Fluorine in Heterocyclic Chemistry Volume 1 · Fluorine in Heterocyclic Chemistry Volume 1 5-Membered Heterocycles and Macrocycles . ... The reaction of N-CH 2 CF 3 pyridinium salt

Valentine Nenajdenko Editor

Fluorine in Heterocyclic Chemistry Volume 1

5-Membered Heterocycles and Macrocycles

Volume 1: ISBN 978-3-319-04345-6 ISBN 978-3-319-04346-3 (eBook)Volume 2: ISBN 978-3-319-04434-7 ISBN 978-3-319-04435-4 (eBook)Set ISBN 978-3-319-06036-1 DOI 10.1007/978-3-319-04346-3DOI 10.1007/978-3-319-04435-4 Springer Cham Heidelberg New York Dordrecht London

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Editor Valentine Nenajdenko Department of Chemistry Moscow State University Moscow, Russia

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157V. Nenajdenko (ed.), Fluorine in Heterocyclic Chemistry Volume 1: 5-Membered Heterocycles and Macrocycles, DOI 10.1007/978-3-319-04346-3_4,© Springer International Publishing Switzerland 2014

Abstract The chapter is devoted to the synthesis and application of indolizines bearing fl uorine atoms, perfl uorinated alkyl (aryl) groups, and COCF 3 fragments.

Keywords Indolizine • Fluorine • Trifl uoromethyl group • Synthesis • Fluorinated heterocycles

Fluorinated Indolizines

Eugene V. Babaev

E. V. Babaev (*) Department of Chemistry , Moscow State University , Leninskie Gory , Moscow 119992 , Russian Federation

Moscow Institute of Physics and Technology , Institutskii per. 9 , 141700 Dolgoprudny, Moscow Region , Russia e-mail: [email protected]

Contents

1 Introduction ........................................................................................................................ 1582 Synthesis of Indolizines with Substituent in Pyrrole Fragment ......................................... 158

2.1 Indolizines with Substituent at Position 3 ................................................................. 1582.2 Indolizines with Substituent at Position 1 ................................................................. 1652.3 Indolizines with Substituent at Position 2 ................................................................. 168

3 Synthesis of Indolizines with Substituent in Pyridine Ring............................................... 1723.1 Indolizines with Substituent at Position 6 and 8 ....................................................... 1723.2 Indolizines with Substituent at Position 7 ................................................................. 1763.3 Indolizines with Substituent at Position 5 ................................................................. 177

4 Conclusion ......................................................................................................................... 177References ................................................................................................................................ 178

mailto:[email protected]

158

1 Introduction

Fluorinated heterocycles have received increasing attention due to their important biological properties. Considerable efforts have been paid to the exploitation of new synthetic routes to these fl uorinated compounds. Indolizine is an important funda-mental ring system in view of its similarity to indole. This heterocycle occurs com-monly as a fully reduced form in natural products. Owing to the increasing importance of fl uorine containing heterocycles in biology, pharmacology, and industrial application, synthesis of fl uorine-containing indolizines became of con-siderable interest. In spite of existence of numerous reviews on the chemistry of indolizines [ 1 ] no attention have been paid to its fl uorinated derivatives. In fact, this area is relatively young (the fi rst research paper on this topic appeared 30 years ago). In spite of many efforts, up to now no indolizines with perfl uorinated groups appeared on the market.

The review is organized in the following way. First, indolizines with substituents at pyrrole fragment are covered. This includes indolizines substituted at positions 3 and 1 (since these positions are most easily substituted), and then 2-substituted derivatives. Then, indolizines substituted at pyridine ring are covered: structures with 6(8)-perfl uorinated groups are reviewed, and fi nally, 7- and 5-derivatives are discussed. Major attention is paid to indolizines; benzo-derivatives are also included.

2 Synthesis of Indolizines with Substituent in Pyrrole Fragment

2.1 Indolizines with Substituent at Position 3

Although the position 3 in the indolizine ring is the most reactive toward electro-philic attack, no direct fl uorination of indolizines have been reported. Instead, in the recent work [ 2 ] 1,3-dipolar cycloaddition was studied to difl uoro-substituted pyri-dinium ylides.

