NOBn

OO

H

NOBn

OO

H

NOBn

O

H

O

HO

HO

HO

HO

HO

HO

NOBn

NH

OBn

OO

O

HN

OHH

HO

HO

NOBn

NH

OBn

OO

O

HN

OHH

HO

HO

NH

OBn

ON

OBn

O

H

O

NOH

HHO

HO

8a

37

6

8a

37

6

7a

7

6

3

NOMe

OBocN

OMe

OBoc

MeO

NOMe

OBoc

O

NOMe

H

ON

H

NOHHO

NOAcHO

Cbz CbzN

OH

H

NOH

O

NOH

O

NOH

H

O

NOH

H

O

NOH

H

O

O

O

NOH

H

O

O

O

NOH

H

O

HO

HO

NOH

H

O

HO

HO

+

+ NOH

HHO

HO

NOH

HHO

HO

NOH

HHO

HO

NOH

HHO

HO

+

+1R

1R 1S

1S

2S

2S 2R

2R

5S5S

6S 6S

8aR 8aR

9aR 9aR

.

RCM strategy for the enantiosynthesis of new polyhydroxylated RCM strategy for the enantiosynthesis of new polyhydroxylated quinolizidines, indolizidines and pyrrolizidinesquinolizidines, indolizidines and pyrrolizidines

Alessia Colombo, Nicola Landoni, Giordano Lesma, Alessandro Sacchetti and Alessandra Silvani

Dipartimento di Chimica Organica e Industriale,Università degli Studi di Milano, via Venezian 21 – 20133 Milano, Italy E-mail: alessia.colombo@unimi.it , nicola.landoni@unimi.it

Glycosidases are enzymes that catalyse the hydrolysis of glycosidic bonds in complex carbohydrates and glycoconjugates. They have been identified as an important class of therapeutic targets with applications in the treatment of influenza infection, cancer, AIDS, and diabetes. Since the beginning of the 1960s, a number of polyhydroxylated alkaloids that are potent glycosidase inhibitors have been found in plants and microorganisms. The first natural polyhydroxylated alkaloid to be detected was the piperidine alkaloid nojirimycin, isolated from a Streptomyces filtrate in 1966 by Inouye et al. The high therapeutic potential of these alkaloids, also called azasugars, has prompted considerable efforts towards their structural modifications and towards the design of new stereocontrolled synthetic routes also for unnatural isomers, which might be of interest for SAR studies. In fact, the specificity of natural alkaloids toward their molecular target remains to be optimized and novel synthetic strategies to expand the repertoire of available analogues are needed.

We are pursuing a noncarbohydrate based approach to various azabicyclic ring skeletons, starting from chemoenzimatically derived or commercially available chiral synthons and relying on RCM reactions in the key steps.

Nojirimycin isolated from Streptomices

THE KEY REACTION: DIHYDROXYLATION WITH OSO4

THE KEY REACTION: ENZIMATIC DESYMMETRIZATION WITH IONIC LIQUIDS

The ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate.

(1) With Candida Antarctica lipase and Vinyl acetate as solvent:

(2) With Candida Cylindracea lipase using stoichiometric vinyl acetate and 1-butyl-3-methylimidazolium hexafluorophosphate as solvent:

NHO OH

CbzOAc N

HO OAc

Cbz

1-butyl-3-methylimidazoliumhexafluorophosphate

, Lipase from CandidaCylindracea, 30°C

NHO OH

CbzOAc

NHO OAc

Cbz

C.A.L

, 30°C

Enzyme Solvent Yield ee Time

CAL Vinyl acetate 62% 90% 48 h

CCLVinyl acetate +

Ionic liquid80% 96% 7h

Heating Catalyst Solvent Temperature Yield Time

Oil bath

Microwave

Grubbs catalyst 2nd generation

5% mol

Toluene

Toluene

120°C

180°C

65%

72%

20h

60min.

T (°C)

10 60

180

40

THE KEY REACTION: RING CLOSING METATHESIS TS relative energies for

Quinolizidine derivatives dr calcd. dr exp.

α-Attack

0.0 Kcal/mol

76 75

β-Attack

0.7 Kcal/mol

24 25

a b

d

c

f

e

h

g

j

i

Reagents and conditions: (a) Vinyl acetate, C.C.L., 1-butyl-3-methylimidazolium hexafluorophosphate, 30°C, (62%), (b) (1) Swern oxidation, (2) t-BuOK, Ph3P(Me)Br, (3) KOH, MeOH, (54%), (c) Na2CO3, Acetone/Water, 2:1, Acryloyl chloride, (73%), (d) EDC, DMAP, 3-butenoic acid, (69%), (e) Grubbs catalyst 2nd generation 5% mol, Toluene, reflux, 20h, (65%), (f) Grubbs catalyst 2nd generation 5% mol, Toluene, reflux, 20h, (87%), (g) (1) OsO4, TMAO, (2) 2,2-dimethoxypropane, H+, (3) Chromatographic separation, (47%), (h) (1) OsO4, TMAO, (2) Chromatographic separation, (64%), (i) (1) BH3

.SMe2, EtOH (2) Acid resin (H+ form), (67%), (j) BH3·SMe2, EtOH, (85%).

