Perfluoroketene Dithioacetals: Versatile Building ... - ACS Publications

Chapter 12

Perfluoroketene Dithioacetals: Versatile Building Blocks toward Trifluoromethyl Heterocycles 1

1,2

Downloaded by CORNELL UNIV on July 18, 2016 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0911.ch012

Charles Portella and Jean-Philippe Bouillon 1

UMR 6519, Réactions Sélectives et Applications, CNRS-Université de Reims Champagne-Ardenne, B.P. 1039, 51687 Reims Cedex 2, France Current address: Sciences et Méthodes Séparatives, EA 2659, Université de Rouen, IRCOF, F-76821 Mont-Saint-Aignan Cedex, France

2

Perfluoroketene dithioacetals with three or four carbon atoms are easily prepared from the corresponding perfluoroaldehydes or esters. They are useful synthons, due to their ability to behave as simple masked carboxylic acid derivatives and to exhibit umpolung reactivity in combination with the vinylfluorinesubstitution. With enolates asnucleophiles,multifunctional building blocks such as α-trifluoromethyl-γ-keto acid derivatives or α-trifluoromethyl -succinic acid derivatives are prepared, opening the access to a wide range of trifluoromethylheterocycles.Two complementary series ofheterocyclesare synthesized according to the starting synthon, which may be regarded as an equivalent of a bis- or tris -electrophilic system.

232

© 2005 American Chemical Society

Soloshonok; Fluorine-Containing Synthons ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

233 Fifteen years ago, we discovered a reaction that was very interesting from a scientific and academic point of view, but was of poor preparative value. This reaction consisted of a selective photoreduction of an α-C-F bond of alkyl perfluorocarboxylic esters (/). The reductive species, excited HMPA, had to be used as solvent because of its low molecular absorption. This was a major drawback for the development of the reaction on a large scale owing to the cost and toxicity of HMPA. Undoubtedly this was also the major reason that this reaction remained ignored. R -CF -C0 R Downloaded by CORNELL UNIV on July 18, 2016 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0911.ch012

F

2

2

J ™ »

R -CHF-C0 R F

2

Nevertheless, this transformation allowed us to explore the use of 2-hydro perfluorocarboxylic esters as intermediates for the elaboration of different types of organofluorine compounds. Indeed, a hydrogen atom on a perfluoroalkyl chain actually behaves as a functional group, and 2-hydro perfluoroesters proved to be valuable precursors towards polyfluorinated 3-oxoesters (2), enaminoesters (3) and heterocycles (4,5). Thus, it became relevant to consider a more practical method to prepare intermediates 1. We thought that 2-hydro perfluoroesters could be considered as the result of the alcoholysis of a perfluoroketene. Perfluoroketenes being difficult to handle, we chose perfluoroketene dithioacetals as more convenient synthetic equivalents. Rp R -CHF-COY

SR

>

F

F

1

SR

2

At the time we undertook this study, only thefirstrepresentative of the series, CF =C(SR) , was known in the literature (6). As this paper is devoted to the synthesis of trifluoromethyl heterocycles, we will not further address these compounds. Peculiar fluorinated ketene dithioacetals of the type (CF ) C=C(SR) (7) and R C(R)=C(SR') (8) are also excluded, despite our significant contribution to the study of the latter family. This review focusses on the applications of synthons 2a,b. After discussing their preparation, a second generation of more elaborated synthons derived from 1 will be described as intermediates in the synthesis of trifluoromethylated heterocycles. 2

2

3

2

2

F

2

Preparation of Perfluoroketene Dithioacetals The main route towards synthons 2 consists of the thioacetalization of perfluoroaldehydes followed by HF elimination (Scheme 1). Both hydrated or


234 hemiketal form of the aldehyde may be used. Initially described for higher homologues (R = H(CF ) ) using concentrated sulfuric acid and potassium hydroxide for the two steps, respectively (9), we found that titanium tetrachloride was the reagent of choice for performing the thioacetalization. Indeed, its reaction with the water or alcohol content releases hydrogen chloride which itself is a good thioacetalization catalyst. The second step is carried out under mild conditions using phase transfer catalysis (10). Well adapted to the first terms of the series owing to commercial availability of the perfluoroaldehydes,this methodology remains general and convenient since the aldehydes may be prepared by L A H reduction of perfluorocarboxylic esters (//). Downloaded by CORNELL UNIV on July 18, 2016 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0911.ch012

