Improved phosphoramidite building blocks for the synthesis of the simplified nucleic acid GNA

Article abstract of DOI:10.1021/jo900365a

(Chemical Equation Presented) An improved synthesis of glycol nucleic acids is reported using new phosphoramidite building blocks in which the exocyclic amino groups of adenine and guanine are protected as N-dimethylformamidines, whereas the amino group o

Full text of DOI:10.1021/jo900365a

Improved Phosphoramidite Building Blocks for
the Synthesis of the Simplified Nucleic Acid GNA
Mark K. Schlegel and Eric Meggers*
Fachbereich Chemie, Philipps-UniVersita¨t Marburg,
Hans-Meerwein-Strasse, 35032 Marburg, Germany
meggers@chemie.uni-marburg.de
ReceiVed February 19, 2009
FIGURE 1. Comparison of the constitution of DNA with the (S)-
enantiomer of GNA.
of side products, long workup times, and suboptimal yields.8
Furthermore, as previously reported, the G phosphoramidite
(Figure 2) is slightly unstable, cannot be purified by chroma-
tography, and slowly decomposes at -20 °C. To address these
issues we set out to modify the protection groups on the
exocyclic amino groups of the GNA building blocks. For
adenine and guanine we chose the N-dimethylformamidine
group9-11 and an acetamide for cytosine (Figure 2). Both of
these protection groups have been reported to be compatible
with quicker deprotection conditions after oligonucleotide
synthesis.12
Accordingly, using methods similar to those previously
described,8 O6-benzylguanine (1)13 was used in the ring-opening
of (S)-glycidyl 4,4′-dimethoxytrityl ether 28 in DMF to afford
compound 3 in 47% yield (Scheme 1). The benzyl group was
subsequently removed using catalytic hydrogenation to produce
compound 4 in 97% yield. The N2-dimethylformamidine deriva-
tive was synthesized by heating a mixture of compound 4 and
dimethylformamide dimethylacetal in DMF at 60 °C for 1 h,
affording compound 5 in 86% yield. Conversion to the phos-
phoramidite G* was then accomplished in 76% yield by the
reaction with 2-cyanoethyl N,N,N′,N′-tetraisopropylphosphor-
diamidite and substoichiometric amounts of 4,5-dicyanoimida-
zole (DCI)14 in CH2Cl2. Gratifyingly, the new phosphoramidite
G* was stable to flash chromatography over silica gel unlike
An improved synthesis of glycol nucleic acids is reported
using new phosphoramidite building blocks in which the
exocyclic amino groups of adenine and guanine are protected
as N-dimethylformamidines, whereas the amino group of
cytosine is protected via an acetamide. Besides a more rapid
synthesis with higher yields, these phosphoramidites allow
the use of a quicker deprotection procedure in the subsequent
solid-phase synthesis of GNA oligonucleotides.
Glycol nucleic acid (GNA) constitutes a minimal solution
for a phosphodiester-containing nucleic acid backbone. The
propylene glycol nucleotide building blocks contain just three
carbon atoms and one stereocenter and are connected by
phosphodiester bonds (Figure 1). Surprisingly, GNA forms
antiparallel Watson-Crick duplexes that significantly exceed
the stabilities of analogous duplexes of DNA and RNA.1-4 The
simplicity of the GNA backbone renders it an interesting scaffold
for future nanoscale architectures.5 However, this requires a
straightforward chemical or enzymatic synthesis of this artificial
nucleic acid. Here we report our progress toward improved GNA
phosphoramidite building blocks for automated solid phase
synthesis of GNA oligonucleotides.
(7) Ti, G. S.; Gaffney, B. L.; Jones, R. A. J. Am. Chem. Soc. 1982, 104,
1316–1319.
(8) Zhang, L.; Peritz, A. E.; Carroll, P. J.; Meggers, E. Synthesis 2006, 645–
653.
(9) McBride, L. J.; Kierzek, R.; Beaucage, S. L.; Caruthers, M. H. J. Am.
Chem. Soc. 1986, 108, 2040–2048.
The traditional amide protection scheme6,7 employed for the
original solid-phase synthesis of GNA suffers from the formation
(10) Vu, H.; McCollum, C.; Jacobsen, K.; Theisen, P.; Vinayak, R.; Spiess,
E.; Andrus, A. Tetrahedron Lett. 1990, 31, 7269–7272.
(11) Vinayak, R.; Anderson, P.; McCollum, C.; Hampel, A. Nucleic Acids
Res. 1992, 20, 1265–1269.