Time (min.)

Second generation Grubbs’s ruthenium catalyst was very suitable for RCM, when the reaction was performed in toluene at reflux. The use of microwave irradiation in this step allowed to complete the reaction in 1 h compared with the 20 h required under conventional oil bath heating.

Second generation Grubbs’s ruthenium catalyst.

TS relative energies for Indolizidine derivatives

dr calcd. dr exp.

α-Attack

0.0 Kcal/mol

96 85

β-Attack

1.98 Kcal/mol

4 15

PM

3 C

alc

. (S

pa

rta

n ’0

6,

Wa

vefu

ctio

n).

5 : 1

3 : 1

NH

HO

OH

HO

Nojirimycin

OH

OH

Glycosidase catalyze the hydrolysis of the glycosidic linkage to generate two smaller

sugars.

(1R,2S,5S,8aR) (1S,2R,5S,8aR)

(1R,2S,6S,9aR) (1S,2R,6S,9aR)

N

OHn1

n2

HOOH

N

OHn1

n2

On3

n1 = 1,2n2 = 1,2

n1 = 1,2n2 = 0,1n3 = 0,1

NH

HO OAc

NHO

COOH

RETROSYNTHETIC STRATEGYFrom enzymatic

desymmetrization

Commercially available

Swainsonine isolated from Swainsona Canescens

N

Swainsonine

HOH

OH

OH

N

Lupinine

H

OH

Lupinine isolated from Maackia Hupehensis

NBnO

O

H

BocCu

RCu

FUTURE DEVELOPMENTS……

Cross Metathesis (CM)Cross Metathesis (CM)

NH

OHO

O

(S)-Pyroglutammic acid

NOBn

O

MeO

Boc

Reagents and conditions: (a) BnBr, iPr2EtN, CH2Cl2, 55°, 4h, (97%), (b) BOC2O, DMAP, CH3CN, r.t, 4h, (98%), (c) LiEt3BH, THF,-78°C, 3h, (98%), (d) pTSA, MeOH, r.t, 3h, (97%), (e) A: allylltrimethylsilane, BF3 etherate, Et2O, -78°C, r.t, 12h, (75%), 4:1 cis:trans, B: allylMgBr, CuBr·Me2S, BF3 etherate, Et2O, -78°C ,5h, r.t, 12h (55%), 96:4 trans:cis: C: vinylllithium, CuBr·Me2S, BF3 etherate, Et2O, -78°C, 6h, r.t, 12h, (42%), 4:1 trans:cis, (f) TFA, CH2Cl2, r.t, 12h, (A:76% cis, B:86% trans, C:57% trans), (g) acryloilchloride, Na2CO3, acetone, 4h, (A:84%, B:80%, C:81%), (h) Grubbs Catalyst 2nd Generation 5% mol, toluene, 120°C, 20h, (A:60%, B:67%, C:45%), (i) OsO4, TMAO, Acetone/Water, 3:1, 40°C, 3h, (A: 87% only (6S,7S), B:72% (6S,7S) 1,5%(6R,7R), C:60% only (6S,7S), (l) LiAlH4,THF, 1.5h, reflux, (A:90%, B:30%, C:40%).

Method

A

Method

B

Method

C

e, f

a, b, c, d

g, h

g, h

g, h

MONOMORINE I

Indolizidine 195B isolated from skin of Dendrobates Auratus

b

Reagents and conditions: (a) allyltrimethylsilane, BF3eterate, ET2O, -78°, r.t, 12h, (80%), (4:1cis/trans), (i) TFA, CH2Cl2, r.t, 3h, (30% cis), (ii) BOC2O, DMAP, TEA , CH3CN, r.t, 12h, (98%), (b) MVK, Grubbs-Hoveyda catalyst 5%mol, CH2Cl2, r.t, 12h, (55%).

a

Castanospermine from seeds of Castanospermum Australe

only (6S,7S)

only (6R,7R)

Nucleophilyc attack on N-acylimminium ion-copper complex

Trans Selectivity

Highly diastereoselective addition of alkylcopper reagents to the optically active N-acylimminium ions derived from proline; the mechanism involving nucleophilic attack on the less hindered face of the ion.

J. Org.Chem., 1995, 60, 5011-5015

N

OHHO

HOH

HO

Castanospermine

N

H

Indolizidine 195B

N

OHH

HO

HO

HO Australine

Australine isolated from Castanospermum Australe

i

i

i

l

l

l

36 : 1 (6S,7S) (6R,7R)

(commercially available)

(3S,6S,7R,8aR)

(3S,6R,7S,8aS)

(3S,6R,7S,7aS)

N

H

Monomorine I

Monomerine I isolated from Monomorium pharaonis