F

2

4

RSH

SR

R - C F - C H O , R'OH F

•

2

^

X

4

SR

SR / = \

2

TiCl 2

aqKOH

RF-CF -\

CH C1 2

F

S

R

2

2

2a

R =CF

2b

R =C F

F

F

2

3

5

Scheme 1

A thiophilic organometallic addition-elimination on perfluorodithiocarboxylic esters is the key reaction sequence in a different approach (Scheme 2) (12). Lithium and magnesium reagents work. In the reaction with a magnesium reagent, the distillation must be carried out after careful elimination of magnesium salts to avoid further reaction. The regioselectivity of this nucleophilic attack is obviously favored by the electron withdrawing effect of fluorine and the concomitant β-fluoride elimination. The prior preparation of the perfluorodithioesters is needed in this approach (13).

2

R M

(M=MgBr, Li)

R

F

SR

2

SR

!

RF-CF -^ 2

SR

1

(-MF)

F

2 Scheme 2


235

Second Generation Synthons Derived from 2 a-Hydroperfluorocarboxylic Derivatives


Perfluoroketene dithioacetals 2 are simply a masked form of a carboxylie acid derivative. Indeed, acid hydrolysis leads to the corresponding 2-hydro thiol ester, which can be transesterified into an ester (Scheme 3) (10). Direct hydrolysis into an α-hydroperfluoroacid under stronger conditions has been exemplified for higher homologues (9). α-Hydroperfluoroacid derivatives have also been prepared by reactions of base with perfluoroalkenes (14). 2 ^

S R

i

Ti(OiPr)

4

Scheme 3 γ-Keto ct-Perfluoroalky! Acid Derivatives The versatility of synthons 2 is mainly due to their ability to exhibit both a normal and an umpolung reactivity. The latter is of particular interest here due to the presence of a vinyl fluorine atom. Indeed, prior to any hydrolysis (normal reactivity), it is easy to convert 2 into a perfluoroalkyl derivative by a nucleophilic addition-elimination process. This fluoride substitution, which can be carried out with simple heteroatom or carbon centered nucleophiles (15), is very useful with functionalized ones such as enolates. In this way access to a second generation of bifimctionalized synthons is possible, opening the field of trifluoromethyl heterocycles. Potassium ketone enolates were effective nucleophiles in a THF solution, giving a variety of γ-keto derivatives 3 (Scheme 4). Compounds 3 may be hydrolyzed into the corresponding γ-keto α-perfluoroalkyl thiol esters 4 or further derivatized before hydrolysis (16)(17)(18)(19).

2a,b

Ο

ΌΚ

Rp

TFA-H 0 2

SEt

R' THF

R'

SEt

SE1

3a,b

Scheme 4


236


α-PerfluoroaIkyI Succinic Acid Derivatives Surprisingly, the reaction of 2 with an ester enolate failed. However, two different routes were found giving access to ct-perfluoroalkyl 1,4-diearboxylic acid derivatives or synthetic equivalents. The first one consists of a chain reaction using ethyl 2-trimethylsilyl acetate as the acetate enolate source. The reaction is initiated by tetramethyl ammonium fluoride (TMAF). A minor amount (5%) is enough for a high yielding reaction where the driving chain transfer step consists of the formation of the strong Si-F bond (Scheme 5) (20), The ketene dithioacetal moiety of the ester 5 is hydrolyzed under standard acidic conditions to give the diester 6. The ester function of 5 may also be selectively hydrolyzed and the resulting acid 7 transformed into the mono esterified succinic derivative 8 (Scheme 5). Curiously, whereas lithium ethyl acetate enolate failed to react with 2, a direct access to acid 7 and 3-substituted 2-trifluoromethyl succinic acid derivatives equivalents 9 was found using lithium enediolates as nucleophiles (Scheme 6) (21). Although such reactions generally allowed simple work-up procedures using pH controlled extractions (22), specific conditions had to be found to overcome some solubility problems inherent to thesefluorinatedspecies (21).

