Polyhydroxylated pyrrolizidines. Part 8. Enantiospecific synthesis of looking-glass analogues of hyacinthacine A5 from DADP

Article abstract of DOI:10.1016/j.tet.2006.04.003

(1R,2S,3S,5R,7aR)-1,2-Dihydroxy-3-hydroxymethyl-5-methylpyrrolizidine[(-)-3-epihyacinthacine A₅, 1a] and (1S,2R,3R,5S 7aS)-1,2-dihydroxy-3-hydroxymethylpyrrolizidine[(+)-3-epihyacinthacine A₅, 1b] have been synthesized either by Witt

Full text of DOI:10.1016/j.tet.2006.04.003

Tetrahedron 62 (2006) 6006–6011
Polyhydroxylated pyrrolizidines. Part 8: Enantiospecific
synthesis of looking-glass analogues of hyacinthacine
A5 from DADP*
*
Isidoro Izquierdo, Marıa T. Plaza, Juan A. Tamayo, Miguel Rodrıguez and Alicia Martos
´
´
Department of Medicinal and Organic Chemistry, Faculty of Pharmacy, University of Granada, Granada 18071, Spain
Received 13 March 2006; revised 29 March 2006; accepted 3 April 2006
Available online 2 May 2006
Abstract—(1R,2S,3S,5R,7aR)-1,2-Dihydroxy-3-hydroxymethyl-5-methylpyrrolizidine[(ꢀ)-3-epihyacinthacine A5, 1a] and (1S,2R,3R,5S
7aS)-1,2-dihydroxy-3-hydroxymethylpyrrolizidine[(+)-3-epihyacinthacine A5, 1b] have been synthesized either by Wittig’s or Horner–
Wadsworth–Emmond’s (HWE’s) methodology using aldehydes 4 and 9, both prepared from (2S,3S,4R,5R)-3,4-dibenzyloxy-20-O-tert-butyl-
diphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine (2, partially protected DADP), and the appropriate ylides, followed by cyclization through
an internal reductive amination process of the resulting a,b-unsaturated ketones 5 and 10, respectively, and total deprotection.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
OH
H
N
H
N
H
N
HO
OH HO
OH
OH
We have recently reported on the stereocontrolled transfor-
mation of D-fructose into a suitably protected derivative of
2,5-dideoxy-2,5-imino-D-allitol (2, DADP),2 which could
be considered as an excellent and versatile key intermediate
for the enantiosynthesis of polyhydroxylated pyrrolizidines.
The necessity for new preparations of enantiomerically pure
looking-glass hyacinthacines such as 1a and 1b arose from
the discovery3 that synthetic L-DMDP (2,5-dideoxy-2,5-
imino-L-mannitol), is a more powerful and more specific
a-glucosidase inhibitor than the enantiomeric natural prod-
uct DMDP (see Fig. 1) one of the most widespread of
secondary metabolite sugar mimics.4 This behaviour occurs
in other polyhydroxylated pyrrolidinic and piperidinic
alkaloids.5
Me
(-)-Hyacinthacine A5
HO
OH
HO
OH
OH
DMDP
L-DMDP
O-Protection interchange
OH
H
OH
H
N
H
N
OTBDPS
HO5'
BnO
2'
OH
OH
N
Me
(+)-3-Epihyacinthacine A5 (1b)
Me
(-)-3-Epihyacinthacine A5 (1a)
OBn
OH
OH
2
Chain lengthening
and cyclization
Figure 1. Synthetic strategy for the preparation of (ꢀ)-3-epi (1a) and (+)-3-
epihyacinthacine A5 (1b) from orthogonally protected DADP (2).
According to Figure 1 below, the pivotal character of
compound 2, would allow the syntheses of (ꢀ)-3-epi (1a)
and (+)-3-epi isomers of hyacinthacine A5, a natural poly-
hydroxylated pyrrolizidinic alkaloid and moderate inhibitor
(IC50¼110 mM) of amyloglucosidase isolated from an
extract of the bulbs of Scilla sibirica (Liliaceae),6 by build-
ing-up the bicyclic skeleton from either C(50) or C(20).
in five steps (28% yield), whereas the application of
the second strategy, consisting in a C(50) O-protection and
C(20) O-deprotection, adequately functionalized chain-
lengthening in this position and finally cyclization to the
pyrrolizidine skeleton, afforded the mirror image (+)-3-epi-
hyacinthacine A5 (1b).
The former synthetic strategy was successfully achieved
and (ꢀ)-3-epihyacinthacine A5 (1a) was obtained from 2
2. Results and discussion
In the synthesis described herein, the starting pyrrolidine 2
was previously N-protected as its Cbz derivative 3 that was
then oxidized (TPAP/NMO) to the pyrrolidinic aldehyde 4
*
For Part 7, see Ref. 1.
