Trialkylsilanes as scavengers for the trifluoroacetic acid deblocking of protecting groups in peptide synthesis

Tetrahedron Letters,Vol.30,No.21,pp Printed in Great Britain

2739-2742,1989

0040-4039/89 $3.00 + .oo Pergamon Press plc

TRIALKYLSILANES AS SCAVENGERS FOR THE TRIFLUOROACETIC ACID DEBLOCKING OF PROTECTING GROUPS IN PEPTIDE SYNTHESIS Daniel 1

Imaging

A. Pearson Agents

1*

, Mary Blanchette', 2

Research

E.I. DuPont de Nemours & Co., Inc. No. Billerica, MA 01862

Mary

Lou Baker,

Cathy A. Guindon

NEN Research Products E.I. DuPont de Nemours Boston, MA 02118

& Co.,

Inc.

Triethylsilane was used as a scavenging agent in peptide synthesis for the removal Abstract: The efficiency of scavenging ability was compared with anisole of protecting groups with TFA. It was also found that triethylsilane/TFA reduces and ethanedithiol in a kinetic experiment. the indole ring of tryptophan. Trialkylsilanes

have been known for some time to be mild reducing agents when used in 1 However, we have found little combination with trifluoroacetic acid as a solvent system. 2 with no mention of efficacy or limitations. We wish to mention of their use as scavengers report

the use of triethylsilane

deblocking

of protecting

their ability We first synthesize sition)

groups

to scavenge became

peptides

interested containing

most of the common

that the crude

ly deprotected

peptide.

amounts

investigation

under

in an ionic hydrogenation

and confers

If this

instance

Further

to triethylsilanol

advantages

(Et3SiH

ethanedithiol by these

contain

group.

group

content

of partialwas used,

results,

hydride

a hydrogen

The reaction

is deblocked if there

an

donors

substituted

is irreversible

and scavenged,

it can not

are on the peptide

group

on the peptide

play a more prominent

can be extremely peptides.

that triethysilyl

in an aqueous

a num-

role.

as will

not looked

trifluoroacetate

as scavengers

point = 108') and the side products 2739

important

Although

would

be expected In this

be seen later

for in the course

is also

formed

and

workup.

in the use of trialkylsilanes

boiling

pro-

Reverse-phase

high levels

can be considered

formed

important

interactions

scavenger

it is presumed

with

scavenger

conditions

time of a protecting

of high-cysteine

acid compo-

to each other which can also react with carbocations.

as intramolecular

the use of an irreversible

later hydrolysed

compounds

Tfi proximity

poor results.

Discouraged

to the protecting

This is especially

to

was initiated.

The side products

1).

the residence

of this investigation,

acidic

that once the protecting

dramatically

in the deprotection

efficient

were in evidence.

when attempting

(up to 30% by amino

were contaminated

as scavengers

the appropriate (Scheme

groups

is the case,

to increase

impurities

the peptide.

ber of functional

produced

of

and anisole.

group was used for sulfhydryl

we tried gave routinely

peptides

group was attached

the advantage

react again with

of cysteines

in the acidic

the efficiency

ethanedithiol

as scavengers

When the trityl

Even when the extremely

of these

the functional

trialkylsilanes

chelators.

scavengers

We have also compared

with the scavengers

a high percentage

into the use of silanes

Trialkylsilanes

where

in using

as carbocation

synthesis.

cation

scavengers

HPLC indicated

pronounced

in peptide

the t-butyl

for use as heavy metal

tection,

and triisopropylsilane

formed

are

that they are volatile

are either

Volatile

enough

2740

or non-polar

enough

Scheme

to be easily

removed

from the desired

peptide

obtained

from the deprotection.

1

Et$iH, 50% TFA/CH,CI,, 1

=

hr, 23’C

H2N’:

+

H2N*QH

+ Et3SiOCOCF,

Kemp, et. al. bony1

3

(Bpoc) amino

most efficient (5% with

, studied carbocation

intermediate

studied, thiol)

carbocation. (0.002M

acid,

we saw no evidence

yl)-propane

produced

was markedly

slowed

into. ducts

tryptophan

Kinetic

Lundt,

of the carbocation

previously

chloride.

Trytophan

and cystine

were

followed

followed

by NMR

the t-butyl

ppm corresponding constant

media

reduced

similar

in an NMR

reduce

It should

indoles4.

