, 595–598. Sammons, R. D.; Frey, P. A. J. Biol.
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Stawinski, J.; Stro¨mberg, R.; Henrichson, C. Tetrahedron Lett. 1986, 27, 4051–
4054. Garegg, P. J.; Lindh, I.; Regberg, T.; Stawinski, J.; Stro¨mberg, R.;
Henrichson, C. Tetrahedron Lett. 1986, 27, 4055–4058.
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M. I.; Malakhova, I. G.; Tsvetkov, E. N.; Shatenshtein, A. I.; Kabachnik, M. I.
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5030 J. Org. Chem. Vol. 73, No. 13, 2008

Generation of Metaphosphate Analogues
the intermediacy of a monomeric metaphosphate has been
implicated in numerous chemical- and enzyme-catalyzed
reactions.16–18 However, despite intense synthetic16–19 and
theoretical20 studies, the monomeric metaphosphate (a meta-
phosphate anion or its monoester) remained an elusive inter-
mediate.21 The dissociative nature of the transition state for the
phosphoryl group transfer reactions has been supported by
product analysis, kinetic studies, thermodynamic activation
parameters, heavy-atom kinetic isotope effects, structure-reactivity
correlation, and stereochemical studies.22,23 At present the
consensus probably exists that in protic media monomeric
metaphosphate most likely is not formed as a diffusible species
(stereochemical studies);24 however, in nonprotic solvents and
under conditions of poor solvation, monomeric metaphosphate
can be formed as a kinetically significant entity.25 Apart from
the parent compounds, some of the metaphosphate analogues,
e.g., metathio-,26,27 metadithio-,23,28 metaamidothiophosphates,29,30
metaphosphonates,31 and others,18 have also been investigated,
but to lesser extent.
Probing the Intermediacy of Metathiophosphate 8
during Oxidation of H-Phosphonothioates. To probe the
formation of metathiophosphate during oxidation of H-phospho-
nothioate monoesters of type 2 with iodine we have chosen ethyl
H-phosphonothioate34 (Chart 2, 2a, triethylammonium salt) as
a model compound. First, the reaction of 2a with iodine in the
presence of different alcohols was examined. When iodine (1.1
equiv) in THF was added to a solution of 2a containing 10 equiv
of an alcohol (MeOH, EtOH, i-PrOH, or t-BuOH) and Et3N (3
equiv) in the same solvent, the brown iodine coloration
immediately vanished. The 31P NMR spectra of the reaction
mixtures showed complete disappearance of the starting material
2a (55.0 ppm, dt, J ) 580 Hz, J ) 9.6 Hz) and a clean
formation of the corresponding phosphorothioate diesters (Chart
2, 10a-d). Chemical identity of the products formed was
additionally confirmed by ESI mass spectrometry (negative
mode).35
1
3
CHART 2
There have been at least two major driving forces for the
continuing interest in monomeric metaphosphate chemistry. As
a low-coordination P(V) phosphorus species (λ5σ3), monomeric
metaphosphate is interesting on theoretical grounds and its
postulated intermediacy in many reactions kindled the hope of
its isolation and characterization. On the synthetic part, high
electrophilicity at the phosphorus center of the monomeric
metaphosphate anion and its monoesters stimulated research
directed toward development of new phosphorylating agents.
Since the reactivity of metaphosphate intermediates can be
modulated by Lewis bases,8,32,33 these species (as well as their
analogues) might be convenient phosphate transferring reagents,
provided that they could be efficiently generated under mild
conditions from easily accessible precursors.
Bellow, we address some mechanistic and synthetic issues
of the reactions of H-phosphonate monoester analogues 2-4
which most likely involve intermediacy of the metathio-,
metadithio-, and metaselenophosphates.
Since it is known that sterically hindered tertiary alcohols
cannot act as nucleophiles in a bimolecular SN2(P) substitution,
phosphorylation of t-BuOH is usually considered a test for the
involvement of three-coordinate metaphosphate species (λ5σ3)
in the reaction.36 In light of this, the above experiments indicate
that during the oxidation of 2a with iodine a reaction pathway
involving formation of ethyl metathiophosphate (structure of
type 8, Scheme 2) may indeed exist. Although this seems to be
the most likely pathway for the formation of the tert-butyl ester
(10d), for 1° and 2° alcohols, the involvement of SN2(P) reaction
(path C, Scheme 2) cannot be ruled out. Since the reactions
studied were too fast to be followed by 31P NMR spectroscopy,37
we addressed this issue by carrying out competition experiments,
in which mixtures of primary and secondary alcohols with
t-BuOH were used.38 If t-BuOH and, for instance, MeOH both
(16) Westheimer, F. H. Chem. ReV. 1981, 81, 313–326.
(17) Quin, L. D. Coord. Chem. ReV. 1994, 137, 525–559.
(18) Romanenko, V. D.; Sanchez, M. Coord. Chem. ReV. 1997, 158, 275–
324.
(19) Banks, M. R.; Gosney, I.; Kilgour, D.; Cadogan, J. I. G.; Hodgson,
P. K. G. Heteroatom Chem. 1996, 7, 503–514.
(20) Florian, J.; Warshel, A. J. Phys. Chem. B 1998, 102, 719–734. Hu, C. H.;
Brinck, T. J. Phys. Chem. A 1999, 103, 5379–5386. SaintMartin, H.; Vicent,
L. E. J. Phys. Chem. A 1999, 103, 6862–6872. Bianciotto, M.; Barthelat, J.-C.;
Vigroux, A. J. Am. Chem. Soc. 2002, 124, 7573–7587.
(21) Hengge, A. C.; Onyido, I. Curr. Org. Chem. 2005, 9, 61–74.
(22) Kirby, A. J.; Jencks, W. P. J. Am. Chem. Soc. 1965, 87, 3209–3216.
Herschlag, D.; Jencks, W. P. J. Am. Chem. Soc. 1986, 108, 7938–7946.
(23) Bourne, N.; Williams, A. J. Am. Chem. Soc. 1984, 106, 7591–7596.
(24) Buchwald, S. L.; Pliura, D. H.; Knowles, J. R. J. Am. Chem. Soc. 1984,
106, 4916–4922. Bla¨ttler, W. A.; Knowles, J. R. J. Am. Chem. Soc. 1979, 101,
510–511. Admiraal, S. J.; Herschlag, D. J. Am. Chem. Soc. 2000, 122, 2145–
21.
(34) Michalski, J.; Tulimowski, Z.; Wolf, R. Chem. Ber. 1965, 98, 3006–
3009.
(25) Lowe, G.; Sproat, B. S. J. Chem. Soc., Chem. Commun. 1978, 783–
785. Cullis, P. M.; Iagrossi, A.; Rous, A. J. J. Am. Chem. Soc. 1986, 108, 7869–
7870.
(35) Isolation not attempted. 31P NMR (THF) and MS data: 10a, δP 59.7
ppm, m, HRMS m/z 154.9930 ([M-H]-, C3H8O3PS- calcd 154.9937); 10b, δP
58.2 ppm, qu, 3J ) 8.2 Hz, HRMS m/z 169.0100 ([M-H]-, C4H10O3PS- calcd
3
(26) Cullis, P. M.; Misra, R.; Wilkins, D. J. J. Chem. Soc., Chem. Commun.
1987, 1594–1596.
169.0094); 10c, δP 55.9 ppm, q, J ) 8.6 Hz, HRMS m/z 183.0270 ([M-H]-,
C5H12O3PS- calcd 183.0250); 10d, δP 49.8 ppm, t, 3J ) 8.0 Hz, HRMS m/z
(27) Quin, L. D.; Hermann, P.; Jankowski, S. J. Org. Chem. 1996, 61, 3944–
3950.
197.0401 ([M-H]-, C6H14O3PS- calcd 197.0407).
(36) Ramirez, F.; Marecek, J. F.; Yemul, S. S. J. Am. Chem. Soc. 1982,
104, 1345–1349.
(28) Yoshifuji, M.; An, D. L.; Toyota, K.; Yasunami, M. Tetrahedron Lett.
1994, 35, 4379–4382.
(37) The most reliable method to check whether more reactive alcohols also
choose to react via metaphosphate or not are kinetic studies. Since an alcohol
molecule is not involved in the formation of metathiophosphate 8, neither in
path A nor in path B (Scheme 2), the rates of these reactions were expected to
be independent of the alcohol concentration. On the other hand, if the bimolecular
displacement occurs (Scheme 2, path C) the overall reaction rate should display
first-order dependence on the alcohol concentration. See, e.g., ref 31.