N+

CF F

X

N

F

XY

YN

X

Y

Y

N

XY

Y

F

CF2Br2/Pb/Bu4NBr

CH2Cl2/ultrasound

X = CN, CO2Et, COPh

Y = CO2Me, CN

MnO2

:CF2

1

E.V. Babaev

159

Pyridine Dipolarophile Yield (%)

4-COPh CO 2 Me 18 ( 1a ) 4-COPh CN 31 ( 1b ) 4-CO 2 Et CO 2 Me 23 ( 1c ) 4-CO 2 Et CN 28 ( 1d ) 4-CN CO 2 Me 48 ( 1e ) 4-CN CN 43 ( 1f ) 3-CN CO 2 Me 16 ( 1g:1h ) (1:1)

Difl uoromethylides were prepared from 4-cyano, 4-benzoyl- and 4-ethoxycarbonyl- substituted pyridines under difl uorocarbene generation condi-tions (ultrasound, CF 2 Br 2 /Pb/Bu 4 NBr) and trapped with dimethyl maleate or fuma-ronitrile. 3-Fluoroindolizines were isolated as fi nal products of the reaction which involves dehydrofl uorination of the primary cycloadducts followed by dehydroge-nation by active MnO 2 .

These ylides where shown to dissociate to carbene and pyridine with low activation barrier. The equilibrium constant of the reaction increases with increasing electron- withdrawing ability of substituents in the pyridine ring. There was no reaction with unsubstituted pyridine or picolinic acid nitrile. In the reaction of nicotinic acid nitrile with the fumaronitrile a mixture of the regioisomeric products was formed:

N

FNC

CN

CN

N+

NC

F

F

N

F

CN

CN

CN

+ 1g

1h

C

The reaction of N-CH 2 CF 3 pyridinium salt with dimethyl acetylenedicarboxylate or perfl uorobut-2-yne in the presence of base (Et 3 N or K 2 CO 3 ) [ 3 ] has provided fi rst formation of indolizines 2 with CF 3 -group in position 3.

N

CF3

X

XN+

CF3

X X

N Et3Nor K2CO3

TfOCH2CF3

90%

X = CO2Me (11%); CF3 (12%)

2TfO-


160

Similarly, the reaction of ylide formed from N-CH 2 COCF 3 pyridinium salt and dimethyl acetylenedicarboxylate or perfl uorobut-2-yne gave rise to 3-COCF 3 - indolizines 3 [ 4 ].

N

X

X

OCF3

N+

CH

O CF3

X X

X = CO2Me (70%)

X = CF3 (20%)

3

The same methodology was applied to N-CH 2 COF 3 γ,γ’-bipyridinium salt lead-ing to intermediate indolizine 4 or fi nal aminosugar 5 with 3-COCF 3 group [ 5 ].

N

O CF3

CO2ArR

N+

O CF3

R

Br

O

OArNH2-Sugar

O

NH-Sugar

R

N

OCF3

ON

Sugar

Et3N/DMF/2h

4 (42%)

DMF/12h7%

DMF/12h20%

5R= γ-Py

It should be mentioned that isoquinoline 2-oxide reacted with perfl uorobut-2-yne similarly forming (among other products) 1,2,3-tris(trifl uoromethyl)- and 1,2-bis(trifl uoromethy1)-3-trifl uoroacethylpyrrolo[2,1-a]isoquinoline [ 6 ].

N

CF3

CF3

OCF3

N+

O

CF3 CF3

N

CF3

CF3

CF3+

traces

Finally, 3-COC 6 F 5 indolizine 6 was obtained together with more saturated prod-uct 7 by cycloaddition of the corresponding ylide and methyl acrylate [ 7 ].

E.V. Babaev

161

N

O

CO2Me CO2Me

N+

O C6F5 C6F5C6F5Br

O

OMe

N

O

Et3N

6 (11%)

+

7 (35%)

Meanwhile, it is much easier to prepare indolizines with perfl uoroacyl substitu-ent at position 3 by perfl uoroacylation reaction. The reaction yields are strongly depended on the basicity of the parent indolizine: thus 2-phenylindolizine under-went trifl uoroacetylation to form 8 in 36 %; the rest (60 % after regeneration) was indolizinium cation formed by protonation of starting material [ 8 ].

