Current Fluoroorganic Chemistry - ACS Publications - American

Chapter 4

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Nitrogen-Containing Organofluorine Compounds through Metathesis Reactions 1,2,

1,2

1,

Santos Fustero *, Juan F. Sanz-Cervera , Carlos del Pozo and José Luis Aceña 2

2

1Departamento de Química Orgánica, Universidad de Valencia, E-46100 Burjassot, Spain Laboratorio de Moléculas Orgánicas, Centro de Investigación Príncipe Felipe, E-46013 Valencia, Spain

The metathesis reaction is a convenient method for the preparation of cyclic and heterocyclic compounds, of which fluorinated nitrogen-containing derivatives are of increasing interest in the search for new biologically active molecules. Several examples have been studied, including fluorinated αand β-amino acids, uracils, piperidines, lactams, and lactones.

Introduction Organofluorine compounds, although practically unknown in Nature, are nowadays essential for the development of new biologically active molecules in both medicinal chemistry and crop protection (1-4). The small size and high electronegativity offluorineatoms endow them with unique properties that make their introduction into organic molecules a standard procedure for modulating the molecules' physical, chemical, and biological characteristics. Particularly interesting are thefluorinatedanalogues of proteins, nucleic acids, and their 54

© 2007 American Chemical Society

In Current Fluoroorganic Chemistry; Soloshonok, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


55 building blocks (5-6) because these analogues can frequently act as mimetics of their parent compounds, but with improved biological activities. For this reason, new methods for the synthesis of nitrogen-containing organofluorinated derivatives are in great demand. At the same time, the metathesis reaction is currently recognized as one of the most potent tools for the creation of carbon-carbon bonds (7), as is clearly reflected by its widespread use in the syntheses of different classes of natural products and compounds of biological interest (8). Specifically, the ring-closing metathesis (RCM) facilitates the preparation of medium- and large-size cyclic compounds, which just a decade ago required both longer and much more complicated syntheses. By the same token, cross metathesis (CM) reactions allow for the preparation of acyclic structures by exchanging alkylidene moieties. The efficiency of this method has been made possible by the development of stable, yet highly reactive ruthenium-based catalysts such as 1-3 (Figure 1), which are compatible with the presence of a variety of functional groups.

M e s — N ^ N - M e s

Mes—N*N-^N-Mes

Figure 1. Structures of Grubbs' first (1) and second generation (2) and GrubbsHoveyda (3) ruthenium catalysts. Metathesis reactions thus constitute a key step in many syntheses of both cyclic and acyclic nitrogenous compounds (9-10). The aim of this review is to show the most recent applications of metathesis processes for preparing organofluorine nitrogen-containing derivatives, with a special focus on the synthesis of fluorinated amino acids, uracils, and other nitrogen heterocycles.

Synthesis of Cyclic Fluorinated Amino Acids Fluorinated a-Amino Acids The insertion of unnatural amino acids is a common strategy for synthesizing modified peptides with more stable secondary and tertiary structures, as well as for improving their bioavailability. For this purpose,



56 fluorinated amino acids, the syntheses of which have been recently reviewed (7772), are widely employed. While several approaches for the preparation of cyclic fluorinated a-amino acids have been reported, only a few have involved RCM processes. One pioneering example is the synthesis of racemic five- to sevenmemberedfluorinateda-amino acid derivatives with the nitrogen atom included in the ring, which was reported by Osipov, Dixneuf et al This synthesis starts from fluorinated imines 4 derived from methyl 3,3,3-trifluoropyruvate or 3chloro-3,3-difluoropyruvate (Figure 2). Regioselective addition of alkenyl Grignard reagents affords amines 5, in which a second unsaturated substituent is introduced on the nitrogen atom to yield dienes 6. The subsequent RCM reaction of 6 in the presence of catalyst 1 affords racemic cyclic a-amino esters 7, which bear a quaternary trifluoromethyl or chlorodifluoromethyl group as a substituent (75-74). CF X 2

M

• t ^

B

*

r

XF C

C0 Me

2

THF,-78°C

'

n

54-79%

4

XF C 2

H

m

r

55-81%

XF C

2

B

NaH, D M F

5

C0 Me

)

