Synthesis of novel tetrahydropyran-based dipeptide isosters by overman rearrangement of 2,3-didehydroglycosides

Article abstract of DOI:10.1002/ejoc.200200710

Differently functionalized tetrahydropyran-based dipeptide isosters have been efficiently synthesized from 3,4,6-tri-Oacetyl-D-glucal. Analogues of the hsp65 p2-13 epitope of Mycobacterium tuberculosis and Mycobacterium leprae were prepared by replacement of the Ala-Tyr or Glu-Glu moiety in the native dodecapeptide with the prepared dipeptide isosters. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).

Full text of DOI:10.1002/ejoc.200200710

FULL PAPER
Synthesis of Novel Tetrahydropyran-Based Dipeptide Isosters by Overman
Rearrangement of 2,3-Didehydroglycosides
Nicole M. A. J. Kriek,[a] Elise van der Hout,[a] Paskal Kelly,[a] Krista E. van Meijgaarden,[b]
Annemieke Geluk,[b] Tom H. M. Ottenhoff,[b] Gijs A. van der Marel,[a] Mark Overhand,[a]
Jacques H. van Boom,[a] A. Rob P. M. Valentijn,[a] and Herman S. Overkleeft*[a]
Keywords: Antigenic peptide / Dipeptide isosters / Overman rearrangement / Solid-phase peptide synthesis / Sugar
amino acids
Differently functionalized tetrahydropyran-based dipeptide
isosters have been efficiently synthesized from 3,4,6-tri-O-
moiety in the native dodecapeptide with the prepared di-
peptide isosters.
acetyl-
D-glucal. Analogues of the hsp65 p2−13 epitope of
Mycobacterium tuberculosis and Mycobacterium leprae
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
were prepared by replacement of the Ala−Tyr or Glu−Glu Germany, 2003)
Introduction
tide isosters, synthetic molecules that incorporate structural
(e.g., conformational) and functional (e.g., amino acid side
chain entities) elements of the native peptide in an essen-
tially non-peptidic structure.[8] In the context of the latter
approach we have recently reported on the transformation
of carbohydrate derivatives into peptide-like building blocks
and their incorporation in a variety of linear and cyclic
oligopeptides.[9Ϫ14] As part of these research efforts, we dis-
closed the facile synthesis of a set of tetrahydropyran-based
2,6-cis- and trans-disposed peptide isosters 3 (Figure 1) and
their incorporation in novel protein:farnesyltransferase[9]
and protein:geranylgeranyltransferase[10] inhibitors. The key
step in the synthetic sequence employed was the treatment
of tri-O-acetyl--glucal 1 with trimethylsilyl cyanide in the
presence of BF3·Et2O. Ensuing functional group manipu-
lations of the resulting Ferrier rearrangement product 2 af-
forded dipeptide isosters 3, possessing an additional
hydroxy function attached to the tetrahydropyran core. We
reasoned that utilization of this hydroxy function, which is
also present in Ferrier rearrangement product 2 as the cor-
responding allylic alcohol, would give rise to a new set of
tetrahydropyran-based dipeptide isosters 4.
Oligopeptides from natural sources form an important
source of lead compounds for the development of new
therapeutics. Important examples include the neuropeptides
Leu- and Met-enkephalin,[1] natural analgesics that exert
their action through binding to the opioid receptor. Other
oligopeptidic leads with therapeutic potential are rep-
resented by a variety of antigenic peptides, oligopeptides
that modulate immune responses to bacterial or viral infec-
tions. Inappropriate antigen presentation can result in the
development of (auto)immune diseases. Disturbance of the
processing and presentation of selected antigenic peptides,
which may be achieved by the application of modified ana-
logues of peptides, is widely accepted as a promising strat-
egy for the treatment of (auto)immune diseases, including
type I diabetes, celiac disease, and multiple sclerosis.[2Ϫ4]
A major drawback of the use of unmodified, naturally
occurring oligopeptides in therapeutic applications is their
intrinsic instability towards proteolytic activities. In this re-
spect, the development of synthetic peptide-like compounds
with enhanced physiological half-lives has attracted wide at-
tention. One area of research comprises the development of
analogues of peptide linkages, such as peptoids,[5] sulfon-
Here we report the synthesis of the four new Fmoc-pro-
amides,[6] and olefins.[7] A second approach entails the re- tected, tetrahydropyran-based 2,5-cis- and 2,5-trans dipep-
placement of selected amino acid residues by so-called pep- tide isosters 5؊8. Key transformations en route to the tar-
get compounds entail the Ferrier rearrangement of tri-O-
[a]
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden
University,
acetyl--glucal with different nucleophiles followed by Mit-
sunobu-mediated inversion of the secondary alcohol in 2
(in the cases of 7 and 8) and final installment of the second-
ary amine through Overman rearrangement. The utility of
dipeptide isosters 5؊8 in solid-phase peptide synthesis is
demonstrated by the preparation of analogues of antigenic
P. O. Box 9502, 2300 RA Leiden, The Netherlands
Fax: (internat.) ϩ 31-71/5274307
E-mail: H.S.Overkleeft@chem.leidenuniv.nl
Leiden University Medical Center,
Dept. of Immunohaematology and Blood Transfusion
P. O. Box 9600, 2300 RC Leiden, The Netherlands
[b]
2418
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200200710
Eur. J. Org. Chem. 2003, 2418Ϫ2427

Synthesis of Novel Tetrahydropyran-Based Dipeptide Isosters
FULL PAPER
Figure 1. Schematic presentation of the synthesis of 2,6- and 2,5-disubstituted dipeptide isosters 3 and 4, respectively, from tri-O-acetyl-
-glucal 1; the novel dipeptide isosters 5؊8 and antigenic peptide 9 from heat shock protein 65 of Mycobacterium tuberculosis and
Mycobacterium leprae.
peptide 9,[15] originating from heat shock protein 65 (hsp65) resulting amine with di-tert-butyl dicarbonate to give Boc-
from Mycobacterium tuberculosis and Mycobacterium lep- protected 14. Hydrogenation of the olefins in 14 was ac-
rae, in which either the AlaϪTyr or the GluϪGlu dipeptide companied by cleavage of the silyl ether[21] to afford alcohol
is substituted.
15, oxidation of which to the corresponding acid 16 was
effected by treatment with a catalytic amount of ru-
thenium() chloride in the presence of sodium periodate.[22]
Subjection of Boc-protected 16 to 50% TFA in DCM and
subsequent treatment with FmocϪOSu gave the desired
(2S,5R,6R)-5-[N-(9-fluorenylmethoxycarbonyl)amino]-6-
propyl-tetrahydropyran-2-carboxylic acid (5) in 14% overall
yield based on 1.
Results and Discussion
As the first research objective, the synthesis of 2,5-trans
dipeptide isoster 5, with a C-6 propyl functionality attached
to the pyran core, was undertaken as follows. Ferrier re-
arrangement[16] of tri-O-acetyl--glucal 1 (Scheme 1) with
(allyl)trimethylsilane (AllϪTMS) in the presence of
BF3·Et2O afforded C-glycoside 10, which was converted
into the known[17] C-glycoside 11 by standard protective
group manipulations (deacetylation, followed by selective
protection of the primary alcohol as the tert-butyldimeth-
ylsilyl ether) in good yield. After formation of the trichlo-
roacetimidate (DBU, trichloroacetonitrile, 11 to 12), the in-
stallation of the secondary amine functionality was now ef-
fected by heating 12 at reflux in 1,2-dichlorobenzene in the
presence of K2CO3,[18] to give 13 through a [3,3]-sigma-
tropic Overman rearrangement.[19] Deacylation of 13
(2S,5R)-5-[N-(9-Fluorenylmethoxycarbonyl)amino]-
tetrahydropyran-2-carboxylic acid (6), with a hydrogen at
the C-6 position, was prepared by a similar synthetic strat-
egy. Application of triethylsilane as the incoming nucleo-
phile in the Lewis acid-mediated Ferrier rearrangement of
1 furnished derivative 17 in 96% yield. Conversion of 17
into 20[23] and ensuing transformations into the target di-
peptide isoster 6 went uneventfully (18% overall yield based
on 1), following the sequence of transformations as outlined
for the synthesis of 5.
Having accomplished the synthesis of 2,5-dipeptide isos-
(KOH, isopropanol)[20] was followed by treatment of the ters 5 and 6, we set out on the preparation of the corre-
Eur. J. Org. Chem. 2003, 2418Ϫ2427
www.eurjoc.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2419

H. S. Overkleeft et al.
FULL PAPER
Scheme 1. Synthesis of dipeptide isosters 5, 6, 7 and 8
Reagents and conditions: (i) BF3·Et2O, AllϪTMS (10) or triethylsilane (17), CH2Cl2, 0 °C; (ii) tBuOK, MeOH; (iii) TBSϪCl, pyridine;
(iv) Cl3CCN, DBU, CH2Cl2, 0 °C; (v) K2CO3, 1,2-DCB, ∆; (vi) KOH (3 equiv.), iPrOH; (vii) Boc2O (1.2 equiv.), CH2Cl2; (viii) H2, cat.