(38) Jankowski, S.; Quin, L. D. J. Am. Chem. Soc. 1991, 113, 7011–7017.
(29) Harger, M. J. P.; Pater, C. Chem. Commun. 1999, 931–932.
(30) Harger, M. P. J.; Jones, D. K. Chem. Commun. 1999, 339–340.
(31) Nonas, T.; Jankowski, S. J. Org. Chem. 2005, 70, 562–566.
(32) Ramirez, F.; Marecek, J. F. Tetrahedron 1979, 35, 1581–1589. Quin,
L. D.; Bourdieu, C.; Quin, G. S. Tetrahedron Lett. 1990, 31, 6473–6476.
(33) Knorre, D. G.; Lebedev, A. V.; Levina, A. S.; Rezvukhin, A. I.; Zarytova,
V. F. Tetrahedron 1974, 30, 3073–3079.
J. Org. Chem. Vol. 73, No. 13, 2008 5031

Bartoszewicz et al.
FIGURE 2. Possible explanation for a partial stereospecificity observed
during oxidative phosphorylations in neat alcohols.
formed. If a planar metaphosphate structure was involved,
racemization of a chiral starting material was expected, while a
bimolecular substitution should lead to inversion of configuration
at the phosphorus center.16
Although separation of ethyl H-phosphonothioate 2a into
enantiomers should be a feasible task,29 we decided to use for
our stereochemical studies another model compound, namely
nucleoside H-phosphonothioate 2b (Chart 2).42 Since 2b
contains a chiral D-deoxyribose moiety, the 31P NMR signals
originating from its stereoisomers possessing RP and SP con-
figurations have distinct chemical shifts and the same is valid
for the corresponding phosphorothioate diesters 11a-d (Chart
2). This should enable very convenient analysis of stereochem-
ical outcomes of oxidative phosphorylations of alcohols with
H-phosphonothioate 2b with use of 31P NMR spectroscopy.
When pure RP and SP diastereoisomers of nucleoside H-
phosphonothioate 2b were reacted separately with iodine (1.1
equiv), in presence of Et3N (3 equiv), in t-BuOH as a solvent,
the reactions showed only negligible stereospecificity for the
formation of phosphorothioate 11d (ca. 10% de). Analogous
experiments carried out in neat MeOH proceeded also with
extensive epimerization, but with noticeably higher stereospeci-
ficity (11a, ca. 25% de).43 The significant erosion of stereo-
chemistry that accompanied these reactions suggested metathio-
phosphate of type 8 as a major intermediate during oxidation
of 2b with iodine.44 However, the observed partial stereospeci-
ficity also deserves some comments.
FIGURE 1. Plot of the ratio of phosphorothioate diesters 10d:10a-c
against the t-BuOH/ROH concentration. The ordinate values were
obtained from integration of 31P NMR signals originating from the
products.
react only with the intermediacy of metathiophosphates of type
8, the relationship between the alcohol composition taken to
the reaction and the ratios of the corresponding products formed
(10d vs 10a) should be linear. In other words, the selectivity of
the reaction in such a case should be independent of the ratio
of the alcohols used. If, however, the more reactive alcohol (e.g.,
MeOH) could form the corresponding phosphorothioate diester
also via the SN2(P) process (path C in Scheme 2), this should
be manifested in a nonlinear plot of the products ratio vs alcohols
ratio.
The competition experiments were carried out with mixtures
of neat alcohols as reaction media, to maximize the probability
of the bimolecular substitution (path C, Scheme 2).27,29,39,40
Negative results of these experiments would then also exclude
the possibility of participation of a SN2(P) pathway in dilute
reaction solutions. To this end MeOH, EtOH, and i-PrOH were
mixed with t-BuOH in three different ratios and these mixtures
were used as reaction media for oxidative couplings with
H-phosphonothioate monoester 2a. The results in Figure 1
clearly show that in all three cases a linear relationship was
obeyed. This indicates that more nucleophilic alcohols also
reacted via the elimination-addition pathway and the contribu-
tion of a direct SN2(P) reaction was apparently negligible even
for MeOH. The inverse of the slopes in Figure 1 represents
relative selectivity for a given alcohol vs t-BuOH, and the values
obtained for the reactions investigated were 8.1, 6.9, and 2.5
for MeOH, EtOH, and i-PrOH, respectively. Such small numbers
point to small steric effects and are in line with little (if any)
involvement of sterically sensitive SN2(P) processes.31,41
To support further the intermediacy of metathiophosphate 8
during oxidation of H-phosphonothioates of type 2 with iodine,
we carried out some stereochemical studies.
Although a contribution of the SN2(P) process (path C,
Scheme 2) could account for certain de of the product with
inverted configuration (at least in the instance of the reaction
with MeOH), a bimolecular substitution at the phosphorus center
for t-BuOH seemed unlikely. A more likely explanation for the
residual stereospecificity observed could be that in the presence
of a high concentration of nucleophiles (neat alcohols), met-
athiophosphate intermediates of type 8 were not formed as
completely free, diffusible species, but were attacked by
nucleophiles also in a solvent cage, on the face opposite to that
still shielded by iodide (path A, Scheme 2) or by the triethy-
lammonium cation (path B, Scheme 2) (Figure 2). This
interpretation is consistent with the fact that a certain degree of
stereospecificity was observed for the reaction in neat t-BuOH,
and is also in line with the results of the competition experiments
that, to high probability, ruled out a direct nucleophilic
displacement at the phosphorus center. Since 2b-RP and 2b-SP
are diastereomers, one cannot completely exclude the possibility
of a certain amount of stereochemical influence from the chiral
nucleoside moiety; however, this phenomenon is usually
By performing these oxidative phosphorylations with an
enantiomerically pure H-phosphonothioate monoester, it should
be possible to distinguish between bimolecular and unimolecular
mechanisms by examining the stereochemistry of the product
(42) Hiresova, R.; Putilova, L.; Kullberg, M.; Stawinski, J. Collect. Symp.
Ser. 2005, 7, 201–205.
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M. H. J. Organomet. Chem. 1997, 529, 23–27.
(43) 31P NMR data: 11a, δP 58.8, 58.7 (neat MeOH), 59.6, 59.4 (THF), 59.0,
58.8 ppm (MeCN); 11d, δP 51.0, 50.9 (neat t-BuOH), 50.7, 50.5 (THF), 51.3,
51.2 ppm (MeCN).
(44) Friedman, J. M.; Freeman, S.; Knowles, J. R. J. Am. Chem. Soc. 1988,
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(41) Harger, M. J. P. Chem. Commun. 2004, 17, 1952–1953.
5032 J. Org. Chem. Vol. 73, No. 13, 2008

Generation of Metaphosphate Analogues
negligible for reactions at the phosphorus center for DNA
derivatives.45,46
283.0657; [M + pyridineH]+, C12H16N2O2PS+ calcd 283.0665).
Compound 14 (R ) Et) appeared to be rather resistant toward
alcohols, and at least 10 equiv of MeOH, EtOH, or i-PrOH had
to be used to cleanly transform it into the phosphorothioate
diesters 10a-c. The reaction with t-BuOH was sluggish and
resulted in significant side-product formation. As was reported
previously, the pyridine adduct of metathiophosphate 14 reacted
efficiently only with good nucleophiles, e.g., fluoride ions8 or
amines.51
Ethyl metathiophosphate-DABCO adduct 15 (R ) Et) was
generated analogously to that of 14, by reacting H-phosphono-
thioate 2a with iodine (1.1 equiv) in THF in the presence of
DABCO (10 equiv). Adduct 15 (R ) Et) was cleanly formed
under these conditions, as indicated by the 31P NMR spectra
and MS analysis (δP ) 65.8 ppm; HRMS m/z 349.1813 ([M +
DABCOH]+, C14H30N4O2PS+ calcd 349.1822). Compound 15
(R ) Et) was rather stable in the reaction mixture; however,
upon treatment with an excess of alcohols (10 equiv), it was
readily transformed into the corresponding phosphorothioate
diesters (10a-d).
It has been postulated on several occasions that highly
electrophilic metaphosphate and its analogues should form
adducts with Lewis bases, including solvent molecules pos-
sessing lone electron pairs.16,17,44,47,48 Formation of such species,
in which fast exchange of the coordinated solvent molecules
may occur prior to the reaction with a nucleophile, was
advocated as an efficient mechanism for racemization in the
reactions involving metaphosphate intermediates.44 Consistent
with these, when oxidative phosphorylation of MeOH or
t-BuOH (10 equiv) with pure diastereoisomers of nucleoside
H-phosphonothioate 2b was carried out in THF or MeCN as
solvents, 100% epimerization at the phosphorus center was
observed.43 Structures of such putative adducts of metathio-
phosphate of type 8 with THF and MeCN are shown in Chart
3 (12 and 13, respectively).
