Full text of DOI:10.1016/S0040-4020(01)00545-2

TETRAHEDRON
Pergamon
Tetrahedron 57 72001) 6531±6538
New design concepts for constraining glycosylated amino acids
Jonathan W. Lane and Randall L. Halcombp
Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA
Received 19 January 2001; revised 23 February 2001; accepted 20 March 2001
AbstractÐIn an effort to probe the mechanism by which glycosyltransferases recognize glycoproteins and assemble the core structures of
O-linked oligosaccharides, two constrained glycopeptides based on the a-N-acetylgalactosaminyl serine substructure were chosen for
synthesis. These compounds representing one of the two possible gauche conformations of the d-serine and l-serine con®gurations of
the parent substructure were successfully prepared in reasonable overall yield through a convergent strategy. q 2001 Elsevier Science Ltd.
All rights reserved.
1. Introduction
fundamental understanding could, in turn, lead to new thera-
peutic strategies for conditions that are characterized by
abnormal glycosylation.
Glycosylation is a ubiquitous posttranslational modi®cation
of proteins and is associated with a number of processes
both within cells and at cell surfaces. These include protein
transport, cell adhesion, and signal transduction.1 Aberrant
glycosylation of cellular proteins and glycolipids is asso-
ciated with various diseases, including cancerous and
in¯ammatory conditions.2 Therefore, an understanding on
a molecular level of the structural features of glycoproteins
that are recognized by various enzymes and receptors would
be valuable in developing inhibitor-based strategies to
control carbohydrate-mediated cellular processes. This
The ground state conformations of small glycopeptide frag-
ments have been studied in some detail by NMR spectro-
scopy;3 however, relatively little is known about the
conformations that are recognized by various enzymes and
receptors. The preferred torsional angle about the exocyclic
C1±O bond 7f) of the carbohydrate moiety in the ground
state is somewhat rigid, as predicted by the exoanomeric
effect.4 However, other torsional angles within the
side chain, including x2, are somewhat more ¯exible, as
Scheme 1. Synthesis strategy for 2/3.
Keywords: glycoproteins; constrained glycopeptides; spiroketalization.
Corresponding author. Tel.: 11-303-492-1043; fax: 11-303-492-5894; e-mail: halcomb@colorado.edu
p
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020701)00545-2

6532
J. W. Lane, R. L. Halcomb / Tetrahedron 57 02001) 6531±6538
Figure 1.
core structures of O-linked oligosaccharides, a series of
conformationally constrained glycopeptides was chosen
for synthesis. These particular targets were identi®ed in an
effort to represent the accessible low energy conformations
about x2 of the native structure 1, and will ultimately be
incorporated into small peptide fragments for biological
evaluation. Among those chosen was compound 2 in
which the a-carbon 7Ca) of the amino acid component of
the structure is incorporated into a six-membered ring, and
the x2 torsional angle is effectively constrained in one of the
two possible gauche conformations 7Fig. 1). Additionally,
the C1±O bond is locked into the low-energy exoanomeric
orientation. The unnatural d-serine con®guration is repre-
sented by the similarly constrained analog 3.
The synthesis strategy for 2 7Scheme 1) was designed with
the focus of preferentially establishing the desired l-serine
con®guration at Ca as well as the necessary anomeric orien-
tation at the spiroketal center. Speci®cally, the desired
anomeric con®guration could arise from a thermodynamic
spiroketalization event,7 and the stereochemistry at Ca
could be established through diastereomeric preferences
inherent within this same process. In a general sense, the
synthesis would commence with a convergent nucleophilic
addition of appropriately protected versions of the carbo-
hydrate 74) and amino acid 75) components of the target
structure. Manipulations to the resulting product 6, inclu-
ding reduction of the internal alkyne, would provide the
spiroketal precursor 7. Selective spiroketalization of one
of the diastereotopic primary hydroxyl groups and subse-
quent oxidation of the other, followed by protecting group
Figure 2. Predicted diastereomeric spiroketalization preferences.
illustrated in the a-N-acetylgalactosaminyl serine sub-
structure 1.
Signi®cant research has gone into designing conformational
constraints into various amino acid and peptide motifs to
probe bioactive conformations.5 However, conformation-
ally restricted glycopeptides are relatively unexplored.6
In an effort to probe the mechanism by which glycosyl-
transferases recognize glycoproteins and assemble the
Scheme 2. Synthesis of 14.

J. W. Lane, R. L. Halcomb / Tetrahedron 57 02001) 6531±6538
6533
Scheme 3. Acetylide addition to 15 and subsequent spiroketalization.
manipulations, should provide the analogs 2 and 3. The
unnatural d-serine analog 3 could also prove useful in our
studies.
Addition of an excess of the dianion of 14 to the lactone 1510
afforded the desired product 16 as a complex mixture of
stereoisomers 7Scheme 3). In order to facilitate puri®cation
of compound 16 from unreacted 14, the ensuing deprotec-
tion step was necessarily performed on the product mixture.
Reaction of the mixture with mild acid under the conditions
shown gave the desired diol 17 as a mixture of the two
presumed anomers in good overall yield from 15. The diol
product resulting from the deprotection of unreacted excess
14 could be recycled if desired. Partial reduction of the
alkyne group of 17 followed by acid-catalyzed spiroketali-
zation proceeded in a one-pot reaction to give a complex
mixture of products 776%), two of which were 18a and 18b.
