Microwave-assisted hydrolysis: efficient synthesis of a-substitutedcysteines on multi-gram scale{

Dominic Fiset and Andre B. Charette*

Received 21st April 2012, Accepted 25th April 2012

DOI: 10.1039/c2ra20744c

A microwave-assisted hydrolysis of thiazolidines was developed

for the synthesis of enantioenriched a-alkylcysteines. Under

optimized conditions, the efficiency of the reaction can be

considerably improved and the scope significantly broadened as

compared to previously described processes. This approach is

shown to be applicable to the multi-gram scale using a high-

throughput microwave system.

Quaternary amino acids play a key role in medicinal chemistry as

they can be used to modify the conformational properties of a

peptide without loss of side-chain functions, while improving its

stability toward biological degradation. More specifically, a-alkyl-

cysteines are of particular interest, as they can be used to constrain

the structure of a peptide via disulfide bonds. Additionally, the (R)-

methylcysteine 5a unit occurs in thiazoline rings of several potent

antitumor or anti-HIV natural products such as (–)-thiangazole,1

cyclothiazomycin,2 largazole (vide infra),3 hoiamides,4 bisebromoa-

mide5 and grassypeptolides.6

Although considerable efforts have been devoted to the develop-

ment of new methodologies for the synthesis of a,a-disubstituted

amino acids,7 available methods for the enantioselective synthesis of

a-alkylcysteines are still limited. The main reported strategies for

their synthesis rely on: (1) hydrolysis of desferrithiocin,8 (2) chiral

auxiliaries,9 (3) ring opening of a chiral aziridine or b-lactone with

thiolates,10 (4) self reproduction of stereocenters (SRS) from

oxazolidinone or thiazolidine derivatives,11 (5) phase transfer

alkylation,12 and (6) enzymatic desymmetrization.13 Among these,

Pattenden’s procedure (Scheme 1),14 which relies on the SRS,

remains the most efficient way to synthesize multi-gram quantities of

(R)-a-methylcysteine 5a, as showcased in the total syntheses of (–)-

didehydromirabazole A,15 (–)-thiangazole,16 micacocidin,17 hoiamide

C,18 bisebromoamide,19 and largazole.20

Recently, Alvarez, Altucci, de Lera and co-workers published a

total synthesis of largazole, a potent histone deacetylase (HDAC)

inhibitor, in which they investigated the influence of different alkyl

groups at the C7 position using Pattenden’s procedure (Scheme 2).21

In their report, they mentioned that the major drawback for this

structure–activity relationship (SAR) study is the small scope of

quaternary amino acids available via this procedure. Even under

harsh acidic conditions (HCl 5N under reflux for 3 days), low

conversion is obtained in most cases for the hydrolysis of the

alkylated thiazolidine 4. Herein, we report a microwave-assisted

hydrolysis that can considerably enhance the reaction efficiency and,

therefore, broaden the scope of this reaction to access a variety of

new a-alkylcysteines. In addition, using MARS, a high-throughput

digestion system, we were able to reproduce this reaction on the

multi-gram scale with short reaction times.

Results and discussion

Since the 1980s,22 microwave heating has appeared as a valuable

alternative to conventional heating processes. In this technique, heat

Centre in Green Chemistry and Catalysis, Department of Chemistry,Universite de Montreal, P.O. Box 6128, Station Downtown, Montreal,Quebec, Canada H3C 3J7. E-mail: andre.charette@umontreal.ca;Fax: (514)-343-7586; Tel: (514)-343-6283{ Electronic Supplementary Information (ESI) available: Characterizationdata and copies of the NMR spectra. See DOI: 10.1039/c2ra20744c

Scheme 1 Pattenden’s procedure14

Scheme 2 Structure–activity relationship study of largazole

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is generated from electromagnetic energy, and its formation strongly

depends on the specific polarity of the molecules. Water is an

excellent solvent for microwave-mediated reactions because it

possesses a polar nature, which accounts for its good potential for

microwave absorption.23 Therefore, we were interested in the

feasibility of the hydrolysis under microwave irradiation to ultimately

enhance the reaction efficiency. To our delight, we found that the

reaction can reach completion within an hour at 160 uC while

maintaining an excellent yield (Scheme 3).

With these conditions in hand, we were keen to investigate the

scope of this reaction (Table 1). In this context, we synthesized

thiazolidines 7 starting with the more affordable cysteine ethyl

ester.24 Satisfyingly, all of the hydrolysis precursors were obtained as

single diastereoisomers.25

The ethyl 7b and benzyl 7c derivatives (entries 1 and 2) can be

respectively hydrolyzed within 1 and 2 h, compared to 3 days under

standard thermal conditions, while maintaining high yields.

p-Methylbenzyl 7d and naphthylmethyl 7e derivatives proved to be

more challenging and thus required longer reaction times (entries 3

and 4). Interestingly, the 4-bromobenzyl 7f derivative could be

obtained in 81% yield, which compares favourably to a 33% yield

reported in the literature (entry 5).21 In fact, halogenated derivatives

are generally not tolerated under standard thermal conditions, as

illustrated with a reported 18% yield for the 3,5-difluorobenzyl

derivative, while no conversion was observed for the 3,5-bis-

trifluoromethylbenzyl.21 Microwave-assisted hydrolysis can be used

to circumvent this limitation, as exemplified with the 2,3,4,5-

pentafluorobenzyl 7g, 4-trifluoromethylbenzyl 7h and 3,5-dichlor-

obenzyl 7i derivatives, which were isolated in good to excellent yields

(entries 6, 7 and 8). In all the examples mentioned above, the

a-substituted cysteines were obtained in a 4-step sequence starting

from commercially available cysteine ethyl ester hydrochloride and

required a single flash chromatography.24

Unfortunately, the allylated 7j derivative was found to be

incompatible under harsh acidic conditions, presumably because of

the reactivity of the olefin (entry 9). Nonetheless, we developed a

2-step sequence using this substrate to expand the scope of this

methodology (Table 2).