CHART 3
To sum up this part, the chemical and stereochemical
experiments discussed above provided strong support for the
intermediacy of monomeric metathiophosphate 8 in the inves-
tigated oxidative phosphorylations, and thus the reaction of
H-phosphonothioate monoesters with iodine may constitute a
novel, efficient method for generation of this electrophilic
phosphorus species.
Toward the Mechanism of Metathio-, Metadithio-, and
Metaselenophosphate Formation. Althoug, the studies de-
scribed so far provided strong support for the intermediacy of
metathiophosphate in the discussed reactions, they did not
address the question on which pathways these species were
formed. We considered two plausible pathways, path A and path
B, that may lead to metathiophosphate 8 (Scheme 2) and in
this context two issues arise: (i) why is a particular mechanism
followed in preference to the other mechanisms and (ii) what
is the nature of the transition from one mechanism to another
as reactants or conditions are changed. In our case, the answer
to these questions should clarify the issue, why H-phosphonate
monoesters of type 1 are resistant to oxidation with iodine, while
the other chalcogen analogues (compounds of type 2-4)
undergo rapid oxidation under such conditions.
As was mentioned in the Introduction, the affinity of iodine
to sulfur is a known phenomenon, which originates from the
softness of both atoms.13,14 This should favor a mechanism
involving initial iodination of the sulfur in compound 2 (path
B, Scheme 2). On the other hand, increased acidity of the
P-H bond in H-phosphonothioates, compared to their oxygen
counterparts,11 may indicate a mechanistic route that com-
mences with the generation of phosphite dianion 5, followed
by its reaction with iodine at the phosphorus center (path A,
Scheme 2).
Although the above results indicated the formation of
structures of type 12 and 13 during the course of the reactions,
we were not able to observe these intermediates by 31P NMR
spectroscopy, even at -80 °C. In the absence of alcohols, the
addition of iodine to a solution of ethyl H-phosphonothioate
2a in THF or MeCN resulted in an intractable mixture of
products (31P NMR experiments) suggesting polymerization of
the metathiophosphate intermediate.
However, since metaphosphates are known to form stable
adducts with tertiary amines,8,16,17,33,49,50 we attempted to
generate analogous species from the putative metathiophosphate
8 and pyridine or 1,4-diazabicyclo[2.2.2]octane (DABCO)
(Chart 3, compounds 14 and 15). To this end, a solution of
iodine (1.1 equiv) in pyridine was added to ethyl H-phospho-
nothioate 2a in the same solvent, at room temperature. The 31
P
NMR spectrum of the reaction mixture showed an immediate
disappearance of the starting material 2a, and a clean formation
of a product resonating at 56.0 ppm, which on the basis of its
chemical shift was assigned to the anticipated ethyl
metathiophosphate-pyridine adduct 14 (R ) Et).8 To verify
this structural assignment, the reaction mixture was analyzed
by ESI mass spectrometry, which indeed, among other signals,
revealed the expected mass peak of this adduct (HRMS: m/z
(51) Kers, I.; Stawinski, J.; Kraszewski, A. Tetrahedron Lett. 1998, 39, 1219–
1222.
(52) Heath, D. F. J. Chem. Soc. 1956, 379, 3796–3804. Heath, D. F. J. Chem.
Soc. 1956, 3804–3809. Cruden, E. W.; Hudson, R. F. J. Chem. Soc. 1962, 3591–
3599. Gerrard, A. F.; Hamer, N. K. J. Chem. Soc. B 1967, 1122–1125. Gerrard,
A. F.; Hamer, N. K. J. Chem. Soc. B 1968, 539–543. Maynard, J. A.; Swan,
J. M. Proc. Chem. Soc. London 1963, 61–62. Maynard, J. A.; Swan, J. M. Aust.
J. Chem. 1963, 16, 596–608. Kenyon, G. L.; Westheimer, F. H. J. Am. Chem.
Soc. 1966, 88, 3557–3561. Kenyon, G. L.; Westheimer, F. H. J. Am. Chem.
Soc. 1966, 88, 3561–3566. Katzhendler, J.; Karaman, R.; Gibson, D.; Breuer,
E.; Leader, H. J. Chem. Soc., Perkin Trans. 2 1989, 2, 589–594. Banks, M. R.;
Cadogan, J. I. G.; Gosney, I.; Hodgson, P. K. G.; Kilgour, D.; Rodger, D. R.
Phosphorus Sulfur Silicon 1993, 76, 37–40. Kehler, J.; Breuer, E. Chem.
Commun. 1997, 1751–1752.
(45) Almer, H.; Stawinski, J.; Stro¨mberg, R. J. Chem. Soc., Chem. Commun.
1994, 1459–1460.
(46) Stawinski, J.; Thelin, M. J. Org. Chem. 1994, 59, 130–136.
(47) Chanley, J. D.; Feageson, E. J. Am. Chem. Soc. 1958, 80, 2686–2691.
(48) Quin, L. D.; Bourdieu, C.; Quin, G. S. Tetrahedron Lett. 1990, 31, 6473–
6476.
(49) Ramirez, F.; Marecek, J. F. Tetrahedron 1979, 35, 1581–1589.
(50) Meisel, M.; Grunze, H. Z. Anorg. Allg. Chem. 1968, 360, 277–283.
J. Org. Chem. Vol. 73, No. 13, 2008 5033

Bartoszewicz et al.
SCHEME 3
SCHEME 4
FIGURE 3. Reaction of ethyl H-phosphonothioate (2a) with EtOH,
promoted by different oxidants.
4) was observed (31P NMR experiments)58 under these reaction
conditions. Since H-phosphonate 1a remained unchanged even
after prolonged reaction time (1 h), it seems that at least one
chalcogen atom, possessing high affinity to a soft electrophile
(iodine), i.e., sufur13,14 or selenium,14,46 must be attached to
the phosphorus center to permit easy oxidation into P(V)
compounds.
To check if the reactions of H-phosphonothioate 2b, H-
phosphonodithioate 3a, and H-phosphonoselenoate 4a with
iodine proceeded via the same type of intermediate, i.e., the
corresponding metathio-, metadithio-, and metaselenophosphate,
in separate experiments we attempted to trap these species in
the form of the corresponding pyridine adducts. Indeed, upon
addition of iodine into solutions of 2b, 3a, and 4a in pyridine,
31P NMR spectroscopy revealed formation of the expected
metaphosphate adducts of type 14 (R ) 5′-O-dimethoxytri-
An intriguing feature of path B is that it would constitute a
new mechanistic pathway in which metaphosphate analogues
could be formed. Usually metaphosphates and their analogues
are generated from P(V) derivatives bearing a good leaving
group and an internal nucleophile as is shown in Scheme
3.16,17,27,39,49,52 For cyclic phosphorus compounds, Z and X are
part of a ring system, and fragmentation to metaphosphates
occurs under pyrolytic conditions.38,53 Other methods of meta-
phosphate generation, such as fragmentations of phosphoryl
radicals,54 mixed phosphoric-carboxylic anhydrides55 or 2,4-
bis(4-methoxyphenyl)-2,4-dithioxo-1,3,2,4-dithiadiphosphet-
ane (Lawesson reagent),38 also have been developed; however,
their applicability is limited to specific metaphosphate analogues.
In contradistinction to all these methods, generation of
metathiophosphate 8 via path B (Scheme 2) occurs in an
oxidation step. Mechanistically, this means that iodine oxidizes
first sulfur in H-phosphonothioate 2 to form iodosulfenyl
derivative 7, and this P(III) intermediate undergoes internal
disproportionation that leads to oxidation of the phosphorus atom
and generation of metathiophosphate 8.
3
tylthymidin-3′-yl, δP ) 58.7 ppm, t, J ) 8.9 Hz), 16 (R )
3
5′-O-dimethoxytritylthymidin-3′-yl, δP ) 118.5 ppm, t, J )
9.7 Hz), and 17 (R ) 5′-O-dimethoxytritylthymidin-3′-yl, δP
3
) 53.5 ppm, t, J ) 9.6 Hz), respectively (Chart 3).