It is important to note that a judicious choice of solvent and
acid concentration were necessary for the success of the
spiroketalization. Overreduction of the internal alkyne func-
tional group of 17 to the corresponding alkane was observed
when methanol was used as a solvent or cosolvent, as well
as when lower acid concentrations were employed. This
alkane could not be cyclized, even under strongly acidic
conditions, and only decomposition was observed in these
attempts. Interestingly, the ole®n groups in 18a and 18b
were resistant to reduction to their respective alkanes even
under high-pressure 71500 psi) catalytic hydrogenation
conditions.
Assuming thermodynamic conditions, the spiroketalization
was predicted to afford an excess of the desired l-dia-
stereomer, 2. This prediction was based on the presumed
inherent preference of an amine bearing a bulky protecting
group, such as a tert-butyl carbamate, over a hydroxy-
methyl, to reside in the pseudo-equatorial position. This
preference is illustrated by the partial half-chair structures
8 and 9 7Fig. 2).
2. Results and discussion
The synthesis of the nucleophilic amino acid component 14
was accomplished in an ef®cient, straightforward manner
7Scheme 2). The synthesis commenced with protection of
the amine group of 108 to give the tert-butyl carbamate 11 in
good yield. Swern oxidation of the free primary alcohol in
11 gave the aldehyde 12. A modi®ed three-step alkynylation
procedure adopted from a recently reported procedure9 was
applied to 12 to give the alkyne 13 in good overall yield. The
acetal 14 was then obtained by a straightforward protecting
group exchange sequence. The alkynyl anion of 13 also
served as a reasonable nucleophile in additions to our carbo-
hydrate acceptor. However, dif®culties associated with the
removal of the acetonide from the resulting product were
encountered. Therefore, the acetonide was replaced with the
more labile p-anisylidine acetal protecting group, producing
compound 14 as a 2:1 mixture of diastereomers. Since both
diastereomers of 14 were found to add to the carbohydrate
acceptor, and since the acetal stereocenter was removed
shortly thereafter, this mixture was carried to the next step
without separation.
HPLC analysis of this spiroketal product mixture revealed
an approximate 2:1:1 mixture of major products in addition
to at least one other inseparable minor product. Partial
separation of the three major compounds could be achieved
by preparative HPLC. Contrary to our initial predictions, the
major constituent of this mixture was shown to be 18a,
which is diastereomeric to the desired product 18b at Ca.
The con®guration of 18a was proven by X-ray analysis of an
analog obtained from further transformations 7unpublished
results). The desired 18b was presumed to be one of the

6534
J. W. Lane, R. L. Halcomb / Tetrahedron 57 02001) 6531±6538
Scheme 4. Synthesis completion.
other major products in the mixture. Unfortunately, resub-
mission of 18a to the original spiroketalization conditions
resulted in no detectable equilibration to the originally
observed product composition. These results seem to
suggest that the spiroketalization did not give a thermo-
dynamic product mixture, and may be re¯ecting kinetic
preferences for the reaction. This notion is reinforced by
the probable presence of other isomers, tentatively assigned
as b-anomers, in the complex product mixture. Preliminary
investigations into a variety of alternate conditions to
equilibrate these products were unsuccessful.
Despite the complex product mixture obtained from the
spiroketalization, 18a and 18b were transformed into the
methyl esters 21a and 21b, respectively, thus meeting
the initial goals of the project 7Scheme 4). In light of the
dif®culties associated with separation of the various spiro-
ketals, it was determined that puri®cation should be
executed at a late stage in the synthesis. Additionally, the
product mixtures obtained following the addition reaction
were complex and often inseparable. Thus, all characteri-
zations prior to 21a and 21b were performed on these
mixtures.
Dess±Martin oxidation11 of the spiroketal mixture including
18a/b, followed by further oxidation under Lindgren condi-
tions12 and subsequent methyl esteri®cation, gave a mixture
of products that included the methyl esters 19a/b in good
overall yield. Removal of the silyl groups of 19a/b and
peracetylation of the resulting free alcohols gave 20a/b in
addition to other minor isomers. Reduction of the azide and
ole®n functionalities in the presence of acetic anhydride
gave 21a/b as the usual product mixture in addition to at
least two byproducts. The identities of the byproducts in this
reaction have yet to be determined, and all attempts to
prevent formation of these byproducts using a variety of
alternate reaction conditions were unsuccessful. Final
HPLC puri®cation successfully facilitated separation of
the two major products from the additional isomers in the
mixture. The two major products 21a and 21b were isolated
in a 6:5 ratio by mass in favor of 21a. X-Ray analysis of
the major diastereomer unambiguously established the
structure of d-serine analog 21a. The structure of the
minor diastereomer or l-serine analog 21b was unambigu-
ously con®rmed by NOE correlations derived from NOESY
experiments. The NOE structural determination protocol
was partially validated through studies performed on 21a
7Fig. 3).
Figure 3. Observed NOE interactions for 21a and 21b.

J. W. Lane, R. L. Halcomb / Tetrahedron 57 02001) 6531±6538
6535
3. Conclusion
was washed with 2£300 mL saturated aq NaHCO3, fol-
lowed by 300 mL H2O, and 300 mL brine. The organic
phase was dried 7MgSO4), ®ltered, and concentrated to
give a tan solid which was recrystallized in a minimal
amount of toluene. Multiple isolations from the mother
liquor gave the product 12 as a powdery white solid
In conclusion, two constrained glycopeptides were success-
fully synthesized in reasonable overall yield through a
convergent strategy. The compounds 21a and 21b, repre-
senting the d-serine and l-serine con®gurations, respec-
tively, were fully characterized and are poised for
incorporation into peptides for subsequent biological
evaluation and structural studies. These compounds illus-
trate a novel concept for constraining torsional angles in
glycopeptides. Studies toward establishing a more ef®cient
and selective synthesis are currently underway.