2

SEt

EtO'

Me SiCH C0 Et 3

TFA-H 0

cat T M A F

+ 2

2

CH C1 2

SEt

EtO'

2

1) K O H a q 2) H C l a q

HO'

(RF=CF ) 3

SEt

TFA-H 0

SEt

2

HO' Ο 8

Scheme 5 Interestingly, compounds 5-9 are all 2-trifluoromethyl or 2-pentafluoroethyl succinic acid derivatives bearing two differentiated carboxylic functions, a useful feature for further applications.


237

Î) LDA

Rl

>-C0 H

SEt

HO'

2

2) 2a

R

l

R

2

SEt

9 Scheme 6


Ethyl β-Bromoperfluorocrotonate While investigating the thiophilic alkylation of perfluorodithiocarboxylic esters as a route to perfluoroketene dithioacetals (/2), it was observed that a subsequent transformation of 2b occured while heating in the presence of magnesium salts. This reaction was studied and optimized to eventually provide the new perhalo unsaturated dithioester 10 (Scheme 7) (23X24), which proved to be an interesting building block for the synthesis of trifluoromethyl thiaheterocycles (vide infra). Compound 10 is the result of a bromide attack on the ethyl group giving a magnesium perfluorodithioester enolate which easily loses the β-fluorine. A subsequent addition-elimination sequence results in the substitution of the vinyl fluorine for bromine (23).

2b

10

Scheme 7

Applications to the Synthesis of Trifluoromethylated Heterocycles The various building blocks described above are divided into two categories bearing either a CF or a C F group. Each of these categories leads to specific applications, which are presented accordingly. 3

2

5


238

The Bis(ethylsulfanyl) Tetrafluoropropene Line


2-Trifluoromethyl /-Lactones The two synthons 3a and 4a are a priori suitable bifunctionalized candidates for heterocycle synthesis. As depicted in Scheme 8, carbonyl methylation or reduction of 3a (R=Me) followed by acid hydrolysis leads to the lactones 12 in high yields (Scheme 8). The intermediate 3a (R=Ph) derivedfromthe reaction of 2a with the acetophenone enolate did not lead to the corresponding lactone under these conditions, because of the different reactivity of the benzylic alcohol 11 (R=Ph) (76). Reversed acid hydrolysis-reduction sequence is preferable in this case, but leads to poor yields of the corresponding lactone and of the overreduced lactol (16). Another approach to 2-trifluoromethyl γ-lactones was proposed later (25).

Ο

A

CF i *Ft 3

N

f

H r

4

R

χΓτ-

3a(R=Me)

,

0 H

CF

V

CF

3

3

1

Conc.HCl,

Π

(R'=H or Me)

J

ν Λ

n

Scheme 8

OrTrifluoromethyl γ-Lactams 2-Trifluoromethyl γ-lactams are easily prepared by reductive amination of 2trifluoromethyl γ-keto thiol ester 4a (R=CH ) (Scheme 9) (17). Excellent yields of W-aryl or 7V-alkyl lactams 13 (R-Ph, Bn) are achieved using the BH -Pyridine complex as reducing agent. The N-unsubstituted compound 14 can also be prepared with the ammonium acetate-NaBH CN system. N-substituted lactams 13 are obtained as a mixture of diastereomers in almost quantitative yields, whereas the γ-lactam 14 is formed as a single diastereomer (undetermined configuration) and is accompanied by a minor amount of the hemi thioaminal 15 (Scheme 9). This seems to be due to a cyclocondensation of the intermediate imine prior to diastereoselective reduction or addition of ethanethiol. The isolation of 15 as a unique product and as a single 3

3

3


239

diastereomer (undetermined configuration) when 4a (R=Me) reacted with ammonium acetate without any reductor is a good argument for such a pathway (17). γ-Lactams are interesting bioactive coupounds (26). There are very few reports of the 2-trifluoromethyl analogues (27). The strategy starting from 2a provides a general and effective access to these compounds.