Although these experiments supported, in principle, path B
(oxidation of the chalcogen atoms with iodine as an initial
reaction step), we could not rule out path A, since the presence
of sulfur or selenium at the phosphorus center usually increases
the acidity of the P-H protons (vide supra).
To evaluate the importance of the affinity of iodine to sulfur
and selenium in these reactions, we turned our attention to other
oxidants, the electrophilic centers of which represented a
spectrum in terms of hardness and softness (Figure 3). Thus, in
addition to bromine, different carbon tetrahalides, which are
known as efficient oxidizing agents used in the Atherton-Todd
reaction, were tested.59
As a model compound, ethyl H-phosphonothioate monoester
2a was chosen, and it was allowed to react with EtOH (10 equiv)
in the presence of Et3N (3 equiv), and the appropriate oxidant
(1.1 equiv of X2 or 3 equiv of CX4) in THF, at room temperature
(Figure 3). Progress of the reactions was followed by 31P NMR
spectroscopy.
Below we present experiments which, collectively, may lend
support for this new mechanism for the generation of metathio-,
metadithio-, and metaselenophosphate.
To have uniform experimental data concerning the suscep-
tibility of H-phosphonate monoesters of type 1, and their thio
(2), dithio (3), and seleno (4) analogues (Chart 1) to oxidation
with iodine, we investigated the reactions of H-phosphonate
(1a),56 H-phosphonothioate (2b), H-phosphonodithioate (3a),7
and H-phosphonoselenoate (4a)57 with EtOH (10 equiv), in the
presence of iodine (1.1 equiv) and Et3N (3 equiv), in THF at
room temperature (Scheme 4). For all H-phosphonate derivates
but 1a, a rapid (<15 s) and clean transformation into the
corresponding phosphodiester analogues (11b, 18, 19; Scheme
As is apparent from data in Figure 3, all the oxidants
investigated, except CCl4, successfully promoted oxidative
thiophosphorylation of ethanol. CCl4 is known as a donor of
hard, electrophilic chlorine, which can efficiently oxidize
H-phosphonate diesters.60 Additionally, it was also reported that
(53) Quin, L. D.; Sadanani, N. D.; Wu, X. P. J. Am. Chem. Soc. 1989, 111,
6852–6853.
(58) Isolation not attempted. 31P NMR (THF) and MS data: 11b, δP 57.3,
57.0 ppm, HRMS m/z 667.1913 ([M-H]-, C33H36N2O9PS- calcd 667.1885);
(54) Avila, L. Z.; Forst, J. W. J. Am. Chem. Soc. 1988, 110, 7904–7906.
(55) Symes, J.; Modro, T. A. Can. J. Chem. 1986, 64, 1702–1708.
(56) Jankowska, J.; Sobkowski, M.; Stawinski, J.; Kraszewski, A. Tetrahedron
Lett. 1994, 35, 3355–3358.
-
18, δP 113.5 ppm, HRMS m/z 683.1623 ([M-H]-, C33H36N2O8PS2 calcd
683.1656);19,δP 52.4,52.2ppm,HRMSm/z715.1278([M-H]-,C33H36N2O9PSe-
calcd 715.1329).
(57) Bollmark, M.; Kullberg, M.; Stawinski, J. Tetrahedron Lett. 2002, 43,
515–518. Kullberg, M.; Stawinski, J. Synthesis 2005, 1668–1674.
(59) Atherton, R. F.; Todd, A. R. J. Chem. Soc. 1947, 674–678.
(60) Steinberg, G. M. J. Org. Chem. 1950, 15, 637–647.
5034 J. Org. Chem. Vol. 73, No. 13, 2008

Generation of Metaphosphate Analogues
SCHEME 5
SCHEME 6
CCl4 is able to oxidize H-phosphonate monoesters of type 1,
provided that the monoalkyl phosphite dianion could be
generated (reflux in neat Et3N).61 Thus, lack of any reaction of
H-phosphonothioate 2a with this oxidant, and a rapid oxidation
of 2a with all soft halogen donors (I2, Br2, CBr4 and CI4),
indicates that sulfur-halogen interaction is probably crucial for
the reaction to proceed.
To have some insight into the acidity of the P-H bond of
H-phosphonothioate monoester 2a, a simple proton-deuterium
exchange experiment in neat D2O, catalyzed by Et3N, was
carried out (Scheme 5). To this end 2a and Et3N (3 equiv) were
dissolved D2O and the proton-deuterium exchange process was
followed with 31P NMR spectroscopy.62 It was expected that
in neat D2O, every ionization of 2a to form dianion 5a will
result in the incorporation of the deuterium. Since after 1 h only
ca. 50% of the starting material 2a became deuterated, this
means that generation of phosphorothioite dianion 5a was
relatively slow. Taking into account that oxidation of H-
phosphonothioate 2a with iodine in THF takes a few seconds,
it is rather unlikely that it may proceed via path A (Scheme 2),
i.e., with the intermediacy of phosphite dianion 5.
Development of Preparative Protocols for Oxidative
Phosphorylations with H-Phosphonothioate, H-Phosphon-
odithioate, and H-Phosphonoselenoate Monoesters. Having
established the mechanistic bases of the investigated reac-
tions, we wanted to determine scope and limitations of these
transformations for the purpose of practical, preparative thio-,
dithio-, and selenophosphorylation. As was shown in the
previous sections, a number of phosphorothioate diesters
could be obtained from ethyl H-phosphonothioate 2a (10a-d;
Chart 2) or from nucleoside H-phosphonothioate 2b (11a,
11b, 11d; Chart 2) via oxidative thiophosphorylation pro-
moted by iodine. Also nucleosides H-phosphonodithioate (3a)
and H-phosphonoselenoate (4a) were successfully used for
oxidative phosphorylation of ethanol, producing the corre-
sponding phosphorodithioate (18) and phosphoroselenoate
(19) diesters (Scheme 4).
Due to the growing interest in nucleotide analogues bearing
single or multiple modifications at the phosphorus center (e.g.,
phosphorothioates, phosphorodithioates, phosphoroselenoates,
etc.) as tools in biochemistry and medicinal chemistry,63 we
decided to focus on optimization of the reaction conditions for
the preparation of such nucleotide analogues via oxidative
phosphorylation. Attractive features of nucleoside H-phospho-
nothioate monoesters of type 2, H-phosphonodithioates 3, and
H-phosphonoselenoates 4 (Chart 1) as substrates for oxidative
phosphorylation are that these compounds are readily available
and, in contrast to H-phosphonate diester derivatives, can
transfer unprotected thiophosphoryl, dithiophosphoryl, and
selenophosphoryl groups to hydroxylic compounds (vide supra).
First, using nucleoside H-phosphonothioate monoester 2b as
a model compound, different solvents were screened for their
efficiency as reaction media for the oxidative phosphorylations
investigated. For this purpose, H-phosphonothioate 2b was
allowed to react with iodine (1.1 equiv), in the presence of
MeOH (10 equiv) and Et3N (3 equiv), in THF, dioxane, MeCN,
pyridine, toluene, and CH2Cl2 as solvents. It was found that in
the instance of THF, dioxane, MeCN, and pyridine, a clean and
rapid formation of phosphorothioate diester 11a (Chart 2)
occurred (31P NMR analysis). In contrast to these, when the
reaction was carried out in toluene or CH2Cl2, the 31P NMR
spectra of the reaction mixtures showed a large number of
signals (∼20) of different intensity, indicating polymerization
of the generated metathiophosphate and/or its reaction with the
thymine nucleobase.16 On this basis we tentatively concluded
that supporting solvents for oxidative phosphorylations in which
metaphosphate-type of intermediates are involved must have a
Lewis base character (e.g., THF, dioxane, MeCN, pyridine).
These solvent are apparently not passive during the course of
the reaction, but play an important stabilizing role for the
generated metathiophosphate intermediates, forming the corre-
sponding adducts (e.g., 12, 13, 14; Chart 3). If such a
stabilization is impossible, which is probably the case for
solvents lacking lone electron pairs (e.g., toluene, CH2Cl2), the
very reactive free metathiophosphate polymerizes before it
“finds” a nucleophile.