70.229 mol, 85%). IR 7thin ®lm): 3338, 2982, 1729 cm21
.
1H NMR 7500 MHz, CDCl3): d 9.61 7s, 1H), 5.54 7s, 1H),
3.99 7AB, Dnˆ11.9 Hz, 4H), 1.44 7s, 15H). 13C NM R
7125 MHz, CDCl3): d 199.5, 155.7, 98.9, 81.2, 62.9, 60.1,
28.4, 27.5, 19.8. TLC 7SiO2, 25% EtOAc/hexanes): Rfˆ
0.41. mp 114±1158C. HRMS: Calcd for C12H21NO5:
260.1498. Found: 260.1510. Anal: calcd for C12H21NO5:
C, 55.58; H, 8.16; N, 5.40. Found: C, 55.64; H, 8.07; N,
5.47.
4. Experimental
4.1.3. 5-Ethynyl-2,2-dimethyl-[1,3]dioxan-+N-tert-butyl-
oxycarbonyl)-5-ylamine +13). A solution of CH2Cl2
7191.0 mmol, 5.0 equiv.) in 150 mL THF was cooled to
2788C and lithium diisopropylamine 70.95 Min THF,
95.8 mmol, 2.5 equiv.), kept at 08C, was added dropwise
over 30 min. The resulting solution was stirred an additional
15 min, and a solution of 12 738.3 mmol, 1.0 equiv.) in
150 mL THF was added dropwise over 25 min. The result-
ing mixture was stirred an additional 1 h at 2788C, and
50 mL H2O was then added. The solution was subsequently
warmed to rt, at which time 50 mL of saturated aq NH4Cl
was added. The resulting layers were separated and the aq
phase was extracted with 2£100 mL EtOAc. The combined
organics were washed with 100 mL brine, dried 7MgSO4),
®ltered, and concentrated to give a colorless solid
736.4 mmol), which was used without further puri®cation.
A portion of the dichloroalcohol product was puri®ed for
characterization. IR 7thin ®lm): 2981, 2360, 2342 cm21. 1H
NMR 7500 MHz, CDCl3): d 5.78 7bs, 1H), 5.25 7s, 1H), 5.12
7bs, 1H), 4.11±3.95 7m, 5H), 1.44 7s, 12H), 1.41 7s, 3H). 13C
NMR 7125 MHz, CDCl3): d 157.4, 98.6, 81.6, 73.1, 65.4,
62.8, 55.3, 28.5, 27.8, 19.6. TLC 7SiO2, 25% EtOAc/
hexanes): Rfˆ0.41. mp 160±1628C. HRMS: Calcd for
C13H23Cl2NO5: 343.0953. Found: 343.0962. Anal: calcd
for C13H23Cl2NO5: C, 45.36; H, 6.73; N, 4.07. Found: C,
45.58; H, 6.69; N, 4.16.
4.1. General methods
Reagents obtained from commercial suppliers were used
without further puri®cation unless otherwise noted. All
solvents were puri®ed and dried by standard distillation
1
methods. H and 13C NMR spectra are reported in parts
per million 7d) relative to CHCl3 77.24 and 77.23 ppm,
respectively) as the internal standard.
4.1.1. +N-tert-Butyloxycarbonyl-5-amino-2,2-dimethyl-
[1,3]dioxan-5-yl)-methanol +11). To a solution of 108
70.310 mol, 1.00 equiv.) in 1000 mL of a 4:1 THF:H2O
solution was added NaHCO3 70.775 mol, 2.50 equiv.),
followed by BOC2O 70.388 mol, 1.25 equiv.). The resulting
solution was stirred at rt for 18 h. The reaction mixture
volume was reduced approximately 75% by aspirator pres-
sure on a rotary evaporator and was subsequently diluted
with 250 mL EtOAc. The layers were separated and the aq
layer was extracted with 3£250 mL EtOAc. The combined
organic extracts were washed with 250 mL brine, dried
7MgSO4), ®ltered, and concentrated to give the crude
product as a white solid. The solid was chromatographed
on silica gel 75:1 hexanes/EtOAc) to yield a powdery white
solid 70.254 mol, 82%). IR 7thin ®lm): 3421, 2979,
1
1714 cm2. H NMR 7500 MHz, CDCl3): d 5.32 7s, 1H),
4.27 7s, 1H), 3.80 7AB, Dnˆ11.6 Hz, 4H), 3.70 7d, Jˆ
6.55 Hz, 2H), 1.43 7s, 12H), 1.41 7s, 3H). 13C NM R
7125 MHz, CDCl3): d 156.6, 98.9, 80.6, 64.9, 64.6, 53.5,
28.5, 27.0, 20.5. TLC 7SiO2, 50% EtOAc/hexanes): Rfˆ
0.42. mp 95±978C. HRMS: Calcd for C12H23NO5:
262.1654. Found: 262.1644. Anal: calcd for C12H23NO5:
C, 55.16; H, 8.87; N, 5.36. Found: C, 55.55; H, 8.68; N,
5.48.