Pd/C, MeOH; (ix) cat. RuCl3·xH2O, NaIO4 (4.5 equiv.), CH2Cl2/H2O/CH3CN (1:1.5:1, v/v/v), 0 °C Ǟ room temp.; (x) TFA/CH2Cl2 (1:1,
v/v); (xi) FmocϪOSu (1.2 equiv.), 10% aq. NaHCO3/1,4-dioxane (1:1, v/v), 0 °C Ǟ room temp.; (xii) p-NO2ϪC6H4CO2H, DEAD, PPh3,
THF; (xiii) KOH (1.5 equiv.), iPrOH.
sponding C-5 epimeric 2,5-cis dipeptide isosters 7 and 8, terms of yield and of stereoselectivity. We therefore elected
respectively. We envisaged that the introduction of the C-5 to follow an alternative route, based on the Mitsunobu-
epimeric amino functionality should be achievable by appli- mediated epimerization of intermediates 11 and 18. As an
cation of 2,3-anhydroglycosides 25 and 32 (that is, with the example, treatment of 11 with p-nitrobenzoic acid under
-galacto instead of the -gluco configuration) in the stereo- Mitsunobu conditions (DEAD, PPh3, THF) furnished ester
selective Overman rearrangement reaction. In theory, these 24, deacylation of which afforded alcohol 25. Through the
intermediates should be accessible by performing the use of the same sequence of reactions as described for the
Ferrier rearrangement of tri-O-acetyl--galactal. However, transformation of alcohol 11 into 5, the 2,5-cis dipeptide
this process is known[24] to be highly inefficient, both in isoster 7 was obtained in 13% overall yield based on 11.
2420
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2003, 2418Ϫ2427

Synthesis of Novel Tetrahydropyran-Based Dipeptide Isosters
FULL PAPER
The preparation of the hydrogen counterpart 8 was ac- used.[25] Cleavage of the immobilized modified peptides
complished in a similar fashion. In this case, subjection of 40aϪf from the resin and simultaneous deblocking of the
alcohol 18 to Mitsunobu reaction conditions at 0 °C re- side chain protecting groups was accomplished by acidoly-
sulted in the formation of an inseparable side product, poss- sis. The obtained crude modified peptides 41؊46 were puri-
ibly originating from SN2Ј attack at the alkene in 18. The fied by RP HPLC to give pure 41 (24%), 42 (26%), 43
amount of side product formed could be minimized by per- (25%), 44 (22%), 45 (28%), and 46 (16%), as indicated by
forming the reaction at Ϫ30 °C and by using only a small LC/MS analysis.
excess of the reagents. Deacylation of 31 resulted in the iso-
lation of the desired allylic alcohol 32, the configuration of
which was ascertained by NOESY NMR analysis. The 2,5-
cis dipeptide isoster 8 was obtained from 32 in 7% overall
Conclusion
yield (based on 18) by means of a sequence of reactions
corresponding to those described for the conversion of al-
cohol 18 into 6.
This paper describes the concise synthesis of 5-[N-(9-
fluorenylmethoxycarbonyl)amino]tetrahydropyran-2-
carboxylic acids 2,5-trans-5 and -6 and 2,5-cis-7 and -8 as
suitably protected building blocks for the incorporation of
novel dipeptide isosters in oligopeptides. The application of
the Ferrier rearrangement allows the installment of differ-
ent carbon, nitrogen, and oxygen functionalities, providing
for a great variety of substituents at the C-6 of the pyran
core. The building blocks 5؊8 were effectively applied in
the solid-phase assembly of modified peptides 41؊46, the
design of which was based on the replacement of either di-
peptide AlaϪTyr or GluϪGlu in the antigenic peptide 9.
The ability of peptides 41؊46 to arrest hsp65-specific T cell
response is currently being investigated and will be reported
in due course.
As the next research objective, the application of dipep-
tide isosters 2,5-trans-5 and -6 and 2,5-cis-7 and -8 in the
solid-phase peptide synthesis of peptidomimetics 41؊46
was undertaken (Scheme 2). Wang resin 38 was loaded with
FmocϪArg(Pbf)ϪOH in the presence of N,NЈ-dicyclohex-
ylcarbodiimide (DCC) and a catalytic amount of 4-(dimeth-
ylamino)pyridine (DMAP), affording immobilized arginine
derivative 39. Deprotection of the amine function in 39, fol-
lowed by condensation of the appropriate amino acids with
(benzotriazol-1-yl)oxy-tris(dimethylamino) phosphonium
hexafluorophosphate (BOP) in the presence of 1-hydroxy-
benzotriazole (HOBt) and N,N-diisopropylethylamine (Di-
PEA) gave immobilized 5-(amino)tetrahydropyran-2-car-
boxylic acid-containing peptides 40aϪc (replacement of the
AlaϪTyr dipeptide) and 40d-f (replacement of GluϪGlu di-
peptide). In all examples, the respective dipeptide isosters
were incorporated uneventfully by means of the standard
Experimental Section
General Methods and Materials: Acetonitrile (Rathburn, HPLC
Fmoc-based solid-phase peptide synthesis procedures grade), 1,2-dichloroethane (1,2-DCE), iPrOH, 1,4-dioxane, and
Scheme 2. Solid-phase synthesis of glycopeptides 41؊46
Reagents and conditions: (i) FmocϪArg(Pbf)ϪOH (3 equiv.), DCC (3.3 equiv.), DMAP (5 mol %), room temp., 2 h; (ii) repeat (10 ϫ)
the following sequence: a. 20% piperidine in NMP, room temp., 5 min, b. FmocϪAAϪOH or building blocks 5؊8 (5 equiv.), BOP (5
equiv.), HOBt (5 equiv.), DiPEA (10 equiv.), NMP, room temp., for amino acids: 1 h and for building blocks 5؊8: 2 h, c. Ac2O/DiPEA/
HOBt/NMP, room temp., 1 min; (iii) 20% piperidine in NMP, room temp., 5 min; (iv) TFA/TIS/H2O (95:2.5:2.5, v/v/v), room temp., 2 h.
Eur. J. Org. Chem. 2003, 2418Ϫ2427
www.eurjoc.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2421

H. S. Overkleeft et al.
FULL PAPER
DCM (all Baker, p.a.) were stored over molecular sieves (4 A).
(Hϩ-form), filtered, and concentrated in vacuo. The crude diol (co-
˚
HPLC grade methanol (Biosolve) was stored over molecular sieves evaporated with pyridine) was dissolved in pyridine (20 mL) and
˚
(3 A). Solvents and reagents used in the solid-phase peptide syn-
the solution was cooled to 0 °C. A solution of tert-butyl(dimeth-
thesis were of peptide synthesis grade (Biosolve) and were used as yl)silyl chloride (0.65 g, 4.3 mmol) in pyridine (10 mL) was added
received. Hyflo, di-tert-butyl dicarbonate, N-9-fluorenylmethoxy-
carbonyl-N-hydroxysuccinimide (FmocϪOsu), and p-nitrobenzoic
acid were bought from Fluka and used as such. Anhydrous 1-
hydroxybenzotriazole (HOBt) was purchased from Neosystem
dropwise, and the reaction mixture was stirred overnight and con-
centrated to dryness. The residue was dissolved in EtOAc (150 mL)
and washed with NaHCO3 (10%) and brine. The organic phase was
dried (MgSO4), filtered, and concentrated. Purification by silica gel
laboratoires (France), and (benzotriazol-1-yl)oxytris(dimethyl- chromatography (toluene/EtOAc, 100:0 Ǟ 90:10, v/v) afforded title
amino)phosphonium hexafluorophosphate (BOP) was obtained from compound 11 (1.0 g, 84%) as an oil. 13C NMR (CDCl3): δ ϭ 134.3,
Senn Chemicals. p-Benzyloxybenzyl alcohol resin (Wang resin) and
129.7, 128.1 (C-4, C-5, ϭCH All), 117.0 (ϭCH2 All), 71.8, 66.1,
65.1 (C-2, C-3, C-6), 64.4 (CH2O), 37.7 (CH2 All), 25.6 [(CH3)3C],
the protected amino acids were bought from NovaBiochem (Switz-
erland). All other reagents were purchased from Acros. Reactions 18.1 [(CH3)3CSi), Ϫ5.5 [(CH3)2Si] ppm. 1H NMR (CDCl3): δ ϭ
were performed under anhydrous conditions at room temperature
unless otherwise stated. TLC analyses were performed on Merck
plastic 60 F254 silica gel sheets. Detection by UV (254 nm) was fol-
lowed by spraying with one of the following solutions: (a) am-
monium molybdate (25 g LϪ1) and cerium() sulfate (10 gLϪ1) in
10% aq. sulfuric acid followed by charring, (b) aq. KMnO4 (2% in
aq. 1% K2CO3), or (c) 3% ninhydrin in ethanol, followed by char-
ring. Column chromatography was performed with Fluka silica gel
(230Ϫ400 mm mesh). Solvents used for column chromatography
5.95Ϫ5.74 (m, 3 H, 4-H, 5-H, ϭCH All), 5.11 (m, 2 H, ϭCH2 All),
4.15 (m, 2 H, 2-H, 3-H), 3.90 (m, 1 H, 6-H), 3.81 (m, 2 H, CH2O),
2.58Ϫ2.26 (m, 2 H, CH2 All), 0.89 [s, 9 H, (CH3)3C], 0.06 [s, 6 H,
(CH3)2Si] ppm.