Since so far all the experiments on oxidative phosphorylations
were performed in the presence of 10 equiv or in neat alcohols,
we investigated if it was possible to decrease the excess of an
alcohol taken to the reaction. We found that in THF as a solvent,
nucleoside H-phosphonothioate 2b could be efficiently coupled
when 3 equiv of 1° alcohols (MeOH and EtOH), 5 equiv of 2°
alcohol (i-PrOH), or 10 equiv of 3° alcohol (t-BuOH) were used
for the reaction, to produce cleanly the corresponding phos-
phorothioate diesters (11a-d). If the amount of a given alcohol
was lower, it resulted in partial polymerization of the metathio-
phosphate and side product formation. Applying this procedure
on a preparative scale enabled isolation of pure 11a-d in
85-93% yield.
As a final stage of these synthetic studies we attempted to
use this newly developed protocol for the preparation of
compounds with modified 3′-5′ internucleotide linkages, i.e.,
dinucleoside phosphorothioate (11e), dinucleoside phospho-
rodithioate (21), and dinucleoside phosphoroselenoate (22), via
oxidative phosphorylation of 3′-O-TBDMS-thymidine (20)
(Scheme 6).
(61) Blackburn, G. M.; Cohen, J. S.; Lord, T. J. Chem. Soc. (C) 1966, 239–
245.
1
(62) 31P NMR data (D2O): 2a, δP ) 50.4 ppm; d-2a, δP ) 50.0 ppm, JP-D
) 224 Hz.
(63) Eckstein, F. Annu. ReV. Biochem. 1985, 54, 367–402. Eckstein, F. Angew.
Chem., Int. Ed. 2003, 14, 160–166. Nawrot, B.; Widera, K.; Wojcik, M.;
Rebowska, B.; Nowak, G.; Stec, W. J. FEBS J. 2007, 274, 1062–1072. Uhlmann,
E.; Peyman, A. Chem. ReV. 1990, 90, 543–584. Kurreck, J. Eur. J. Biochem.
2003, 270, 1628–1644. Wilson, C.; Keefe, A. D. Curr. Opin. Chem. Biol. 2006,
10, 607–614. Wilds, C. J.; Pattanayek, R.; Pan, C. L.; Wawrzak, Z.; Egli, M.
J. Am. Chem. Soc. 2002, 124, 14910–14916.
Somewhat surprisingly, when we added iodine (1.1 equiv)
to a THF solution containing nucleoside H-phosphonothioate
J. Org. Chem. Vol. 73, No. 13, 2008 5035

Bartoszewicz et al.
2b, 3′-protected thymidine derivative 20 (3 equiv),64 and Et3N
(3 equiv), the expected dinucleoside phosphorothioate 11e was
not formed (TLC and 31P NMR analysis). Instead, a familiar
pattern of signals, originating from polymerization of the
generated metathiophosphate intermediate, could be observed
in the 31P NMR spectra of the reaction mixture. Increasing the
excess of nucleoside 20 up to 10 equiv resulted in no
improvements.
and H-phosphonoselenoate monoesters toward iodine pointed
to the involvement of analogous species, i.e., the corresponding
metadithiophosphate and metaselenophosphate in these reac-
tions. Formation of metathiophosphates, metadithiophosphates,
and metaselenophosphates via oxidation of a phosphorus-bound
chalcogen atom by iodine constitutes a new mechanistic pathway
for generation of these types of tricoordinate P(V) phosphorus
species (λ5σ3), under exceedingly mild reaction conditions.
The potential of the developed method for generation of
different metaphosphate analogues, for preparative phosphory-
lation of hydroxylic compounds, was demonstrated in the
synthesis of various nucleoside phosphorothioate, phospho-
rodithioate, and phosphoroselenoate diesters. Since H-phospho-
nothioate, H-phosphonodithioate, and H-phosphonoselenoate
monoesters are readily available, this experimentally simple
reaction can be of practical use in the synthesis of various
phosphate analogues. As these reactions proceed with the
intermediacy of planar intermediates, i.e., the corresponding
metaphosphate derivatives, they are expected to be inherently
nonstereospecific.
A possible explanation why this reaction failed could be that
effective nucleophilicity of nucleoside 20 was too low to secure
a successful attack on a short-living metathiophosphate inter-
mediate, even if this was partially stabilized by THF. A large
size of the nucleoside molecule implies both low diffusion
coefficient (slow diffusion) and long correlation time (slow
rotation),65 which together with a steric hindrance can lower
the ability of 20 to capture the metathiophosphate intermediate.
To remedy this problem, we attempted to increase a lifetime
of the metathiophosphate adduct by carrying out the reaction
in pyridine, which forms much more stable adducts with
metathiophosphates (Chart 3, 14). Under such reaction condi-
tions, dinucleoside phosphorothioate 11e was indeed formed,
but unfortunately, the reaction was slow (ca. 3 h) and resulted
in significant side product formation. Using DABCO in THF
as the reaction media (intermediate of type 15; Chart 3) did not
improve the outcome of the reaction. These experiments
indicated that the reaction proceeding via metaphosphate-type
intermediates requires fine-tuning of the reactivity of the
metaphosphate adducts formed to achieve satisfactory results.
Since metaphosphate adducts of type 14 and 15 were apparently
not reactive enough toward nucleoside 20, we investigated once
again the THF adduct of type 12, but to lower its reactivity,48
this time we performed the oxidative thiophosphorylation of
20 at -78 °C. It was rewarding to find that under such reaction
conditions dinucleoside phosphorothioate 11e was rapidly
formed (ca. 30 s), but for the reaction to proceed cleanly, it
was necessary to use 10 equiv of nucleoside 20. The same
protocol was applied to nucleoside H-phosphonodithioate mo-
noester 3a and nucleoside H-phosphonoselenoate monoester 4a,
which upon the reaction with nucleoside 20 furnished cleanly
dinucleoside phosphorodithioate 21 and dinucleoside phospho-
roselenoate 22, respectively, in good isolated yields.66
Experimental Section
General Procedure for Oxidative Esterification of 5′-O-
Dimethoxytritylthymidin-3′-yl H-Phosphonothioate (2b, Tri-
ethylammonium Salt) with Simple Alcohols. To a solution of
2b42,67 (145 mg, 0.2 mmol), an alcohol (MeOH: 24 µL, 19 mg,
0.6 mmol; EtOH: 35 µL, 28 mg, 0.6 mmol; i-PrOH: 76 µL, 60
mg, 1 mmol; t-BuOH: 190 µL, 148 mg, 2 mmol), and Et3N (83
µL, 61 mg, 0.6 mmol) in THF (3 mL) was added dropwise a
solution of iodine (56 mg, 0.22 mmol) in the same solvent (3 mL)
with vigorous stirring. After the addition was complete (ca. 1 min),
the solvent was evaporated, the residue was partitioned between
CH2Cl2 and 5% aq NaHCO3, and the aqueous phase was once more
extracted with CH2Cl2. The combined organic layers were dried
over Na2SO4 and evaporated to dryness. The products were purified
by silica gel column chromatography, using a stepwise gradient of
MeOH (0-5%) in CH2Cl2, containing 0.02% Et3N.
5′-O-Dimethoxytritylthymidin-3′-yl Methyl Phosphorothio-
ate, Triethylammonium Salt (11a). White powder, 141 mg (93%),
1
1:1 mixture of diastereoisomers. H NMR (400 MHz, CDCl3): δ
11.87 (1H, b, (CH3CH2)3NH+), 8.81 and 8.78 (1H, 2 × s, H3),
7.62 and 7.60 (1H, 2 × s, H6), 7.43-6.77 (13H, m, DMT protons),
6.44 (1H, m, H1′), 5.29 (1H, m, H3′), 4.36 and 4.31 (1H, 2 × m,
H4′), 3.77 (6H, s, 2 × OCH3), 3.60 and 3.54 (3H, 2 × d, CH3OP,
JCH3-P ) 13.0 Hz), 3.46-3.29 (2H, m, H5′, H5′′), 3.08 (6H, q,
Conclusions
(CH3CH2)3NH+, JCH -CH ) 7.4 Hz), 2.77-2.59 (1H, m, H2′),
3
2.48-2.23 (1H, m,2 H2′′), 1.36 (3H, s, CH3), 1.32 (9H, t,
In summary, the mechanism of oxidation of H-phosphono-
thioate monoesters with iodine was examined in detail. The
results of different diagnostic experiments, including reaction
with tertiary alcohol, competition experiments, stereochemical
studies, and trapping the intermediates in the form of observable
adductswithamines,wereconsistentwiththeelimination-addition
process and supported the notion that these reaction proceeded
with the intermediacy of a metathiophosphate monoester.