* Corresponding author. Tel.: +34 958 249583; fax: 34 958 243845; e-mail:
isidoro@ugr.es
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.04.003

I. Izquierdo et al. / Tetrahedron 62 (2006) 6006–6011
6007
and finally allowed to react with 1-triphenylphosphoranyl-
H7α
H1
H7
H1
α
H7a
H7a
N
H7β
H6β
H6
H7β
H6β
H6α
Me
idene-2-propanone to afford, in a highly stereoselective
manner, 4-[(3E,20R,30R,40S,50S)-30,40-dibenzyloxy-N-benzyl-
oxycarbonyl-50-tert-butyldiphenylsilyloxymethylpyrrolidin-
20-yl]but-3-en-2-one (5), in accordance with the J3,4 values of
16 and 15.6 Hz, showed by H-3 in the mixture of rotamers
(Scheme 1).
OBn
H2
OH
H2
N
α
OBn
OH
OH
OPG
Me
H5 H3
H5 H3
6
1a
H
Cbz
N
H
OTBDPS
OBn
-Face
β
a
R
H
2
N
Me
OBn
OPG
BnO
OBn
H
α
-Face
H
3 R = CH2OH (79%)
b
c
4 R = CHO (88%)
B
5 R = (E)-HC=CHCOMe (58%)
Figure 2. NOE interactions in 6 and 1a and hydrogenation pathway of
intermediate D5-pyrrolizine (B).
Scheme 1. Synthesis of pyrrolidinic a,b-unsaturated ketone 5. Reagents and
˚
conditions: (a) CbzCl/Me2CO/K2CO3; (b) TPAP/NMO/CH2Cl2/4 A MS
and (c) Ph3P]CHCOCH3/MePh, 80 ꢁC.
peculiar shape of D5-pyrrolizine B,6,8 where it is appreciated
that the b-face is less hindered for hydrogen attack that the
a-face is, affording only compound 6.
Catalytic hydrogenation (10% Pd–C)–cyclization of 5
afforded, in only one step, the fully protected (1R,2S,3S,5R,
7aR)-1,2-dibenzyloxy-3-tert-butyldiphenylsilyloxymethyl-
5-methylpyrrolizidine (6).
The above mentioned pivotal chiral character of 3, allows the
synthesis of looking-glass molecules, was probed in the syn-
thesis of the mirror image of 1a, (+)-3-epihyacinthacine A5
(1b). Thus, conventional benzoylation of 3 gave the fully pro-
tected derivative 7 (see Scheme 3). O-desilylation of 7 to the
corresponding partially protected pyrrolidine 8 and subse-
quent oxidation (TPAP/NMO) afforded the pyrrolidinic alde-
hyde 9 that was not investigated, but used in the next step. In
order to explore new synthetic possibilities for chain-length-
ening andfunctionalizationatC(50) in 9, theformerC(20) in3,
the HWE’s methodology was applied. Thus, aldehyde 9 read-
ily reacted with diethyl (2-oxopropyl)phosphonate giving
4-[(20S,30S,40R,50R)-50-benzoyloxymethyl-30,40-dibenzyloxy-
N-benzyloxycarbonylpyrrolidin-20-yl]but-3-en-2-one (10).
On the contrary of compound 5, the stereochemistry at the
carbon–carbon double bond in 10 could not be determined
in this case, due to an extensive broadening of the resonance
signals.
According to Scheme 2, formation of 6 must take place as
follows: concomitant hydrogenation and N-deprotection of
5 gave the saturated ketone A, not isolated, which on sub-
sequent intramolecular condensation gave the intermediate
D5-pyrrolizine B that was finally hydrogenated to 6.
OBn
OBn
OBn
OBn
OPG
H
H
N
a
-H2O
5
HN
O
Me
OPG
B
A
a
OR
H
N
OR
OR1
PG = TBDPS
Me
Cbz
N
R1
a
6 R = Bn; R1 = TBDPS
1a
R
b
3
BnO
OBn
Scheme 2. Synthesis of (ꢀ)-3-epihyacinthacine A5 (1a). Reagents and
conditions: (a) 10% Pd–C/H2/MeOH and (b) (i) 10% Pd–C/H2/HCl, then
Amberlite IRA-400 (OHꢀ form), (ii) TBAF$3H2O/THF.
7 R = CH2OBz; R1 = CH2OTBDPS
8 R = CH2OBz; R1 = CH2OH
b
c
9 R = CH2OBz; R1 = CHO
d
10 R = CH2OBz; R1 = -HC=CHCOMe
The stereochemistry of the new C(5) stereogenic centre was
established on the basis of extensive NOE experiments. The
NOE interactions are shown in Figure 2. The definite NOE
effects between C(3)H and C(5)H, and Me(5)H and C(8)H
were crucial in order to establish the R-configuration at C-5.
In addition, the rest of the NOE interactions also confirmed
the total stereochemistry of 6 and made possible to assign the
resonance signals for H-6a,6b,7a,7b.
Scheme 3. Synthesis of pyrrolidinic a,b-unsaturated ketone 10. Reagents
and conditions: (a) BzCl/CH2Cl2/TEA/DMAP (cat.); (b) n-Bu4N+Fꢀ$3H2O/
˚
THF and (c) TPAP/NMO/CH2Cl2/4 A MS; (d) (EtO)2P(O)CH2COCH3/
NaH/THF, rt.
