A novel neoglycopeptide building block

Article abstract of DOI:10.1016/S0968-0896(03)00182-2

The synthesis of a neoglycopeptide building block is described. The key step is a cycloaddition where the chemistry is orthogonal to standard glycosyl transfer methodology. Also described is some exploratory chemistry of the building block.

Full text of DOI:10.1016/S0968-0896(03)00182-2

Bioorganic & Medicinal Chemistry 11 (2003) 3021–3027
A Novel Neoglycopeptide Building Block
Alessandra Bartolozzi,y Baoqing Li{ and Richard W. Franck*
Department of Chemistry, Hunter College of CUNY, 695 Park Ave., NewYork, NY 10021, USA
Received 30 July 2002; accepted 23 December 2002
Abstract—The synthesis of a neoglycopeptide building block is described. The key step is a cycloaddition where the chemistry is
orthogonal to standard glycosyl transfer methodology. Also described is some exploratory chemistry of the building block.
# 2003 Elsevier Science Ltd. All rights reserved.
The synthesis of N-linked glycopeptides is an important
activity in present-day glycoscience.1 There are a variety
of methods for introducing the anomeric primary amino
group which, in the native N-glycopeptides, serves as
the link to the aspartic acid g carboxyl and thence to the
peptide or protein via the aspartic acid a-amino acid
functionality. Interestingly, except for the method to be
described below, there is only one other approach that
introduces a pre-functionalized amino group at the
anomeric carbon, namely the modified Ritter reaction
reported by Fraser-Reid.2 There is also great interest in
‘neo’ glycopeptides where the link between carbo-
hydrate and peptide is not N-aspartyl, but is N-other or
where even the N is omitted.3 Our method is based on
the general concept of heterocycloaddition (Scheme 1)
to glycals (represented by 1) with a diene that contains
both the functionality desired in the aglycone (peptide)
and the required hetero atoms which make up the diene
framework (shown as 2) and thus merge the sugar and
the aglycone in one step, for example, when Y=N and
Z=‘peptide component’ for the specific example of a
glycopeptide (depicted as 3).4 Our approach requires
some preliminary synthetic construction of the neces-
sary diene, but avoids manipulations of the anomeric
functionality (other than deprotection). We describe in
detail the case where X=S, Y=N and Z=‘aspartate
residue’.5
In order to prepare an N-linked aspartic acid by our
method it was necessary to prepare heterodiene 4. The
precursor to diene 4 was prepared as shown in Scheme 2
by first applying our version6 of the C-acetylation of
amino acids to the BOC benzyl ester of aspartic acid 5
to produce keto diesters 8a,b. This transformation rou-
tinely takes place in >90% yield. Preparation of the
oximes 9a,b under conditions carefully defined to avoid
oxazolidinone formation was then followed by appli-
cation of the Hudson rearrangement to afford hetero-
diene precursors 4a and 4b.7
Sulfonimines 4a,b were then phthalimidosulfenylated to
produce the ultimate heterodiene precursors 11a,b. It
Scheme 1.
*Corresponding author. Tel.: +1-212-772-5340; fax: +1-212-772-5332; e-mail: rfranck@huntercuny.edu
yTaken in part from the PhD thesis of A.B., 2000, Universita di Firenze; the experiments described were done while A.B. was at Hunter College as
a visiting researcher sponsored by Dipartimento di Chimica Organica, Universita di Firenze, Firenze, Italy.
{Taken in part from the PhD thesis of B.L., 1999, City University of New York.
0968-0896/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0968-0896(03)00182-2

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A. Bartolozzi et al. / Bioorg. Med. Chem. 11 (2003) 3021–3027
Scheme 2.
should be noted that the four-step sequence from 5 to
4a,b is tolerated by the BOC group and causes no epi-
merization of the a-amino carbon. The dienes 12a and
12b were then generated in the presence of tribenzyl
galactal 13 and glucal 14 by addition of lutidine and
pyridine, respectively. The cycloadditions proceeded
smoothly over a period of 12h at room temperature to
afford the adduct 15 in 75% yield and for the same time
at 40 ꢀC to afford adduct 16 in 66% yield (88% yield
based on recovered glucal) after purification by flash
chromatography (Scheme 3). Again, it should be noted
that the a-amino carbon is tolerant of the conditions for
the conversions of Scheme 3.
At this point, we had in hand a masked galactoaspartate
and a masked glucoaspartate mimic, each with two dif-
ferentiated carboxyls, a protected a-amino function and
a protected anomeric vinylogous amide. Since the
toluenesulfonyl group in the galacto series was very
robust and not selectively cleavable in the presence of
Scheme 3.

A. Bartolozzi et al. / Bioorg. Med. Chem. 11 (2003) 3021–3027
3023
Scheme 4.
Experimental
the other protecting groups, we chose to carry out
our exploratory chemistry with the TMS-ethylsulfonyl
protecting group in the gluco series, as follows
(Scheme 4).