NPh N

Ph

COCF3

N+ Ph+(CF3CO)2O

CF3CO2-

8

Nitroindolizines having 6- and 8-nitro group in the same reaction led to trifl uo-roacetyl derivatives 9 – 12 in quantitative yield [ 9 ]. It was shown that their basicity is decreased.

NR

R'

X

YN

R

COCF3

R'

X

Y

(CF3CO)2O

9-12

Indolizine Yield (%)

9 3-COCF 3 -6-NO 2 -2-Ph-7-Me 97 10 3-COCF 3 -6-NO 2 -2-Me 96 11 3-COCF 3 -8-NO 2 -2-Me 100 12 3-COCF 3 -8-NO 2 -2-Ph 100

5-Bromoindolizine underwent trifl uoroacetylation selectively at position 3 lead-ing to compound 13 ; the yield was 83 % [ 10 ]. 2-Carbomethoxyindolizine is trans-formed to 3-COCF 3 derivative 14 in 85 % yield [ 11 ].

N

Br

N

Br COCF3

(CF3CO)2O

13


162

N

O

OMeN

O

OMe

COCF3

(CF3CO)2O

14

There is only one example of trifl uoroacylation in the series of benzoindolizines. 3-Perfl uoroacyl derivatives 15 and 16 were obtained in high yield [ 12 ] using trifl uo-roacetic and perfl uorosuccinic anhydride as acylating agents.

NAr'

Ar

OMe

MeO

NAr'

Ar

OMe

MeO

COCF3

N

Ar'Ar

MeO

MeOO

FF F

F

OOH

OO

O

FFF

F

(CF3CO)2O

16 (95%)15 (73%)

Ar=3,4-(MeO)2C6H3

Ar'=4-ClC6H4

Pyrrolo[1,2-a]pyrazinum (7-azaindolizinium) cations may underwent ring open-ing and transformation of the pyrazinium fragment under the action of MeNH 2 . The rearrangement is known as Kost-Sagitullin (enamine) rearrangement.

N+

N

RMeNH2

N

N

R

MeNH

NH

N

R

MeNH

HN

NHR

On the other hand, 7-azaindolizinium cations with COCF 3 group may be involved in haloformic recyclization leading to oxo-derivatives of pyrrolo[1,2-a]pyrazines.

Here the NHMe-amino group was originated from the reagent.

N

N

O

R

O

N+

N

R

F3COC

MeNH2

NN

MeN

R

HO CF3

MeN

N

R

F3COC

NH

- MeNH2

- CHF3

E.V. Babaev

163

At lower temperature the product 17 of enamine rearrangement to pyridine ring predominated, whereas at higher temperature (and in water solution) the ring trans-formation occurred with haloformic reaction to form 18 [ 13 ].

MeN

N

R

COCF3

HN

N

N

R

F3COC

MeNH

N

F3COC

NHMe

R

N

N

O R

O

N+

N

R

F3COC

MeNH217a-c

18a,b

№ R

17:18

30 °C 170 °C

a Et 10:39 45:6 b n-Pr 15:50 44:10 c PhCH 2 60:0 70:0

Electrophilic nature of 3-COCF 3 group is displayed by the reaction of indolizine 14 with hydrazine forming pyridazinone derivative 19 in the yield 79 % [ 11 ].

N

O

OMeCOCF3

N

O

NN

CF3

N2H4

14 19

3-Trifuoroacetyl indolizines underwent removal of 3-COCF 3 group in good yield by the reaction with cold alkali [ 9 , 4 , 12 ].

NPh

NPh

COCF3

N

X

X

COCF3

N

X

X

NAr'

Ar

OMe

MeO

NAr'

Ar

OMe

MeO

COCF3

X=CF3

OHOHOH

8 163

58% 95%83%


164

It should be mentioned that similar pyrroles, indoles and azulenes bearing COCF 3 group all reacted with alkali with haloformic removal of CHCF 3 ; the “strange” behavior of 3-COCF 3 indolizines was explained by their higher basicity and possi-bility of substitution of trifl uoroacetate ion by ipso-protonation [ 14 ].