In Scheme 4 below and as for 5, catalytic hydrogenation
of 10 afforded a single isomeric pyrrolizidine identified as
(1S,2R,3R,5S,7aS)-3-benzoyloxymethyl-1,2-dibenzyloxy-5-
methylpyrrolizidine (11).
Removal of the protecting groups in 6 gave the target mole-
cule (ꢀ)-3-epihyacinthacine A5 (1a), in accordance with its
analytical and spectroscopic data.
The absolute configuration of the new stereogenic centre
C(5) was established on the basis of the NOE effects found
(see Fig. 3). Thus, the definite NOE effects between C(3)H–
C(5)H, C(3)H–C(7a)H, C(5)H–C(7a)H and Me(5)H–C(8)H
The high stereoselective formation of 6 can be attributed,
according to our previous results7 and to Figure 2, to the

6008
I. Izquierdo et al. / Tetrahedron 62 (2006) 6006–6011
Wittig’s or HWE’s methodologies are both suitable for the
OR
OBn
OBn
OPG
H
N
H
N
enantiosynthesis of complex polyhydroxylated pyrrolizi-
dines alkaloids; (ii) the configuration at C(5) in 5-methylpyr-
rolizidines is controlled by that existing at C(7a), in such
a way that C(5)Me group and C(7a)H are in a trans-disposi-
tion and (iii) finally, that (ꢀ)-3-epihyacinthacine A5 (1a) and
its (+)-enantiomer (1b) were shown as moderate inhibitors
towards Jack bean a-mannosidase (IC50 253 and 417 mM,
respectively), whereas the former was specific of bovine
liver b-galactosidase.
a
a
OR
OR1
10
Me
Me
11 R = Bn; R1 = Bz
[12 R = Bn; R1 = H]
1b
A
b
c
Scheme 4. Synthesis of (+)-3-epihyacinthacine A5 (1b). Reagents and
conditions: (a) 10% Pd–C/H2/MeOH; (b) MeONa (cat.)/MeOH; (c) 10%
Pd–C/H2/HCl, then Amberlite IRA-400 (OHꢀ form).
4. Experimental
4.1. General
H7
H
H1
α
H7a
N
H7
β
OBn
H2
H
H6
β
β
-Face
OBn
OBz
α
-Face
H
OBn
OBz
H6
α
N
Solutions were dried over MgSO4 before concentration
under reduced pressure. The 1H and 13C NMR spectra were
recorded with Bruker AMX-300, AM-300 and ARX-400
spectrometers for solutions in CDCl3 (internal Me4Si). IR
spectra were recorded with a Perkin–Elmer FTIR Spectrum
One instrument, UV–vis measurements in a SpectronicÒ
Genesys 5 spectrophotometer and mass spectra were
recorded with a Hewlett–Packard HP-5988-A and Fisons
mod. Platform II and VG Autospec-Q mass spectrometers.
Optical rotations were measured for solutions in CHCl3
(1-dm tube) with a Jasco DIP-370 polarimeter. TLC was per-
formed on precoated silica gel 60 F254 aluminium sheets and
detection by employing a mixture of 10% ammonium molyb-
date (w/v) in 10% aqueous sulfuric acid containing 0.8%
cerium sulfate (w/v) and heating. Column chromatography
was performed on silica gel (Merck, 7734). The noncrystal-
line compounds were shown to be homogeneous by chroma-
tographic methods and characterized by NMR, MS and
HRMS.
Me
Me
OBn
H
H5 H3
H
A
11
Figure 3. NOE interactions in 11 and hydrogenation pathway of intermedi-
ate D5-pyrrolizine (A) in Scheme 4.
were essential in order to establish the S-configuration at
C-5. In addition, the rest of the NOE interactions also con-
firmed the total stereochemistry of 11 and made possible
to assign the resonance signals for H-6a,6b,7a,7b (Fig. 3).
As above, the configuration at the new stereogenic centre
C(5), was again controlled by that existing at C(7a) in
such a way that Me(5) and C(7a)H had a trans-disposition.9
Finally, compounds 1a and 1b were tested on a range of gly-
cosidases and their IC50 values are included in the Table 1.
Thus, 1a was inhibitor to b-galactosidase (from bovine liver)
at 62.4 mM, whereas both (1a and 1b) were shown to be
inhibitors to a-mannosidase (from jack beans) at 62.4 mM
(1a) and 89.6 mM (1b). Compound 1a was a more potent
inhibitor (Ki¼250 mM) of jack beans a-mannosidase than
1b (Ki¼420 mM). However, only 1a inhibited of bovine liver
b-galactosidase activity (Ki¼300 mM). The alkaloids were
not inhibitors of a-glucosidase (from baker’s yeast), b-
glucosidase (from almonds), a-galactosidase (green coffee),
b-galactosidase (from Aspergillus oryzae) at 62.4 mM (1a)
and 89.6 mM (1b).