All reactions were carried out under a dry argon or
nitrogen atmosphere at ambient temperature unless
otherwise stated. Low temperatures were recorded as
bath temperatures. Chromatography was carried out on
silica gel 60, 230–400 mesh, using flash chromatography
techniques. Analytical thin-layer chromatography
(TLC) was performed on E. Merck precoated silica 60
F254 plates. Petroleum ether, hexane, pentane, dichloro-
methane, and ethyl acetate used as eluants were ACS
reagent grade solvent. The following reaction solvents
were purified by distillation: dichloromethane and
chloroform (from P2O5), diethyl ether (from benzophe-
none and sodium, N2), benzene (from CaH2, N2) and
THF (from benzophenone and sodium, N2), triethyl-
amine (from CaH2, N2), acetonitrile (from P2O5). NMR
spectra were measured with a GE QE 300 MHz instru-
ment. Chemical shifts are reported in d units, coupling
constants in Hz. TMS (d=0.0) was used as internal
reference for spectra measured in CDCl3. Infrared
spectra were recorded on a Perkin–Elmer 1310 spectro-
photometer and a BOMEM FTIR.
The o carboxyl was subjected to LiI and LiBr in DMF
and pyridine at several temperatures; and the
demethylation was successfully achieved with lithium
bromide and refluxing pyridine to afford 17 in 70%
yield. The BOC-protected a-amino function of 16 was
easily deprotected by TFA/CH2Cl2 to afford 18 which
then spontaneously cyclized with the o ester carbonyl to
afford lactam 19 in >95% yield. This tricyclic deriva-
tive permitted the simple deprotection of the TMS-
ethylsulfonyl group by TBAF in THF to produce 20 in
96% yield as a 3/1 mixture of epimers. In the more
flexible material 16, fluoride catalyzed liberation of the
anomeric N with either TBAF or CsF resulted in lactam
formation (21) with the a-ester carbonyl, even when
acetic acid was used with the fluoride reagent so as to
protonate the supposed anionic N intermediate. Lactam
21 was obtained as a 1.5/1 mixture of epimers.
In conclusion, we have demonstrated that a ‘neo’
glycopeptide building block can be constructed by a
novel heterocycloaddition. The rigid structure of this
sugar-peptide-type molecule represents a novel and ver-
satile scaffold for the construction of glycopeptides
through peptide chain elongation at either the N- or
C-terminal functions of 17, 20 and 21.8 However, it is
our belief that the most promising future development
will be to elaborate our synthesis of 4b and 11b to an
aspartate already incorporated into a small peptide
since small peptides should be inert to all the chemical
steps required. Then, a cycloadduct analogous to 16
would have the amino function and the acid carboxyl of
aspartate embedded in a robust peptide linkage. Thus
the adverse ring-formations and epimerizations, which
are due to the ester functions in our model, should be
minimized.
Benzyl 2-NHBOC-5-carbomethoxy-4-oxopentanoate (8).
A solution of benzyl N-BOC aspartate (150 mg, 0.46
mmol), Meldrum’s acid (74 mg, 0.51 mmol) and DMAP
(125 mg, 1.02 mmol) in 2.5 mL of anhydrous CH2Cl2
was cooled at ꢁ5 ꢀC. A solution of isopropenyl chloro-
formate (64 mg, 0.53 mmol) in 1.2mL of anhydrous
CH2Cl2 was added very slowly to the reaction mixture
under vigorous magnetic stirring. After the addition was
complete, stirring was continued for 1 h at ꢁ5 ꢀC until
the complete disappearance of the starting material was
detected. Then, 1.0 mL of 10% aq solution of KHSO4
was added to the solution, the cooling bath was
removed and an additional 1.0 mL of the KHSO4 solu-
tion was added. After dilution with CH2Cl2, the two
phases were separated and the organic phase was
washed with brine and then dried over anhydrous

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A. Bartolozzi et al. / Bioorg. Med. Chem. 11 (2003) 3021–3027
sodium sulfate. The evaporation of the solvent gave the
Meldrum’s acid adduct (petroleum ether/ethyl acetate
1/4, Rf=0.5) that was used for the following reaction
without any purification.
(B part of an ABX system, JAB=13.5 Hz), 1.413 (s, 9H,
ter-but). 13C NMR (CDCl3, 75 MHz): d 172.0, 170.4 (Cq
esters); 155.9, 151.5 (Cq t-Boc+Cq oxime); 135.8 (Cq
arom); 129.2, 129.0, 128.9, 128.8 (CH arom.); 80.8 (Cq
ter-but); 68.1 (CH2Ph); 52.9 (OCH3); 51.7 (CHCH2);
40.3 (CH2); 30.9 (CHCH2); 29.0 (ter-but). Spectral data
for oxime 9a, crude yield 92% from 8a: 1H NMR
(CDCl3), d ppm 8.04 (br, 1H), 7.33 (m, 5H), 5.62(d,
J=9.0 Hz, 1H), 5.16 (s, 2H), 4.60 (m, 1H), 4.11 (q,
J=7.2 Hz, 2H), 3.34 (m, 2H), 2.84 (m, 2H), 1.28 (s, 9H),
1.20 (t, J=7.2Hz, 3H); 13C NMR (CDCl3), d ppm
172.0, 168.9, 155.9, 150.5, 135.6, 128.8, 128.5, 80.3, 67.5,
61.4, 51.2, 36.8, 33.9, 28.5, 14.4, 14.3.