NPh

NPh

COCF3

NPh

HOO

CF3

NPh

O OCF3

H

OH−

- CF3CO2-

+

Particularly, this statement was confi rmed by reaction of trifl uoroacetyl deriva-tives of 6- and 8-nitroindolizines with alkali. Being less basic these compounds underwent haloformic reaction to form nitroindolizine-2-carboxilic acids. The reac-tion, however, did not stop at this point and fi nalized with transformation of pyridine ring (of indolizines) to benzene ring of indoles [ 9 ].

N

COCF3

Me

O2N

PhN

Me

O2N

Ph

CO2HNH

O2NCOOH

Me

Ph

NR

COCF3

NO2

NR

NO2

COOH

NH

R

NO2

COOH

OH-OH-

OH-OH-

9 20 21

11,12 22 23,24

Indolizine Indole (acid) Yield of indole (acid), %

9 21 ( 20) 98 (67) 11 23 (22) 72 (42) 12 24 80

It should be mentioned that conversion of indolizines to indole 2-carboxylic acids proceeded in higher yields and in milder conditions (0 °C) than for indolizines without COCF 3 group. The overall mechanism is of the ANRORC type:

E.V. Babaev

165

N

R

NO2

HOOC

HN

R

O2N

HOOC

N

R

NO2-

HOOC

HON

R

HOOCHO

NO2

NR

COOH

~H

OHO2NH

H

OH-

- OH-


Position 1 of the indolizine ring is the second one (after position 3) that can be attacked by electrophiles. However, there are no examples of direct fl uorination of indolizine ring at position 1. Among the benzoindolizines there is such an example [ 15 ] where the desired F-containing scaffold 25 was obtained by use of Selectfl uor.

N

OO

Oi-Pr

OMe

MeO O-i-Pr

N

OO

Oi-Pr

OMe

MeOO-i-Pr

FN+

N+F

Cl 2BF4-

53%25

Another example of direct incorporation of perfl uorinated substituent at the posi-tion 1 is the insertion of CF 3 S group using CF 3 SCl as electrophile [ 16 ]. Reaction proceeded at positions 1 and 3 even in the case of deactivated 3-COMe indolizine quantitatively. In the case of 3-benzylindolizine substitution at C-1 is accompanied by insertion of electrophile in the benzyl fragment as well.

NR

R'

NR

SCF3

SCF3

N

SCF3

CF3S

27a-c26a-d27d

CF3SClCF3SCl

26a R=R’=H 27a 26b R=H, R’=Me 27b 26c R=Me, R’=Ac 27c 26d R=Me, R’=PhCH 2 27d


166

As it was shown recently [ 17 ], perfl uorophenyl group can be inserted at position 1 and 3 under the cross-coupling conditions. This regioselective reaction took place with pyridine, potassium phosphate, copper (I) iodide, 1,10-phenanthroline and iodine in 1,4-dioxane at 120–130 °C during 74 h.

N

CN

N

CN

FF

F

FF

F

FF

F

FF

F

F

F

F

CuI/I2

78%

Investigation of the direction of trifl uoroacetylation of 2-methylindolizine has shown minor amounts of the 1-substituted isomer formed in addition to the “usual” product of substitution in the 3 position [ 18 ].

N N

F3COC

N COCF3+

(CF3CO)2O

28 (93%) 29 (3.5%)

Na2CO3

Furthermore, 1-COCF 3 isomer is formed when 3-COCF 3 indolizine was heated in CH 3 COOH (together with 1- and 3-acetylindolizines) [ 14 ]. The reaction mecha-nism seemed to be intramolecular.

N

F3COC

N COCF3 N COMe N

MeOC

+

29 (7%)28

AcOH +

30 (13%) 26c (11%)

N

R

Rf

PO(OR')2N+

CH

R

Rf PO(OR')2

31

E.V. Babaev

167

Compound Rf R’ R Yield (%)

31a CF 3 Et CN 74 31b CF 3 Pr CN 70 31c C 2 F 5 Et CN 73 31d C 2 F 5 Pr CN 49 31e CF 3 Et CO 2 Et 47 31f CF 3 Pr CO 2 Et 51 31g C 2 F 5 Et CO 2 Et 51 1 h CF 3 Et COPh 77 31i C 2 F 5 Et COPh 65 31j C 3 F 7 Et COPh 70

In addition to direct insertion of perfl uorinated group at position 1, there are plenty of methods how to introduce such a group via cycloaddition reaction. Thus, conve-nient method to perfl uoroalkylated indolizinyl-phosphonates 31 was reported [ 19 ]. The reaction proceeded regioselectively via the 1,3-cycloaddition of pyridinium N-ylide and perfl uoroalkynyl phosphonate in 49–77 % yields. Similar reaction was proposed to obtain pyrrolo[1,2-a]isoquinolinyl phosphonates in 48–78 % yields [ 20 ].