(2R,3S,6R)-6-Allyl-2-[(tert-butyldimethylsilanyloxy)methyl]-3-O-
(trichloroacetimidoyl)-3,6-dihydro-2H-pyran (12): Alcohol 11 (4.8 g,
16.9 mmol) was coevaporated with 1,2-DCE (2 ϫ 50 mL) and dis-
solved in DCM (70 mL). Under nitrogen, the solution was cooled
to 0 °C and DBU (3.2 mL, 21.6 mmol) and Cl3CCN (2.2 mL,
22.2 mmol) were added. After the mixture had been stirred for 1
hour, TLC analysis (EtOAc/toluene, 5:95, v/v) showed complete
conversion of the alcohol. The reaction mixture was diluted with
EtOAc (100 mL) and poured into aq. NH4Cl (10%, 50 mL). The
layers were separated, the organic layer was washed with water and
brine, dried (Na2SO4), and filtered, and the solvents were evapo-
rated. The residue was loaded onto a silica gel column, and elution
with toluene/EtOAc (99:1 Ǟ 95:5, v/v), after concentration of the
appropriate fractions, furnished title compound 12 (5.2 g) in 72%
yield. 13C NMR (CDCl3): δ ϭ 161.5 (CϭNH), 134.1, 133.1, 122.8
(C-4, C-5, ϭCH All), 117.2 (ϭCH2 All), 91.3 (CCl3), 72.1, 71.5,
69.5 (C-2, C-3, C-6), 62.5 (CH2O), 37.7 (CH2 All), 25.8 [(CH3)3C],
18.1 [(CH3)3CSi), Ϫ5.5 [(CH3)2Si] ppm. 1H NMR (CDCl3): δ ϭ
8.36 (s, 1 H, NH), 5.99Ϫ5.79 (m, 3 H, 4-H, 5-H, ϭCH All), 5.29
(m, 1 H, 3-H), 5.09 (m, 2 H, ϭCH2 All), 4.30 (m, 1 H, 6-H), 3.92
(m, 1 H, 2-H), 3.81 (m, 2 H, CH2O), 2.58Ϫ2.26 (m, 2 H, CH2 All),
0.89 [s, 9 H, (CH3)3C], 0.06 [s, 6 H, (CH3)2Si] ppm.
1
were of technical grade and were distilled prior to use. H and 13C
NMR spectra were recorded with a Bruker AC 200 instrument.
Chemical shifts (delta) are given in ppm relative to TMS as an
internal standard for 1H NMR and 13C NMR spectroscopy. The
HPLC purifications were performed on a BioCAD ‘‘Vision’’ auto-
mated HPLC system with an Alltima C18 column (Alltech, 10.0
mm D ϫ 250 mm L, 5µ particle size) with use of the following
buffers: A: H2O, B: CH3CN, and C: 1% aq. TFA. The system was
eluted at 4.0 mLminϪ1 and detection was performed at 214 nm
unless stated otherwise. LC/MS analysis was performed on a Jacso
HPLC system (detection simultaneously at 214 and 254 nm). An
analytical Alltima C18 column (Alltech, 4.0 mm D ϫ 250 mm L,
5µ particle size) was used. Buffers: A: H2O, B: CH3CN, and C:
0.5% aq. TFA, linear gradient 5 Ǟ 50% B in 20 min. Mass spectra
were recorded with a Sciex API 165 mass instrument equipped with
a custom-made Electrospray (ESI).
(2R,3S,6R)-3-O-Acetyl-2-(acetyloxymethyl)-6-allyl-3,6-dihydro-2H-
pyran (10): Boron trifluorideϪdiethyl ether (3.8 mL, 30 mmol) was
added dropwise to a cooled (Ϫ30 °C), stirred solution of 3,4,6-tri-
O-acetyl--glucal (8.2 g, 30 mmol, dried by co-evaporation with
1,2-DCE, 2 ϫ 15 mL) and (allyl)trimethylsilane (3 mL, 36 mmol)
in DCM (100 mL). After TLC analysis showed the reaction to be
complete, the reaction mixture was quenched by addition of
NaHCO3 (10%, 30 mL) and EtOAc (150 mL). The layers were
separated, and the organic layer was washed with water and brine,
dried (MgSO4), filtered, and concentrated. Purification (silica gel,
toluene/EtOAc, 100:0 Ǟ 75:25, v/v) afforded 10 (7.7 g, 94%) as a
colorless oil. 13C NMR (CDCl3): δ ϭ 169.7 (2 ϫ CϭO acetyl),
132.3, 131.7 (C-4, C-5), 123.4 (ϭCH All), 116.9 (CH2 All), 70.8,
62.9, 64.5 (C-2, C-3, C-6), 62.4 (CH2O), 37.3 (CH2 All), 20.1 (CH3
(2S,5R,6R)-6-Allyl-2-[(tert-butyldimethylsilanyloxy)methyl]-5-(tri-
chloroacetamido)-2,5-dihydro-6H-pyran (13): A solution of imidate
12 (2.9 g, 6.8 mmol, dried by coevaporation with 1,4-dioxane) and
K2CO3 (100 mg) in 1,2-dichlorobenzene (50 mL) was heated at re-
flux temperature (180 °C). After 5 h the [3,3]-sigmatropic re-
arrangement was complete, as shown by formation of a more polar
compound (TLC analysis, EtOAc/toluene, 1:9, v/v). The reaction
mixture was cooled to room temperature, filtered through Hyflo,
and concentrated in high vacuum. Purification by silica gel column
chromatography (light petroleum/EtOAc, 100:0 Ǟ 85:15, v/v)
yielded 13 (2.8 g, 96%) as an oil. 13C NMR (CDCl3): δ ϭ 161.4
(CϭO), 133.4, 131.2, 125.0 (C-3, C-4, ϭCH All), 117.2 (ϭCH2
All), 92.5 (CCl3), 74.6, 71.7 (C-2, C-6), 63.7 (CH2O), 45.9 (C-5),
35.6 (CH2 All), 25.8 [(CH3)3C], 18.0 [(CH3)CSi], Ϫ5.5 [(CH3)2Si]
ppm. 1H NMR (HHCOSY, 300 MHz, CDCl3): δ ϭ 6.75 (d, J ϭ
9.3 Hz, 1 H, NH), 6.06 (m, 2 H, 3-H, 4-H), 5.85 (m, 1 H, ϭCH
All), 5.15 (m, 2 H, ϭCH2 All), 4.34 (m, 2 H, 2-H, 5-H), 4.11 (m,
1 H, 6-H), 3.77 (m, 2 H, CH2O), 2.33 (m, 2 H, CH2 All), 0.89 [s,
9 H, (CH3)3C], 0.10 [s, 6 H, (CH3)2Si] ppm.
1
acetyl) ppm. H NMR (CDCl3): δ ϭ 5.92Ϫ5.71 (br. m, 3 H, 4-H,
5-H, ϭCH All), 5.14 (m, 2 H, ϭCH2 All), 4.22 (m, 4 H, 2-H, 6-H,
CH2O), 3.90 (m, 1 H, 3-H), 2.37 (m, 2 H, CH2 All), 2.01 (s, 6 H,
2 ϫ CH3 acetyl) ppm.
(2R,3S,6R)-6-Allyl-2-[(tert-butyldimethylsilanyloxy)methyl]-3,6-di-
hydro-3-hydroxy-2H-pyran (11): Ferrier rearrangement product 10
(0.7 g, 4.3 mmol) was dissolved in MeOH (25 mL), tBuOK (25 mg)
was added, and the reaction mixture was stirred for 2 h, after which
TLC analysis (toluene/EtOAc, 1:1, v/v) showed the reaction to be
complete. The reaction mixture was neutralized with DOWEX
(2S,5R,6R)-6-Allyl-5-[(tert-butoxycarbonyl)amino]-2-[(tert-butyl-
dimethylsilanyloxy)methyl]-2,5-dihydro-6H-pyran (14): KOH (1.9 g,
34.2 mmol) was added to a solution of Overman rearrangement
product 13 (4.9 g, 11.4 mmol) in iPrOH (50 mL). The reaction mix-
2422
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2003, 2418Ϫ2427

Synthesis of Novel Tetrahydropyran-Based Dipeptide Isosters
FULL PAPER
ture was stirred for 3 d (TLC analysis EtOAc/light petroleum, 1:9, centration, crude 5 was isolated by silica gel column chromatogra-
v/v). Subsequently, the reaction mixture was diluted with DCM phy (light petroleum/EtOAc/AcOH, 3:1/0 Ǟ 0:98:2, v/v/v). Concen-
(100 mL), and di-tert-butyl dicarbonate (3.0 g, 13.7 mmol) was ad-
tration of the appropriate fractions yielded dipeptide isoster 5
ded. The reaction mixture was stirred overnight and the solvents (0.49 g, 71%). 13C NMR (CDCl3): δ ϭ 173.2 (CϭO acid), 156.2
were evaporated. The residue was purified by column chromatogra-
(CϭO Fmoc), 143.6, 141.1 (Cq Fmoc), 127.5, 126.8, 124.7, 119.7
phy (silica gel, light petroleum/EtOAc, 100:0 Ǟ 90:10, v/v) to yield
(Carom Fmoc), 74.2, 72.0 (C-2, C-6), 66.2 (CH2 Fmoc), 47.4 (C-5),
14 as a colorless oil (3.0 g, 70%, 2 steps). 13C NMR (CDCl3): δ ϭ 47.0 (CH Fmoc), 25.1, 25.0 (C-3, C-4), 18.0 (CH3CH2), 13.7 (CH3)
155.2 (CϭO), 134.5, 129.0, 126.6 (C-3, C-4, ϭCH All), 116.4 (ϭ
CH2 All), 78.7 (Cq Boc), 73.8 (C-6), 72.1 (C-2), 63.5 (CH2O), 45.0
(C-5), 35.3 (CH2 All), 28.0 [(CH3)3C Boc], 27.0 [(CH3)3C TBS],
17.8 [(CH3)CSi], Ϫ5.8 [(CH3)2Si] ppm. 1H NMR (CDCl3): δ ϭ 5.90
(m, 3 H, 3-H, 4-H, ϭCH All), 5.04 (m, 2 H, ϭCH2 All), 4.67 (d,
1 H, NH), 4.34 (m, 2 H, 2-H, 5-H), 4.12 (m, 1 H, 6-H), 3.87 (m, 2
H, CH2O), 2.25 (m, 2 H, CH2 All), 1.48 [s, 9 H, (CH3)3C Boc],
0.84 [s, 9 H, (CH3)3C TBS], 0.04 [s, 6 H, (CH3)2Si] ppm.
ppm. ESI-MS: m/z ϭ 410.0 [M ϩ H]ϩ; 432.0 [M ϩ Na]ϩ; 448.1
[M ϩ K]ϩ.