A solution of the Meldrum adduct in 4.0 mL of benzene
and 1.0 mL of anhydrous MeOH was heated under
reflux for 9 h. After evaporation of the solvent the crude
product was purified by silica gel column chromato-
graphy (petroleum ether/ethyl acetate 2/1, Rf=0.4)
1
to afford 8b (169 mg, 96%) as a yellow oil. H NMR
(CDCl3, 300 MHz): d 7.36–7.31 (m, 5H, Ph), 5.48 (bd,
J=8.4 Hz, 1H, NH), 5.151 (s, 2H, CH2Ph), 4.59–4.53
(m, 1H, CHNH), 3.706 (s, 3H, OCH3), 3.432(s, 2H,
COCH2CO), 3.31–3.23 (A part of an ABX system,
JAB=18.3 Hz, JAX=4.4 Hz, 1H of CH2CH), 3.14–3.06
(B part of an ABX system, JAB=18.3 Hz, JBX=4.4 Hz,
1H of CH2CH), 1.417 (s, 9H, ter-but). 13C NMR
(CDCl3, 75 MHz): d 201.2 (CO); 171.4, 167.5 (Cq
esters); 156.0 (Cq t-Boc); 135.9 (Cq arom); 129.2, 129.0,
128.8 (CH arom); 80.8 (Cq ter-but); 68.1 (CH2Ph); 53.1,
50.3, 49.6, 45.5 (OCH3, COCH2CO, CH2CH, CH2CH);
29.0 (ter-but).MS (ES): calcd for C19H25NO7+Na, 402;
found m/z 402.2 [M+Na]+); 280.1 ; 248.1.
The crude oxime 9a was converted to the title com-
pound 4a as follows using material prepared from 200
mg (0.49 mmol) of 8a. A solution of oxime (3 mmol, 1
equiv) in anhydrous CCl4 (30 mL, 0.10 M) was cooled
to 0 ꢀC and treated with triethylamine (4.5 equiv). The
solution was stirred for 5 min at such temperature
before a suspension of p-toluenesulfonyl cyanide (2.5
equiv) in 1 mL of CCl4 was added. The resulting reac-
tion mixture was stirred at 0 ꢀC for 1 h, allowed to warm
to room temperature over 30 min and further stirred at
room temperature for 10 h. Concentration of the reac-
tion mixture afforded the crude product. Purification by
Similarly, 8a was produced by using ethanol in place of
methanol. Yield: 92%; mp: 53–56 ꢀC (lit.113 54–57 ꢀC);
FTIR (film): 3374, 2979, 1738, 1713, 1501, 1367, 1164;
1H NMR (CDCl3), d ppm 7.32(m, 5H), 5.43 (d, J=8.9,
1H), 5.18 (s, 2H), 4.60 (m, 1H), 4.2 (q, J=7.2, 2H), 3.41
(s, 2H), 3.20 (m, 2H), 1.42 (s, 9H), 1.23 (t, J=7.2, 3H).
13C NMR ( CDCl3), d ppm: 201.2, 171.3, 166.8, 155.8,
135.6, 128.9, 128.7, 128.5, 80.5, 67.8, 61.2, 49.9, 49.5,
45.1, 28.6, 14.4.
1
a silica column afforded the product. Yield: 56%; H
NMR (CDCl3), d ppm 10.96 (br, 1H), 7.84 (d, J=8.1
Hz, 2H), 7.70 (d, 7.9, 1H), 7.38 (m, 6H), 5.64 (d, J=8.1,
1H), 5.20 (m, 3H), 4.8 (m, 1H), 4.14 (m, 2H), 3.65 (m,
1H), 2.40 (s, 3H), 2.30 (m, 3H), 1.42 (s, 9H), 1.32 (m,
3H). 13C NMR (CDCl3), d ppm 171.2, 169.0, 152.3,
144.7, 137.5, 135.7, 130.4, 130.2, 129.0, 128.8, 128.7,
127.5, 100.5, 67.8, 60.8, 53.1, 35.0, 30.1, 28.6, 22.0, 14.5.