(1) Zhang, L.; Peritz, A.; Meggers, E. J. Am. Chem. Soc. 2005, 127, 4174–
4175.
(2) Schlegel, M. K.; Peritz, A. E.; Kittigowittana, K.; Zhang, L.; Meggers,
E. ChemBioChem 2007, 8, 927–932.
(3) Schlegel, M. K.; Essen, L.-O.; Meggers, E. J. Am. Chem. Soc. 2008,
130, 8158–8159.
(4) Schlegel, M. K.; Xie, X.; Zhang, L.; Meggers, E. Angew. Chem., Int.
Ed. 2009, 48, 960–963.
(12) For DNA, the N2-isobutyryl amide protection group of the guanosine
nucleoside takes approximately 10 h in aqueous ammonia at 55 °C to be removed.
See, for example: (a) Koster, H.; Kulikowski, K.; Liese, T.; Heikens, W.; Kohli,
V. Tetrahedron 1981, 37, 363–369. (b) Schulhof, J. C.; Molko, D.; Teoule, R.
Nucleic Acids Res. 1987, 15, 397–416.
(5) Zhang, R. S.; McCullum, E. O.; Chaput, J. C. J. Am. Chem. Soc. 2008,
130, 5846–5847.
(6) Schaller, H.; Weimann, G.; Lerch, B.; Khorana, H. G. J. Am. Chem. Soc.
1963, 85, 3821–3827.
(13) Lembicz, N. K.; Grant, S.; Clegg, W.; Griffin, R. J.; Heath, S. L.;
Golding, B. T. J. Chem. Soc., Perkin Trans. 1 1997, 185–186.
(14) Rosenbohm, C.; Christensen, S. M.; Sørensen, M. D.; Pedersen, D. S.;
Larsen, L.-E.; Wengel, J.; Koch, T. Org. Biomol. Chem. 2003, 1, 655–663.
10.1021/jo900365a CCC: $40.75
Published on Web 05/14/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 4615–4618 4615

SCHEME 2. Synthesis of Adenine Phosphoramidite A*
SCHEME 3. Synthesis of Cytosine Phosphoramidite C*
FIGURE 2. Previous (A, G, C, T) and optimized new (A*, G*, C*)
GNA phosphoramidite building blocks.
SCHEME 1. Synthesis of Guanine Phosphoramidite G*
benzoyl protection group of C against an acetyl group in C*
(Figure 2). N4-Acetylcytosine DNA phosphoramidites are widely
used in mild and ultramild oligonucleotide synthesis. Accord-
ingly, epoxide ring-opening of compound 2 using commercially
available N4-acetylcytosine (9) in DMF with catalytic amounts
of NaH afforded compound 10 in 57% yield (Scheme 3).
Subsequent conversion to phosphoramidite C* was achieved
using 2-cyanoethyl N,N,N′,N′-tetraisopropylphosphordiamidite
and DCI in CH2Cl2 proceeded in 77%. It should be noted that
attempts to purify compound C* using flash chromatography
over silica gel were unsuccessful. However, we found that pure
phosphoramidite C* can be isolated using basic alumina
(Brockmann type II). This new synthesis has an overall yield
of 44% over three steps for C* which is slightly better than the
39% over the same number of steps for the previous phos-
phoramidite C.
Having the three new phosphoramidites A*, G*, and C*
along with the previously reported T phosphoramidite8 in hand,
GNA oligonucleotides were prepared using general procedures
for DNA oligonucleotides, except that the coupling time was
extended to 3 min. Subsequent cleavage from the solid support
and deprotection of the exocyclic amino protection groups was
accomplished in only 15-20 min at 55 °C using a 1:1 mixture
of 40% aqueous methylamine and 25% aqueous ammonium
hydroxide (AMA).15 After cooling to room temperature, the
entire solution of crude tritylated oligonucleotide was applied
directly to a Sep-Pak Classic reversed-phase column and
subsequently washed, detritylated with 1.5% aqueous TFA, and
then eluted from the column using 20% aqueous acetonitrile.
its N2-isobutyryl counterpart G. In addition, this new synthesis
of G* has a significantly improved overall yield of 30% over
five steps compared to 8% over the same number of steps for
the previous phosphoramidite G.