Additional studies have shown that the key step of the
transformation was oxidation of the sulfur atom of the H-
phosphonothioate monoester by iodine to produce a iodosulfenyl
intermediate that collapsed to the corresponding metathiophos-
phate derivatives. A similar behavior of H-phosphonodithioate
(CH3CH2)3NH+, JCH -CH ) 7.4 Hz). 13C NMR (100 MHz, CDCl3):
3
2
δ 163.8 and 163.7 (C4), 158.6 (C4DMT, C4′DMT), 150.6 and 150.5
(C2), 144.37 and 144.35 (C1′′DMT), 135.8, 135.7, 135.6, 135.5, 135.4
and 135.3 (C6, C1DMT, C1′DMT), 130.1 (C2DMT, C6DMT, C2′DMT
,
C6′DMT), 128.2 and 127.9 (C2′′DMT, C3′′DMT, C5′′DMT, C6′′DMT),
127.0 (C4′′DMT), 113.2 (C3DMT, C5DMT, C3′DMT, C5′DMT), 111.3 and
111.2 (C5), 87.01 and 86.97 (Ar3CDMT), 85.5 and 85.1 (2 × d, C4′,
J4′-P ) 6.0 Hz), 84.6 and 84.5 (C1′), 76.6 and 76.4 (2 × d, C3′,
J3′-P ) 5.8 Hz), 64.1 and 63.8 (C5′), 55.20 and 52.18 (2 × OCH3),
52.96 and 52.93 (2 × d, CH3OP, JCH -P ) 6.1 Hz), 45.7
3
((CH3CH2)3NH+), 39.7 and 39.5 (2 × d, C2′, J2′-P ) 4.2 Hz), 11.6
and 11.5 (CH3), 8.6 ((CH3CH2)3NH+). 31P NMR (162 MHz,
CDCl3): δ 57.3 and 57.1. HRMS: m/z 695.2181 ([M - H]-,
C35H40N2O9PS- calcd 695.2198).
(64) Boehringer, M. P.; Graff, D.; Caruthers, M. H. Tetrahedron Lett. 1993,
34, 2723–2726.
5′-O-Dimethoxytritylthymidin-3′-yl Ethyl Phosphorothioate,
Triethylammonium Salt (11b). White powder, 134 mg (87%), 1:1
(65) Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry;
University Science Books: Sausalito, CA, 2006; pp 155-157.
(66) Excess of nucleosidic component 20 could be efficiently recovered during
chromatography (see the Supporting information).
(67) Stawinski, J.; Thelin, M.; Westman, E.; Zain, R. J. Org. Chem. 1990,
55, 3503–3506.
5036 J. Org. Chem. Vol. 73, No. 13, 2008

Generation of Metaphosphate Analogues
1
mixture of diastereoisomers. H NMR (400 MHz, CDCl3): δ 8.96
C6′DMT), 128.18, 128.13, and 127.9 (C2′′DMT, C3′′DMT, C5′′DMT,
(1H, b, H3), 7.61 and 7.60 (1H, 2 × q, H6, J6-CH ) 1.1 Hz),
C6′′DMT), 126.9 (C4′′DMT), 113.2 (C3DMT, C5DMT, C3′DMT, C5′DMT),
111.1 and 111.0 (C5), 86.9 (Ar3CDMT), 85.6 and 85.0 (2 × d, C4′,
J4′-P ) 6.7 Hz), 84.72 and 84.68 (C1′), 79.6 and 79.5 (2 × d,
(CH3)C, JC-P ) 9.2 Hz), ∼76.8 and 76.4 (2 × d, C3′, J3′-P ) 5.3
Hz), 64.2 and 64.1 (C5′), 55.2 (2 × OCH3), 45.5 ((CH3CH2)3NH+),
39.7 and 39.3 (2 × d, C2′, J2′-P ) 3.8 Hz), 30.22 and 30.15 (2 ×
d, (CH3)3C, JCH -P ) 4.4 Hz), 11.43 (CH3), 8.1 ((CH3CH2)3NH+).
31P NMR (1623 MHz, CDCl3): δ 51.0 and 50.7. HRMS: m/z
695.2181 ([M - H]-, C35H40N2O9PS- calcd 695.2198).
General Procedure for Oxidative Esterification of 5′-
Dimethoxytritylthymidin-3′-yl H-Phosphonothioate (2b), 5′-
Dimethoxytritylthymidin-3′-yl H-Phosphonodithioate (3a), and
5′-Dimethoxytritylthymidin-3′-yl H-Phosphonoselenoate (4a)
(All Triethylammonium Salts) with 3′-O-(tert-Butyldi-
methylsilyl)thymidine (20). To a solution of 2b,42,67 3a7 or 4a57
(0.2 mmol), 2064 (714 mg, 2 mmol), and Et3N (83 µL, 61 mg, 0.6
mmol) in THF (3 mL) was added a solution of iodine (56 mg,
0.22 mmol) in the same solvent (3 mL) dropwise, at -78 °C with
vigorous stirring. After the addition was complete, the mixture was
allowed to warm to room temperature and the solvent was
evaporated. The residue was partitioned between CH2Cl2 and 5%
aq NaHCO3 and the aqueous phase was once more extracted with
CH2Cl2. The combined organic layers were dried over Na2SO4 and
evaporated to dryness. The products were isolated by silica gel
column chromatography, using a stepwise gradient of MeOH
(0-5%) in CH2Cl2, containing 0.02% Et3N. A yield of 1.3-1.5
mmol (450-550 mg) of 3′-O-(tert-butyldimethylsilyl)thymidine 20
could be recovered during the chromatography.
3
7.32-6.77 (13H, m, DMT protons), 6.44 and 6.43 (1H, 2 × dd,
H1′, J1′-2′ ) 8.1 Hz, J1′-2′′ ) 5.3 Hz), 5.28 (1H, m, H3′), 4.36 and
4.32 (1H, 2 × m, H4′), 4.07-3.84 (2H, m, CH3CH2), 3.76 (6H, s,
2 × OCH3), 3.50 (0.5H, dd, H5′, J5′-5′′ ) 10.5 Hz, J5′-4′ ) 2.8
Hz), 3.45-3.36 (1.5H, m, H5′, H5′′), 2.97 (6H, m, (CH3CH2)3NH+,
JCH -CH ) 7.3 Hz), 2.66 (1H, m, H2′), 2.35 (1H, m, H2′′), 1.36
and2 1.33 (3H, 2 × d, CH3, JCH -6 ) 1.1 Hz), 1.28-1.21 (10.5H,
3
3
apparent t, (CH3CH2)3NH+ and CH3CH2, J(CH -CH ) NH ) 7.2 Hz),
+
3
2 3
1.14 (1.5H, CH3CH2, JCH -CH ) 7.1 Hz). 13C NMR (100 MHz,
CDCl3): δ 163.9 (C4), 1583.56 2and 158.53 (C4DMT, C4′DMT), 150.45
and 150.41 (C2), 144.33 and 144.32 (C1′′DMT), 136.75 and 135.73
(C6), 135.50, 135.47, and 135.3 (C1DMT, C1′DMT), 139.1 (C2DMT
C6DMT, C2′DMT, C6′DMT), 128.1 and 127.9 (C2′′DMT, C3′′DMT
C5′′DMT, C6′′DMT), 126.0 (C4′′DMT), 113.2 (C3DMT, C5DMT, C3′DMT
,
,
,
C5′DMT), 111.11 and 111.06 (C5), 86.9 (Ar3CDMT), 85.4 and 85.1
(2 × d, C4′, J4′-P ) 5.9 Hz), 84.62 and 84.57 (C1′), 76.6 and 76.3
(2 × d, C3′, J3′-P ) 5.5 Hz), 63.9 (C5′), 61.86 and 61.83 (2 × d,
CH3CH2, JCH-P ) 5.8 Hz), 55.2 (2 × OCH3), 45.7 ((CH3CH2)3NH+),
39.7 and 39.5 (2 × d, C2′, J2′-P ) 3.7 Hz), 16.3 and 16.2 (2 × d,
CH3CH, JCH3-P
) 5.9 Hz), 11.52 and 11.46 (CH3), 9.2
((CH3CH2)3NH+). 31P NMR (162 MHz, CDCl3): δ 57.3 and 57.1.
HRMS: m/z 667.1913 ([M - H]-, C33H36N2O9PS- calcd 667.1885).