To a stirring solution of the above dichloroalcohol
735.8 mmol, 1.0 equiv.) in 225 mL CH2Cl2 at rt was added
triethylamine 753.7 mmol, 1.5 equiv.), followed by methane
sulfonyl chloride 753.7 mmol, 1.5 equiv.). The resulting
solution was stirred for 18 h and was subsequently quenched
with 75 mL H2O. The layers were separated, and the aq
phase was extracted with 2£75 mL CH2Cl2. The combined
organics were washed with 100 mL saturated aq NH4Cl,
100 mL H2O, and 100 mL brine. The organics were dried
7MgSO4), ®ltered, and concentrated to give a dark orange
foam 737.0 mmol), which was used without further puri®-
cation. A portion of the dichloromesylate product was puri-
4.1.2. N-tert-Butyloxycarbonyl-5-amino-2,2-dimethyl-[1,3]-
dioxane-5-carbaldehyde +12).
A solution of oxalyl
chloride 70.534 mol, 2.0 equiv.) in 525 mL CH2Cl2 was
cooled to 2788C in a 5 L ¯ask equipped with a mechanical
stirrer, and a solution of DMSO 70.801 mol, 3.0 equiv.) in
350 mL CH2Cl2 was added dropwise over 45 min. The
resulting mixture was stirred an additional 15 min at
2788C. A solution of 11 70.267 mol, 1.0 equiv.) in
525 mL of CH2Cl2 was then added dropwise over 45 min
producing a cloudy mixture which was allowed to stir at
2788C for 2 h. Triethylamine 71.602 mol, 6.0 equiv.) was
then added slowly over 30 min, and the thick solution
was gradually warmed to rt over 2 h. The reaction mixture
®ed for characterization. IR 7thin ®lm): 2984, 1710 cm21
.
1H NMR 7500 MHz, CDCl3): d 6.25 7s, 1H), 5.69 7s, 1H),
5.15 7s, 1H), 4.18 7AB, Dnˆ12.4 Hz, 2H), 4.02±3.92 7m,
2H), 3.30 7s, 3H), 1.44 7s, 12H), 1.39 7s, 3H). 13C NM R
7125 MHz, CDCl3): d 155.6, 99.2, 83.7, 81.1, 69.9,
62.9, 62.0, 55.2, 39.7, 28.4, 26.7, 20.5. TLC 7SiO2,
25% EtOAc/hexanes): Rfˆ0.33. HRMS: Calcd for
C14H25Cl2NO7S: 421.0729. Found: 421.0737. Anal: calcd

6536
J. W. Lane, R. L. Halcomb / Tetrahedron 57 02001) 6531±6538
for C14H25Cl2NO7S: C, 39.82; H, 5.97; N, 3.32. Found: C,
40.97; H, 5.89; N, 3.49.
brine, dried 7MgSO4), ®ltered, and concentrated to give a
crude oil consisting of an inseparable mixture of product
and p-anisaldehyde. This oil was then dissolved in
100 mL MeOH, cooled to 08C, and treated with sodium
borohydride 720.7 mmol, 3.0 equiv.). Upon completion of
the addition, the resulting mixture was allowed to warm to
rt and stirred for 3 h. The mix was then quenched with
25 mL saturated aq NaHCO3, and following the usual aq
workup, a mixture of product and 4-methoxybenzyl alcohol
was isolated as a crude oil. The oil was chromatographed on
silica gel 76:1 hexanes/EtOAc) to give the product as a 2:1
To a stirring solution of the dichloromesylate intermediate
736.50 mmol, 1.0 equiv.) in 350 mL THF at 2518C was
added methyllithium 71.4 Min diethyl ether, 200.75
mmol, 5.5 equiv.) dropwise over 30 min. The resulting
solution was slowly warmed to 08C over 1 h and was subse-
quently quenched with 25 mL H2O, followed by 50 mL
saturated aq NH4Cl. The mixture was allowed to warm to
rt, the layers were separated, and the aq phase was extracted
with 3£75 mL EtOAc. The combined organics were washed
with 100 mL brine, dried 7MgSO4), ®ltered, and concen-
trated to give a crude tan solid. The solid was chromato-
graphed on silica gel 75:1 hexanes/EtOAc) to give 13 as a
powdery white solid 724.40 mmole, 67%) from 12. IR 7thin
®lm): 3311, 2980, 1712 cm21. 1H NMR 7500 MHz, CDCl3):
d 5.13 7s, 1H), 4.02±3.94 7m, 4H), 2.40 7s, 1H), 1.45 7s, 9H),
1.43 7s, 3H), 1.40 7s, 3H). 13C NMR 7125 MHz, CDCl3): d
154.5, 98.8, 80.5, 80.4, 73.5, 66.5, 47.4, 28.6, 28.4, 18.8.
TLC 7SiO2, 25% EtOAc/hexanes): Rfˆ0.32. mp 113±
1158C. HRMS: Calcd for C13H21NO4: 256.1549. Found:
256.1557.
1
mixture of diastereomers by H NMR as a powdery white
solid 75.3 mmol, 76%) from 13. A portion of the major dia-
stereomer was isolated for characterization: IR 7thin ®lm):
1
2977, 2360, 1717 cm21. H NMR 7500 MHz, CDCl3): d
7.39 7d, Jˆ8.7 Hz, 2H), 6.89 7d, Jˆ8.7 Hz, 2H), 5.45 7s,
1H), 5.23 7bs, 1H), 4.40 7d, Jˆ11.3 Hz, 2H), 3.95 7d, Jˆ
11.3 Hz, 2H), 2.45 7s, 1H), 1.46 7s, 9H). 13C NM R
7125 MHz, CDCl3): d 160.5, 154.5, 129.9, 127.58, 113.9,
102.0, 80.5, 79.6, 74.2, 73.2, 55.5, 47.3, 28.6. TLC 7SiO2,
25% EtOAc/hexanes): Rf'sˆ0.22 and 0.30. mp 130±1328C.