Benzyl 2-NHBoc-5-carbomethoxy-4-(trimethylsilylethyl-
sulfonamido)-3-pentenoate (4b). The crude oximes 9a,b
were synthesized from 8a,b as follows. A solution of
NH2OHꢂ HCl (2.0 equiv) and Na2CO3 (2.0 equiv) in
EtOH (20 mL) was heated under reflux. After 1 h, 1.14 g
(3.0 mmol) of keto compound 8 was added at room
temperature. The resulting solution was heated under
reflux until the disappearance of the starting material
was detected. The solvent was evaporated, the mixture
was diluted with ethyl acetate (10 mL) and washed twice
with brine (2ꢃ8.0 mL). Drying of the organic phase
over anhydrous sodium sulfate followed by evaporation
of the solvent gave the crude product as a mixture of
syn–anti isomers (dichloromethane/methanol 70/1,
Rf=0.2, 95%) which was pure enough to be used in the
following step. Attempts to further purify the product
by silica gel column and florosil column chromato-
graphy showed that the product was not stable during
purification and the yield of the chromatographed pro-
duct was very low. Spectral data of one of the two 9b
The crude oxime 9b was converted to the title com-
pound 4b as follows using material prepared from 815
mg (2.07 mmol) of 8b. A solution of the oxime (approx.
2mmol) and triethylamine (2mmol) in anhydrous ether
(4 mL) was cooled to ꢁ30 ꢀC and a solution of the
TMSethylsulfinyl chloride in diethyl ether (2mmol in
4.0 mL) was added. The reaction mixture was stirred for
1 h at ꢁ30 ꢀC and then at room temperature for 45 min.
The crude product was purified by silica gel column
chromatoghraphy with petroleum ether/ethyl acetate 5/1,
Rf=0.48) to obtain 4b (792mg, 70%) as a pale yellow
oil. The 1H NMR spectrum revealed the presence of two
compounds. One of these two (possibly an isomer at the
nitrogen center) was present only in traces and was not
1
separable from the major product. H NMR (CDCl3,
300 MHz): d 10.590 (s, 1H, NH), 7.42–7.34 (m, 5H, Ph),
5.29–5.10 (m, 4H, CHCOOMe+NHtꢁBoc+CH2Ph),
4.61–4.55 (m, 1H, CHCH2), 3.700 (s, 3H, OCH3), 3.24–
3.09 (m, 3H, CH2SO2+1H of CH2CH), 2.84–2.76 (m,
1H of CH2CH), 1.395 (s, 9H, ter-but), 1.08–1.02(m,
2H, CH2Si(CH3)3), 0.031 (s, 9H, Si(CH3)3).
1
oximes: H NMR (CDCl3, 300 MHz): d 8.46 (bs, 1H,
OH), 7.42–7.27 (m, 5H, Ph), 5.44 (bd, J=8.1 Hz, 1H,
NH), 5.204 (s, 2H, CH2Ph), 4.65–4.57 (m, 1H of
CHCH2), 3.71 (s, 3H, OCH3), 3.31–3.26 (A part of an
AB system, JAB=16.1 Hz, 1H of COCH2CO), 3.24–3.18
(B part of an AB system, JAB=16.1 Hz, 1H of
COCH2CO), 3.04–2.96 (A part of an ABX system,
JAB=13.5 Hz, JAX=9.1 Hz, 1H of CH2CH), 2.89–2.83
13C NMR (CDCl3, 75 MHz): d 171.4, 169.6 (Cq esters);
155.7, 152.9 (Cq enam.+Cq t-Boc); 135.9 (Cq arom);
129.3, 128.9 (CH arom.); 100.2 (CHCOOMe); 81.2(C
ter-but); 68.3 (CH2Ph); 53.5, 52.5, 52.3 (OCH3+CH2-
SO2+CHCH2); 37.6 (CHCH2); 29.3 (ter-but); 10.8
(CH2Si(CH3)3);ꢁ0.88 (Si(CH3)3).
q

A. Bartolozzi et al. / Bioorg. Med. Chem. 11 (2003) 3021–3027
3025
Synthesis of galactal cycloadduct 15. First, the tosyl
imine 4a is phthalimidosulfenylated as follows. To a
solution of sulfonyl imine 76 mg (0.15 mmol) in di-
chloromethane was added PhthN-S-Cl (38 mg, 1.2
equiv) in portions at 0 ꢀC during a period of 15 min. The
reaction mixture was stirred at such temperature for an
additional 20 min, allowed to warm up to room tem-
perature in 30 min. Cold n-pentane was added. A white
precipitate formed which was filtered and then washed
with cold n-pentane to afford the desired product. To a
solution of the phthalimidosulfenyl imine and tri-O-
benzyl-d-galactal (54 mg, 0.13 mmol, 1 equiv) in
chloroform was added a catalytic amount of 2,6-lutidine
(2mol%). The resulting solution was stirred at room
temperature until the reaction was complete as mon-
itored by TLC. The solution was dissolved in dichloro-
methane and washed with saturated ammonium chloride
and brine, dried over Na2SO4. The organic solvent was
removed under reduced pressure. The crude materials
were purified by a silica gel column to give the desired
product (50 mg, 0.09 mmol). Yield: 75%; 1H NMR
(CDCl3), d ppm 7.83 (d, J=11.1 Hz, 2H), 7.35 (m, 20H),
6.87 (d, J=11.1 Hz, 2H), 6.13 (d, J=6.9 Hz, 1H), 6.22 (m,
1H), 5.15 ( m, 1H), 4.97 (d, 12.9, 1H), 4.85 (d, J=13.2Hz,
1H), 4.53 (m, 5H), 4.21 (m, 2H), 4.0–3.7 (m, 5H), 3.54 (m,
2H), 3.28 (m, 2H), 2.21 (s, 3H), 1.29 (s, 9H), 1.20 (m, 3H).