^ > ^

2

2

C0 Me 2

C H ^ r t 45-98%

n= 0 , 1 , 2

PG= S0 Ph, Cbz, Boc

m = 1,2

X=F,C1

2

Figure 2. Synthesis of five- to seven-memberedfluorinated a-amino esters. Our research group has described the preparation of enantiomerically pure fluorinated p-amino alcohols startingfrom2,2-difluoro-4-pentenoic acid 8 (75) (Figure 3). Conversion of this acid into imidoyl chloride 9 with Uneyama's method (16) was followed by condensation with the lithium carbanion derived from optically pure sulfoxide 10; the subsequent reduction of the imino function gave P-amino sulfoxide 11 in a 99:1 diastereomeric ratio. After changing the amino protecting group, transformation of sulfoxide 12 into p-amino alcohol 13 was accomplished by means of a non-oxidative Pummerer reaction, and subsequent JV-allylation or homoallylation was accompanied by oxazolidinone ring formation. Finally, an RCM of compounds 14 provided bicyclic P-amino alcohol derivatives 15 in high yields. These, in turn, served as direct precursors to the corresponding seven- and eight-membered cyclic a-amino acids 16, which incorporate a gem-difluoro unit within thering(7 7).


57 1) L D A , T H F , - 7 8 ° C

PNfl\

PPh F

3 f

CI

Et N,CCl4

2) B u N B H 4

3

F

F

F

8

80%

9

D

NHPMP

•5

4

THF, M e O H , - 7 0 ° C

' 80%

•5

(10)

S A r ^

PMP-NH, HO

#

0

NHCbz


la) b)

TFAA K

2

C 0

c) N a B H

3

4

88%

l)BzOH NHCbz HO. 2) N a H , F

O

A N ^ V ^

DCCDMAP

^ Q DMF

'

C H j C l j , rt

F F

13

F

75-87%

14

45-84%

H H 0 C 2

15

1

N

V

^

\

n=

1,2

A r = 1-naphtyl

16

Figure 3. Synthesis ofbicyclic fluorinated p-amino alcohol derivatives. More recently, we have developed a different synthetic route to cyclic ocamino acids that also starts from fluorinated acid 8, which is first converted into chiral imidoyl chloride 17 derived from 0-(methyl)-a-phenylglycinol (Figure 4). Palladium-catalyzed alkoxycarbonylation of the derived imidoyl iodide in the presence of 2-(trimethylsilyl)ethanol (TMSE-OH) gives imino ester 18, and addition of allylzinc bromide affords amino ester 19 in a fully regio- and diastereoselective manner. A subsequent R C M reaction furnishes compound 20, and then removal of the chiral auxiliary and double bond reduction through hydrogenation yields 21. Finally, TBAF-assisted ester hydrolysis produces cyclic amino acid 22, the absolute configuration of which was determined by subjecting its acetamide derivative 23 to X-ray analysis (18).


58 ^OMe OMe

N'

O H

l)Nal

OH F

PPh Et N, 3

F

"Ph

N ^ P h

2

3

CCl

CI

2) C O , T M S E - O H

4

F

F

Pd (dba) K C0

67%

2

8

3

17

2

3

40%


sOMc m N ^ P h

ZnBr

•

OTMSE

N H C0 TMSE 2

F

2

V

FC H ^

THF,-40°C F

6

F

83%

ds>99:l

18

A

87%

19

„OMe

Ph

H N C0 TMSE

N HC0 TMSE 2

2

H

2

wouh

Tj

89% 20

^

p

F

H N C0 H

2

O C

2

TBAF^

Tj

p

F

65%

2

F y X

~Tj

^^"^ (S)-21

(S)-22

AcHN AczO

F

£

0 TMSE 2

V

70% (5)-23

Figure 4. Synthesis of enantiomerically pure fluorinated cyclic a-amino acids. Fluorinated p-Amino Acids The P-analogues o f proteinogenic amino acids are interesting target compounds that can be found in the structure o f biologically active natural products (19-21). In addition, they can form oligomers (P-peptides) with defined folded structures (22). While syntheses of fluorinated acyclic P-amino acids are relatively common in the literature (23-26), those of their cyclic derivatives are much rarer; in fact, only our group has reported their synthesis with the aid of metathesis reactions. To begin, we prepared racemic seven-membered p-amino esters containing a ge/w-difluoro unit, again starting from imidoyl chloride 9 (Figure 5). Reaction with the lithium enolate derived from ethyl 4-pentenoate