Hydrogenation followed by microwave-assisted hydrolysis furn-

ished a propyl 7j analogue in 76% overall yield (entry 1). This

strategy could be further extended to the synthesis of butyl 7k and

3-methylbutyl 7l derivatives (entries 2 and 3) in good yields.

Multi-gram synthesis of (R)-a-methylcysteine hydrochloride

Until very recently, a major limitation for the microwave-assisted

organic reaction (MAOS) had been the preparation of a substantial

amount of material. Considerable efforts have been devoted to

address this issue, which have resulted in the development of

several large batch reactors, high throughput and flow systems.26

Consequently, we investigated whether our methodology could be

reproduced on multi-gram quantities (Table 3).

To initiate this study, we selected the synthesis of (R)-a-methyl-

cysteine 5a due to its occurrence in natural products and its synthetic

utility. First, it should be noted that our conditions are compatible

with a standard microwave system for the generation of up to one

gram of the amino acid (entry 1). However, we tested our reaction on

a Voyager stop-flow system, since it can be used for significantly

larger scales.27 Although our first attempt was successful (entry 2),

this system was found to be incompatible with the use of corrosive

HCl 5N (entry 3).

We then turned our attention to a MARS digestion system, which

is typically used for protein hydrolysis. As a proof of concept, we

performed our reaction on a four-gram scale (four closed vessels each

Scheme 3 A comparison between thermal and microwave conditions

Table 1 Reaction scope

Entry R Time (h) Yield (%)a

1 1 95

2 2 81

3 10 84

4 10 73

5 10 81

6 10 67b

7 10 79b

8 10 82b

9 1 0c

a Isolated yield. b Product is 95% pure (contains disulfide dimer).c Only decomposition was observed.

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holding one gram). This method proved to be highly representative

of the results obtained on a standard microwave system, as

showcased with the excellent reaction efficiency achieved (entry 4).

Satisfyingly, this reaction could be further reproduced on a 10 gram

scale by running 5 simultaneous reactions (entry 5). Based on these

results and on the loading capacity of the MARS system (40 closed

vessels, 50 mL each), this methodology is amenable to an 80 gram

scale synthesis of 5a on a single run. Given that the sequence for the

synthesis of the hydrolysis precursor could be achieved industrially,28

we believe that this process is a significant improvement over existing

methods for the preparation of substantial amounts of enantioen-

riched a-alkylcysteines.

Conclusions

In summary, a microwave-assisted hydrolysis of thiazolidines was

developed for the synthesis of a-alkylcysteines. Under microwave

irradiation, the reaction efficiency and its scope were significantly

enhanced. The effectiveness of these conditions was showcased in the

synthesis of halogenated derivatives, paving the way to a new variety

of a-substituted cysteines, which can potentially be used in SAR

studies of various natural products. Moreover, the feasibility of this

method was illustrated on the multi-gram scale using MARS, a high

throughput system.

Experimental section

General procedure for the microwave-assisted hydrolysis

Substrate 7 (0.20 mmol) was added in a 5 mL vial and purged with

argon for 15 min. A solution of aqueous HCl 5N (0.66 mL, 0.30 M),

previously degassed by bubbling argon for 30 min, was added. After

purging with argon for 15 min, the vial was capped and heated under

microwave irradiation (high absorbance) at 160 uC. After 1 h, the

reaction was cooled down to room temperature, the pressure was

released with a needle and the vial uncapped. The aqueous phase was

washed with EtOAc (3 6 15 mL) then the acidic aqueous phase

was concentrated under reduced pressure to afford the desired

(R)-a-alkylcysteine 8 as a highly hygroscopic solid.

General procedure with the MARS digestion system

Thiazolidine 7a (10.0 g, 38.55 mmol) was added to 5 different closed

vessels (5 6 2.0 g) and purged with argon for 15 min. A solution of

aqueous HCl 5N (5 6 25 mL), previously degassed by bubbling

argon for 30 min, was added to each vessel which were rapidly sealed

under an atmosphere of argon. The solutions were heated at 160 uCfor 1 h using the MARS digestion system. The combined aqueous

phases were washed with EtOAc (3 6 75 mL) then the acidic

aqueous phases were concentrated under reduced pressure to afford

the desired (R)-a-alkylcysteine 5a as a highly hygroscopic beige solid

(6.60 g, 99%).

Acknowledgements

This work has been supported by NSERC (Canada), the Canada

Research Chair Program, the Canada Foundation for

Innovation, the Centre in Green Chemistry and Catalysis

(CGCC) and the Universite de Montreal. The authors would

like to thank A. Lemire, F. Galaud and P. Lavallee (Universite

de Montreal) as well as J. O9Donnell (CEM) for helpful

discussions; P. Lapointe (IRIC) is also acknowledged for

supplying some of the reagents used in this study.

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