5′-O-Dimethoxytritylthymidin-3′-yl Isopropyl Phosphorothio-
ate, Triethylammonium Salt (11c). White powder, 141 mg (90%),
1
1:1 mixture of diastereoisomers. H NMR (400 MHz, CDCl3): δ
8.79 and 8.72 (1H, 2 × s, H3), 8.01 (1H, b, (CH3CH2)3NH+), 7.63
and 7.62 (1H, 2 × q, H6, J6-CH ) 1.2 Hz), 7.43-6.79 (13H, m,
3
DMT protons), 6.46 and 6.44 (1H, 2 × dd, H1′, J1′-2′ ) 7.4 Hz,
J1′-2′′ ) 5.5 Hz), 5.28 (1H, m, H3′), 4.64 and 4.55 (1H, 2 × dsep,
3′-O-(tert-Butyldimethylsilyl)thymidin-5′-yl 5′-O-Dimetho-
xytritylthymidin-3′-yl Phosphorothioate, Triethylammonium
Salt (11e). White powder, 175 mg (81%), 1:1 mixture of diaste-
reoisomers. 1H NMR (400 MHz, CDCl3): δ 11.89 (1H, b,
(CH3CH2)3NH+), 8.90, 8.89, 8.81, and 8.73 (2H, 4 × s, H3a, H3b),
7.78, 7.77, 7.64, and 7.59 (2H, 4 × s, H6a, H6b), 7.45-6.81 (13H,
m, DMT prot.), 6.48-6.34 (2H, m, H1′a, H1′b), 5.36 (1H, m, H3′a),
4.62-4.31 (1H, 3 × m, H3′b), 4.39 and 4.31 (1H, 2 × m, H4′a),
4.19-3.88 (3H, m, H4′b H5′b, H5′′b), 3.81, 3.80, and 3.79 (6H, 3
× s, 2 × OCH3), 3.56-5.36 (2H, m, H5′a, H5′′a), 3.10 and 3.09
(6H, 2 × q, (CH3CH2)3NH+, JCH -CH ) 7.3 Hz), 2.68 and 2.56
(CH3)2CH, JCH-P ) 10.5 Hz, JCH-(CH )2 ) 6.3 Hz), 4.40 and 4.35
3
(1H, 2 × m, H4′), 3.77 (6H, s, 2 × OCH3), 3.52 (0.5H, dd, H5′,
J5′-5′′ ) 10.5 Hz, J5′-4′ ) 2.8 Hz), 3.45-3.36 (1.5H, m, H5′, H5′′),
2.91 (6H, q, (CH3CH2)3NH+, JCH
) 7.2 Hz), 2.71 and 2.67
2-CH3
(1H, 2 × ddd, H2′, J2′-2′′ ) 13.4 Hz, J2′-1′ ) 5.5 Hz, J2′-3′ ) 1.1
Hz), 2.35 (1H, m, H2′′), 1.34 and 1.33 (3H, 2 × d, CH3, JCH -6
)
1.2 Hz), 1.28 (1.5H, d, (CH3)2CH, JCH -CH ) 6.3 Hz), 1.24 3(9H,
t, (CH3CH2)3NH+, JCH -CH ) 7.2 Hz),3 ∼1.23 (1.5H, (CH3)2CH),
1.20 (1.5H, d, (CH3)23CH, JCH -CH ) 6.3 Hz), 1.12 (1.5H, d,
2
(CH3)2CH, JCH -CH ) 6.3 Hz). 13C3NMR (125 MHz, CDCl3): δ 163.6
2
3
(1H, 2 × dd, H2′a, J2′a-2′′a ) 13.3 Hz, J2′a-1′a ) 5.3 Hz), 2.43-2.10
(3H, m, H2′′a, H2′b, H2′′b), 1.98, 1.95, 1.48, and 1.39 (6H, 4 × s,
2 × CH3), 1.33 (9H, t, (CH3CH2)3NH+, JCH -CH ) 7.3 Hz), 0.90
3
(C4), 158.63 and 158.60 (C4DMT, C4′DMT), 150.22 and 150.19 (C2),
144.41 and 144.36 (C1′′DMT), 136.9 and 135.5 (C6), 135.38 and
135.34 (C1DMT, C1′DMT), 130.1 (C2DMT, C6DMT, C2′DMT, C6′DMT),
128.2 and 127.9 (C2′′DMT, C3′′DMT, C5′′DMT, C6′′DMT), 127.0
(C4′′DMT), 113.2 (C3DMT, C5DMT, C3′DMT, C5′DMT), 111.10 and
3
2
and 0.88 (9H, 2 × s, (CH3)3CSi), 0.09 and 0.06 (6H, 2× s,
(CH3)2Si). 13C NMR (100 MHz, CDCl3): δ 163.90, 163.85, 163.74,
and 163.71 (C4a, C4b), 158.69, 158.66, 158.64, and 158.58 (C4DMT
,
111.07 (C5), 87.0 (Ar3CDMT), 85.6 and 85.3 (2 × d, C4′, J4′-P
)
C4′DMT), 150.39 and 150.38 (C2a, C2b), 144.32 and 144.30
(C1′′DMT), 136.4, 135.7, 135.6, 135.5, 135.4, and 135.1 (C6a, C6b,
C1DMT, C1′DMT), 130.1 and 130.0 (C2DMT, C6DMT, C2′DMT, C6′DMT),
6.2 Hz), 84.75 and 84.71 (C1′), ∼76.6 (C3′), 70.1 and 69.9 (2 ×
d, (CH3)2CH, JCH-P ) 6.0 Hz), 64.09 and 64.03 (C5′), 55.2 (2 ×
OCH3), 45.8 ((CH3CH2)3NH+), 39.7 and 39.5 (2 × d, C2′, J2′-P
)
)
128.2, 128.1, 128.0, 127.8, and 127.7 (C2′′DMT, C3′′DMT, C5′′DMT
C6′′DMT), 127.1 and 127.0 (C4′′DMT), 113.3 and 113.1 (C3DMT
,
,
3.9 Hz), 24.1, 23.9, 23.76, and 23.73 (4 × d, (CH3)2CH, JCH -P
3
5.0 Hz), 11.49 and 11.46 (CH3), 9.5 ((CH3CH2)3NH+). 31P NMR
(202 MHz, CDCl3): δ 52.2 and 52.1. HRMS: m/z 681.1999 ([M -
H]-, C34H38N2O9PS- calcd 681.2041).
C3′DMT, C5DMT, C5′DMT), 111.3, 111.2, 111.1, and 111.0 (C5a, C5b),
87.0 and 86.9 (Ar3CODMT), 86.5, 86.4, 86.1, 85.7, 85.6, 83.3, 85.2,
85.1, 85.0, 84.9, 84.7, and 84.6 (C1′a, C1′b, C4′a, C4′b), ∼77.2
(C3′a), 72.84 and 72.80 (C3′b), 65.8 and 65.1 (d, C5′b, J5′-P ) 7.9
Hz), 64.0 and 63.9 (C5′a), 55.2 (2 × OCH3), 45.6 ((CH3CH2)3NH+),
40.9 and 40.8 (C2′b), 39.8 and 39.1 (d, C2′a, J2′-P ) 3.9 Hz), 25.7
((CH3)3CSi), 17.9 ((CH3)3CSi), 12.45, 12.39, 11.6, and 11.5 (CH3a,
CH3b), 8.6 ((CH3CH2)3NH+), -4.68, -4.71, –4.74, and -4.76
((CH3)2Si). 31P NMR (202 MHz, CDCl3): δ 57.5 and 57.3. HRMS:
m/z 977.3330 ([M - H]-, C47H58N4O13PSSi- calcd 977.3233).