13C NMR (CDCl3), d ppm 173.1, 167.0, 155.7, 144.2,
138.1, 138.0, 135.6, 129.8, 128.6, 128.3, 128.2, 128.0, 127.8,
127.6, 127.5, 127.2, 119.2, 86.6, 77.7, 77.6, 77.5, 77.2, 74.5,
73.4, 72.7, 67.0, 62.3, 44.7, 44.6, 33.6, 29.7, 28.4, 21.5, 13.9.
CHCH2), 3.79–3.74 (m, 4H, OCH3+H3), 3.60–3.57 (m,
1H, H5), 3.35–3.31 (m, 8H, CH2SO2+CHCH2+
H6a+H6b+H2+H4), 1.198 (s, 9H, ter-but), 1.16–1.07
(m, 2H, CH2Si(CH3)3), ꢁ0.045 (s, 9H, Si(CH3)3). 13C
NMR (CDCl3, 75 MHz): d 171.5, 166.9, 155.9, 145.1
(Cq esters+Cq t-Boc+Cq olef); 138.0, 137.7, 135.4 (Cq
arom); 128.5, 128.4, 128.3, 128.1, 128.0, 127.9, 127.7,
127.6, 127.5 (CH arom); 118.6 (Cq olef); 80.0 (Cq ter-
but); 86.0, 78.7, 78.2, 77.2 (C1+C3+C4+C5); 76.2,
75.1, 73.3, 68.6, 67.2(4 CH2Ph+ C6 ); 73.4 (CH2CH);
54.4 (CH2CH); 53.0 (OCH3); 48.1(C2); 33.4 (CH2SO2);
28.3 (ter-but); 9.8 (CH2Si(CH3)3);ꢁ2.0 (Si(CH3)3). IR:
3360, 2950, 1718, 1595, 1497, 1453, 1352, 1253, 1153,
1072cm ꢁ1. MS (ES): m/z 1006 (100%, [M+NH+4 ]+).
Anal. calcd: (%) for C51H64N2O12S2Si (989.28): C,
61.92; H, 6.52; N, 2.83; S, 6.48. Found: C, 61.65; H,
6.54; N, 2.70; S, 6.45. [a]2D3 +189ꢀ (c 0.28, CHCl3).
Synthesis of carboxylic acid 17
A solution of 16 (100 mg, 0.10 mmol) and LiBr (35 mg,
0.40 mmol) in anhydrous pyridine (3.0 mL) was refluxed
for 3.5 h. The solvent was evaporated and silica gel col-
umn cromatography (dichloromethane/methanol 9/1;
with dichloromethane/methanol 9/1, Rf=0.4) gave 17
1
(68 mg, 70%) as a pale yellow oil. The H NMR is
1
characterized by very broad signals. H NMR (CDCl3,
300 MHz): 7.43–7.04 (m, 20H); 6.16–6.13 (m, 1H); 5.03–
4.96 (m); 4.97–4.70 (m); 4.53–4.43 (m); 3.61–3.01 (m);
1.2–0.86 (m); 0.025–0.014 (s, 9H, t-but) d. MS (ES):
calcd for C50H62N2O12S2Si: 974 found m/z 875.4 (100%,
[MꢁtꢁBoc+H]+), 997.3 (25%, [M+Na]+).
Synthesis of glucal cycloadduct 16
To an ice-cooled solution of 4b (350 mg, 0.646 mmol) in
CHCl3 (3.0 mL), propylene oxide (45 mg, 0.77 mmol)
and phthalimidosulphenyl chloride, 98% (154 mg, 0.71
mmol) were added slowly. After 20 min, the ice bath
was removed and the mixture was stirred for 90 min at
Synthesis of lactam 21
TBAF (0.24 mmol) was added to an ice-cooled solution
of 16 (120 mg, 0.12 mmol) in THF (2.0 mL). The ice
bath was immediately removed and the solution was
stirred at room temperature until the complete dis-
appearance of the starting material was observed (about
6 h). The solution was diluted with CH2Cl2 and washed
with saturated solution of NH4Cl (3ꢃ3.0 mL). The
organic phase was dried over anhydrous sodium sulfate
and after evaporation of the solvent silica gel column
chromatography of the crude product (petroleum ether/
ethyl acetate 2.5/1) gave 21 (75 mg, 80%) as a yellow oil
as a mixture of isomers (1.5/1, isomer with higher Rf/
isomer with lower Rf).