O

CF

1)R'NH

3

K ^ A ^ E t

¥ 3

2

r

/

Hex-AcOH ^

Ô

R

(

=

p

h

>

B

n

2)BH -Pyr 3


Me 4a (R=Me) 13 NH OAc 4

NaBH CN 3

MeOH

F

Λ Ρ

c 3

N

NH

F +

Mf

"lie 14

Ρ

NH I N

Me

SEt

15 (minor)

Scl eme 9

4- Trifluoromethyl Pyridazine Derive ives Heterocyclic compounds of the pyridazine family constitute an important class of biologically active derivatives (28). One of the main preparative methods starts from γ-keto acid derivatives (29). It was natural that 2trifluoromethyl analogues were intermediates of choice to access 4trifluoromethyl pyridazine derivatives. The unique representative found in the literature (a fluorinated analogue of minaprine) (30) came from a completely different building block: trifluoroethylidenacetophenone. The perfluoroketene dithiocetal 2a, via the γ-keto thiolesters 4a, proved to be an effective route towards a variety of 4-trifluoromethyl pyridazine derivatives, including the 4,5dihydropyridazinone series 16, the pyridazinones 17, and the 3chloropyridazines 18 as precursors of 3-aminopyridazines 19 (Scheme 10) (19). The versatility of perfluoroketene dithioacetals is particularly well illustrated with these pyridazine derivatives. Indeed, a wide structural diversity may be envisaged, depending on the starting ketone enolate and the final nucleophile (amine in these examples).


240 FC

Ο

3

Ο FaC^A^R*

R'NHNH

2>

g

0

PTSA

k^N

4 a


Q

C

1

3

Ρ

k^N

(R'=H,Me,Ph)

R

17

Ci P

Ffi^A^

2>

16

Ο

^SAt"

uCl

MeCN

R

(R=Me, AT)

Ο C

NHR" R"NH ^

3 ° γ ^ Ν

2

^ΝΛ

ΙΐγΝ

ll^N

R

R

R

18

19

Ν

(R»= ik,Ar) A

Scheme 10 2-Trifluoromethylsuccinimides Whereas the succinic diester 6a was supposed to be converted into pyridazinediones by reaction with hydrazines, the ethyloxycarbonyl group proved to be highly resistant to condensation, and only displacement of the ethylsulfanyl group was observed. In contrast to 6a, the acid 8a behaves as a bis(electrophilic) species, but fails to react with hydrazines to give pyridazine diones. Instead, reaction of 8a with amines (hydrazines included) gives 2-trifluoromethylsuccinimides 21 via the acid 20 (Scheme 11). N-Alkyl, N-aryl

Scheme 11


241 and N-aminosuccinimides may be prepared (20). It is noteworthy that phenyl hydrazine gave the same product as aniline, corresponding to the formal lost of ammonia. A similar reaction occured with T^^V-dimethylhydrazine, giving Nmethylamino succinimide (20).

The Bis(ethylsulfanyl) Hexafluorobutene Line


2-Trifluoromethyl Furans and Pyrroles The β-elimination of a fluoride anion is much easier from a pentafluoroethyl group than from a trifluoromethyl group. Hence the behavior of the pentafluoroethyl building blocks 4b with basic reagents such as amines is different, the first step being generally the elimination of HF. As a consequence, the reactions of 4b with amines followed by subsequent cyclocondensation give trifluoromethyl heterocycles different than from 4a, according to the reaction pathways depicted in Scheme 12 (18). The result of HF elimination is the new highly fiinctionalized intermediate 22, which in turn can react with amines according to a basic or a nucleophilic pathway (Scheme 12). The amines strong enough to abstract the highly

Scheme 12


242


activated α-proton of 22 convert it into the enolate 23. An intramolecular Michael type addition followed by a fluoride elimination provide 2trifluoromethyl furan derivatives 24. On the other hand, amines can act as nucleophiles to directly displace the vinyl fluorine. This leads to the βenaminoester type intermediate 25, the cyclocondensation of which gives 2trifluoromethyl pyrrole derivatives 26. The chemoselectivity of the reaction of 4b with amines correlates with both their basicity and steric hindrance. Taking into account that fiirane can react with primary amines to be converted in pyrrole, under suitable conditions, good yields of either pyrrole or furan derivatives are obtained, as examplified in Scheme 13 (18).