Amidine protection has been previously investigated in the
context of adenosine nucleosides in an attempt to minimize acid
catalyzed depurination during oligonucleotide synthesis.10 We
thus next explored N6-dimethylformamidine protection of (S)-
9-(3-(4,4′-dimethoxytrityloxy)-2-hydroxypropyl)adenine (7),8
obtained from adenine (6) in one step by the reaction with the
tritylated glycidol 2 in the presence of catalytic amounts of
NaH.8 The reaction of compound 7 with dimethylformamide
dimethyl acetal in DMF afforded compound 8 in 99% yield
(Scheme 2). The following conversion to phosphoramidite A*
using 2-cyanoethyl N,N,N′,N′-tetraisopropylphosphordiamidite
and DCI in CH2Cl2 could be accomplished in 83% yield. This
new synthesis has a vastly improved overall yield of 51% over
four steps for A* compared to only 26% over the same number
of steps for the previous phosphoramidite A.
Next, in order to render the cytosine GNA building block
more amenable toward milder deprotection we replaced the
(15) Reddy, M. P.; Hanna, N. B.; Farooqui, F. Tetrahedron Lett. 1994, 35,
4311–4314.
4616 J. Org. Chem. Vol. 74, No. 12, 2009

a hydrogen atmosphere. After 3 h, TLC showed completion of the
reaction, and the mixture was filtered through Celite and washed
with 5:1 CH2Cl2/MeOH to afford compound 4 as a tan solid (2.73
g, 97%): 1H NMR (300 MHz, DMSO-d6) δ (ppm) 10.69 (br, 1H),
7.59 (s, 1H), 7.42 (d, J ) 7.5 Hz, 2H), 7.35-7.17 (m, 7H), 6.88
(dd, J ) 8.7, 1.6 Hz, 4H), 6.48 (br, 2H), 5.40 (d, J ) 4.0 Hz, 1H),
4.11-3.90 (m, 3H), 3.74 (s, 6H), 2.98 (dd, J ) 8.6, 3.7 Hz, 1H),
2.90 (dd, J ) 8.6, 3.4 Hz, 1H); 13C NMR (75 MHz, DMSO-d6) δ
(ppm) 158.0, 156.9, 153.4, 151.3, 144.9, 138.0, 135.6, 129.7, 127.7,
126.6, 116.4, 113.1, 85.3, 68.0, 65.7, 55.0, 46.3; IR (film) ν (cm-1
)
) 3412 (br), 3127, 2949, 2832, 1699, 1607, 1578, 1541, 1508,
1481, 1462, 1443, 1412, 1379, 1302, 1250, 1173, 1152, 1113, 1074,
1024, 986, 899, 828, 789, 777, 752, 725, 692, 629; HRMS calcd
for C29H29N5O5Na (M + Na)+ 550.2061, found (M + Na)+
550.2061.
FIGURE 3. Crude HPLC trace of the 13-mer GNA strand 3′-
CATGTCGTGCGTA-2′ after synthesis and deprotection. The solid
support for the 2′-end A nucleotide was synthesized from compound 8
as previously described.8 The strand was eluted (1 mL/min) over a
Waters XTerra column (MS C18, 4.6 × 50 mm, 2.5 µm) at 60 °C with
aqueous triethylammonium acetate buffer (50 mM, pH ) 7.0) and a
linear gradient of 2-8% acetonitrile over 30 min. See the Supporting
Information for a MALDI-TOF spectrum of this crude GNA strand.
(S)-9-(3-(4,4′-Dimethoxytrityloxy)-2-hydroxypropyl)-N2-[(dim-
ethylamino)methylene]guanine (5). To a solution of 4 (2.63 g,
4.99 mmol) in anhydrous DMF (16.0 mL) was added dimethyl-
formamide dimethyl acetal (2.35 mL, 17.4 mmol) and the mixture
heated to 60 °C for 1 h. After cooling and removal of the DMF,
the residue was redissolved in methylene chloride, washed once
with saturated aqueous NaHCO3, dried over Na2SO4, and finally
concentrated. The product was purified via column chromatography
starting with 100:1 EtOAc/Et3N then eluting with 40:3:0.01 EtOAc/
MeOH/Et3N to afford compound 5 as a white foam (2.50 g, 86%):
1H NMR (300 MHz, CDCl3) δ (ppm) 9.21 (br, 1H), 8.48 (s, 1H),
7.51-7.45 (m, 3H), 7.39-7.15 (m, 7H), 6.83 (dd, J ) 9.0, 2.3 Hz,
4H), 4.44 (m, 2H), 4.01 (dd, J ) 14.4, 8.2 Hz, 1H), 3.78 (s, 6H),
3.36 (dd, J ) 9.5, 4.8 Hz, 1H), 3.08 (dd, J ) 9.4, 7.3 Hz, 1H),
3.01 (s, 3H), 2.89 (s, 3H); 13C NMR (75 MHz, CDCl3) δ (ppm)
158.7, 158.2, 157.5, 156.5, 150.3, 145.1, 139.5, 136.2, 136.1, 130.1,
128.2, 128.0, 126.9, 120.0, 113.33, 113.30, 86.4, 69.3, 65.0, 55.4,
48.6, 41.3, 35.2; IR (solid) ν (cm-1) ) 2929, 2836, 1630, 1558,
1506, 1444, 1416, 1399, 1345, 1326, 1300, 1245, 1174, 1110, 1066,
1024, 981, 827, 755, 726, 701, 644, 581; HRMS calcd for
C32H34N6O5Na (M + Na)+ 605.2483, found (M + Na)+ 605.2477.