(2R,3S)-3-O-Acetyl-2-(acetyloxymethyl)-3,6-dihydro-2H-pyran (17):
Triethylsilane (7.7 mL, 40 mmol) and BF3·Et2O (3.4 mL, 52 mmol)
were added to a cooled solution (0 °C) of 3,4,6-tri-O-acetyl--glu-
cal (10.9 g, 40 mmol, dried by co-evaporation with 1,2-DCE) in
DCM (125 mL). The reaction mixture was stirred for 1 hour, after
which TLC analysis (toluene/EtOAc, 1:1, v/v) showed complete
conversion of the starting material. The reaction was quenched by
addition of NaHCO3 (1 , 20 mL), the layers were separated, and
the organic phase was washed with H2O and brine, dried (MgSO4),
filtered, and concentrated. Purification of the residue by silica gel
column chromatography (light petroleum/EtOAc, 75:25 Ǟ 60:40,
v/v) afforded 17 as an oil (8.7 g, 96%). 13C NMR (CDCl3): δ ϭ
169.9, 169.5 (CϭO acetyl), 128.9, 123.5 (C-4, C-5), 73.1, 64.6 (C-
(2S,5R,6R)-5-[(tert-Butoxycarbonyl)amino]-2-(hydroxymethyl)-6-
propyl-tetrahydropyran (15): Olefin 14 (3.1 g, 8.0 mmol) was dis-
solved in MeOH (50 mL), and palladium on carbon (10%, 80 mg)
was added. The suspension was repeatedly degassed under Ar. The
reaction mixture was stirred overnight (16 h) under H2, after which
TLC analysis (EtOAc/toluene, 1:9, v/v) showed the complete con-
sumption of the olefin. The catalyst was removed by filtration
through Hyflo, and the filtrate was concentrated. The residue was
loaded onto a silica gel column, and elution with toluene/EtOAc
(95:5 Ǟ 50:50, v/v) afforded alcohol 15 (1.7 g, 78%) as an oil. 13C
NMR (CDCl3): δ ϭ 155.5 (CϭO), 79.0 (Cq Boc), 72.3 (C-6), 70.2
(C-2), 62.7 (CH2O), 48.5, 46.4 (C-5), 33.8 (CH2 Pr), 29.1, 28.5,
24.2, 23.4 (C-3, C-4), 17.4 (CH3CH2), 13.5 (CH3) ppm.
1
2, C-3), 64.2, 62.6 (C-6, CH2O), 20.0, 19.8 (CH3 acetyl) ppm. H
NMR (CDCl3): δ ϭ 5.95, 5.73 (2 ϫ m, 2 H, 4-H, 5-H), 5.74 (m, 1
H, 3-H), 4.24 (m, 4 H, 6a-H, 6b-H, CH2O), 3.74 (m, 1 H, 2-H),
2.10 (m, 6 H, 2 ϫ CH3 acetyl) ppm.
(2R,3S)-2-[(tert-Butyldimethylsilanyloxy)methyl]-3-hydroxy-3,6-
dihydro-2H-pyran (18): Ferrier rearrangement product 17 (8.2 g,
38 mmol) was transformed into 18 (8.3 g, 89%) as described for 11.
13C NMR (CDCl3): δ ϭ 128.0, 127.0 (C-4, C-5), 77.3, 66.1 (C-
2, C-3), 65.0 (C-6, CH2O), 25.7 [(CH3)3C], 18.1 [(CH3)3C], Ϫ5.7
(2S,5R,6R)-5-[(tert-Butoxycarbonyl)amino]-6-propyl-tetra-
hydropyran-2-carboxylic Acid (16): Alcohol 15 (0.68 g, 2.5 mmol)
was dissolved in a mixture of DCM/CH3CN/H2O (1:1:1.45, v/v/v,
19 mL), and the mixture was cooled to 0 °C. With vigorous stirring,
NaIO4 (2.5 g, 11.7 mmol) and a catalytic amount of RuCl3·xH2O
(6 mg) were added. After 3 h the reaction mixture was diluted with
EtOAc (25 mL) and the solids were removed by filtration. The ex-
cess of NaIO4 was removed by addition of MeOH (5 mL), and the
obtained suspension was filtered. A solution of aq. NaHSO3 (10%,
10 mL) was added to the filtrate, which resulted in the decoloriz-
ation of the mixture. The pH was adjusted to 2 by addition of aq.
HCl (1 ). The layers were separated and the aqueous layer was
extracted with EtOAc (3 ϫ 15 mL). The combined organic layers
were dried (MgSO4), filtered, and concentrated. The residue was
dissolved in aq. NaHCO3 (10%, 30 mL) and washed with EtOAc
(2 ϫ 10 mL). The water layer was acidified to pH 2 with HCl (1
) and extracted with EtOAc (4 ϫ 20 mL). The organic phase was
dried over MgSO4, filtered, and concentrated in vacuo. The ob-
tained carboxylic acid 16 (0.48 g, 68%) was used in the next step
without further purification. 13C NMR (CDCl3): δ ϭ 176.3 (CϭO
acid), 156.2 (CϭO Boc), 79.5 (Cq Boc), 74.8, 72.4 (C-2, C-6), 46.7
(C-5), 33.6 (CH2 Pr), 28.9 [(CH3)3C], 25.8, 21.3 (C-3, C-4), 18.3
(CH3CH2), 13.9 (CH3) ppm.
1
[(CH3)2Si] ppm. H NMR (CDCl3): δ ϭ 5.81 (m, 2 H, 4-H, 5-H),
4.23 (m, 1 H, 3-H), 4.15 (m, 2 H, 6a-H, 6b-H), 3.95 (m, 1 H,
CHHO), 3.68 (m, 1 H, CHHO), 3.37 (m, 1 H, 2-H), 3.08 (br. s, 1
H, OH), 0.91 [s, 9 H, (CH3)3C], 0.12 [s, 6 H, (CH3)2Si] ppm.
(2R,3S)-2-[(tert-Butyldimethylsilanyloxy)methyl]-3-O-(trichloro-
acetimidoyl)-3,6-dihydro-2H-pyran (19): Alcohol 18 (2.4 g,
10 mmol) was treated with DBU (1.9 mL, 13 mmol) and Cl3CCN
(1.3 mL, 13 mmol) according to the procedure described for 12.
Silica gel column chromatography (light petroleum/EtOAc/Et3N,
94:5:1 Ǟ 90:9:1, v/v/v) afforded pure 19 (3.0 g, 76%). 13C NMR
(CDCl3): δ ϭ 161.5 (CϭNH), 130.0, 123.3 (C-4, C-5), 76.6, 69.8
(C-2, C-3), 65.3 (C-6), 63.0 (CH2O), 25.8 [(CH3)3C], 18.3
1
[(CH3)CSi], Ϫ5.4 [(CH3)2 Si] ppm. H NMR (CDCl3): δ ϭ 8.39 (s,
1 H, NH), 5.96 (m, 2 H, 4-H, 5-H), 5.42 (dd, J ϭ 2.9, J ϭ 8.0 Hz,
1 H, 3-H), 4.25 (m, 2 H, 6a-H, 6b-H), 3.80 (m, 2 H, CH2O), 3.69
(m, 1 H, 2-H), 0.89 [s, 9 H, (CH3)3C], 0.07 [s, 6 H, (CH3)2Si] ppm.
(2S,5R)-2-[(tert-Butyldimethylsilanyloxy)methyl]-5-(trichloro-
acetamido)-2,5-dihydro-6H-pyran (20): An Overman rearrangement
of imidate 19 (2.0 g, 5.1 mmol) as described for 13 furnished tri-
chloroacetamide 20 (1.8 g, 89%). 13C NMR (CDCl3): δ ϭ 161.1
(CϭO), 131.1, 124.4 (C-3, C-4), 73.9 (C-2), 64.9 (C-6), 64.1
(CH2O), 44.7 (C-5), 25.5 [(CH3)3C], 17.9 [(CH3)CSi], Ϫ5.7
(2S,5R,6R)-5-[N-(9-Fluorenylmethoxycarbonyl)amino]-6-propyl-
tetrahydropyran-2-carboxylic Acid (5): Carboxylic acid 16 (0.48 g,
1.7 mmol) was added to a mixture of TFA/DCM (1:1, v/v, 20 mL),
and the solution was stirred for 1 hour. The solvents were removed,
and the residue was coevaporated twice with toluene (25 mL). The
crude amino acid was taken up in 10% aq. NaHCO3/1,4-dioxane
(2:1, v/v, 30 mL) and cooled to 0 °C. A solution of FmocϪOSu
1
[(CH3)2Si] ppm. H NMR (CDCl3): δ ϭ 6.76 (d, J ϭ 8.0 Hz, 1 H,
NH), 5.99 (m, 2 H, 3-H, 4-H), 4.43 (m, 1 H, 5-H), 4.16 (m, 2 H,
6a-H, 6b-H), 3.61 (m, 3 H, 2-H, CH2O), 0.90 [s, 9 H, (CH3)3C],
0.08 [s, 6 H, (CH3)2Si] ppm.