HRMS: Calcd for C18H23NO5: 333.1576. Found: 333.1573.
Anal: calcd for C18H23NO5: C, 64.85; H, 6.95; N, 4.20.
Found: C, 64.54; H, 6.97; N, 4.21.
4.1.4. Addition precursor +14). To a stirring solution of 13
720.60 mmol, 1.0 equiv.) in 400 mL MeOH at rt was added
4.1.5. Spiroketalization precursor +17). To a solution of
14 74.48 mmol, 3.0 equiv.) in 40 mL anhydrous THF at rt
was added hexamethylphosphoramide 717.90 mmol,
12.0 equiv.). The mixture was then cooled to 2788C, and
n-butyllithium 71.6 Min hexanes, 9.39 mmol, 6.3 equiv.)
was added slowly. The resulting yellow mixture was
allowed to stir for 1.5 h at 2788C, at which time a solution
of 1510 71.49 mmol, 1.0 equiv.) in 15 mL anhydrous THF
was added slowly by canula. The yellow mixture was
allowed to stir for an additional 20 min at 2788C and was
then quenched with 5 mL H2O, followed by 15 mL satu-
rated aq NH4Cl. The heterogeneous mixture was allowed
to warm to rt and was then diluted with 25 mL EtOAc.
The phases were separated, and the aq phase was extracted
with 2£25 mL EtOAc. The combined organic phases were
washed with 50 mL brine, dried 7MgSO4), ®ltered, and
concentrated. The concentrated solution was chromato-
graphed on silica gel 76:1 then 3:1 hexanes/EtOAc) to
give a moderately separable mixture of recovered 14 and
product. The entirety of recovered 14 and product were
subsequently combined and redissolved in 100 mL of a
78:1:1) AcOH:H2O:MeOH solution. The resulting solution
was stirred for 12 h at rt and was then diluted with excess
toluene. The corresponding biphasic mixture was concen-
trated by high-vacuum 70.1 torr) rotary evaporator at a
temperature not exceeding 358C using additional toluene
azeotropes as necessary. The resulting crude oil was imme-
diately chromatographed on silica gel 75:1 then 2:1 then 1:1
hexanes/EtOAc) to give the product as an inseparable 7~2:1)
p-toluenesulfonic
acid
monohydrate
74.12 mmol,
0.2 equiv.) in one portion. The resulting solution was stirred
for 12 h, and was subsequently quenched with excess
triethylamine. The mixture was allowed to stir for an
additional 20 min, and the volume of the solution was
reduced approximately 90% by aspirator pressure rotary
evaporator. The concentrated solution was diluted with
200 mL EtOAc, and was then washed with 75 mL saturated
aq NaHCO3. After separation of phases, the aq phase was
extracted with 2£75 mL EtOAc. The combined organics
were dried 7MgSO4), ®ltered, and concentrated to give the
diol as a crude white solid 720.60 mmol, 100%), which was
used without further puri®cation. IR 7thin ®lm): 3399, 2978,
1
1696 cm21. H NMR 7500 MHz, CDCl3): d 5.29 7s, 1H),
3.89 7dd, Jˆ11.5, 6.4 Hz, 2H), 3.79 7dd, Jˆ11.5, 7.3 Hz,
2H), 3.08 7bs, 2H), 2.44 7s, 1H), 1.44 7s, 9H). 13C NM R
7125 MHz, CDCl3): d 155.8, 81.5, 81.1, 73.9, 66.6, 57.2,
28.5. TLC 7SiO2, 50% EtOAc/hexanes): Rfˆ0.23. mp
116±1188C. HRMS: Calcd for C10H17NO4: 216.1236.
Found: 216.1240.
To a solution of 4-methoxybenzaldehyde dimethyl acetal
720.7 mmol, 3.0 equiv.) in 20 mL DMF at rt was added
the above diol 76.9 mmol, 1.0 equiv.), followed by
camphorsulfonic acid 70.7 mmol, 0.1 equiv.). The resulting
mixture was then placed on an aspirator pressure rotary
evaporator and allowed to mix at 408C for 4 h. The reaction
was subsequently treated with excess triethylamine and
concentrated to give a crude, orange residue consisting of
an inseparable mixture of product, p-anisaldehyde, and
excess 4-methoxybenzaldehyde dimethyl acetal. The resi-
due was then redissolved in 100 mL of a 4:1 THF:H2O
solution, and 5 mL glacial AcOH was added. This solution
was allowed to stir for 2 h and was then quenched with
25 mL saturated aq NaHCO3. The resulting phases were
separated, and the aq phase was extracted with 2£75 mL
EtOAc. The combined organics were washed with 75 mL
1
mixture of presumed anomers by H NMR. A third minor