Phosphoramidite G*. To a solution of 5 (1.80 g, 3.09 mmol)
in 15.5 mL of anhydrous methylene chloride under nitrogen was
added a 1 M solution of dicyanoimidazole (2.20 mL in acetonitrile).
2-Cyanoethyl N,N,N′,N′-tetraisopropylphosphordiamidite (1.03 mL,
3.24 mmol) was then added dropwise and the solution stirred at
room temperature. After 2 h, the reaction mixture was diluted with
methylene chloride, washed twice with saturated aqueous NaHCO3,
dried over Na2SO4, and then concentrated. The product was purified
via column chromatography starting with 1:1:0.01 hexanes/acetone/
Et3N,then eluting with 1:2:0.01 hexanes/acetone/Et3N to afford
compound G* as a white foam (1.85 g, 76%): 31P NMR (162 MHz,
CDCl3) δ (ppm) 150.3, 150.0; HRMS calcd for C41H52N8O6P (M
+ H)+ 783.3742, found (M + H)+ 783.3736.
This improved deprotection and workup procedure represents
a saving of at least 12 h for the removal of the protection groups
and one HPLC purification compared to the conventional
procedure previously reported for GNA oligonucleotides.8 The
crude HPLC trace of a representative oligonucleotide solution
obtained according to this protocol is shown in Figure 3 and
demonstrates the high quality of the crude product.
In conclusion, we have presented an improved protection
group scheme which provides a more economical route for the
synthesis of GNA phosphoramidite building blocks and also
quicker access to GNA oligonucleotides. These synthetic routes
should be applicable to the large scale synthesis of GNA
phosphoramidites and work along these lines is in progress.
Experimental Section
(S)-9-(3-(4,4′-Dimethoxytrityloxy)-2-hydroxypropyl)-O6-ben-
zylguanine (3). Compound 1 (3.10 g, 12.8 mmol) was partially
dissolved in anhydrous DMF (25.0 mL) under a nitrogen atmo-
sphere. NaH was added (105 mg, 2.63 mmol, 60% in mineral oil),
and the solution was allowed to stir under nitrogen for 1 h. In a
separate flask, compound 2 (4.60 g, 12.2 mmol) was dissolved in
26.0 mL of DMF, added to the first solution, and then heated to 90
°C overnight. The next morning, the solution was cooled, all solvent
removed, and the resulting oil coevaporated with toluene, redis-
solved in ethyl acetate, and concentrated again. The product was
purified via column chromatography starting with 2:1:0.01 hexanes/
acetone/Et3N, then eluting with 3:2:0.01 hexanes/acetone/Et3N to
afford compound 3 as a light yellow foam (3.73 g, 47%): 1H NMR
(300 MHz, CDCl3) δ (ppm) 7.53 (s, 1H), 7.51 (d, J ) 7.1 Hz,
2H), 7.42 (d, J ) 7.5 Hz, 2H), 7.35-7.25 (m, 9H), 7.21 (t, J ) 7.3
Hz, 1H), 6.82 (d, J ) 8.8 Hz, 4H), 5.52 (s, 2H), 5.20 (br, 1H),
4.85 (s, 2H), 4.28 (m, 1H), 4.16 (m, 2H), 3.78 (s, 6H), 3.21 (dd, J
) 9.5, 4.3 Hz, 1H), 3.03 (dd, J ) 9.4, 5.6 Hz, 1H); 13C NMR (125
MHz, CDCl3) δ (ppm) 161.0, 158.8, 158.7, 154.0, 144.8, 140.8,