4.1.1. (2S,3S,4R,5R)-3,4-Dibenzyloxy-N-benzyloxy-
carbonyl-20-O-tert-butyldiphenylsilyl-2,5-bis(hydroxy-
methyl)pyrrolidine (3). To a well stirred solution of 22
(1.8 g, 3 mmol) in dry acetone (20 mL), anhydrous potas-
sium carbonate (3 g) and a solution of benzyl chloroformate
(600 mL, 4.2 mmol) in the same solvent (10 mL) were added
and the mixture kept at rt for 30 min. TLC (Et2O) then
revealed the presence of a faster-running compound. The
mixture was filtered and the solid thoroughly washed with
acetone and the filtrate and washings concentrated to a resi-
due that was submitted to chromatography (Et2O–hexane,
1:2) to give 3 as colourless syrup. Yield: 1.7 g (79%);
[a]2D5 +10 (c, 1.8). IR (neat): 3448 (OH), 3067 and 3031
(aromatic), 1704 (C]O, Cbz), 740 and 700 cmꢀ1 (aro-
Table 1. IC50 values for compounds 1a and 1b versus different glycosidasesa
Enzyme
IC50 (mM)
1a
1b
a-Glucosidase (baker’s yeast)
b-Glucosidase (almond)
a-Galactosidase (green coffee)
b-Galactosidase (bovine liver)
b-Galactosidase (A. oryzae)
a-Mannosidase (Jack bean)
NI
NI
NI
329
NI
253
NI
NI
NI
NI
NI
417
1
matic). NMR data (300 MHz, inter alia): H, d 7.68–7.12
(m, 25H, 5Ph), 5.09 and 5.01 (2br d, 2H, J¼12 Hz,
CH2Ph), 4.61–4.51 (br m, 4H, 2CH2Ph), 4.28–3.60 (3br m,
8H, H-2,20a,20b,3,4,5,50a,50b), 1.05 (s, 9H, CMe3); 13C (inter
alia), d 155.73 (C]O, Cbz), 77.66 and 76.74 (C-3,4), 71.96
and 71.79 (2CH2Ph), 67.57 (CH2Ph, Cbz), 64.63 and 62.82
(C-20,50), 64.56 and 63.59 (C-2,5), 26.98 (CMe3) and 19.23
(CMe3). HRMS (LSIMS): m/z 738.3221 [M++Na]. For
C44H49NO6NaSi 738.3227 (deviation +0.9 ppm).
a
NI¼inhibition not observed under assay conditions.
3. Conclusions
Three conclusions can be stated from the above results: (i)
that partially protected polyhydroxylated pyrrolidines,
derived from common hexuloses, together with classical
4.1.2. 4-[(3E,20R,30R,40S,50S)-30,40-Dibenzyloxy-N-benzyl
oxycarbonyl-50-tert-butyldiphenylsilyloxymethylpyrroli-
din-20-yl]but-3-en-2-one (5). To a stirred solution of 3

I. Izquierdo et al. / Tetrahedron 62 (2006) 6006–6011
6009
(835 mg, 1.17 mmol) in dry CH2Cl2 (10 mL) were added
˚
activated 4 A molecular sieves (0.6 g), N-oxide-N-methyl-
m/z 628.3227 [M++Na]. For C39H47NO3NaSi 628.3223
(deviation ꢀ0.6 ppm).
morpholine (NMO, 213 mg, 1.82 mmol) and tetra-n-propyl-
ammonium perruthenate (TPAP, 50 mg) and the reaction
mixture was kept at rt for 15 min. TLC (Et2O/hexane, 1:1)
then indicated the absence of the starting material and the
presence of a faster-running compound. The reaction was
diluted with ether (30 mL), filtered through a bed of Silica
gel 60 (Scharlau, 230–400 mesh) and thoroughly washed
with ether. The combined filtrate and washings were concen-
trated to aldehyde 4 (730 mg, 88%); [a]2D5 ꢀ9 (c, 0.85). IR
(neat): 3068 and 3032 (aromatic), 1735 (CHO), 1710
(C]O, Cbz), 738 and 700 cmꢀ1 (aromatic). This material
was used in the next step.
4.1.4. (1R,2S,3S,5R,7aR)-1,2-Dihydroxy-3-hydroxy-
methyl-5-methylpyrrolizidine [(L)-3-epihyacinthacine
A5, 1a]. A solution of 6 (260 mg, 0.43 mmol) in methanol
(30 mL) was acidified (concd HCl) and hydrogenated
(10% Pd–C, 110 mg) at 60 psi for 15 h. The catalyst was fil-
tered off, washed with methanol and the filtrate and wash-
ings neutralized with Amberlite IRA-400 (OHꢀ form) and
1
concentrated. H NMR of the residue showed the absence
of benzyl group and that the TBDPS group still remains.