compound observed by H NMR was presumed to be a
1
rapidly equilibrating ring-opened structure. Product was
isolated as a foam 71.11 mmol, 79%) from 15 based
on recovered 15. Diol arising from deprotected 14 was
recovered cleanly as the white solid 72.65 mmol). IR 7thin
®lm): 2930, 2857, 2114 cm21. 1H NMR 7500 MHz, CDCl3):
d 7.65±7.58 7m, 4H), 7.42±7.32 7m, 6H), 5.28±5.26 77bs
7minor), dˆ5.28); 7bs 7minor), dˆ5.27); 7bs 7major),

J. W. Lane, R. L. Halcomb / Tetrahedron 57 02001) 6531±6538
6537
dˆ5.26), totalˆ1H), 4.06±3.98 7m, 1H), 3.82±3.58 7m,
11H), 1.39 7s, 9H), 1.045±1.034 77s 7minor), dˆ1.045); 7s
7major), dˆ1.038); 7s 7minor), dˆ1.034), totalˆ9H),
0.951±0.944 77s 7major), dˆ0.951); 7s 7minor), dˆ0.948);
7s 7minor), dˆ0.944), totalˆ9H), 0.837±0.811 77s 7minor),
dˆ0.837); 7s 7major), dˆ0.828); 7s 7minor), dˆ0.811),
totalˆ9H), 0.195±0.120 77s 7minor), dˆ0.195); 7s 7major),
dˆ0.175); 7s 7minor), dˆ0.170); 7s 7minor), dˆ0.166); 7s
7major), dˆ0.153); 7s 7minor), dˆ0.145); 7s 7minor),
dˆ0.141); 7s 7major), dˆ0.132); 7s 7minor), dˆ0.120),
totalˆ9H), 0.030±0.015 77s 7minor), dˆ0.030); 7s 7major),
dˆ0.024); 7s 7minor), dˆ0.015), totalˆ3H). 13C NM R
7125 MHz, CDCl3): d 155.8, 155.6, 135.9, 135.8, 133.7,
133.6, 133.5, 130.1, 130.0, 128.0, 127.9, 94.7, 92.2, 85.0,
82.1, 81.2, 81.0, 75.9, 73.8, 73.4, 71.5, 71.2, 70.8, 67.9,
67.0, 65.7, 62.6, 62.3, 57.2, 52.4, 28.5, 27.2, 27.1, 26.6,
26.5, 26.4, 26.3, 19.4, 19.3, 18.8, 18.7, 23.4, 23.5, 23.6,
23.7, 24.3, 24.4, 24.5. TLC 7SiO2, 50% EtOAc/hexanes):
Rf'sˆ0.45 and 0.53. HRMS: Calcd for C44H72N4O9Si3
7M1Na)1: 907.4505. Found: 907.4494.
18a/b 70.783 mmol, 1.00 equiv.) in 26 mL CH2Cl2 at rt was
added NaHCO3 70.790 mmol, 1.00 equiv.) followed by
Dess±Martin periodinane 71.570 mmol, 2.00 equiv.). The
reaction mixture was stirred for 4.5 h at which time it was
quenched with 10 mL of a saturated aq NaHCO3 solution
containing Na2S2O3 725g per 100 mL). After stirring for
15 min, the mixture was diluted with 50 mL EtOAc and
25 mL H2O. The phases were separated, and the aq phase
was extracted with 2£25 mL EtOAc. The combined organic
phases were washed with 50 mL brine, dried 7MgSO4),
®ltered, and concentrated. The resulting product was
dissolved in 25 mL of a 4:1 tBuOH:H2O solution.
NaH2PO4 70.822 mmol, 1.05 equiv.) followed by 2-meth-
yl-2-butene 72.0 Min THF, 3.290 mmol, 4.20 equiv.), and
®nally NaClO2 72.349 mmol, 3.00 equiv.) were successively
added, and the solution was allowed to stir at rt for 14 h. The
reaction mixture was then diluted with 25 mL H2O and
50 mL EtOAc. The phases were separated, and the aq
phase was extracted with 2£25 mL EtOAc. The combined
organic phases were washed with 50 mL brine, dried
7MgSO4), ®ltered, and concentrated to give a clear oil.
The oil was dissolved in 19.5 mL of a 7:2 benzene:MeOH
solution and 7trimethylsilyl)diazomethane 72.0 Min hex-
anes, 1.566 mmol, 2.00 equiv.) was added. The yellow
solution was allowed to stir for 4 h at rt, at which time it
was concentrated by aspirator pressure rotary evaporator to
give a crude, yellow oil. The oil was immediately chro-
matographed on silica gel 79:1 hexanes/EtOAc) to give
the product as the usual mixture of diastereomers as well
as possible anomers by 1H NMR. Product was isolated as a
clear foam 70.683 mmol, 87%) from 18a/b. IR 7thin ®lm):
4.1.6. Spiroketals +18a/b). To
a
solution of 17
71.055 mmol, 1.0 equiv.) in 53 mL EtOAc was added
Pd/C 710%, 0.62 g) followed by camphorsulfonic acid
70.949 mmol, 0.9 equiv.). The ¯ask was evacuated under
aspirator pressure and re®lled with hydrogen three times,
and then stirred under hydrogen 71 atm) for 18 h. The
mixture was then ®ltered through a bed of Celite and
quenched with excess triethylamine. The solution was
concentrated to give a clear, crude oil. The oil was imme-
diately chromatographed on silica gel 73:1 then 1:1 hexanes/
EtOAc) to give the product mixture as a clear foam