3′-O-(tert-Butyldimethylsilyl)thymidin-5′-yl 5′-O-Dimetho-
xytritylthymidin-3′-yl Phosphorodithioate, Triethylammonium
tert-Butyl 5′-O-dimethoxytritylthymidin-3′-yl Phosphorothio-
ate, Triethylammonium Salt (11d). White powder, 136 mg (85%),
1
1:1 mixture of diastereoisomers. H NMR (400 MHz, CDCl3): δ
8.96 (1H, b, H3), 7.63 (1H, s, H6), 7.48-6.68 (13H, m, DMT
protons), 6.47 (1H, m, H1′), 5.33 (1H, m, H3′), 4.42 (1H, m, H4′),
3.78 (6H, s, 2 × OCH3), 3.53 (0.5H, dd, H5′, J5′-5′′ ) 10.6 Hz,
J5′-5′′ ) 2.9 Hz), 3.46 (0.5H, dd, H5′, J5′-5′′ ) 10.6 Hz, J5′-4′
)
2.3 Hz), 3.43-3.38 (1H, m, H5′′), 3.06 (6H, q, (CH3CH2)3NH+,
JCH -CH ) 7.3 Hz), 2.72 (1H, dd, H2′, J2′-2′′ ) 13.3 Hz, J2′-1′
)
3
5.62Hz), 2.36 (1H, m, H2′′), 1.50 and 1.40 (9H, 2 × s, (CH3)3C),
Salt (21). White powder, 167 mg (76%). H NMR (500 MHz,
1
1.36 and 1.34 (3H, 2 × s, CH3), 1.31 (9H, t, (CH3CH2)3NH+,
CDCl3): δ 10.29 (1H, b, (CH3CH2)3NH+), 8.56 and 8.48 (2H, 2 ×
s, H3a, H3b), 7.83 and 7.59 (2H, 2 × s, H6a, H6b), 7.43-6.88 (13H,
m, DMT prot.), 6.43 (1H, dd, H1′a, J1′a-2′a ) 8.6 Hz, J1′a-2′′a ) 5.4
Hz), 6.40 (1H, dd, H1′b, J1′b-2′b ) 7.9 Hz, J1′b-2′′b ) 6.1 Hz), 5.47
(1H, dd, H3′a, J3′a-P ) 12.5 Hz, J3′a-2′′a ) 5.8 Hz), 4.56 (1H, m,
JCH -CH ) 7.3 Hz). 13C NMR (100 MHz, CDCl3): δ 163.8 (C4),
3
158.542and 158.51 (C4DMT, C4′DMT), 150.4 and 150.3 (C2), 144.39
and 144.35 (C1′′DMT), 135.90 and 135.85 (C6), 135.57, 135.51,
135.35, and 135.31 (C1DMT, C1′DMT), 130.1 (C2DMT, C6DMT, C2′DMT
,
J. Org. Chem. Vol. 73, No. 13, 2008 5037

Bartoszewicz et al.
H3′b), 4.40 (1H, m, H4′a), 4.26 (1H, m, H5′b), 4.06 (1H, m, H5′′b),
4.03 (1H, m, H4′b), 3.78 (6H, s, 2 × OCH3), 3.48 (1H, dd, H5′a,
3.98 (1H, 2 × m, H4′b), 3.91 (1H, m, H5′′b), 3.78 (6H, s, 2 ×
OCH3), 3.58-5.29 (2H, m, H5′a, H5′′a), 3.11 (6H, q,
J5′a-5′′a ) 11.5 Hz, J5′a-4′a ) 2.8 Hz), 3.41 (1H, dd, H5′′a, J5′′a-5′a
)
(CH3CH2)3NH+, JCH -CH ) 7.4 Hz), 2.75-2.00 (4H, m, H2′a, H2′′a,
2
11.5 Hz, J5′′a-4′a ) 2.4 Hz), 3.19 (6H, q, (CH3CH2)3NH+, JCH -CH
H2′b, H2′′b), 1.96, 1.95 3and 1.38 (6H, 3 × s, 2 × CH3), 1.32 (9H,
3
) 7.3 Hz), 2.67 (1H, dd, H2′′a, J2′′a-2′a ) 13.6 Hz, J2′′a-1′a )2 5.4
Hz), 2.35 (1H, m, 2′a), 2.20-2.12 (2H, m, H2′b, H2′′b), 1.96 and
t, (CH3CH2)3NH+, JCH -CH ) 7.4 Hz), 0.88 and 0.87 (9H, 2 × s,
3
2
(CH3)3CSi), 0.08 and 0.05 (6H, 2 × s, (CH3)2Si). 13C NMR (125
MHz, CDCl3): δ 163.8, 163.7, 163.63 and 163.58 (C4a, C4b), 158.67
and 158.65 (C4DMT, C4′DMT), 150.3 (C2a, C2b), 144.3 (C1′′DMT),
∼1.35 (6H, 2 × s, 2 × CH3), 1.37 (9H, t, (CH3CH2)3NH+, JCH -CH
2
) 7.3 Hz), 0.87 (9H, 2 × s, (CH3)3CSi), 0.07 and 0.06 (6H, 32 ×
s, (CH3)2Si). 13C NMR (125 MHz, CDCl3): δ 163.8 and 163.6 (C4a,
C4b), 158.64 and 158.60 (C4DMT, C4′DMT), 150.4 and 150.2 (C2a,
C2b), 144.4 (C1′′DMT), 136.5, 135.7, 135.5, and 135.3 (C6a, C6b,
136.2 135.7, 135.6, 135.4, 135.4, and 135.2 (C6a, C6b, C1DMT
,
C1′DMT), 130.1 (C2DMT, C6DMT, C2′DMT, C6′DMT), 128.2 and 128.0
(C2′′DMT, C3′′DMT, C5′′DMT, C6′′DMT), 127.1 (C4′′DMT), 113.3
(C3DMT, C3′DMT, C5DMT, C5′DMT), 111.2 and 111.1 (C5a, C5b), 87.1
(Ar3CODMT), 86.4 and 86.3 (2 × d, C4′b, J4′b-P ) 8.6 Hz), 85.5,
85.2, 85.06, 85.01, 84.9, 84.7, 84.6 and 84.5 (C1′a, C1′b, C4′a),
77.6 (d, C3′a, JC3′a-P ) 5.2 Hz), 72.9 (C3′b), 66.0 and 65.3 (d, C5′b,
J5′-P ) 6.1 Hz), 64.1 and 63.9 (C5′a), 55.2 (2 × OCH3), 45.8
((CH3CH2)3NH+), 40.8 (C2′b), 39.8 and 39.1 (C2′a), 25.7
((CH3)3CSi), 17.9 ((CH3)3CSi), 12.52, 12.45, 11.57 and 11.51 (CH3a,
CH3b), 8.2 ((CH3CH2)3NH+), -4.66, -4.68, and -4.71 ((CH3)2Si).
31P NMR (202 MHz, CDCl3): δ 52.2 and 52.0. HRMS: m/z
1025.2657 ([M - H]-, C47H58N4O13PSeSi- calcd 1025.2678).
C1DMT, C1′DMT), 130.16 and 130.12 (C2DMT, C6DMT, C2′DMT
,
C6′DMT), 128.1 and 128.0 (C2′′DMT, C3′′DMT, C5′′DMT, C6′′DMT),
127.0 (C4′′DMT), 113.2 and 113.1 (C3DMT, C3′DMT, C5DMT, C5′DMT),
111.3 and 111.1 (C5a, C5b), 87.0 (Ar3CODMT), 86.6 (d, C4′b, J4′b-P
) 9.3 Hz), 85.2 (d, C4′a, J4′a-P ) 5.5 Hz), 84.9 and 84.7 (C1′a,
C1′b), ∼76.8 (C3′a), 72.2 (C3′b), 65.6 (d, C5′b, J5′-P ) 7.5 Hz), 63.8
(C5′a), 55.2 (2 × OCH3), 46.1 ((CH3CH2)3NH+), 40.8 (C2′b), 39.5
(d, C2′a, J2′-P ) 3.9 Hz), 25.8 ((CH3)3CSi), 17.9 ((CH3)3CSi), 12.5
and 11.5 (CH3a, CH3b), 8.6 ((CH3CH2)3NH+), -4.7 ((CH3)2Si). 31
P
NMR (162 MHz, CDCl3): δ 113.0. HRMS: m/z 993.3116 ([M -
H]-, C47H58N4O12PS2Si- calcd 993.3005).
3′-O-(tert-Butyldimethylsilyl)thymidin-5′-yl 5′-O-Dimetho-
xytritylthymidin-3′-yl Phosphoroselenoate, Triethylammonium
Salt (22). White powder, 174 mg (77%), 1:1 mixture of diastere-
oisomers. 1H NMR (500 MHz, CDCl3): δ 11.58 (1H, b,
(CH3CH2)3NH+), 8.62 and 8.49 (2H, 2 × s, H3a, H3b), 7.74 and
7.62 (2H, 2 × s, H6a, H6b), 7.44-6.78 (13H, m, DMT prot.),
6.46-6.32 (2H, m, H1′a, H1′b), 5.43 (1H, m, H3′a), 4.52-4.40 (1H,
2 × m, H3′b), 4.29 (1H, m, H4′a), 4.13 (1H, m, H5′b), 4.06 and
Acknowledgment. Financial support from the Swedish
Research Council is gratefully acknowledged.
Supporting Information Available: NMR spectra of com-
pounds 11a-e, 21, and 22. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO8006072
5038 J. Org. Chem. Vol. 73, No. 13, 2008