1
room temperature. After an H NMR of the reaction
mixture showed the total disappearance of the starting
material, 3,4,6-tri-O-benzyl glucal 14 (269 mg, 0.65
mmol), and pyridine (41 mL, 0.52mmol) were added
and the solution was heated to 40 ꢀC overnight. After
evaporation of the solvent the crude product was pur-
ified by silica gel column chromatography (petroleum
ether/ethyl acetate 9/1; with petroleum ether/ethyl ace-
tate 4/1, Rf=0.3) to obtain 16 (421 mg, 66%) as a pale
yellow oil and recovering 66 mg (conversion yield: 88%)
1
of starting material. The H NMR shows traces of an
1
isomer, probably the b-cycloadduct, not separable from
the major product. 1H NMR (CDCl3, 500 MHz): d
7.34–7.01 (m, 20H, Ph), 6.453 (d, J=8.0 Hz, 1H, NH),
6.371 (d, J1,2=7.5 Hz, 1H, H1), 5.31–5.29 (A part of an
AB system, JAB=12.5 Hz, 1H of CH2Ph), 5.05–5.03 (B
part of an AB system, JAB=12.5 Hz, 1H of CH2Ph),
4.78–4.76 (A part of an AB system, JAB=10.5 Hz, 1H
of CH2Ph), 4.72–4.70 (A part of an AB system,
JAB=11.0 Hz, 1H of CH2Ph), 4.63–4.61 (B part of an
AB system, JAB=10.5 Hz, 1H of CH2Ph), 4.51–4.49 (B
part of an AB system, JAB=11.0 Hz, 1H of CH2Ph),
4.48–4.46 (A part of an AB system, JAB=12.5 Hz, 1H
of CH2Ph), 4.43–4.40 (B part of an AB system,
JAB=12.5 Hz, 1H ofCH2Ph), 4.39–4.36 (m, 1H,
Spectral data of the first isomer (higher Rf): H NMR
spectra at different temperatures show sharpening of the
peaks.
1H NMR (CDCl3, 300 MHz): d 7.36–7.16 (m, 15H, Ph);
5.97 (bm, 1H, H1); 5.13–5.09 (bm, 1H, CHCH2); 4.68–
4.51 (m, 6H, 3 CH2Ph); 4.18–4.11 (m, 1H, CHCH2);
4.02–3.98 (m, 1H, H5); 3.86–3.70 (m, 8H,
H6a+H6b+OCH3+H3+H4+1H of CHCH2); 3.28
(bm, 1H, H2); 3.153 (m, 1H); 1.430 (s, 9H, ter-but). 13C
NMR (CDCl3, 75 MHz): d 172.9 (Cq lact); 164.4 (Cq
esters); 155.0, 145.1 (Cq t-Boc+Cq olef); 138.0, 137.6,
137.1 (Cq arom); 128.4, 128.3, 128.2, 128.1, 127.9, 127.7,
127.6, 127.5 (CH arom); 96.8 (Cq olef); 80.7; 77.2; 74.6;

3026
A. Bartolozzi et al. / Bioorg. Med. Chem. 11 (2003) 3021–3027
74.0, 73.8, 73.6, 73.5 (broad signals); 68.4; 68.2; 53.4;
52.1; 50.1; 41.1; 34.0; 28.3.MS (ES): calcd for
C39H44N2O9S: 716 Found: m/z 717.3 (35%, [M+H]+),
734.3 (100%, [M+NH4+]+), 739.2(30%, [M+Na] +).
with saturated solution of NH4Cl (3ꢃ3.0 mL). The
organic phase was dried over anhydrous sodium sul-
fate and after evaporation of the solvent the crude
product was purified by silica gel column chromato-
graphy (petroleum ether/ethyl acetate 1/2, Rf=0.30,
Rf=0.22 visualized with UV and phosphomolybdic
acid) to afford 20 (109 mg, 96%) as a yellow oil as a
mixture of isomers (3/1, isomer with Rf=0.30/isomer
with Rf=0.22).
Spectral data of the second isomer (lower Rf): 1H NMR
(CDCl3, 300 MHz): d 7.36–7.15 (m, 15H, Ph); 5.866 (bd,
J=3.3 Hz, 1H, H1); 5.12–5.10 (m, 1H, NH); 4.74–4.49
(m, 6H, 3 CH2Ph); 4.19–4.12(m, 1H, CHCH2); 4.06–
4.03
(m,
1H,
H5);
3.99–3.69
(m,
8H,
1
H6a+H6b+OCH3+H3+H4+1H of CHCH2); 3.259
(bdd, J2,1=4.0 Hz, J2,3=6.6 Hz, 1H, H2); 3.10–3.07 (A
part of an ABX system, JAB=16.8 Hz, JAX=7.3 Hz,
1H of CHCH2); 3.04–3.02(B part of an ABX system,
JAB=16.8 Hz, JBX=7.3 Hz, 1H of CHCH2); 1.426 (s,
9H, ter-but). MS (ES): calcd for C39H44N2O9S: 716.