(0.69 g, 2.0 mmol) in 1,4-dioxane (10 mL) was slowly added. After (2S,5R)-5-[(tert-Butoxycarbonyl)amino]-2-[(tert-butyldimethyl-
the reaction mixture had been stirred overnight at room tempera-
ture, its pH was adjusted to 2 (HCl, 1 ) and the mixture was
silanyloxy)methyl]-2,5-dihydro-6H-pyran (21): Deacylation of the
trichloroacetamide 20 (2.3 g, 6.0 mmol) and subsequent protection
extracted with EtOAc (4 ϫ 20 mL). After drying (MgSO4) and con- of the amine with a Boc group according to the procedure de-
Eur. J. Org. Chem. 2003, 2418Ϫ2427
www.eurjoc.org  2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2423

H. S. Overkleeft et al.
FULL PAPER
scribed for 14 furnished Boc-protected amine 21 (1.5 g, 74%). 13C
dissolved in iPrOH (60 mL), KOH (1.1 g, 20.1 mmol) was added,
NMR (CDCl3): δ ϭ 155.1 (CϭO), 130.0, 127.2 (C-3, C-4), 79.2 and the reaction mixture was stirred overnight. The volume was
(Cq Boc), 74.1 (C-2), 66.7 (C-6), 64.2 (CH2O), 44.0 (C-5), 28.2 reduced, and the remaining solution was diluted with ethyl acetate
[(CH3)3C Boc), 25.7 [(CH3)3C TBS), 18.1 [(CH3)CSi], Ϫ5.5 (150 mL) and washed with water and brine. After drying (MgSO4),
1
[(CH3)2Si] ppm. H NMR (CDCl3): δ ϭ 5.87 (m, 2 H, 3-H, 4-H), filtration, and concentration, the residue was purified by silica gel
4.55 (m, 1 H, 5-H), 4.11 (m, 2 H, 6a-H, 6b-H), 3.71 (m, 1 H,
column chromatography (light petroleum/EtOAc, 99:1 Ǟ 96:4, v/
CHHO), 3.57 (m, 1 H, CHHO), 3.44 (m, 1 H, 2-H), 1.45 [(CH3)3C v). Concentration of the appropriate fractions gave 25 in 80% yield
Boc), 0.89 [s, 9 H, (CH3)3C TBS), 0.06 [s, 6 H, (CH3)2Si] ppm.
(3.0 g). 13C NMR (CDCl3): δ ϭ 134.0 (ϭCH All), 131.8, 126.4 (C-
4, C-5), 116.7 (ϭCH2 All), 76.4, 63.1 (C-2, C-3, C-6), 62.6 (CH2O),
36.3 (CH2 All), 25.4 [(CH3)3C], 17.8 [(CH3)3CSi], Ϫ5.7 [(CH3)2Si]
(2S,5R)-5-[(tert-Butoxycarbonyl)amino]-2-(hydroxymethyl)-
tetrahydropyran (22): Reduction of 21 (2.4 g, 6.0 mmol) by the pro-
cedure described for 15 afforded alcohol 22 in 75% yield (1.0 g).
13C NMR (CDCl3): δ ϭ 155.3 (CϭO), 78.9 (Cq Boc), 77.4 (C-2),
70.4 (C-6), 64.7 (CH2O), 46.1 (C-5), 29.2, 26.3 (C-3, C-4), 27.8
[(CH3)3C] ppm. 1H NMR (CDCl3): δ ϭ 4.40 (d, 1 H, NH), 4.11
(m, 1 H, 2-H), 3.47 (m, 4 H, 6a-H, 6b-H, CH2O), 3.23 (m, 1 H, 5-
1
ppm. H NMR (CDCl3): δ ϭ 5.92 (m, 3 H, 4-H, 5-H, ϭCH All),
5.12 (m, 2 H, ϭCH2 All), 4.18 (m, 1 H, 6-H), 3.78 (m, 4 H, 2-H,
3-H, CH2O), 2.51Ϫ2.18 (br. m, 2 H, CH2 All), 0.91 [s, 9 H,
(CH3)3C], 0.09 (s, 3 H, CH3Si), 0.06 (s, 3 H, CH3Si) ppm.
(2R,3R,6R)-6-Allyl-2-[(tert-butyldimethylsilanyloxy)methyl]-3-O-
H), 2.06, 1.72 (2 ϫ m, 4 H, 3a-H, 3b-H, 4a-H, 4b-H), 1.44 [s, 9 H, (trichloroacetimidoyl)-3,6-dihydro-2H-pyran (26): Conversion of al-
(CH3)3C] ppm.
cohol 25 (3.0 g, 10.7 mmol) into imidate 26 by the procedure de-
scribed for 12 and purification by silica gel column chromatography
(toluene/EtOAc, 99:1 Ǟ 95:5, v/v) afforded 26 (4.2 g, 90%). 13C
NMR (CDCl3): δ ϭ 161.9 (CϭNH), 135.7 (ϭCH All), 134.2, 121.0
(C-4, C-5), 117.3 (ϭCH2 All), 72.4, 71.1, 68.1 (C-2, C-3, C-6), 61.7
(CH2O), 36.8 (CH2 All), 25.8 [(CH3)3C], 18.1 [(CH3)3CSi], Ϫ5.4
[(CH3)2Si] ppm. 1H NMR (CDCl3): δ ϭ 8.26 (br. d, 1 H, NH),
6.23Ϫ5.78 (m, 3 H, 4-H, 5-H, ϭCH All), 5.17 (m, 2 H, CH2 All),
4.37 (m, 1 H, 3-H), 4.01 (m, 1 H, 6-H), 3.83 (m, 3 H, 2-H, CH2O),
2.54Ϫ2.22 (br. m, 2 H, CH2 All), 0.87 [s, 9 H, (CH3)3C], 0.05 (s, 3
H, CH3Si), 0.03 (s, 3 H, CH3Si) ppm.
(2S,5R)-5-[(tert-Butoxycarbonyl)amino]tetrahydropyran-2-
carboxylic Acid (23): Alcohol 22 (1.0 g, 4.5 mmol) was oxidized to
the corresponding acid 23 (0.79 g, 71%) as described for 16. 13C
NMR (CDCl3): δ ϭ 174.6 (CϭO acid), 157.4 (CϭO Boc), 81.7 (Cq
Boc), 74.8 (C-2), 70.6 (C-6), 46.6, 45.4 (C-5), 29.3, 27.6, 27.2 (C-3,
C-4), 28.2 [(CH3)3C Boc) ppm. ESI-MS: m/z ϭ 246.1 [M ϩ H]ϩ;
267.8 [M ϩ Na]ϩ.
(2S,5R)-5-[N-(9-Fluorenylmethoxycarbonyl)amino]tetrahydropyran-
2-carboxylic Acid (6): (2S,5R)-5-[(tert-butoxycarbonyl)amino]te-
trahydropyran-2-carboxylic acid (0.66 g, 2.7 mmol) was subjected
to TFA/DCM (1:1, v/v) and subsequent treatment with
FmocϪOSu as described for 5. After silica gel column chromatog-
raphy, 6 was obtained in 78% yield over 2 steps (0.77 g). 13C NMR
(CDCl3/MeOD/D2O): δ ϭ 174.2 (CϭO acid), 157.2 (CϭO Fmoc),
144.4, 141.9 (Cq Fmoc), 128.3, 127.7, 125.5, 120.5 (Carom Fmoc),
75.7 (C-2), 73.4 (C-6), 70.6 (CH2 Fmoc), 47.7 (CH Fmoc), 46.7 (C-
5), 28.2, 25.9 (C-3, C-4) ppm. ESI-MS: m/z ϭ 368.1 [M ϩ H]ϩ;
390.4 [M ϩ Na]ϩ; 406.3 [M ϩ K]ϩ.
(2S,5S,6R)-6-Allyl-2-[(tert-butyldimethylsilanyloxy)methyl]-5-(tri-
chloroacetamido)-2,5-dihydro-6H-pyran (27): The Overman re-
arrangement of 26 (4.2 g, 9.7 mmol) was performed as described
for compound 13. Silica gel column chromatography (toluene/
EtOAc, 100:0 Ǟ 95:5, v/v) furnished 27 (3.2 g, 78%). 13C NMR
(CDCl3): δ ϭ 161.0 (CϭO), 133.6, 131.6, 123.2 (ϭCH All, C-3, C-
4), 117.4 (ϭCH2 All), 92.3 (CCl3), 74.2, 70.3 (C-2, C-6), 64.7
(CH2O), 47.3 (C-5), 35.0 (CH2 All), 25.7 [(CH3)3C], 18.1
[(CH3)3CSi), Ϫ5.5 [(CH3)2Si] ppm. 1H NMR (CDCl3): δ ϭ 6.76
(br. d, J ϭ 8.0 Hz, 1 H, NH), 6.03Ϫ5.77 (m, 3 H, 3-H, 4-H, ϭCH
All), 5.15 (m, 2 H, ϭCH2 All), 4.18 (m, 2 H, 2-H, 5-H), 3.92 (m,
1 H, 6-H), 3.73 (m, 2 H, CH2O), 2.56Ϫ2.27 (br. m, 2 H, CH2 All),
0.90 [s, 9 H, (CH3)3C], 0.07 [s, 6 H, (CH3)2Si] ppm.