70.810 mmol, 76%). IR 7thin ®lm): 2931, 2111,
1
2955, 2858, 1749 cm21. H NMR 7500 MHz, CDCl3): d
7.64±7.59 7m, 4H), 7.44±7.32 7m, 6H), 6.37±5.93 7m,
1H), 5.83±5.62 7m, 1H), 5.44±5.16 7m, 1H), 4.38±3.38
7m, 11H), 1.417±1.382 77s 7major), dˆ1.417); 7s 7minor),
dˆ1.398); 7s 7minor), dˆ1.394); 7s 7minor), dˆ1.382),
totalˆ9H), 1.047±1.034 77s 7minor), dˆ1.047); 7s 7major),
dˆ1.039); 7s 7minor), dˆ1.034), totalˆ9H), 0.953±0.925
77s 7major), dˆ0.953); 7s 7minor), dˆ0.933); 7s 7minor),
dˆ0.925), totalˆ9H), 0.842±0.808 77s 7minor), dˆ0.842);
7s 7minor), dˆ0.831); 7s 7major), dˆ0.816); 7s 7minor),
dˆ0.808), totalˆ9H); 0.179±0.117 7s 7minor), dˆ0.179);
7s 7minor), dˆ0.172); 7s 7major), dˆ0.156); 7s 7minor),
dˆ0.131); 7s 7major), dˆ0.122); 7s 7minor), dˆ0.117),
totalˆ9H), 0.024±0.011 77s 7minor), dˆ0.024); 7s 7major),
dˆ0.019); 7s 7minor), dˆ0.011), totalˆ3H). 13C NM R
7125 MHz, CDCl3): d 170.7, 135.8, 135.7, 133.8, 133.7,
133.6, 133.5, 130.8, 130.2, 130.1, 130.0, 129.9, 129.8,
129.2, 128.0, 127.9, 115.5, 96.6, 96.3, 96.1, 80.6, 77.5,
77.2, 77.0, 73.8, 73.7, 72.7, 72.6, 72.3, 71.8, 71.5, 71.4,
71.3, 65.1, 64.7, 64.4, 63.6, 62.8, 55.3, 53.1, 52.9, 28.5,
28.4, 28.3, 27.1, 26.6, 26.5, 26.4, 26.3, 26.2, 26.1, 19.4,
19.3, 19.2, 18.8, 18.7, 23.0, 23.3, 23.4, 23.5, 23.6,
24.0, 24.3, 24.4, 24.6. TLC 7SiO2, 25% EtOAc/hexanes):
Rfˆ0.48. MS: Calcd for C45H72N4O9Si3 7M1Na)1: 919.45.
Found: 919.48.
1
1698 cm21. H NMR 7500 MHz, CDCl3): d 7.64±7.58 7m,
4H), 7.44±7.32 7m, 6H), 6.09±5.93 77dd 7major), dˆ6.08,
Jˆ9.9, 1.4 Hz); 7dd 7minor), dˆ6.03, Jˆ10.3, 1.2 Hz); 7dd
7minor), dˆ5.96, Jˆ9.9, 1.4 Hz), totalˆ1H), 5.72±5.60 77d
7minor), dˆ5.65, Jˆ10.3 Hz); 7d 7major), dˆ5.61, Jˆ
9.9 Hz); 7d 7minor), dˆ5.57, Jˆ9.9 Hz), totalˆ1H), 5.39±
5.08 77bs 7major), dˆ5.39); 7bs 7minor), dˆ5.17); 7bs
7minor), dˆ5.08), totalˆ1H), 4.66±4.38 7m, 1H), 4.20±
3.40 7m, 10H), 1.43±1.38 77s 7major), dˆ1.43); 7s 7minor),
dˆ1.41); 7s 7minor), dˆ1.38), totalˆ9H), 1.048±1.037 77s
7minor), dˆ1.048); 7s 7major), dˆ1.044); 7s 7minor), dˆ
1.037), totalˆ9H), 0.963±0.940 77s 7major), dˆ0.963); 7s
7minor), dˆ0.944); 7s 7minor), dˆ0.940), totalˆ9H),
0.849±0.817 77s 7minor), dˆ0.849); 7s 7minor), dˆ0.837);
7s 7major), dˆ0.827); 7s 7minor), dˆ0.817), totalˆ9H);
0.188±0.125 7s 7major), dˆ0.188); 7s 7minor), dˆ0.178);
7s 7major), dˆ0.167); 7s 7minor), dˆ0.159); 7s 7minor),
dˆ0.143); 7s 7major), dˆ0.134); 7s 7minor), dˆ0.125),
totalˆ9H), 0.05±0.03 77s 7minor), dˆ0.05); 7s 7major), dˆ
0.04); 7s 7minor), dˆ0.03), totalˆ3H). 13C NMR 7125 MHz,
CDCl3): d 157.0, 135.9, 135.8, 133.7, 133.6, 131.5, 130.1,
130.0, 128.9, 128.0, 127.9, 127.8, 96.4, 95.9, 80.6, 77.5,
77.2, 77.0, 73.6, 72.1, 71.9, 71.5, 65.8, 65.5, 65.3, 62.7,
53.3, 28.5, 28.4, 27.1, 26.6, 26.5, 26.4, 26.3, 26.2, 19.4,
18.8, 18.7, 23.1, 23.3, 23.7, 23.8, 24.3, 24.4, 24.5.
TLC 7SiO2, 25% EtOAc/hexanes): Rf'sˆ0.37 and 0.47.
HRMS: Calcd for C44H72N4O8Si3 7M1Na)1: 891.4556.
Found: 891.4520.
4.1.8. d-Serine and l-serine constrained glycopeptides
+21a/b). To a solution of 19a/b 70.551 mmol, 1.0 equiv.)
in 11 mL THF at rt was added tetrabutylammonium ¯uoride
71.0 Min hexanes, 2.755 mmole, 5.0 equiv.). The reaction
mixture was stirred for 5 h at which time it was quenched
with 10 mL saturated aq NH4Cl and diluted with 25 mL
4.1.7. Spiroketal methyl esters +19a/b). To a solution of

6538
J. W. Lane, R. L. Halcomb / Tetrahedron 57 02001) 6531±6538
EtOAc. The phases were separated, and the aq phase was
extracted with 2£15 mL EtOAc. The combined organic
phases were washed with 10 mL brine, dried 7MgSO4),
®ltered, and concentrated. The resulting crude oil was
chromatographed on silica gel 7EtOAc then 9:1 EtOAc/
MeOH) to give a clear oil which was immediately taken
up in 9 mL of a 2:1 pyridine:Ac2O solution. The mixture
was stirred for 18 h at rt, at which time the solution was
concentrated by high-vacuum 70.1 torr) rotary evaporator.