The residue was dissolved in THF (5 mL) and treated with
a solution of TBAF$3H2O (350 mg) in the same solvent
(5 mL) at rt overnight. TLC (ether–methanol–aq 30%
NH4OH, 5:1:0.1) then revealed a new compound with Rf
0.46. The solvent was eliminated and the residue chromato-
graphed (ether/ether–methanol–aq 30% NH4OH, 5:1:0.1)
To a solution of 4 (730 mg, 1 mmol) in dry toluene (20 mL)
was added 1-triphenylphosphoranylidene-2-propanone
(1.07 g, 3.36 mmol) and the mixture was heated at 80 ꢁC
for 3 h. TLC (ether/hexane, 2:1) then revealed the presence
of a slightly slower-running compound. The reaction mix-
ture was filtered and supported on silica gel, then chromato-
graphed (ether/hexane, 1:2) to afford 5 (730 mg, 83%) as
a thick syrup; [a]2D6 +27 (c 1). IR (neat): 3068 and 3032
(aromatic), 1705, 1679 and 1633 (C]O, conjugated ketone,
Cbz and C]C conjugated), and 700 cmꢀ1 (aromatic). NMR
to afford pure 1a (75 mg, 93%), which had [a]2D9 ꢀ15 and
29
405
[a] ꢀ20 (c 0.44, methanol). NMR data (400 MHz, meth-
1
anol-d4): H, d 4.10 (dd, 1H, J1,2¼7.0, J2,3¼4.8 Hz, H-2),
0
3.73 (dd, 1H, J3,8¼4.4, J8,8 ¼11.9 Hz, H-8), 3.70 (dd, 1H,
0
0
J3,8 ¼4.6 Hz, H-8 ), 3.62 (dd, 1H, J1,7a¼8.5 Hz, H-1), 2.77
(ddd, 1H, J7a,7b¼5.8, J7a,7a¼10.4 Hz, H-7a), 2.53 (br sex,
1H, J5,6a¼J5,6b¼J5,Me¼6.3 Hz, H-5), 2.43 (q, 1H, H-3),
2.23 (ddt, 1H, J6b,7b¼9.0, J6b,7a¼7.9, J6a,6b¼12.8 Hz,
H-6b), 1.79 (dddd, 1H, J6a,7b¼2.6, J7a,7b¼11.7 Hz, H-7b),
1.65 (dddd, 1H, J6a,7a¼10.7 Hz, H-6a), 1.45 (br dq, 1H,
H-7a) and 1.19 (d, 3H, Me); 13C, d 78.26 (C-2), 76.14
(C-7a), 72.45 (C-1), 71.70 (C-3), 63.22 (C-8), 56.95 (C-5),
37.69 (C-6), 24.98 (C-7) and 21.19 (Me). Mass spectrum
(LSIMS): m/z 156.1025 [M+ꢀCH2OH]. For C8H14NO2
156.1025 (deviation ꢀ0.1 ppm).
1
data (400 MHz): H, d 7.57–7.14 (m, 25H, 5Ph), 6.64 and
0
6.56 (2br dd, J2 ,4¼6.5 and 7.1 Hz, H-4, two rotamers),
6.26 and 6.12 (2br d, J3,4¼16 and 15.6 Hz, H-3, two
rotamers), 5.20–4.97 and 4.64–3.74 (4br m, 12H, 3PhCH2
and H-20,30,40,50,500a,500b), 2.05 and 1.90 (2br s, 3H,
H-1,1,1, two rotamers) and 1.01 (s, 9H, CMe3). 13C (inter
alia), d 198.16 (C-2), 155.62 (Cbz), 81.52 and 80.47 (C-
30,40), 72.27, 71.66 and 67.30 (2PhCH2 and Cbz), 64.21,
63.56, 62.49 and 62.26 (C-20,50, two rotamers), 62.72 (C-
500), 27.01 (C-1 and CMe3) and 19.33 (CMe3). HRMS
(LSIMS): m/z 776.3381 [M++Na]. For C47H51NO6NaSi
776.3383 (deviation +0.3 ppm).
4.1.5. (2R,3R,4S,5S)-20-O-Benzoyl-3,4-dibenzyloxy-N-
benzyloxycarbonyl-50-O-tert-butyldiphenylsilyl-2,5-
bis(hydroxylmethyl)pyrrolidine (7). To a stirred solution
of 3 (835 mg, 1.17 mmol) in dry dichloromethane (10 mL)
were added triethylamine (TEA, 150 mL, 1.8 mmol),
DMAP (50 mg) and benzoyl chloride (150 mL, 1.4 mmol)
and the mixture left at rt for 20 h. TLC (ether/hexane 2:1)
then revealed a faster-running compound. Conventional
work-up of the reaction mixture and column chromato-
4.1.3. (1R,2S,3S,5R,7aR)-1,2-Dibenzyloxy-3-tert-butyl-
diphenylsilyloxymethyl-5-methylpyrrolizidine (6). Com-
pound 5 (690 mg, 0.92 mmol) in methanol (30 mL) was
hydrogenated at 60 psi over 10% Pd–C (200 mg) for 18 h.