59 afforded the imino ester 24. Next, an R C M reaction in the presence of catalyst 2 afforded the cyclic imino ester 25, which in turn was stereoselectively reduced with N a C N B H to yield racemic cw-p-amino ester 26. Reversing the order of steps (imine reduction before the metathesis reaction) produced a 1:1 mixture of cis and trans P-amino esters and also caused a decrease in the yield (27%) of the metathesis process. Compound 26 could be transformed into saturated derivative 27 either through hydrogenation or by removing the /?-(methoxy)phenyl (PMP) protecting group with eerie ammonium nitrate (CAN) to yield 28 (27). Downloaded by UNIV MASSACHUSETTS AMHERST on September 27, 2012 | http://pubs.acs.org Publication Date: January 11, 2007 | doi: 10.1021/bk-2007-0949.ch004

3

PMPs .PMP

OLi

N

O

OEt

F

F

o

T H F , -78 ° C

CH Cl ,40 C

85%

75%

2

2

24

Figure 5. Synthesis of seven-membered P-amino esters by means of ring-closing metathesis. Despite its effectiveness in the aforementioned synthesis, this synthetic strategy was not suitable for the preparation of five- and six-membered p-amino acid derivatives. For this reason, we developed an alternative route, employing a cross-metathesis (CM) reaction instead of an R C M (Figure 6). In this manner, imidoyl chlorides 9, 29, and 30, along with varying lengths of their unsaturated chains, were reacted with ethyl acrylate in the presence of catalyst 2 to give compounds 31-33. To the best of our knowledge, this is the first reported example of a metathesis reaction in the presence of an imidoyl halide functionality. After the double bond was hydrogenated, the resulting imidoyl chlorides were reacted with lithium diisopropylamide ( L D A ) to produce an intramolecular Dieckmann-type condensation that led to enamino esters 34-36. These compounds were efficiently reduced under several different conditions in


60 sequence to obtain selectively cw-P-amino esters 37, a protected difluorinated analogue of the fungicide agent c/s-pentacin, and 38, along with the previously obtained 27. In addition, an enantioselective version of this synthesis has also been carried out with a chiral auxiliary in the ester group in order to prepare enantiomerically pure P-amino acid derivatives (28).


P

M

%

^Y

0 E t

P

M

C

2,PhMe,95«X: F

PMP p

F

P

-

l)H ,Pd-C

N

2

I

X F

93-95%

J

9

n=l

31

n=2 n=3

32 33

PMP N H

T 0

29 30

N

^

F

n=l

40-80%

n=2 n=3

N

p

cis: trans ratio

N H [HI

F

i

^

c

o

E

2

t

"

^

r

F

" t ^

c

2) L D A THF,-78°C

°

2

E

t

NaCNBH

3

n= 1

n= 2

n= 3

52:48

88:12

96:4

94:6

85:15

97:3

TFA, THF 34

n=l

(±>37

n=l

35 36

n=2 n=3

(±)-38

n=2

(±)-27

n=3

Znl

2

NaCBH )2 4

dry'CH Cl 2

2

Figure 6. Synthesis offive- to seven-membered p-amino esters by means ofcross metathesis.

Synthesis of Fluorinated Uracils The metathesis reaction has been widely employed for the synthesis of nucleoside derivatives, usually to build the sugar ring. Only occasionally has it been used to prepare fluorinated analogues (29). We became interested in the synthesis of uracils with a fluorinated group at C-6 due to their potential as agrochemicals (30-31). In this context, we prepared two new families of bicyclic difluorinated uracils starting from nitrile 39, which was obtained in three steps from acid 8 (Figure 7). Addition of the lithium enolates of acetate derivatives to 39 afforded enamino esters 40, which were further reacted with several different isocyanates to yield uracils 41. When a suitable olefin substituent was present in the molecule ( R vinyl or allyl), direct RCM cyclization produced six- and seven-membered bicyclic uracils 42. In contrast, palladium-mediated Nallylation of 41 ( R H) furnished diene 43, which was then cyclized to give 44 (32). 1=

1=


61 OLi f ^ O E t ^

-

^

C

N

N H

/ N

p/ \ p

TH T U

O

2

^ ^ V ^ O E t

R'

or*

F V, F -J

FT, - T7 O8 ° C

N

«

Ri

30-95%

H

-

R

j

N

C

0

40-84%

39

40

!

O


a

DMF,0°C

( R = vinyl, allyl)

HN

O

1

N

H N

N n=0,1 2

R = alkyl, aryl F

F

Ri

64-82%

(

41

42

OAc

Q

O

(R»=H) Pd (dba)3 dppb 2

t

H O,THF,60°C 2

60-77%

^

^

N

^

N

'

V N ^ A Q p

F

* CH#1

^ 8

2 I

50 C

77-93% 43

V V

%

,

^

p

' ^

Q

¥

44

Figure 7. Synthesis of bicyclic fluorinated uracils.