136.5, 136.0, 135.9, 130.1, 128.5, 128.4, 128.2, 128.1, 128.0, 127.0,
(S)-9-(3-(4,4′-Dimethoxytrityloxy)-2-hydroxypropyl)-N6-[(dim-
ethylamino)methylene]adenine) (8). To a solution of 7 (1.32 g,
2.58 mmol) in 7.5 mL of anhydrous DMF was added dimethyl-
formamide dimethyl acetal (1.21 mL, 9.04 mmol) and the mixture
heated to 60 °C for 1 h. After cooling and removal of the DMF,
the residue was redissolved in methylene chloride, washed once
with saturated aqueous NaHCO3, dried over Na2SO4, and finally
concentrated. The product was purified via column chromatography
starting with 100:1 EtOAc/Et3N then eluting with 40:3:0.01 EtOAc/
MeOH/Et3N to afford compound 8 as a white foam (1.45 g, 99%):
1H NMR (300 MHz, CDCl3) δ (ppm) 8.89 (s, 1H), 8.44 (s, 1H),
7.84 (s, 1H), 7.43 (m, 2H), 7.35-7.17 (m, 7H), 6.82 (m, 4H), 4.98
(br, 1H), 4.45 (m, 1H), 4.32-4.17 (m, 2H), 3.79 (s, 6H), 3.29 (dd,
J ) 9.4, 4.7 Hz, 1H), 3.23 (s, 3H), 3.21 (s, 3H), 3.10 (dd, J ) 9.4,
5.7 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ (ppm) 159.5, 158.7,
158.4, 152.1, 151.8, 144.8, 143.0, 135.91, 135.83, 130.1, 128.11,
128.00, 127.0, 125.8, 113.3, 86.5, 69.6, 64.8, 55.3, 48.7, 41.4, 35.2;
IR (solid) ν (cm-1) ) 2929, 2836, 1630, 1558, 1506, 1444, 1416,
1399, 1345, 1326, 1300, 1245, 1174, 1110, 1066, 1024, 981, 827,
115.4, 113.3, 86.4, 69.5, 68.3, 64.7, 55.3, 48.5; IR (film) ν (cm-1
)
) 3515, 1401, 3341, 3212, 3065, 3034, 2934, 2834, 1616, 1589,
1510, 1456, 1410, 1385, 1356, 1333, 1300, 1252, 1175, 1154, 1101,
1061, 1028, 907, 828, 789, 756, 727, 698, 633, 581; HRMS calcd
for C36H35N5O5Na (M + Na)+ 640.2530, found (M + Na)+
640.2529.
(S)-9-(3-(4,4′-Dimethoxytrityloxy)-2-hydroxypropyl)gua-
nine (4). Compound 3 (3.30 g, 5.3 mmol) and Pd/C (1.70 g, 10%
on carbon) were suspended in EtOAc (125 mL), and the solution
was purged with nitrogen, then hydrogen, and allowed to stir under
J. Org. Chem. Vol. 74, No. 12, 2009 4617

755, 726, 701, 644, 581; HRMS calcd for C32H35N6O4 (M + H)+
567.2714, found (M + H)+ 567.2709.
Phosphoramidite C*. To a solution of 10 (1.06 g, 2.00 mmol)
in 10.0 mL of anhydrous methylene chloride under nitrogen was
added a 1 M solution of dicyanoimidazole (1.40 mL in acetonitrile).
2-Cyanoethyl N,N,N′,N′-tetraisopropylphosphordiamidite (0.67 mL,
2.1 mmol) was then added dropwise and the solution stirred at room
temperature. After 2 h, the reaction mixture was diluted with
methylene chloride, washed once with saturated aqueous NaHCO3,
dried over Na2SO4, and then concentrated. The product was purified
via column chromatography (basic alumina, Brockmann type II)
starting with 100:1 EtOAc/Et3N then eluting with 50:1:0.01 EtOAc/
MeOH/Et3N to afford compound C* as a white foam (1.12 g, 77%):
31P NMR (162 MHz, CDCl3) δ (ppm) 150.3, 150.1; HRMS calcd
for C39H49N5O7P (M + H)+ 730.3364, found (M + H)+ 730.3358.