Found: m/z 717.3 (35%, [M+H]+), 734.3 (100%,
[M+NH+4 ]+), 739.2(30%, [M+Na] +).
Spectral data of the first isomer (Rf=0.30): H NMR
(CDCl3, 300 MHz): d 7.39–7.14 (m, 20H, Ph); 5.79 (bs,
1H, NH); 5.334 (d, J1,2=4.4 Hz, 1H, H1); 5.25–5.21 (A
part of an AB system, JAB=12.1 Hz, 1H of CH2 of
COOBn); 5.19–5.15 (B part of an AB system, JAB=12.1
Hz, 1H of CH2 of COOBn); 5.03–5.00 (A part of an AB
system, JAB=10.2Hz, 1H of CH2Ph); 4.88–4.84 (A part
of an AB system, JAB=10.6 Hz, 1H of CH2Ph); 4.77–
4.74 (B part of an AB system, JAB=10.2Hz, 1H of
CH2Ph); 4.59–4.51 (m, 2H, CH2Ph); 4.51–4.47 (B part
of an AB system, JAB=10.6 Hz, 1H of CH2Ph); 4.43
(bs, 1H, NH); 4.18–4.11 (m, 1H,CHCH2); 3.94–3.91 (m,
1H); 3.65–3.63 (m, 2H, H6a+H6b); 3.62–3.51 (m, 2H);
3.162(dd, J2,1=4.0 Hz, J2,3=9.2Hz, H 2); 2.70–2.67 (m,
2H, CHCH2). 13C NMR (CDCl3, 75 MHz): d 169.6,
Synthesis of lactam 19
Compound 16 (51 mg, 0.05 mmol) was dissolved at 0 ꢀC
in 0.8 mL of a 10% solution of CF3COOH in anhyd-
rous CH2Cl2. Ten min after the addition the ice bath
was removed and the solution was left for 2h at room
temperature with magnetic stirring. The solvent was
evaporated and Et2O was added to the crude product to
precipitate the salt of the amine as a white solid. After
evaporation of the solvent the product was dissolved in
anhydrous DMF (2.0 mL) and NEt3 (8.5 mL, 0.06
mmol) was added. The solution was left at room tem-
perature for 4 h and the solvent was removed by eva-
poration and silica gel column chromatography of the
crude product (petroleum ether/ethyl acetate 1/1, visua-
lized with UV and phosphomolibdic acid) gave 19 (44
165.2(C esters+Cq lact); 144.7 (Cq olef); 137.9, 137.8,
q
137.7, 134.8 (Cq arom); 128.9, 128.8, 128.6, 128.5, 128.4,
128.3, 128.2, 128.1, 127.9, 127.8, 127.7 (CH arom); 88.0
(Cq olef.); 78.9; 78.7; 78.0; 77.2; 75.3; 73.6; 72.2; 69.2;
67.7; 51.9; 41.7; 31.3. MS (ES): calcd for C40H40N2O7S:
692. Found m/z 693.5 (100%, [M+H]+), 715.5 (30%,
[M+Na]+).[a]D23 +214ꢀ (c 0.13, CHCl3).
Spectral data of the second isomer (Rf=0.22): 1H NMR
(CDCl3, 300 MHz): d 7.39–7.14 (m, 20H, Ph); 5.796 (bd,
J=1.8 Hz, 1H, NH); 5.384 (d, J1,2=4.8 Hz, 1H, H1);
5.22–5.18 (A part of an AB system, JAB=12.1 Hz, 1H
of CH2 of COOBn); 5.16–5.12(B part of an AB system,
JAB=12.1 Hz, 1H of CH2 of COOBn); 4.99–4.95 (A
part of an AB system, JAB=9.9 Hz, 1H of CH2Ph);
4.88–4.84 (A part of an AB system, JAB=10.6 Hz, 1H
of CH2Ph); 4.69–4.66 (B part of an AB system,
JAB=9.9 Hz, 1H of CH2Ph); 4.57–4.52(m, 2H); 4.50–
4.47 (B part of an AB system, JAB=10.6 Hz, 1H of
CH2Ph); 4.17–4.12(m, 1H, CHCH2); 3.69–3.60 (m, 2H);
3.54–3.43 (m, 2H); 2.84–2.76 (A part of an ABX system,
JAB=15.7 Hz, JAX=8.4 Hz, 1H of CHCH2); 2.74–2.66
(B part of an ABX system, JAB=15.7 Hz, JBX=5.9 Hz,
1H of CHCH2). 13C NMR (CDCl3, 75 MHz): d 170.1,
165.7 (Cq esters+Cq lact); 145.6 (Cq olef); 138.1, 137.9,
137.8; 134.7 (Cq arom); 128.7, 128.6, 128.5, 128.3, 128.2,
128.0, 127.9, 127.8, 127.7, 127.6 (CH arom); 87.6 (Cq
olef); 79.2, 78.7, 78.4, 76.8, 75.1, 73.6, 72.1, 69.3, 67.7,
51.2, 41.3, 31.3. MS (ES): calcd for C40H40N2O7S: 692.