Synthesis of Fluorinated Piperidines Nitrogen heterocycles, including pyrrolidines and piperidines, constitute structural features of many natural products and drug candidates, and are now readily accessible through R C M reactions (55). Consequently, several fluorinated nitrogen heterocycles have been prepared by means of R C M processes, although with several limitations. Thus, when Brown et al. tried to prepare both pyrrolidines and piperidines starting from fluoroolefins 45 (Figure 8), the approach was effective only in the latter case (n= 2), yielding fluorinated piperidine 46. In contrast, the allyl amine (n= 1) underwent a double bond isomerization followed by enamine hydrolysis, which led to the formation of compound 47 (34). This isomerization is probably mediated by a ruthenium hydride species formed as a result of the decomposition of catalyst 2 (55). Billard et al. employed a fluoral hemiketal as starting material for the preparation of an R C M precursor (Figure 9). Thus, allylation of compound 48 followed by imine hydrolysis and protection afforded trifluoromethylated homoallylamine 49. JV-Allylation gave diene 50, and its R C M cyclization furnished 51, which was hydrogenated and deprotected to yield racemic trifluoropipecoline hydrochloride 52 (36).


62

83%


75% 47

Figure 8. Synthesis offluorinated piperidines.

1 ) ^ / T M S

F C

N

3

Ph

2

)

H

C

1

5

>

o

0

C

F

3)CbzCl,NaHC0 48

7

— ^

3

0

/

C

C ^ N ^ Cbz

H

^

C

A

go%

i ) H 2

r ^ i r

NaH, D M F

49

o

1

f F

3

j

3 3

t F

3

c ^ N ^

91%

2

>

H

Cbz

50

p d

c

- " ; C

1

1^1 F

3

C ^ N ^ H

99%

51

, C

2

1

(±)-52

Figure 9. Synthesis of trifluoropipecoline.

More recently, Bonnet-Delpon et al. also made use of imines derived from fluoral in order to prepare an RCM precursor (Figure 10). To this end, chiral imine 53 was bis-allylated in a one-pot process to obtain diene 54 with a high degree of diastereoselectivity, although the configuration of the new stereocenter was not determined. A subsequent RCM reaction produced trifluoromethyl piperidine 55 with no loss of optical purity (57).

CF |[

3

1) ^

^

B

F

C

v

* ^ v

•

P h ^ . N .

2

) A

V

54%

s

^

F

n

x

s

n

9

54

^

8

j

J ,

*

^ J

P h ^ j

%

*Y ^

OMe >98%de

^

1

CHzCl^rt k

OMe 53

3

jf

P h ^ N V

r

Zn,CuI,DMF,rt

OMe >98%de

55

Figure 10. Synthesis of enantiomerically pure trifluoromethylated piperidines.


63


Synthesis of Fluorinated Lactams, Lactones and Aza-Lactones The synthesis of lactam rings by means of R C M reactions is quite straightforward since the acyclic precursors are readily available through acylation of secondary alkenyl amines. In this way, Haufe et al prepared fluorinated five- and six-membered lactams starting from amines 56 (Figure 11). Acylation with a-fluoroacrylic acid produced 57, and subsequent R C M with catalyst 2 afforded five- and six-membered lactams 58. The higher homologies (n= 3 and 4) failed to yield the corresponding seven- and eight-membered lactams under the same conditions (38). Similar results were obtained by Rutjes et al (Le. 59 —> 60), but the synthesis of a five-membered trifluoromethylated lactam was unsuccessful (39-40).

Bn

R = H , n= 1 n=2 n=3,4 R = M e , n= 1

79% 86% 0% 46%

58

Figure 11. Synthesis of fluorinated lactams.