4b (R=Me)

Q

C H NH-^ 5

U

C H„NH excess neat 5

Ν

Ο

iPr NH

EtS—\^

2

2

Et 0 5

CH

3

C Hj, 5

Scheme 13 Trifluoromethyl Pyrazoles and Pyrimidines The reactivity of the succinic type synthon 6b is currently under investigation. However, we can disclose here that, as observed for 6a, the ethoxycarbonyl moiety exhibits the same resistance to condensation reactions. Under basic conditions, 6b easily loses HF to convert into the corresponding α,β-unsaturated intermediate, which has a high tendency to tautomerize into a higher conjugated form. According to preliminary experiments with hydrazines and amidines, there are three electrophilic centers competing for giving two isomeric heterocycles (Scheme 14) (31).

CF CF ^

Pyrazoles

3

6b ("hf)

Eto e 2

ο

^

Pyrimidines

Scheme 14


243 5-Membered CF -Heterocycles from 2-Hydroperfluorobutanoates 3


The 2-hydroperfluorobutanoates derived from 2b are synthetic equivalents of perfluorocrotonic acid derivatives and of 2-hydro-3-oxo-perfluoroesters, which are interesting precursors for pyrazole 27 (4) and oxazo- or imidazolidines 28 (5) type heterocycles (Scheme 15). The electron-withdrawing effect of the trifluoromethyl group favors an intramolecular Michael attack. It also stabilizes the hydrated form of pyrazolidinones (4).

RN

Ζ

28

Scheme 15

5-Trifluoromethyl l 2-Dithiole~3-Thione t

During the optimization of the preparation of perhalodithiocrotonic ester 10 it was observed that some decomposition occurred with over heating, giving minor amounts of the dithiolethione 29. This compound is indeed the result of a thermal reaction between molecular sulfur and the dithiocrotonic ester. The conditions were optimized to achieve a high yielding preparation of the 4-fluoro5-trifluoromethyl-l,2-dithiole-3-thione 29 in a one-pot process from the corresponding perfluoroketene dithioacetal 2b (Scheme 16) (24). Compounds 29 behave as effective dienophile via the C=S bond, as reported earlier for perfluorodithioesters (/5a).

1,3-Dipolar Cycloaddition of Dithiocarboxylic Dimethylacetylendicarboxylate

Derivatives

As in non-fluorinated series, l,2-dithiole-3-thione 29 reacts as a 1,3-dipole with dimethylacetylene dicarboxylate (DMAD) to give the adduct 30 which in turn reacts as an heterodiene with DMAD to give the 1:2 adduct 31


with

244

(Scheme 16). In contrast to the non-fluorinated series, where this transformation is known to be a thermal process, the second [4+2] cycloaddition is a chain process initiated by a photoinduced single electron transfer to oxygen (32). The 1:2 adduct reacts with water by simple filtration over silica gel, leading to the trithianaphtalene type compound 32. The detailed study of the reaction conditions gave access to an effective direct synthesis of either 31 or 32fromthe dithiolethione 29 (52). F MgBr S , Downloaded by CORNELL UNIV on July 18, 2016 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0911.ch012

2>

8

210 °C Ε

29

30

Scheme 17


245

The perhalodithiocrotonate 10 reacts similarly with DMAD in a 1,3-dipolar cycloaddition, but owing to the perhalo character of the substrate, an unexpected pathway is followed. The interesting vinylogue of tetrathiafulvalene (TTF) 33 is the result of a multistep process involving the condensation of a primary adduct with the substrate 10 and a subsequent cycloaddition with DMAD (33). This TTF derivative is quantitatively converted into the bis(spiro) compound 34 which under stronger heating may be transformed into the benzodithiine 35. The latter may be directly preparedfromthe crotonic derivative 10 (Scheme 17).


Conclusion Perfluoroketene dithioacetals 2a,b are versatilefluorinatedsynthons for the synthesis of a wide range of trifluoromethyl heterocycles including oxygen, nitrogen and sulfur, via more elaborated intermediates 4-10. Each of the two basic synthons open their own line of applications. Compounds 2a and 2b behave as equivalents of a polyelectrophile bearing two or three contiguous positive charges adjacent to a trifluoromethyl group. The ethylsulfanyl groups allow the "umpolung" reactivity, but the sulfur atoms play a more essential role since they can also be involved in the applications to thia-heterocycles. CF, l'î