TLC (ether/hexane 1:2) then showed the presence of a new
compound of higher mobility. The catalyst was filtered off,
washed with methanol and the filtrate and washings concen-
trated to a residue that was submitted to column chromato-
graphy (ether/hexane 1:2) to afford pure syrupy 6 (290 mg,
52%), which had [a]2D5 +15 (c 1.3). IR (neat): 3068, 3030,
graphy (ether/hexane 1:3) afforded pure 7 (910 mg, 95%)
as a colourless syrup, which had [a]2D5 +10 and [a] +27
26
405
(c 1.8). IR (neat): 3088 and 3067 (aromatic), 1722 (COPh
and >NCO2Bn), 740 and 700 cmꢀ1 (aromatic). NMR data
1
(400 MHz): H, d 8.14–7.18 (m, 30H, 6Ph), 5.21–5.01 and
1
738 and 700 cmꢀ1 (aromatic). NMR data (400 MHz): H,
4.69–3.68 (2m, 14H, 3CH2Ph, H-2,20a,20b,3,4,5,50a,50b),
and 1.03 (s, 9H, CMe3); 13C (inter alia), d 166.03 (COPh),
155.74 (>NCO2Bn), 76.97, 76.07, 75.78 and 75.07 (C-3,4,
two rotamers), 71.85, 71.71 and 71.43 (2OCH2Ph, two
rotamers), 67.35 and 67.13 (two rotamers), 63.58, 62.99,
60.41 and 59.88 (C-2,5, two rotamers), 62.57, 62.23, 62.02
and 61.69 (C-20,50, two rotamers), 26.98 (CMe3) and 19.26
(CMe3). Mass spectrum (LSIMS): m/z 842.3485 [M++Na].
For C51H53NO7NaSi 842.3489 (deviation +0.4 ppm).
d 7.70–7.25 (2m, 20H, 4Ph), 4.70 and 4.63 (2d, 2H,
J¼11.8 Hz, CH2Ph), 4.60 and 4.57 (2d, 2H, J¼12.8 Hz,
CH2Ph), 4.06 (dd, 1H, J1,2¼6.2, J2,3¼3.4 Hz, H-2), 3.75
0
(dd, 1H, J3,8¼5.0, J8,8 ¼10.5 Hz, H-8), 3.60 (dd, 1H, H-1),
0
0
4.56 (dd, 1H, J3,8 ¼6.6 Hz, H-8 ), 3.11 (dt, 1H, J7b,7a¼5.4,
J1,7a¼J7a,7a¼10.0 Hz, H-7a), 2.83 (m, 1H, H-3), 2.52 (sex,
1H, J5,6b¼J5,6a¼J5,Me¼6.2 Hz, H-5), 2.18 (dq, 1H,
J6b,7b¼J6b,7a¼8.3, J6a,6b¼13 Hz, H-6b), 1.77 (m, 1H,
H-7b), 1.58 (m, 1H, H-6a), 1.39 (dq, 1H, J6a,7a¼7.6,
J7a,7b¼10.7 Hz, H-7a), 1.08 (s, 9H, CMe3) and 0.98 (d,
3H, Me); 13C (inter alia), d 84.91 (C-2), 79.36 (C-1), 72.20
and 71.73 (2CH2Ph), 72.09 (C-7a), 68.27 (C-3), 65.86 (C-
8), 55.03 (C-5), 37.10 (C-6), 26.99 (CMe3), 25.21 (C-7),
21.33 (Me) and 19.34 (CMe3). Mass spectrum (LSIMS):
4.1.6. (2R,3R,4S,5S)-20-O-Benzoyl-3,4-dibenzyloxy-N-
benzyloxycarbonyl-2,5-bis(hydroxymethyl)pyrrolidine
(8). To a stirred solution of 7 (840 mg, 1.03 mmol) in THF
(15 mL) was added TBAF$3H2O (490 mg, 1.55 mmol)
and the mixture was kept at rt. TLC (ether/hexane 3:1)

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I. Izquierdo et al. / Tetrahedron 62 (2006) 6006–6011
then showed a new compound of lower mobility. The mixture
was neutralized with acetic acid, concentrated to a residue
that was dissolved in ether, washed with brine, concentrated,
and then submitted to column chromatography (ether/hexane
2:1) to yield pure 8 (500 mg, 84%) as a colourless syrup,
which had [a]2D6 ꢀ12 (c 0.8). IR (neat): 3470 (OH), 3064
and 3032 (aromatic), 1719 (COPh and >NCO2Bn), 712
and 698 cmꢀ1 (aromatic). NMR data (300 MHz): 1H, d 7.93–
7.27 (m, 20H, 4Ph), 5.26 and 5.12 (2d, 2H, J¼12.3 Hz,
OCH2Ph), 4.62–3.85 (m, 11H, 2CH2Ph, H-2,20a,20b,3,4,5,50a)
2:1) to afford pure syrupy 11 (280 mg, 74%), which had
25
[a]2D5 +8.5 and [a] +19 (c 1). IR (neat): 3063 and 3031
405
(aromatic), 1720 (CO benzoate), 712 and 697 cmꢀ1 (aro-
matic). NMR data (400 MHz): 1H, d 8.03 (d, 2H, Jo,m¼7.5 Hz,
H-ortho Bz), 7.58 (t, 1H, Jm,p¼7.5 Hz, H-para Bz), 7.44 (t,
2H, H-meta Bz), 7.39–7.27 (m, 10H, 2CH2Ph), 4.77 and
4.58 (2d, 2H, J¼11.7 Hz, CH2Ph), 4.65 and 4.59 (2d, 2H,
0
J¼12.0 Hz, CH2Ph), 4.43 (dd, 1H, J3,8¼4.8, J8,8 ¼11.4 Hz,
0
0
H-8), 4.30 (dd, 1H, J3,8 ¼6.3 Hz, H-8 ), 4.14 (dd, 1H,
J1,2¼6.4, J2,3¼4.2 Hz, H-2), 3.68 (dd, 1H, J1,7a¼8.7 Hz,
H-1), 3.10 (dt, 1H, J7a,7a¼5.5, J7a,7b¼9.9 Hz, H-7a), 3.00
(br q, 1H, H-3), 2.62 (br sex, 1H, J5,6a¼J5,6b¼7.0 Hz, H-5),
2.22 (dq, 1H, J6a,7a¼J6a,7b¼8.5, J6a,6b¼13.0 Hz, H-6a),
1.81 (dddd, 1H, J7a,6b¼2.2 Hz, H-7a), 1.65 (dddd, 1H,
J6b,7b¼10.0 Hz, H-6b), 1.47 (dq, 1H, J7a,7b¼10.9 Hz, H-7b)
and 1.16 (d, 3H, JMe,5¼6.0 Hz, Me); 13C (inter alia),
d 166.46 (COPh), 84.82 (C-2), 79.07 (C-1), 72.73 (C-7a),
72.73 and 71.83 (2CH2Ph), 65.93 (C-8), 65.07 (C-3), 54.81
(C-5), 37.13 (C-6), 25.36 (C-7) and 21.47 (Me). Mass spec-
trum (LSIMS): m/z 494.2309 [M++Na]. For C30H33NO4Na
494.2307 (deviation ꢀ0.4 ppm).