Found m/z 693.5 (100%, [M+H]+), 715.5 (30%,
[M+Na]+).
1
mg, quantitative yield) as a pale yellow oil. H NMR
(CDCl3, 300 MHz): d 7.39–7.14 (m, 20H, Ph); 6.38 (bs,
1H, NH); 6.152(d, J1,2=5.9 Hz, 1H, H1); 5.27–5.29 (A
part of an AB system, JAB=12.1 Hz, 1H of CH2 of
COOBn); 5.17–5.13 (B part of an AB system, JAB=12.1
Hz, 1H of CH2 of COOBn); 4.88–4.82(A part of an AB
system, JAB=10.0 Hz, 1H of CH2Ph); 4.85-4.78 (m, 1H
of CH2Ph); 4.73–4.70 (B part of an AB system,
JAB=10.0 Hz, 1H of CH2Ph); 4.58–4.51 (m, 3H); 4.24–
4.19 (m, 1H, CHCH2); 3.78–3.26 (m, 8H,
H2+H3+H4+H5+H6a+H6b+1H of CH2SO2+1H of
CHCH2); 3.05–2.92 (m, 2H, 1H of CH2SO2+1H of
CHCH2); 1.11–1.05 (m, 2H, CH2Si(CH3)3); 0.00 (s, 9H,
Si(CH3)3). 13C NMR (CDCl3, 75MHz): d 169.2, 163.2 (Cq
est.+Cq lact); 143.4 (Cq olef); 137.5, 137.4, 137.3 (Cq
arom); 134.5 (Cq arom); 128.6, 128.4, 128.3, 128.1, 127.9,
127.8, 127.7, 127.6 (CH arom); 116.3 (Cq olef); 85.8 (CH);
78.2, 77.9 (2 CH); 76.0, 75.2, 73.6 (3 CH2); 72.4 (CH);
68.4, 68.1 (2CH 2); 52.6 (CH2); 52.2 (CH); 46.3 (CH); 32.0
(CH2SO2); 9.9 (CH2Si(CH3)3); ꢁ1.92(Si(CH 3)3).
Synthesis of desulfonylated lactam 20
TBAF (0.16 mmol) was added to a solution of 19 (140
mg, 0.16 mmol) in anhydrous THF (2.5 mL) and the
solution was stirred until the complete disappearance
of the starting material was detected (about 4 h).
After dilution with CH2Cl2 the solution was washed
Acknowledgements
Financial support for this work at Hunter College of
CUNY came from NIH grant GM 51216 and PSC/
CUNY funds. The Chemistry Department infrastructure

A. Bartolozzi et al. / Bioorg. Med. Chem. 11 (2003) 3021–3027
3027
is supported by an RCMI grant NIH RR 03037. The
NMR laboratory has received support from the NY
State GRI and HEAT initiatives. The mass spectro-
metry facility has also received funding from GRI and
NSF grant CHE-9708881. We thank the Department of
Organic Chemistry, Universita di Firenze and Professor
G. Capozzi, supported by MURST, for sponsoring the
visit of A.B. to Hunter College.
3. (a) Lundquist, J. J.; Debenham, S. D.; Toone, E. J. Org.
Chem. 2000, 65, 8245. (b) Herzner, H.; Reipen, T.; Schultz,
M.; Kunz, H. Chem. Rev. 2000, 100, 4495. (c) Dondoni, A.;
Marra, A. Chem. Rev. 2000, 100, 4395.
4. (a) For papers describing this approach for O-glycosides,
see: Capozzi, G.; Falciani, C.; Menichetti, S.; Nativi, C.; Raf-
faelli, B. Chem. Eur. J. 1999, 5, 1748. (b) Capozzi, G.; Dios,
A.; Franck, R. W.; Geer, A.; Marzabadi, C.; Menichetti, S.;
Nativi, C.; Tamarez, M. Angew. Chem., Int. Ed. Engl. 1996,
35, 777.
5. Li, B.; Franck, R. W.; Capozzi, G.; Menichetti, S.; Nativi,
C. Org. Lett. 1999, 1, 111.
References and Notes
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add the direct Staudinger reaction linking of an anomeric
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standard peptide-coupling conditions.