Chemoenzymatic synthesis of A C5-chiral building block: A substrate modification approach.

Tetrahedron Letters,Vo1.28,No.42,pp Printed in Great Britain

CHEMOENZYMATIC

SYNTHESIS

4935-4938,1987

OF A C,-CHIRAL

0040-4039/87 $3.00 + .oO Pergamon Journals Ltd.

BUILDING BLOCK: A SUBSTRATE

MODIFICATION

APPROACH.

RenG Roy* and Allan W. Rey Ottawa-Carleton Department

Chemistry

of Chemistry,

Ottawa,

Ontario,

Institute

Ottawa

University

Canada KlN 984

Abstract: The enantioselectivity of a-chymotrypsin hydrolysis of prochiral aimethyl-3-hydroxyglutarates was controlled by the proper choice of the hydroxyl protecting groups. This strategy allows the synthesis of a versatile C,-chiral building block.

In an effort

to establish

and anti-1,3-diol2 natural

and 1,3-polyol

products,

envisaged

we were

1,5-functionality

prochiral

dimethyl

be totally

building

general

(&I'+.

However,

et al.6, the ee was very dependent

enough

to be of value for synthetic

studies

have already

Unfortunately,

involved

precursor.

purposes.

led to striking

of la, were disappointingly

variations

in the reaction

conditions.

constant

in enzyme-catalyzed

hydrolysis

of the

had been previously conditions

It was therefore hydrolysis

improvements

It was

disposed

reported

by Rosen et al.5 and more recently

to reinvestigate

to Optimize

the ee.

SitUatiOnS7.

excesses,

obtained

from the

and low (- 61%) even after exhaustive

The eels were determined

by capillary

GC or by

1H-NMR on the diastereomeric configuration

for 2

enantioselective

(R)-a-methylbenzylamide 3a (Scheme 1). The i absolute agreed with literature information and corresponded to an

pro-S - ester hydrolysis4. Scheme 1

Me02C&

chymotrypsin

CO,Me

MeO,C w

OR 5

Za-e -_

la-e m.., (El-H,NCH(CH,)P~

_

Me02C&CONHCH(CH3iPh

c

EDC,t-BuOH 3a-e -u a, R=H; b,R=Bn; c, R=Bz; d,R=MOM,

e, R=TBDMS 4935

to

by

and was never high

decided

in order

in similar

as shown in Table 1, the enantiomeric

hydrolysis

Our interests

upon the reaction

in the enzymatyic

and in numerous

@-hydroxyketones.

enzymatic

This reaction

as reported

arooks

conditions

of both the syn-1

of an unsymmetrically

the a-chymotrypsin

3-hydroxyglutarate

reaction

of chiral

fragment composed

led us towards

enantioselective".

for the synthesis

units found in some polyene macrolides

would be a suitable

synthesis3

strategy

faced with the elaboration

that a C,-chiral

asymmetric

a reiterative

CO,H

the Such

4936

TABLE

1.

Chymotrypsin-catalyzed

Enz. Subst. (w/w)

1:2 1:l 1:5 1:2

6.7 7.0 7.0 7.0 7.8 7.8

1:l 1:2 2:l 70d

7.8 7.8 7.0 7.0 NazHPO,

Reactions bYields

A A A A A A A B C A

Yieldb (91

eec

100 92 86 95 83 99 61 100 67 100

64 62 58 65 57 63 63 55 62 15

Conf.

R R R R R R R R R S

buffer'+; 3=A + 20% MeOH; C = PBS.

performed

of isolated

'Determined

of Ja_.

Solventa

PH

1:2 1:5

aA=O,OIM

Hydrolysis

at room temperature. 2a based on recuperated

by GC an?lH-NMR

la.

on 3e.

dPLE units.

Since influence order

none of the variations

to take better

hypothesized

unprotected 2).