and 3.63 (dd, 1H, J4,5 b¼4.9, J5 a,5 b¼11.6 Hz, H-50b); 13C
(inter alia), d 77.10 and 76.68 (C-3,4), 72.15 and 71.90
(2OCH2Ph), 67.80 (>NCO2CH2Ph), 64.32 and 61.30
(C-2,5), 63.82 and 63.17 (C-20,50). Mass spectrum (LSIMS):
m/z 604.2312 [M++Na]. For C35H35NO7Na 604.2311 (devi-
ation ꢀ0.1 ppm).
0
0
0
4.1.7. 4-[(20S,30S,40R,50R)-50-Benzoyloxymethyl-30,40-
dibenzyloxy-N-benzyloxycarbonylpyrrolidin-20-yl]but-
3-en-2-one (10). To a solution of 8 (1.08 g, 1.9 mmol) in dry
dichloromethane (10 mL) were added activated powdered
˚
4 A molecular sieve (700 mg), N-methylmorpholine N-
4.1.9. (1S,2R,3R,5S,7aS)-1,2-Dihydroxy-3-hydroxy-
methylpyrrolizidine [(+)-3-epihyacinthacine A5, 1b].
Conventional debenzoylation of 11 (270 mg, 1 mmol) in
0.2 N sodium methoxide in dry methanol (7 mL) gave after
work-up compound 12 (280 mg, 0.8 mmol) that was dis-
solved in dry methanol (30 mL) and hydrogenated (10%
Pd–C, 170 mg) in acid medium (concd HCl, four drops) at
50 psi for 48 h. TLC (ether/methanol 3:1) then showed a
not mobile compound. The catalyst was filtered off, washed
with methanol and the filtrate and washings neutralized with
Amberlite IRA-400 (OHꢀ form), then concentrated. Column
chromatography (ether/methanol/TEA 5:1:0.1) of the resi-
due gave pure 1b (110 mg, 80%), which had [a]2D6 +14.5
(c 0.8, methanol) and NMR data identical to 1a.
oxide (325 mg, 2.8 mmol) and TPAP (40 mg) and the reac-
tion mixture kept at rt for 1 h. TLC (ether/hexane 4:1)
then showed a faster-running compound. The reaction was
diluted with ether (30 mL), filtered through a bed of Silica
gel 60 (Scharlau, 230–400 mesh) and thoroughly washed
with ether. The combined filtrate and washings were concen-
trated to aldehyde 9, that was used in the next step.
To a well stirred suspension of sodium hydride (60% 150 mg,
3.7 mmol) in anhydrous THF (15 mL), diethyl(2-oxopro-
pyl)phosphonate (660 mL, 3.7 mmol) was added and the mix-
ture was left at rt for 1 h, when a solution of aldehyde 9 in
THF (10 mL) was added. After 5 min TLC (ether/hexane
4:1) revealed the presence of a new compound of slightly
lower mobility. The solvent was eliminated and the residue
was partitioned into ether/water. The organic phase was sep-
arated and concentrated to a residue that was submitted to
column chromatography with ether/hexane (3:1) as eluent
4.2. Glycosidase inhibitory activities
All pNP-pyranoside substrates, a-glucosidase (from baker’s
yeast), b-glucosidase (from almonds), a-galactosidase (from
green coffee), b-galactosidase (from bovine liver), b-galac-
tosidase (from A. oryzae) and a-mannosidase (from Jack
beans) were purchased from Sigma Chemical Company.