GNA Oligonucleotide Synthesis and Purification. GNA oli-
gonucleotides were prepared on an ABI 394 DNA/RNA synthesizer
on a 1 µmol scale. GNA phosphoramidites (A*, G*, C*, and T)
were used at a concentration of 100 mM with a standard protocol
for 2-cyanoethyl phosphoramidites, except that the coupling time
was extended to 3 min. After the trityl-on synthesis, the resin was
incubated with AMA solution (1.5 mL) for 15-20 min at 55 °C.
After cooling, the entire solution was applied directly to a Sep-
Pak Classic reversed-phase column (Waters, 360 mg) and washed
sequentially with 3% NH4OH (15 mL), water (10 mL), 1.5%
aqueous TFA (10 mL), and finally water (10 mL). The oligo was
then eluted with 20% aqueous acetonitrile and further purified using
a Waters XTerra column (MS C18, 4.6 × 50 mm, 2.5 µm) at 60
°C with aqueous TEAA (50 mM) and acetonitrile as the eluent.
All identities were confirmed by MALDI-TOF MS.
Phosphoramidite A*. To a solution of 8 (1.82 g, 3.21 mmol)
in 16.0 mL of anhydrous methylene chloride under nitrogen was
added a 1 M solution of dicyanoimidazole (2.20 mL in acetonitrile).
2-Cyanoethyl N,N,N′,N′-tetraisopropylphosphordiamidite (1.07 mL,
3.37 mmol) was then added dropwise and the solution stirred at
room temperature. After 2 h, the reaction mixture was diluted with
methylene chloride, washed twice with saturated aqueous NaHCO3,
dried over Na2SO4, and then concentrated. The product was purified
via column chromatography starting with 3:2:0.01 hexanes/acetone/
Et3N then eluting with 1:1:0.01 hexanes/acetone/Et3N to afford
compound A* as a white foam (2.05 g, 83%): 31P NMR (162 MHz,
CDCl3) δ (ppm) 150.4, 149.9; HRMS calcd for C41H52N8O5P (M
+ H)+ 767.3793, found (M + H)+ 767.3781.
(S)-1-(3-(4,4′-Dimethoxytrityloxy)-2-hydroxypropyl)-N4-ace-
tylcytosine (10). To a suspension of 9 (1.16 g, 7.57 mmol) in 15.0
mL of anhydrous DMF was added NaH (60 mg, 1.5 mmol, 60%
in mineral oil) and the mixture allowed to stir under nitrogen for
1 h. A solution of 2 (2.71 g, 7.20 mmol) in 15.0 mL of anhydrous
DMF was added to the above solution, and the reaction was heated
to 110 °C overnight. The next morning, the solution was cooled,
all solvent removed, and the resulting oil coevaporated with toluene,
redissolved in ethyl acetate, and concentrated again. The product
was purified via column chromatography starting with 3:2:0.01
hexanes/acetone/Et3N then eluting with 1:1:0.01 hexanes/acetone/
Et3N to afford compound 10 as a light yellow foam (2.30 g, 57%):
1H NMR (300 MHz, CDCl3) δ (ppm) 9.81 (br, 1H), 7.57 (d, J )
7.3 Hz, 1H), 7.41 (m, 2H), 7.34-7.17 (m, 8H), 6.82 (m, 4H), 4.33
(dd, J ) 13.5, 2.6 Hz, 1H), 4.21 (br, 1H), 4.07 (br, 1H), 3.83-3.73
(m, 7H), 3.24 (dd, J ) 9.6, 5.1 Hz, 1H), 3.08 (dd, J ) 9.6, 6.0 Hz,
1H), 2.22 (s, 3H). 13C NMR (75 MHz, CDCl3) δ (ppm) 171.1,
162.9, 158.7, 157.4, 150.5, 144.7, 135.75, 135.71, 130.1, 128.09,
128.03, 127.0, 113.3, 96.7, 86.4, 68.9, 64.5, 55.3, 54.6, 24.9. IR
(solid) V (cm-1) ) 3256, 2963, 2929, 2836, 1630, 1558, 1507, 1445,
1417, 1347, 1299, 1245, 1174, 1112, 1069, 1030, 827, 789, 755,
727, 701, 645, 582; HRMS calcd for C30H31N3O6Na (M + Na)+
552.2105, found (M + Na)+ 552.2104.
Acknowledgment. We gratefully acknowledge support from
the Philipps-Universita¨t Marburg.
Supporting Information Available: 1H and 13C NMR
spectra of compounds 3-5, 8, and 10; 31P NMR spectra for
compounds G*, A*, and C*. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO900365A
4618 J. Org. Chem. Vol. 74, No. 12, 2009