The resulting crude oil was chromatographed on silica gel
72:1 hexane/EtOAc) to give a clear foam 70.436 mmol,
80%) from 19a/b. This product was observed as the usual
mixture, and full characterization was not performed. The
majority of the product 70.399 mmol, 1.0 equiv.) was subse-
quently dissolved in 10 mL MeOH, and acetic anhydride
71.995 mmol, 5.0 equiv.) followed by Pd/C 710%, 0.145 g)
were added in succession. The ¯ask was evacuated under
aspirator pressure and re®lled with hydrogen three times,
and then stirred under hydrogen 71 atm) for 12 h. The
mixture was then ®ltered through a bed of Celite, and
concentrated to give a crude oil. The oil was immediately
chromatographed on silica gel 71:1 hexanes/EtOAc then
EtOAc) to give a product mixture including the two major
diastereomers 21a/b. The two major diastereomers 21a and
21b were puri®ed by preparative HPLC on a silica gel
column using EtOAc as the eluent. The d-isomer 721a)
was isolated as a white, crystalline solid 70.101 mmol,
26%) from 20a/b in greater than 95% purity. The l-isomer
721b) was isolated as a white foam 70.085 mmol, 22%) from
20a/b in greater than 95% purity.
Henry Dreyfus Foundation 7Camille Dreyfus Teacher±
Scholar Award), P®zer 7Junior Faculty Award), and
Novartis 7Young Investigator Award) for support. This
work was greatly facilitated by a 500 MHz NMR spectro-
meter that was purchased partly with funds from an NSF
Shared Instrumentation Grant 7CHE-9523034). Bob
Barkley 7University of Colorado) is thanked for obtaining
mass spectra. Bruce Noll 7University of Colorado) is
thanked for the determination of the X-ray crystal struc-
tures. Neil Whittemore is thanked for performing the
NMR studies. Jennifer Koviach is thanked for her direction
and helpful suggestions.
References
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4.1.9. d-Isomer +21a). IR 7thin ®lm): 2978, 1748,
1
1663 cm21. H NMR 7500 MHz, CDCl3): d 5.64 7bs, 1H),
5. 7a) Degrado, W. F. In Design of Peptides and Proteins, Vol.
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5.34 7d, Jˆ2.6 Hz, 1H), 5.15 7dd, Jˆ11.1, 3.4 Hz, 1H), 4.87
7bs, 1H), 4.34 7t, Jˆ10.5 Hz, 1H), 4.16 7dd, Jˆ10.9, 7.1 Hz,
1H), 4.10±4.05 7m, 3H), 3.85 7d, Jˆ11.5 Hz, 1H), 3.71 7s,
3H), 2.23 7dt, Jˆ13.9, 5.0 Hz, 1H), 2.15 7s, 3H), 2.05 7s,
3H), 1.99 7s, 3H), 1.97 7s, 3H), 2.00±1.93 7m, 1H), 1.82 7dt,
Jˆ13.9, 4.6 Hz, 1H), 1.66 7ddd, Jˆ14.0, 4.6, 2.2 Hz, 1H),
1.42 7s, 9H). 13C NMR 7125 MHz, CDCl3): d 170.7, 170.6,
98.4, 69.2, 67.5, 67.4, 62.0, 52.7, 28.4, 25.4, 21.0, 20.9,
20.8. TLC 7SiO2, EtOAc): Rfˆ0.38. HRMS: Calcd for
C25H38N2O13 7M1H)1: 575.2452. Found: 575.2446.
6. 7a) Bock, K.; Duus, J. O.; Hindsgaul, O.; Lindh, I. Carbohydr.
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4.1.10. l-Isomer +21b). IR 7thin ®lm): 1748, 1669,
1
1369 cm21. H NMR 7500 MHz, CDCl3): d 5.85 7d, Jˆ
10.0 Hz, 1H), 5.34 7dd, Jˆ3.3, 0.9 Hz, 1H), 5.13 7dd, Jˆ
10.8, 3.3 Hz, 1H), 4.68 7bs, 1H), 4.38 7d, Jˆ11.0 Hz, 1H),
4.29 7t, Jˆ10.5 Hz, 1H), 4.12 7dd, Jˆ11.0, 6.6 Hz, 1H)
4.08±4.00 7m, 2H), 3.71 7s, 3H), 3.37 7d, Jˆ11.0 Hz, 1H),
2.13 7s, 3H), 2.04 7s, 3H), 2.02±1.93 7m, 1H), 1.96 7s, 3H),
1.95 7s, 3H), 1.89 7dt, Jˆ12.8, 6.2 Hz, 1H), 1.79±1.65 7m,
2H), 1.39 7s, 9H). 13C NMR 7125 MHz, CDCl3): d 172.1,
170.9, 170.8, 170.7, 170.6, 97.6, 69.5, 67.5, 67.4, 64.9, 61.9,
55.4, 52.4, 50.7, 28.4, 27.3, 23.5, 21.0, 20.9. TLC 7SiO2,
EtOAc): Rfˆ0.30. MS: Calcd for C25H38N2O13 7M1H)1:
575.2. Found: 575.3.
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Acknowledgements
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This research was supported by an NSF Career Award
7CHE-9733765). R. L. H. also thanks the Camille and