However, Using

reduction as

that the use of

of indole above,

this

(5 equiv.)

the same conditions

be mentioned

the decomposition

We

and triisopropylsi-

with triethlsilane

85% reduction.

to the conditions

chloride

rate constants.

were not looked

there were no pro-

and triisopropylsilane

The same protocol

A 0.2M solution

decrease

of the singlet

and the increase

were monitored

from the following

formula

of t-butyl

acid.

with

as set forth

and to 0.50 ml of this was added

of 0.50 ml of trifluoroacetic

trifluoroacetate

to P-methylpropane

was calculated

triethylsilane

and ethanedithiol.

was prepared

by the addition

of t-butyl

to compare

anisole

at 20' and the gradual

ester

in acidic

as the scavenger.

retard

under

saw only E-(p-biphenyl-

to triethylsilane

was rapidly

in evidence.

were performed

used scavengers

in deuteromethylene

trialkylsilane

and instead

reduction

subjected

glycine

of the

seen.

et. a1.5 was used in determining

acetate

of N-d-Bpoc

trapping

scavenging.

indicated

that might

incomplete

0.5% TFA/dichloromethane)

that trialkylsilanes

of tryptophan

that even with the

of 2-(p-biphenylyl)-propene

indicates

the deblocking

triethylsilane,

of 2-(biphenylyl)-propene

derivatives

experiments

the two commonly

This

of N-b(-biphenyloxycar-

It was found

a reservoir

in solution.

was only 4% reduction

of reduction

chloride.

by using triisopropylsilane

When histidine

in the deblocking

mercaptan,

2 hours at 23', NMR studies

those above,there protected

O.OIOM

the sensitivity

lane in 90% TFA/methylene such that after

existed

as a result

It has been reported checked

benzyl

When we studied

amino

experiment,

therefore

scavengers

acids with 0.5% TFA/methylene

scavenger

10 equivalents

conditions

&

trifluoro0.1 mm01 of

The reaction

at i.XI ppm corresponding of the doublet

by integration. and theresults

centered

The second-order tabulated

below.

by

was to

at 0.87 rate

2741

1

k2 (M-’mill-9 ethanedithiol

4.9 x 1o-2

triethylsilane

6.1 x 10”

anisole

4.2 x 10”

triisopropylsilane

2.3 x 10”

Finally, on a RaMPS

the peptide

peptide

from glycine

protection.

teine

side chains

were

of nitrogen

have synthesized products

protected

acid

side chains

as S-trityls.

other

scavengers

solid-phase

and the residue

by this method. from partial (even thiol

up in water

and washed

of other

containing

These

scavengers)

were

and the cys-

was performed

in

per protecting

volatiles

were

with ether.

removed

group under

Recoveries (product

high cysteine-content

pronounced

used.

flow rate = 1.0 ml/min A = 0.1% TFA taq), B = 0.1% TJTA/90% CH,CN (aq) O-5 min 0% 8; 5-45 min O-15% B; 45-60 min 15% B; 60-70 min 50% B

Crude GEC-4 after deprotection and cleavage from resin

were

is peak

peptides

time of 55 to 60 minutes

peaks were much more

starting

for N-terminal

esters

the crude product

The peaks with a retention detritylation.

group

of triethylsilane

was synthesized

methodology

as t-butyl

The cleavage/deprotection

HPLC shown below,

is typical

(GEC-4)

the Fmoc protecting

were protected

2 equivalents

taken

This example

>

synthesis

At the end of this time the TFA and other

pure.

resulting

stepwise

on Wang resin and utilizing

high and, as seen from the reverse-phase 5) was relatively

= t

acid for 1 hour with

used as the scavenger. a stream

a-x

a = initial cont. (mol/ 1) (a-x) = cont. at time t (min) k, = second order rate constant

by standard

The glumatic

neat trifluoroacetic

k2 =

H-CysGlyGluCysGlyGluGlyGlyCysGlyGluCysGly-OH

synthesizer

supported

amine

_- 1

(-

we

are side when

2742

In conclusion,

for the deprotection

proved

to be a better

latter

is approximately

ibility

scavenger

using this system hindered

than ethanedithiol

an order

of the scavenging

of magnitude

reaction

in the presence

triisopropylsilane

of high cysteine-content

when

triethylsilane

of tryptophan

or careful

even

higher.

titration

peptides,

triethylsilane

thouigh the rate constant This

is probably

is used.

containing

Care should

peptides.

of carbocations

of the

due to the irreversbe taken when

The use of the more

with triethylsilane

is

warranted.

References 1. 2. 3. 4.

D.N. Kursanov, Z.N. Parnes, N.M. Loim, Synthesis 1974, 633 A. Davison, D. Brenner, J. Lister-James, A.G. Jones, U.S. Patent 4,673,562; D. Brenner, A. Davison, J. Lister-James, A.G. Jones, Inorg. Chem. 1984, 23, 3793 D.S. Kemp, N. Fotouhi, J.G. Boyd, R.I. Carey, C. Ashton, J. Hoare, m. 2. Peptide Protein Res., 1988, 31, 359 A.E. Lanzilottl, R. Littell, W.J. Fanshawe, T.C. McKenzie, M.F. Lovell, J. Orq. Chem.

1979, 44, 4809 5. B.F. Lundt, N.L. Johansen, 12, 258

A. Volund,

(Received in USA 16 February 1989)

J. Markussen,

-Int. 2. Peptide

Protein

Res.,

1978,