R

PO(OR')2

Rf

N

CH

R

N+

Rf PO(OR')2

In the reaction of 2-bromo-3,3,3-trifl uoropropene with pyridinium ylides cyclo-addition occurred readily leading to 1-CF 3 derivatives of indolizines [ 21 ]. Similar reaction took place in the cases of pyridazinium and isoquinolinium ylides.

N

R'CF3

R

N+

R

R'

Br

FFF

N

CF3

R

N+

R

Br

FFF

CF3

R

NN

R'

RN+

NR'

Br

FF

F

base

32

base base


168

R R’ Yield (%) R R’ Yield (%)

COPh H 40 COPh Me 49 COMe H 24 COMe Me 27 CO 2 Et H 35

The reaction of 3-COCF 3 pyridinium ylide with 1,1,1-trifl uoropropyne pro-ceeded similarly giving 1-CF 3 indolizine in low yield [ 4 ]. Similarly behaved other pyridinium ylides with benzoyl and ester groups [ 22 ].

N

CF3

OR

N+

CH

O R

CF3 R = CF3 33 (4%)

Ph 34 (14%)EtO 35 (22%)

Pyridinium ylides reacted with 4-ethoxy-1,1,1-trifl uorobut-3-en-2-one to give the corresponding 1-trifl uoroacetyl-substituted indolizines [ 23 ]. Isoquinolinium ylides behaved similarly.

N

R

COCF3N+

CHR

OMe

CF3 O

N+

CHR

N

R

COCF3

OMe

CF3 O

36

70-75%

R Yield (%)

COPh 76 % CN 74 % CO 2 Et R=CO 2 Et/Me


2-Fluoroindolizines are easily available via cycloaddition of fl uorinated vinyl tosyl-ates and pyridinium ylides [ 24 ]. Using β-substituted pyridinium ylides both isomers (6 and 8) were formed with clear predominance of 8-isomers. One product with 3-CO 2 Et group was recently patented as the intermediate [ 25 ]. The reaction also proceeded with isoquinolinium and benzimidazolium ylides.

N

FR

R'

R"

N+

R

R'

R" F

F

OTos

N

F R

R'

R"

Et3N/K2CO3

DMF/70 �C

+

37

E.V. Babaev

169

R R’ R” Yield (%) R R’ R” Yield (%)

COPh H H 34 CO 2 Et H COPh 8 (1:1.5) COPh CH 3 H 23 CN H H 59 COPh H CH 3 37 (1:2) CN CH 3 H 33 COPh H Br 27 (1:1.5) CN H CH 3 67 (1:10) CO 2 Et H Br 40 (1:6) CN H Br 58 (1:1.7)

N

R

F

N+

R R

NN

R'

FR

N+N

R'

F

FOTos

base base10-68% 12-61%

Further modifi cation of the strategy was proposed; 1-chloro-2,2,2- trifl uoroethane (bp 6 °C) or 1,1,1,2-tetrafl uoroethane (bp –27 °C) gave the corresponding 2- fl uoroindolizines 38 via 1,3-dipolar cycloaddition at 80–100 °C in DMSO at atmospheric pressure in normal glassware [ 26 ]. The reaction started with the elimi-nation of HF from CF 3 CH 2 X and can be applied to isoquinolinium ylides.

NF

R

R'

N+

R

R'F

FXF

Et3N / K2CO3

38

X R, R’ Yield of 38 X R, R’ Yield of 38

Cl COPh, H 45 F COPh, H 37 Cl COPh, Me 67 F CO 2 Et, H 47 Cl CO 2 Et, H 82 F CO 2 Et, Me 28 Cl CO 2 Et, Me 76

In the presence of base, 2,2-dihydropolyfl uoroalkanoates of the type R F CF 2 CH 2 CO 2 Et reacted with N-(cyanomethyl)pyridinium ylides to give the corre-sponding indolizine derivatives carrying both a fl uoroalkyl and a cyano group [ 27 ].