(2R,3R,6R)-6-Allyl-2-[(tert-butyldimethylsilanyloxy)methyl]-3-O-(p-
nitrobenzoyl)-3,6-dihydro-2H-pyran (24): A solution of DEAD
(2.7 mL, 17.3 mmol) in THF (20 mL) was slowly added to a cooled
(0 °C) solution of alcohol 11 (3.3 g, 11.5 mmol) in THF (50 mL)
containing triphenylphosphane (4.50 g, 17.3 mmol) and p-nitroben-
zoic acid (2.1 g, 12.7 mmol). After the mixture had been stirred
overnight, the solvent was removed and the residue was dissolved
in EtOAc (100 mL) and washed with aq. NaHCO3 (10%), water,
and brine. The organic layer was dried over MgSO4, filtered, and
concentrated. The residue was dissolved in diethyl ether/light petro-
leum (9:1, v/v) and stored at 4 °C overnight. The solids were re-
moved by filtration and the filtrate was concentrated in vacuo.
Crude 24 was loaded onto a silica gel column, and elution with
toluene/EtOAc (99:1 Ǟ 98:2, v/v) afforded p-nitrophenyl ester 24
(4.5 g, 91%). 13C NMR (CDCl3): δ ϭ 163.7 (CϭO), 150.2 (Cq
(2S,5S,6R)-5-[(tert-Butoxycarbonyl)amino]-2-(hydroxymethyl)-6-
propyl-tetrahydropyran (29): The Boc-protected Overman re-
arrangement product 28 (2.6 g, 88%, two steps) was obtained from
27 (3.2 g, 7.5 mmol) as described for 14. Subsequent hydrogenation
of 28 by the procedure performed for 15 yielded saturated alcohol
29 (1.3 g, 70%).13C NMR (CDCl3): δ ϭ 154.4 (CϭO), 78.4 (Cq
Boc), 76.0, 68.6 (C-2, C-6), 64.4 (CH2O), 46.5 (C-5), 30.8 (CH2 Pr),
27.4 [(CH3)3C Boc), 22.4, 21.1 (C-3, C-4), 17.8 (CH2CH3), 13.0
(CH3) ppm.
(2S,5S,6R)-5-[(tert-Butoxycarbonyl)amino]-6-propyl-tetra-
PhϪNO2), 135.4 (Cq Ph), 135.5 (ϭCH All), 133.9, 130.4, 123.1, hydropyran-2-carboxylic Acid (30): Alcohol 29 (0.67 g, 2.5 mmol)
121.5 (Carom, C-4, C-5), 117.2 (ϭCH2 All), 72.5, 70.0, 64.5 (C-2, was oxidized in a fashion similar to that described for compound
C-3, C-6), 61.4 (CH2O), 36.4 (CH2 All), 25.4 [(CH3)3C], 17.7
16 to yield Boc-protected dipeptide isoster 30 (0.41 g, 55%). 13C
[(CH3)CSi], Ϫ5.9 [(CH3)2 Si] ppm. 1H NMR (CDCl3): δ ϭ 8.34 NMR (CDCl3): δ ϭ 172.8 (CϭO), 152.9 (CϭO), 78.8 (Cq Boc),
(m, 4 H, Harom), 6.23 (m, 2 H, 4-H, 5-H), 5.99 (m, 1 H, ϭCH All), 74.7, 73.9 (C-6), 68.1, 67.5 (C-2), 47.7; 46.0 (C-5), 30.4 (CH2 Pr),
5.43 (m, 1 H, 3-H), 5.24 (m, 2 H, ϭCH2 All), 4.50 (m, 1 H, 2-H), 25.8 [(CH3)3C Boc), 23.3, 22.4 (C-3, C-4), 15.9 (CH2CH3), 11.3
4.16 (m, 1 H, 6-H), 3.89 (m, 2 H, CH2O), 2.67Ϫ2.35 (br. m, 2 H,
CH2 All), 0.90 [s, 9 H, (CH3)3C], 0.08 (s, 3 H, CH3Si), 0.06 (s, 3
H, CH3Si) ppm.
(CH3) ppm.
(2S,5S,6R)-5-[N-(9-Fluorenylmethoxycarbonyl)amino]-6-propyl-
tetrahydropyran-2-carboxylic Acid (7): Boc-protected acid 30
(406 mg, 1.35 mmol) was transformed into title compound 7
(417 mg, 76%) by the procedure described for 5. 13C NMR
(2R,3R,6R)-6-Allyl-2-[(tert-butyldimethylsilanyloxy)methyl]-3-
hydroxy-3,6-dihydro-2H-pyran (25): Ester 24 (5.8 g, 13.4 mmol) was
2424
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2003, 2418Ϫ2427

Synthesis of Novel Tetrahydropyran-Based Dipeptide Isosters
(CDCl3): δ ϭ 172.8 (CϭO acid), 156.0 (CϭO Fmoc), 143.3, 141.1 130.9, 125.5 (C-3, C-4), 79.2 (Cq Boc), 74.8 (C-2), 68.7, 65.1 (C-6,
FULL PAPER
(Cq Fmoc), 127.5, 126.9, 124.7, 119.6 (Carom Fmoc), 74.3, 72.1 (C-
CH2O), 43.7 (C-5), 28.1 [(CH3)3C Boc), 25.7 [(CH3)3C TBS), 18.2
2, C-6), 65.9 (CH2 Fmoc), 47.5 (C-5), 47.0 (CH Fmoc), 25.1, 25.0 [(CH3)3CSi], Ϫ5.6 [(CH3)2Si] ppm. 1H NMR (CDCl3): δ ϭ 5.86
(C-3, C-4), 17.9 (CH3CH2), 13.8 (CH3) ppm. ESI-MS: m/z ϭ 410.0 (m, 2 H, 3-H, 4-H), 4.07 (m, 2 H, 2-H, 5-H), 3.61 (m, 4 H, 6a-H,
[M ϩ H]ϩ; 432.0 [M ϩ Na]ϩ; 448.1 [M ϩ K]ϩ.
6b-H, CH2O), 1.39 [s, 9 H, (CH3)3C Boc], 0.86 [s, 9 H, (CH3)3C
TBS], 0.03 [s, 6 H, (CH3)2Si] ppm.
(2R,3R)-2-[(tert-Butyldimethylsilanyloxy)methyl]-3-O-(p-nitro-
benzoyl)-3,6-dihydro-2H-pyran (31): A solution of alcohol 18 (6.1 g,
25 mmol), triphenylphosphane (9.8 g, 37.5 mmol), and p-nitroben-
zoic acid (4.6 g, 27.5 mmol) in THF (100 mL) was cooled to Ϫ30
(2S,5S)-5-[(tert-Butoxycarbonyl)amino]tetrahydropyran-2-car-
boxylic Acid (37): Olefin 35 (6.1 mmol) was converted into alcohol
36 (0.83 g, 58%) by the procedure described for 22. Oxidation of
36 (0.46 g, 2.0 mmol) according to the procedure described for 23
gave acid 37 (0.3 g, 60%). 13C NMR (CDCl3): δ ϭ 173.8 (CϭO
acid), 155.2 (CϭO Boc), 79.1 (Cq Boc), 74.8 (C-2), 70.3 (C-6), 44.1
(C-5), 27.9 [(CH3)3C], 27.0, 23.6 (C-3, C-4) ppm.
°C.
A
solution of diisopropyl azodicarboxylate (5.4 mL,
27.5 mmol) in THF (20 mL) was added dropwise, and the reaction
mixture was allowed to warm up slowly to room temperature. After
stirring overnight, the mixture was diluted with EtOAc (150 mL)
and poured into aq. NaHCO3 (10%, 75 mL), and the layers were
separated. The organic layer was washed with brine (75 mL), dried
(MgSO4), filtered, and concentrated to dryness. Purification by sil-
ica gel column chromatography (toluene) afforded a mixture of es-
ter 31 and an unidentified side product (6.3 g, 64%). 13C NMR
(CDCl3): δ ϭ 164.2 (CϭO), 150.4 (Cq PhϪNO2), 134.6 (Cq Ph),
134.6, 122.2 (C-4, C-5), 130.7, 123.2 (Carom), 74.6 (C-3), 67.1 (C-
6), 66.6 (C-2), 64.9 (CH2O), 25.7 [(CH3)3C], 18.1 [(CH3)CSi], Ϫ5.5
[(CH3)2Si] ppm.
(2S,5S)-5-[N-(9-Fluorenylmethoxycarbonyl)amino]tetrahydropyran-
2-carboxylic Acid (8): Carboxylic acid 37 (0.20 g, 0.80 mmol) was
subjected to TFA/DCM (1:1, v/v) followed by introduction of the
Fmoc group to furnish 8 (0.19 g, 63%) as described for 6. 13C NMR
(CDCl3): δ ϭ 174.0 (CϭO acid), 156.0 (CϭO Fmoc), 143.6, 141.1
(Cq Fmoc), 127.5, 126.9, 124.9, 119.8 (Carom Fmoc), 75.1 (C-2),
70.4 (C-6), 66.6 (CH2 Fmoc), 46.9 (CH Fmoc), 44.9 (C-5), 27.2,
23.6 (C-3, C-4) ppm. ESI-MS: m/z ϭ 368.1 [M ϩ H]ϩ; 390.0 [M
ϩ Na]ϩ; 406.1 [M ϩ K]ϩ.