SR or

F

SR

π ν CF

QD SR

2

5

F

SR

r-jactones γ-lactams pyridazines succinimides

m u

@

@

" ~

pyrroles furans pyrimidines dithiolthiones trithianaphtalenes

benzodithiines

The chemistry of perfluoroketene dithioacetals and derived synthons undoubtedly remains an open field for other extensions confirming their high versatility. Acknowledgments. We warmly thank Dr Murielle Muzard, Dr Jean-François Huot, Dr Cédric Brûlé and Béatrice Hénin, for their contribution to this work. We also thank Professor Yuriy Shermolovich and Dr Vadim Timoshenko for a fruitful collaboration in the thia-heterocycle syntheses. Dr Karen Pie has kindly offered his expertise for English correction. This work has been supported by CNRS, the Ministry of Education and Research, and by CEREP company.


246

References 1. 2. 3. 4. 5.


6.

7.

8.

9.

10. 11. 12. 13.

14.

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247

Rendal, J. L.; Pearlson, W. H. US 1957, 2795601 19570611; Chem. Abstr. 1957, 51, 90875. (h) Rendall, J. L.; Pearlson, W. H. GB 1955, 737164 19550921; Chem. Abstr. 1956, 50, 74163. 15. Muzard, M . PhD Dissertation, Reims, 1992. 16. Huot, J.-F.; Muzard, M.; Portella, C. Synlett 1995, 247-248. 17. Hénin, Β.; Huot, J.-F.; Portella, C. J. Fluorine Chem 2001, 107, 281-283. 18. Bouillon, J.-P.; Hénin, Β.; Huot, J.-F., Portella, C. Eur. J. Org. Chem. 2002, 1556-1561. 19. Brulé, C.; Bouillon, J.-P.; Nicolaï, Ε.; Portella, C. Synthesis 2003, 436-442. 20. Brulé, C.; Bouillon, J.-P.; Portella, C. Tetrahedron, 2004, in press. 21. Sotoca, E.; Bouillon, J.-P.; Gil, S.; Parra, M.; Portella, C. Tetrahedron Lett. 2004, in press. 22. (a) Brun, Ε. M.; Gil, S.; Mestres, R.; Parra, M. Synthesis 2000, 1160-1165. (b) Brun, E. M.; Gil, S.; Mestres, R.; Parra, M. Tetrahedron Lett. 1998, 54, 15305-15320. 23. Bouillon, J. -P.; Shermolovich, Y. G.; Portella, C. Tetrahedron Lett. 2001, 42, 2133-2135. 24. Timoshenko, V. M.; Bouillon, J. -P.; Shermolovich, Y. G.; Portella, C. Tetrahedron Lett. 2002, 43, 5809-5812. 25. Tellier, F.; Audouin, M.; Sauvêtre, R. J. Fluorine Chem. 2002, 113, 167175. 26. Baldwin, J. E.; Lynch, G.; Pitlik, J. J. Antibiot. 1991, 44, 1-50. 27. (a) Suzuki, M.; Okada, T.; Taguchi, T.; Hanzana, Y.; Iitoka, Y. J. Fluorine Chem. 1992, 57, 239-243. (b) Paulini, K.; Reissig, H. -U. J. Prakt. Chem./Chem.-Ztg 1995,337,55-61. 28. (a) Steiner, G.; Gries, J.; Lenke, D. J. Med. Chem. 1981, 24, 59-72. (b) Wermuth, C. G. Farmaco 1993, 48, 253-259. 29. Albright, J. D.; Moran, D. B.; Wright, W. B.; Collins, J. B.; Beer, B.; Lippa, A. S.; Greenblatt, E. N. J. Med. Chem. 1981, 24, 592-601. 30. Contreras, J. M.; Rival, Y. M.; Chayer, S.; Bourguignon, J. J.; Wermuth, C. G. J. Med. Chem. 1999, 42, 730-741. 31. Brulé, C.; Bouillon, J.-P.; Portella, C. unpublished results. 32. Timoshenko, V. M.; Bouillon, J.-P.; Chernega, A. N.; Shermolovich, Y. G.; Portella, C. Eur. J. Org. Chem. 2003, 2471-2474. 33. Timoshenko, V. M.; Bouillon, J.-P.; Chernega, A. N.; Shermolovich, Y. G.; Portella, C. Chem. Eur. J. 2003, 9, 4324-4329.


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