We have recently shown that regioselective preparation of unsaturated fluorinated lactams containing a gew-difluoro unit in the ring system is possible by R C M and carefully choosing of the appropriate ruthenium catalyst, solvent, and reaction conditions (Figure 12). For this purpose we used dienyl amide 61, which was prepared in two steps from acid 8 (n=l,2). Treatment of 61 with catalyst 1 in refluxing CH C1 afforded exclusively the normal R C M product 62 whereas the use of catalyst 2 in toluene at reflux gave the isomeric lactams 63. It was also possible to isomerize the double bond starting from 62 under the same 2

2



64 reaction conditions. As in the case described above, this double bond isomerization was probably promoted by a ruthenium hydride species formed as a result of the decomposition of catalyst 2 at high temperatures (55). Interestingly, the other possible regioisomer was not detected due to a stereoelectronic effect exerted by the difluoromethylene group that compels the double bond to migrate exclusively towards the nitrogen atom. This observation was supported by the fact that non-fluorinated analogues produced mixtures of isomers towards both sides under the same reaction conditions. For the synthesis of a six-membered lactam, we chose a different strategy: 61 was first isomerized to 64 in the presence of a ruthenium hydride, with subsequent RCM cyclization furnishing lactam 65 (41). Fluorinated analogues of naturally occurring macrolactones can be a source of derivatives with improved biological activities, as was demonstrated with the preparation of a trifluoromethylated analogue of the antitumoral compounds epothilones. Moreover, their synthesis is usually performed by means of RCM cyclizations (Le. 66 —• 67) (Figure 13) (42). Accordingly, we decided to prepare different classes of fluorinated macrolactones (Figure 14).

O O

F

95%

N

—

/

la)(COCl)2

F

8(n=l,2)

O

N'

B N

RuHCl(COXPPh )3^ 3

PhMe, A

F - V ^ N ^ " y

^

2

F A ^ N ' *

CH Cl2,A 2

71%

Figure 12. Tandem RCM-isomerization or isomerization-RCM for the synthesis of unsaturatedfluorinated lactams.


65


o

o

Figure 13. Synthesis of a precursor of a trifluoromethylated analogue of the epothilones by ring-closing metathesis.

1) ^ T - C 0 ^

H

C

b

Z

2

H

F

75-95%

C b z H N ^ y ^ ^ ^

HO, F

2) 2 , C H C 1 , A 2

F

^

2

75-98%

Z-68

E- a n d

l)TBSOTf

NHCbz

C F

70

I

2

^

^

N

C

'

)

b

Z

C F

3

6

o.86%

" - ^ C F ,

73

n=l

74

n=3

^

n

Z-69

n=l n=3

1)TFA

3

71 n = l 72

'

N

O

13

^

/

n=3

CH C1 ,A

Vxfl^/

54-93%

O

2

2

Z-75

E- a n d

Z-76

n=l n=3

Figure 14. Synthesis of 10- and 12-memberedfluorinated lactones and aza-lactones.



66 Thus, acylation of enantiomerically pure fluorinated amino alcohol 13 followed by an RCM reaction with catalyst 2 afforded 10- and 12-membered lactones 68 and 69, respectively, both with a difluoromethylene unit. While 68 was obtained as the Z isomer only, the higher homologue 69 proved to be an equimolecular mixture of E and Z olefins. In contrast, the synthesis of the corresponding 11-membered lactone was much less effective since ringcontraction products were predominant in the reaction mixture. A different class of aza-macrolactones was prepared from trifluoromethylated P-amino alcohol 70, which was obtained in a similar manner to compound 13. Thus, hydroxyl protection and JV-alkylation with ailyl bromide or 4-pentenyl bromide led to 71 and 72, respectively, and further deprotection and acylation with 4-pentenoyl chloride afforded 73 and 74. Finally, an RCM reaction in the presence of catalyst 2 produced 10- and 12-membered trifluorometylated aza-lactones 75 and 76, the latter as a 3:1 mixture of E and Z isomers (43).

Conclusions In summary, metathesis reactions have been used quite intensively in the last few years for the preparation of organofluorinated nitrogen-containg compounds. These methods have made possible the preparation of numerous systems that otherwise would have been much more difficult to obtain. While the vast majority of examples of the application of metathesis reactions correspond to RCM, we also believe that CM reactions will follow in their development in the immediate future.

Acknowledgements The authors wish to thank the Ministerio de Educacion y Ciencia (BQU2003-01610) and the Generalitat Valenciana of Spain (GR03-193 and GV05/079) for financial support. C. P. thanks the MEC for a Ram6n y Cajal contract and J. L. A. thanks the CIPF for a postdoctoral fellowship.

References 1.

Organo-Fluorine Compounds, Houben-Weyl vol. E10 a-c; Baasner, B.; Hagemann, H.; Tatlow, J. C.; Eds.; Georg Thieme Verlag KG: Stuttgart-New York, 2000.


67 2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18. 19. 20. 21. 22. 23.

24.

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