(2R,3R)-2-[(tert-Butyldimethylsilanyloxy)methyl]-3-hydroxy-3,6-
dihydro-2H-pyran (32): Ester 31 (3.2 g, 8.1 mmol) was dissolved in
methanol (75 mL), and a catalytic amount of tBuOK (100 mg) was
added. The resulting solution was stirred for 16 h and neutralized
by addition of acetic acid. The solvent was removed by evapor-
ation, and the residue was purified by silica gel column chromatog-
raphy (toluene/EtOAc, 100:0 Ǟ 90:10, v/v) to give alcohol 32 (1.5 g,
76%). 13C NMR (CDCl3): δ ϭ 129.5, 126.6 (C-4, C-5), 76.3, 61.9
(C-2, C-3), 65.7 (C-6), 62.4 (CH2O), 25.6 [(CH3)3C], 18.1
[(CH3)3CSi], Ϫ5.5 [(CH3)2Si] ppm. 1H NMR (HHCOSY,
General Procedure for Solid-Phase Peptide Synthesis: The 5-[N-(9-
fluorenylmethoxycarbonyl)amino]tetrahydropyran-2-carboxylic
acid-containing peptides 41؊46 were synthesized on an automatic
peptide synthesizer (ABI, 443A, Applied Biosystems, division of
PerkinϪElmer). The synthesis was performed by use of the
FastMoc protocol supplied with the synthesizer. Each peptide was
synthesized on a 50 µmol scale starting from Wang resin loaded
with Nα-(9-fluorenylmethoxycarbonyl)-Nω-2,2,4,6,7-pentamethyl-
dihydrobenzofuran-5-sulfonyl-arginine (vide infra). The Fmoc-am-
ino acids used in the synthesis were as follows: FmocϪAlaϪOH,
400 MHz, CDCl3): δ ϭ 6.01 (m, 1 H, 4-H), 5.90 (ddd, 1 H, J4,5
ϭ
1.5, J5,6b ϭ 3.5, J5,6a ϭ 10.1 Hz, 5-H), 4.15 (m, 2 H, 6a-H, 6b-H),
3.91 (m, 1 H, 3-H), 3.80 (m, 2 H, CH2O), 3.50 (dt, 1 H, J2,3 ϭ 1.9,
J2,CHHO ϭ J2,CHHO ϭ 6.3 Hz), 2.00 (br. s, OH), 0.87 [s, 9 H,
(CH3)3C], 0.59 (s, 3 H, CH3Si), 0.53 (s, 3 H, CH3Si) ppm.
FmocϪArg(Pbf)ϪOH,
Glu(OtBu)ϪOH, FmocϪIleϪOH,
FmocϪThr(tBu)ϪOH and FmocϪTyr(tBu)ϪOH.
FmocϪAsp(OtBu)ϪOH,
FmocϪ
FmocϪLys(Boc)ϪOH,
Generally, the following steps were performed for each cycle:
(2R,3R)-2-[(tert-Butyldimethylsilanyloxy)methyl]-3-O-(trichloro-
acetimidoyl)-3,6-dihydro-2H-pyran (33): Trichloroacetimidate 33
(2.2 g, 90%) was obtained from allylic alcohol 32 (1.5 g, 6.1 mmol)
by the procedure described for 19. 13C NMR (CDCl3): δ ϭ 161.1
(CϭO), 132.4, 120.4 (C-4, C-5), 76.2, 67.8 (C-2, C-3), 64.9 (C-6),
61.1 (CH2O), 25.1 [(CH3)3C], 17.4 [(CH3)3CSi], Ϫ6.2 [(CH3)2Si]
(a) Removal of the Fmoc group by treatment with 20% piperidine
in NMP (5 ϫ 1 min).
(b) Coupling of the suitably protected amino acids by application
of a fivefold excess. The amino acid was dissolved in NMP (0.5 mL)
and 1 equiv. of BOP/HOBt (0.5  BOP/0.5  HOBt in NMP/DMF,
1:1, v/v, 0.5 mL) and 2.5 equiv. of DiPEA (1.25  DiPEA in NMP,
0.5 mL) were added. The resulting solution was transferred to the
reaction vessel and the suspension was shaken for 1 hour. In the
case of the 5-[N-(9-fluorenylmethoxycarbonyl)amino]tetrahydropy-
ran-2-carboxylic acids 5؊8 the time of the coupling reaction was
set to 2 h.
1
ppm. H NMR (CDCl3): δ ϭ 8.27 (s, 1 H, NH), 6.17 (m, 2 H, 4-
H, 5-H), 5.28 (m, 1 H, 3-H), 4.27 (m, 2 H, 6a-H, 6b-H), 3.81 (m,
3 H, 2-H, CH2O), 0.88 [s, 9 H, (CH3)3], 0.06 (s, 3 H, CH3Si), 0.04
(s, 3 H, CH3Si) ppm.
(2S,5S)-2-[(tert-Butyldimethylsilanyloxy)methyl]-5-(trichloro-
acetamido)-2,5-dihydro-6H-pyran (34): Imidate 33 (2.2 g, 5.4 mmol)
was subjected to thermal rearrangement as depicted for 20 to give
trichloroacetamide 34 (1.6 g, 76%). 13C NMR (CDCl3): δ ϭ 160.1
(CϭO), 132.8, 123.5 (C-3, C-4), 92.2 (CCl3), 75.1 (C-2), 67.8, 64.9
(C-6, CH2O), 44.9 (C-5), 25.7 [(CH3)3C], 18.2 [(CH3)3CSi], Ϫ5.5
[(CH3)2Si] ppm. 1H NMR (CDCl3): δ ϭ 6.90 (d, 1H. NH), 5.99
(m, 2 H, 3-H, 4-H), 4.28 (m, 1 H, 5-H), 4.13 (m, 1 H, 2-H), 3.72
(m, 4 H, 6a-H, 6b-H, CH2O), 0.89 [s, 9 H, (CH3)3C], 0.06 [s, 6 H,
(CH3)2Si] ppm.
(c) Capping of the remaining amine functions. A capping solution
(0.5  acetic anhydride, 0.125  DiPEA and 0.015  HOBt in
NMP, 1 mL) was added and the resulting suspension was shaken
for 1 min.
When the synthesis was complete, the resin was removed from the
reaction vessel and placed in a round-bottomed flask. A mixture
of TFA/H2O/TIS (95:2.5:2.5, v/v/v, 10 mL) was added, and the sus-
(2S,5S)-5-[(tert-Butoxycarbonyl)amino]-2-[(tert-butyldimethyl- pension was shaken for 2 h. Subsequently the resin was filtered and
silanyloxy)methyl]-2,5-dihydro-6H-pyran (35): Overman product 34 washed with TFA. The filtrate was diluted with toluene, and the
(2.4 g, 6.1 mmol) was converted into Boc-protected 35 (2.4 g, qu- solvents were evaporated to dryness. The residue was dried in high
ant.) as described for 21. 13C NMR (CDCl3): δ ϭ 154.9 (CϭO),
vacuum, dissolved in little TFA, and precipitated by dropwise ad-
Eur. J. Org. Chem. 2003, 2418Ϫ2427
www.eurjoc.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2425

H. S. Overkleeft et al.
FULL PAPER
dition to diethyl ether. The crude pyran-containing peptide was
purified by RP HPLC.
analysis: Rt ϭ 10.68 min, ESI-MS: m/z ϭ 1291.9 [M ϩ H]ϩ; 646.7
[M ϩ 2H]2ϩ; 431.5 [M ϩ 3H]3ϩ; calcd. 1290.7.
Fmoc؊Arg(Pbf)-Wang Resin (39): In a silylated flask (20% trimeth-
ylsilyl chloride in CHCl3), p-benzyloxybenzyl alcohol resin 38,
(Wang resin, 0.5 g, 430 µmol, 0.86 mmol/g) was dried by coevapor-
ation with 1,4-dioxane (2 ϫ 15 mL). The resin was suspended in
DCM (40 mL), Nα-Fmoc-NG-2,2,4,6,7-pentamethyl-dihydrobenzo-
furan-5-sulfonyl--arginine (FmocϪArg(Pbf)ϪOH, 0.84 g, 1.3
mmol), dicyclohexylcarbodiimide (DCC, 0.29 g, 1.4 mmol), and 4-
(dimethylamino)pyridine (DMAP, 8 mg, 0.07 mmol) were added,
and the flask was rotated for 2 h. The resin was filtered and sub-
sequently washed with DCM (2 ϫ 15 mL), MeOH (15 mL), and
DCM (2 ϫ 15 mL), and the loading of the resin was repeated once
more. The resin was dried in high vacuum and the loading of the
resin (0.48 mmol/g, 56%) was determined by UV absorption by the
following procedure. A sample of the resin (4Ϫ5 mg) was exactly
weighed in a volumetric flask (10 mL). A solution of piperidine/
DMF (1:4, v/v, 1 mL) was added, and the mixture was left for
10 min. The volume was adjusted to 10 mL by addition of MeOH
(HPLC grade) and the UV absorption was measured between 250
and 350 nm. The loading of the resin was calculated from the for-
mula: L (mmol/g) ϭ (A ϫ vol)/(7.8 ϫ wt), where A is the absorp-
tion value at 300 nm (secondary maximum), vol is the volume of
the sample (10 mL), and wt is the weight of the resin (mg).
H-Ala-Lys-Thr-Ile-Ala-Tyr-Asp-[(2S,5S,6R)-5-Amino-2-carboxy-
6-propyl-tetrahydropyran]-Ala-Arg-Arg-OH (46): The crude peptide
46 (35 mg, Ϯ29 µmol) was purified by RP HPLC (linear gradient
14 Ǟ 23% B in 14.7 min, 3.0 CV) to afford 46 (7.67 mg, 16%, TFA
salt). LC/MS analysis: Rt ϭ 11.88 min, ESI-MS 1334.0 [M ϩ H]ϩ;
667.5 [M ϩ 2H]2ϩ; calcd. 1332.8.