This

explain

advantage

domain

orientations

conditions

is not sterically

3-hydroxy undesired

of the enzyme's

of the substrate

provided

ar (aromatic)

domain.

would

Thus,

enough

for the binding

of 2a.

of the hydroxyl

Therefore,

active

in the binding

the binding

to the am (amide) to be hydrolyzed to prevent

function could constrain

that the pro-E ester group would

protecting sitea'a.

it seems likely

to prevent

allow the pro-R - ester

of the S monoacid

derivatization

the am domain

congested

group in competition

orientation

with la had a dramatic

the size of the hydroxyl

for the lack of enantioselectivity.

the formation

a suitable

we varied

of the dimension

that two different

been responsible (hydrogen)

in the reaction

on the enantioselectivity,

group in

It was

site could have that the h of the domain

(Scheme

and would

this mode of binding,

the substrate

to bind to

still have a higher affinity

for the

Scheme 2

Asp

102

-

A new strategy was therefore found to successfully this substrate

modification

an ee well above

undertaken.

Various

raise the ee of the products. approach.

the unprotected

For example,

substrate

(93%).

bulky hydroxyl

protecting

groups were

Table 2 shows a number of results the water

soluble MOM derivative

Interestingly,

the configuration

using

fi gave of

4937 TABLE 2.

Chymotrypsin-Catalyzed Enz. Subst. (w/w)

Hydrolysis

of lb-d - _I

Solventa

PH

Yieldb (%)

eec

Conf.

l:l(lb)

7.8

C

68

84

R

3686i;,)

7.0

C

78

12

S

1:2(k)

7.8

Ce

42

84

R

1:1(S)

7.8

C

86

92

R

1:2(lc)

7.8

Cf

68

91

R

160d:c)

7.0

D

79

33

S

180d(-_)

7.0

C

77

59

S

1:2(E)

7.8

Af

100

88

R

A

95

93

R

1:2(E)

7.0

1:1(S)

7.8

A

100

93

R

2:l(ld)

7.8

Ag

92

93

R

33OdKJ)

7.0

A

100

14

S

aA=O,O.lM

Na,HPO,

buffer;

B=A+20%

CHsCN;

C=A+20%

dioxane;

D=A+ZO%

MeOH.

Reactions

performed

at room temperaturell. bYields

of isolated

2b-d based on recuperated lb-d. Yields may vary depending rnN -N which was followed by the equivalent of base consumed.

of completion 'Determined

by GC and/or

dPLE units; eAt 36-C; gSame

results

the major within

1H-NMR on 3b-dls.

fMembrane-Enzoied

enantiolner remained 1,

not to be general

However,

Catalysis

(MEEC)l'+.

in 2 hrs).

suggesting

site was as predicted

the n(active),site.

data with

Enzymatic

at 0°C and 36°C (complete

the active

on the extent

that the focalized

and corresponded

the results

orientation

of Table 2 demonstrate

tnat the strategy

since it does not apply to the other enzymatic

pig liver esterase

(PLE) are included

of the substrate

to the pro-S - ester group binding system studied.

and remain comparable

with

to

appears Hence,

previous

the

results

on lalo.

In conclusion, enzymes

the present

in asymnetric

undertaken

complementary Typical

synthesis

before abandoning

and that a closer

their usage.

to the existing

another

facet for the utilization

look at the enzyme's

The above

strategy

active

of

site should be

should therefore

be considered

ones7.

Experiment:

u-Chymotrypsin dissolved

(EC 3.4.21.1,

acetate,

in the same buffer.

throughout

the reaction

was estimated

from bovine

The pH was adjusted

with a Radiometer

automatic

by the volume of base consumed

was complete

the remaining

II

buffer was equally

pancreas,

distributed

1.00 g, 15 pal)

in four dialysis

M.W. cut off 12-14 kDa) which were added to a solution

4.55 mmol)

reaction

Sigma Type

in 16 mL of O.OlM Na*HPO,

(cellulose

remove

report establishes

after overnight starting

material

during

contact.

of 2

bags

(1.00 9,

to 7.8 with 0.4N NaOH and kept constant titrator.