Kinetic studies were performed at 37 ꢁC in 50 mM sodium
citrate/phosphate buffer. Enzyme concentrations ranging
from 0.5 mg mLꢀ1 to 0.1 mg mLꢀ1 were used, depending
on the substrate studied. The activities of enzymes were
determined using p-nitrophenyl glycosides as substrates at
the optimum pH of each enzyme. Substrates, suitably diluted
enzyme solutions and inhibitors were incubated together for
30 min at 37 ꢁC. Reactions were followed in an UV–vis
spectrophotometer by measuring the change in the absor-
bance of light at 400 nm. Data were analyzed using the
programme GraFit.10
to give pure 10 (520 mg, 45% from 8) as a colourless syrup,
24
405
which had [a]2D3 ꢀ15 and [a] ꢀ46 (c 2.1). IR (neat): 3088
and 3063 (aromatic), 1718 and 1678 (PhCO2, >NCO2Bn and
a,b-unsaturated ketone), 713 and 699 cmꢀ1 (aromatic).
NMR data (300 MHz): 1H, d 7.85–7.25 (m, 20H, 4Ph),
6.52 (br m, 1H, H-4), 6.20–6.07 (br m, 1H, H-3), 5.22–4.25
(2m, 10H, 3CH2Ph, H-30,40,500a,500b), 4.01 (br t, 1H,
J¼4 Hz) and 3.91 (br t, 1H, J¼5.2 Hz) for H-20,50 and 1.90
(br s, 3H, H-1,1,1); 13C (inter alia), d 197.58 (C-2), 166.09
(COPh), 155.72 (>NCO2Bn), 72.11 (2OCH2Ph), 67.58
(>NCO2CH2Ph), 62.56 (C-500) and 27.41 (C-1). Mass spec-
trum (LSIMS): m/z 642.2470 [M++Na]. For C38H37NO7Na
642.2468 (deviation ꢀ0.3 ppm).
4.1.8. (1S,2R,3R,5S,7aS)-3-Benzoyloxymethyl-1,2-dibenz-
yloxy-5-methylpyrrolizidine (11). Compound 10 (500 mg,
0.8 mmol) in dry methanol (15 mL) was hydrogenated at
60 psi over 10% Pd–C (100 mg) for 24 h. TLC (ether/hexane
4:1) then showed the presence of a new compound of lower
mobility. The catalyst was filtered off, washed with methanol
and the filtrate and washings concentrated to a residue that
was submitted to column chromatography (ether/hexane
Acknowledgements
The authors are deeply grateful to Ministerio de Ciencia y
´
Tecnologıa (Spain) for a grant (A. Martos) and Junta de
Andalucıa (Group CVI-250) for financial support.
´

I. Izquierdo et al. / Tetrahedron 62 (2006) 6006–6011
6011
Gretzke, D.; Krulle, T. M.; Butters, T. D.; Dwek, R. A.;
¨
Supplementary data
Nash, R. J.; Asano, N.; Fleet, G. W. J. Carbohydr. Res. 2005,
340, 2713–2718.
6. Yamashita, T.; Yasuda, K.; Kizu, H.; Kameda, Y.; Watson,
A. A.; Nash, R. J.; Fleet, G. W. J.; Asano, N. J. Nat. Prod.
2002, 65, 1875–1881.
1H and 13C NMR (six pages) for compounds 1a, 6 and 11.
Supplementary data associated with this article can be found
in the online version, at doi:10.1016/j.tet.2006.04.003.
7. (a) Izquierdo, I.; Plaza, M.-T.; Franco, F. Tetrahedron: Asym-
metry 2002, 13, 1581–1585; (b) Izquierdo, I.; Plaza, M.-T.;
Franco, F. Tetrahedron: Asymmetry 2004, 15, 1465–1469.
8. (a) Kato, A.; Adachi, I.; Miyauchi, M.; Ikeda, K.; Komae, T.;
Kizu, H.; Kameda, Y.; Watson, A. A.; Nash, R. J.; Wormald,
M. R.; Fleet, G. W. J.; Asano, N. Carbohydr. Res. 1999, 316,
95–103; (b) Asano, N.; Kuroi, H.; Ikeda, H.; Kizu, H.; Kameda,
Y.; Kato, A.; Adachi, I.; Watson, A. A.; Nash, R. J.; Fleet,
G. W. J. Tetrahedron: Asymmetry 2000, 11, 1–8; (c) Kato, A.;
Kano, E.; Adachi, I.; Molyneux, R. J.; Watson, A. A.; Nash,
R. J.; Fleet, G. W. J.; Wormald, M. R.; Kizu, H.; Ikeda, K.;
Asano, N. Tetrahedron: Asymmetry 2003, 14, 325–331.
9. Izquierdo, I.; Plaza, M.-T.; Robles, R.; Franco, F. Tetrahedron:
Asymmetry 2001, 12, 2481–2487.
References and notes
1. Part 7 of this series: Izquierdo, I.; Plaza, M.-T.; Tamayo, J. A.
J. Carbohydr. Chem., in press.
´
2. (a) Izquierdo, I.; Plaza, M.-T.; Rodrıguez, M.; Franco, F.;
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I.; Plaza, M.-T.; Rodrıguez, M.; Franco, F.; Martos, A. Tetra-
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Wormald, M. R.; Dwek, R. A.; Winters, A. L.; Nash, R. J.;
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´
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