[1]
P. Clapes, J. L. Torres, P. Adlercreutz, Bioorg. Med. Chem.
1995, 3, 245Ϫ255.
[2]
B. Bielekova, R. Martin, J. Mol. Med. 2001, 79, 552Ϫ565.
[3]
S. M. Anderton, S. P. Manickasingham, C. Buckhart, T. A.
Luckuck, S. J. Holland, A. G. Lamont, D. C. Wraith, J. Immu-
nol. 1998, 161, 3357Ϫ3364.
L. B. Nicholson, A. Murtaza, B. P. Hafler, A. Sette, V. K.
[4]
Kuchroo, Proc. Natl. Acad. Sci. USA 1997, 94, 9279Ϫ9284.
J. A. W. Kruijtzer, L. J. F. Hofmeyer, W. Heerma, C. Versluis,
[5]
R. M. J. Liskamp, Chem. Eur. J. 1998, 4, 1570Ϫ1580.
D. B. A. de Bont, K. M. Sliedregt-Bol, L. J. F. Hofmeyer, R.
[6]
M. J. Liskamp, Bioorg. Med. Chem. 1999, 7, 1043Ϫ1047.
[7]
K. M. Bol, R. M. J. Liskamp, Tetrahedron 1992, 48,
6425Ϫ6438.
[8]
S. A. W. Gruner, E. Locardi, E. Lohof, H. Kessler, Chem. Rev.
2002, 102, 491Ϫ514.
H-Ala-Lys-Thr-Ile-[(2S,5S,6R)-5-Amino-2-carboxy-6-propyl-
tetrahydropyran]-Asp-Glu-Glu-Ala-Arg-Arg-OH (41): The synthesis
of the pyran-containing peptide 41 was according to the general
procedure for solid-phase peptide synthesis, followed by RP HPLC
purification (linear gradient, 10 Ǟ 22% B in 19.6 min, 4.0 CV),
furnishing 41 (20.6 mg, 24.4%, TFA salt). LC/MS analysis: Rt ϭ
[9]
H. S. Overkleeft, S. H. L. Verhelst, E. Pieterman, N. J.
Meeuwenoord, M. Overhand, L. H. Cohen, G. A. van der Ma-
rel, J. H. van Boom, Tetrahedron Lett. 1999, 40, 4103Ϫ4106.
F. El Oualid, L. Bruining, I. M. Leroy, L. H. Cohen, J. H. van
Boom, G. A. van der Marel, H. S. Overkleeft, M. Overhand,
Helv. Chim. Acta 2002, 85, 3455Ϫ3472.
C. M. Timmers, J. J. Turner, C. M. Ward, G. A. van der Marel,
M. L. C. E. Kouwijzer, P. D. J. Grootenhuis, J. H. van Boom,
Chem. Eur. J. 1997, 3, 920.
R. M. van Well, H. S. Overkleeft, M. Overhand, E. V. Carst-
enen, G. A. van der Marel, J. H. van Boom, Tetrahedron Lett.
2000, 41, 9331Ϫ9335.
B. M. Aguilera, G. Siegal, H. S. Overkleeft, N. J.
Meeuwenoord, F. P. J. T. Rutjes, J. C. M. van Hest, H. E. Scho-
emaker, G. A. van der Marel, J. H. van Boom, M. Overhand,
Eur. J. Org. Chem. 2001, 1541.
[10]
[11]
11.08 min, ESI-MS: m/z ϭ 1358.2 [M ϩ H]ϩ; 679.7 [M ϩ 2H]2ϩ
453.5 [M ϩ 3H]3ϩ; 340.3 [M ϩ 4H]4ϩ; calcd. 1356.7.
;
[12]
H-Ala-Lys-Thr-Ile-[(2S,5R)-5-Amino-2-carboxytetrahydropyran]-
Asp-Glu-Glu-Ala-Arg-Arg-OH (42): Pyran-containing peptide 42
was synthesized as described in the general procedure for solid-
phase peptide synthesis. HPLC purification (linear gradient 13 Ǟ
17% B in 12.3 min, 2.5 CV) afforded 21.7 mg (26.4%, TFA salt).
LC/MS analysis: Rt ϭ 7.90 min, ESI-MS: m/z ϭ 1316.0 [M ϩ H]ϩ;
658.9 [M ϩ 2H]2ϩ; 439.6 [M ϩ 3H]3ϩ; calcd. 1314.7.
[13]
[14]
R. M. van Well, H. S. Overkleeft, M. Overhand, G. A. van der
Marel, D. Bruss, P. G. de Groot, J. H. van Boom, Bioorg. Med.
Chem. Lett. In pres.
H-Ala-Lys-Thr-Ile-[(2S,5S)-5-Amino-2-carboxytetrahydropyran]-
Asp-Glu-Glu-Ala-Arg-Arg-OH (43): The solid-phase synthesis of
peptide 43 was performed according to the general procedure for
SPPS. After RP HPLC purification (linear gradient 10 Ǟ 18% B
in 14.7 min, 3.0 CV) the 2,5-cis-pyran-containing peptide 43 was
obtained in 25% yield (20.6 mg, TFA salt). LC/MS analysis: Rt ϭ
[15]
Antigenic peptide 9, the p2Ϫ13 residues of the heat shock pro-
tein 65 (hsp65), binds specifically to the human class II allele
HLA-DR3, which is reported to be a mediator in the occurrence
of tuberculoid leprosy as well as the autoimmune diseases myas-
thenia gravis and insulin-dependent diabetes mellitus. See in this
respect: W. C. A. van Schooten, D. G. Elferink, van J. Embden,
D. C. Anderson, R. R. P. de Vries, Eur. J. Immunol. 1989, 19,
2075Ϫ2079; T. H. H. Ottenhof, J. B. A. G. Haanen, A. Geluk,
T. Mutis, B. K. Ab, J. E. Thole, W. C. A. van Schooten, P. J. van
den Elsen, R. R. P. de Vries, Immunol. Rev. 1991, 121, 171Ϫ191;
J. L. Tiwari, P. I. Terasaki in: HLA and Disease Associations,
Springer-Verlag, New York, 1985, p 4Ϫ17.
8.19 min, ESI-MS: m/z ϭ 1316.2 [M ϩ H]ϩ; 659.0 [M ϩ 2H]2ϩ
439.6 [M ϩ 3H]3ϩ; 329.8 [M ϩ 4H]4ϩ; calcd. 1314.7.
;
H-Ala-Lys-Thr-Ile-Ala-Tyr-Asp-[(2S,5R,6R)-5-Amino-2-carboxy-
6-propyl-tetrahydropyran]-Ala-Arg-Arg-OH (44): Purification of
the crude peptide (29 mg, 21.7 µmol) by RP HPLC (linear gradient
14 Ǟ 23% B in 14.7 min, 3.0 CV) furnished 44 (7.99 mg, 22.1%,
TFA salt). LC/MS analysis: Rt ϭ 8.07 min, ESI-MS: m/z ϭ 1334.1
[M ϩ H]ϩ; 667.5 [M ϩ 2H]2ϩ; 445.4 [M ϩ 3H]3ϩ; calcd. 1332.8.
[16]
S. Danishefsky, J. F. Kerwin Jr., J. Org. Chem. 1982, 47,
3803Ϫ3805.
I. Hanna, P. Wlodyka, J. Org. Chem. 1997, 62, 6985Ϫ6990.
T. Nishikawa, M. Asai, N. Ohyabu, M. Isobe, J. Org. Chem.
[17]
[18]
1998, 63, 188Ϫ192.
H-Ala-Lys-Thr-Ile-Ala-Tyr-Asp-[(2S,5R)-5-Amino-(2-carboxy)-
tetrahydropyran]-Ala-Arg-Arg-OH (45): A sample of the crude pep-
tide 45 (45 mg, Ϯ 25 µmol) was loaded onto a semipreparative RP
HPLC column. Elution with buffer A: H2O; B: CH3CN, C aq. 1%
TFA with application of a linear gradient in B (13 Ǟ 19%, in
14.7 min, 3.0 CV) gave 45 (11.46 mg, 28%, TFA salt). LC/MS
[19]
L. E. Overman, J. Am. Chem. Soc. 1976, 98, 2901.
[20]
The use of methanol as the solvent in the removal of the trich-
loroacetyl function resulted in partial migration of the TBS
group.
K. Hattori, H. Sajiki, K. Hirota, Tetrahedron Lett. 2000, 41,
5711Ϫ5714.
[21]
2426
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2003, 2418Ϫ2427

Synthesis of Novel Tetrahydropyran-Based Dipeptide Isosters
FULL PAPER
A. A-H. Abdel-Rahman, G. A. Winterfeld, M. Takhi, R. R.
[22]
[24]
[25]
P. H. J. Carlsen, T. Katsuki, V. S. Martin, K. B. Sharpless, J.
Org. Chem. 1981, 46, 3939Ϫ3940.
The synthesis of intermediate 20 has been reported by K. C.
Schmidt, Eur. J. Org. Chem. 2002, 713Ϫ717.
[23]
The best results were obtained when two consecutive couplings
of 2 h each were applied.
Nicolaou, C. K. Hwang, B. E. Marron, S. A. Defrees, E. A.
Couladouros, Y. Abe, P. J. Carroll, J. P. Snyder, J. Am. Chem.
Soc. 1990, 112, 3040Ϫ3054.
Received December 2, 2002
Eur. J. Org. Chem. 2003, 2418Ϫ2427
www.eurjoc.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2427