The extent of the reaction

the reaction

The solution

(40 mg) after which

performed

was extracted

the reaction

at 23°C.

The

with ether to

mixture

was

4938

acifified usual.

This solution was extracted

to 2 with 2.5N HCl. The products

remaining

steps after dialyzing

in the dialysis

the contents

was thus isolated

in a yield

on the derivative

3d obtained

bags were

with a fresh solution

with EtOAc and processed isolated

The enantioselectivity

of 91%.

by the coupling

by repeating

of the buffer.

as

the above

The i monoacid

of the reaction

of (R)-(r-methylbenzylamine -

2

was determined

with a carbodiimide

(EDC). A reactor product

such as the one just described

and compensates

enantioselectivity

is repeatedly

utilized

for the hi$l ,~,izy:le-slrilstrate ratio used.

the time of the reaction

could be decreased

for a scale-up of the

Without

a decrease

in the

3-4 fold when the reactions

are perforl:ledat 36°C. Acknowledgements The authors University

gratefully

acknowledge

and the Natural

Sciences

a Grant-in-aid

and Engineering

from the Research

Research

Council

Services

of Canada

of Ottawa

for financial

support. References 1.

K.-M. Chen, G.E. Hardtmann, E,

K. Prasad,

D.A. Evans and K.T. Chapnan,

3.

See for example: Synthesis",

ed. by

Morrison,

J.D.

Natural

4.

S.G. Cohen

5.

T. Rosen, M. Watanabe

6.

D.W. Brooks,

7.

L.Y.P.

S.G.

10.

and C.J. N.Y.

66,

3657 (19843.

J. Org. Chem., 52,

192 (1987).

J. Org. Chem., -51, 2047 (1986); A. Zaks and 2767 (1986); B.-nan J. Am.

Zhou, A.S. Gopalan,

Sot., 105,

F. Van

(1983).

(1969).

C. Tamm, K. Gawronska

groups on k

for the benzyl ether E.

12.

T. Iversen

13.

All compounds

It was incorporated

and D.R. Bundle, gave physical

and M.L. Bender, Act. Chem.

and

Gawronski,

J.K.

M.D. Bednarski,

Helv. Chim.

H.K. Chenault,

13

by an acid catalyzed property

(61% yield,

reaction

except

with benzyl

ref. 12).

1240 (1981). GC were taken on a BP1

3300 instrument.

XL-300

in CDCl,.

ppn for the (3!)- and (32)-g

USA

standard methodologies

data in accord with the structure.

The CY, of the MOM groups were at 6 3.313 diastereoisomers

E.S. Simon and G.M. Whitesides,

1283 (1987). in

following

J. Chem. Sot., Chem. Conun.,

column with a Varian

ppm and 3.256

were introduced

due to its base labile

1H-NMR were run on a Varian

(Received

therein.

2501 (1983).

All the protecting

capillary

(1985); S. Butt

cited

146 (1987).

trichloroacetamidate

14.

J. Org. Chem., s,

Sci., 2,

P. Mohr, N. Warspe-Sarcevic, e,

11.

Jones,

in "Asymmetric

Jones

(1961).

Act. Chem. Res., 2, 145 (1976); V.T. O'Souza 0,

J.B.

Press, Vol. 5, pp. 309-344

and C.S. Cooper,

J. Am. Chem. Sot., 108,

Trans.

D.M.

Academic

J. Am. Chem. Sot., 83, 4228

Hui and J.B.

W.-R.

E.,

Lett.,

5939 (1986).

42, 3351 (1986);

and C.H. Heathcock,

R.P. Kellogg

A.M. Klibanov,

Lett., 7,

Prod. Rep., 489 (1986) and references

and E. Khedouri,

Lam, R.A.H.F.

Tetrahedron

Jones, Tetrahedron,

J.B.

and S.M. Roberts,

9.

Tetrahedron

155 (1987).

2.

8.

and Notes

0. Repic and M.J. Shapiro,

1987)

respectively. J. Am. Chem. SOC., 109,