N

R

CN

CO2Et

N+

R

CN

K2CO3

Cl(CF2)nCH2CO2Et(CF2)mX

39

R n m X Yield R n m X Yield

H 2 1 H 76 Me 2 1 H 90 H 4 3 Cl 70 Me 4 3 Cl 83 H 6 5 Cl 61 Me 6 5 Cl 88 H 8 7 Cl 52


170

Ethyl 2,2-dihydropoly(per)fl uoroalkanoates reacted with N-phenacylpyridinium ylides in DMF to give poly(per)fl uoroalkyl-substituted indolizines 40 and 41 [ 28 ]. Origin of the products 41 is explained by the adduct aromatization.

N

RCO2Et

COPh

N+

R

COPh

K2CO3 N

R

CO2Et

COPh

F

F

Cl(CF2)nCH2CO2Et(CF2)mX

40 (0-82%)

(CF2)kX

41 (0-63%)

+

R=H,Me,CN

N-Benzylpyridinium ylides (generated in situ from the N-benzylpyridinium bro-mide and alkali) reacted with ethyl 3-fl uoro-3-fl uoroalkyl acrylates to give one or two fl uoroalkylated indolizine derivatives through 1,3-dipolar cycloaddition fol-lowed by an oxidative aromatization or 1,3-H-shift aromatization process [ 29 ].

NCF2CF2CF2Cl

RCO2Et

X

N+

R X

NCHFCF2CF2Cl

RCO2Et

X

Cl(CF2)3CF=CHCO2Et

42 43

+

NaOH

Pyridinium salt Products and yield (%)

R=H, X=H 42a (18) 43a (14) R=H, X=OMe 42b (31) 43b (18) R=H, X=NO 2 42c (31) 43c (29) R=CO 2 Et, X=H 42d (35) 43d (8) R=Me, X=NO 42e (5) – R=CO 2 Et, X=NO 2 42f (33) –

In the presence of K 2 CO 3 and Et 3 N, pyridinium N-ylides, generated in situ from their halides, reacted with gaseous fl uoroalkenes CF 2 =CFX (X=Cl, Br) in DMF under atmospheric pressure in normal glassware at 70 °C to give the corresponding 1,2-fl uorinated indolizines. Similar results were obtained with tetrafl uoroethene in an autoclave [ 30 ].

E.V. Babaev

171

N+

R

R'

NF

R

FR'

F

F

X

F

K2CO3

Et3N 44

Alkene/X Ylide, R,R’ Yield

Cl COPh, H 66 Cl COPh, Me 64 Cl CO 2 Et, H 37 Br COPh, H 57 Br COPh, Me 77 Br CO 2 Et, H 75 F COPh, Me 32

The reaction proceeded also with quinolinium, isoquinolinium and benzimidazo-lium ylides. In the reaction of ylide obtained from β-picoline the mixture of 6- and 8-isomers was formed.

N+

CORF

F

X

FK2CO3

Et3N

K2CO3

Et3NK2CO3

Et3N

CORN

FF

N+

COPh

COPh

NF

F

NN+

COR

Ph

N

COR

Ph

N

F

F

F

F

X

F

F

F

X

F

N+

Me

COPh

NF

F

MeCOPh

F

F

X

F

NF

FMe

COPh

K2CO3

Et3N

+

X=Cl 60%, X=F 37%

1:1

Similar reaction took place for hexafl uoropropene to form 1-CF 3 indolizines. The result was similar to the early one by Banks with NCH 2 CO 2 Bu t [ 31 ], [ 32 ], NCH 2 COCF 3 [ 4 ] and NCH 2 COR [ 22 ] pyridinium salts.


172

N+

R

R'

NF

R

CF3R'

F

F

CF3

F

K2CO3

Et3N 45

R, R’ Yield of 45 (%) R Yield of 46 (%)

COPh, H 62 CO 2 Bu t 37 COPh, Me 71 COCF 3 10 CO 2 Et, H 57 COPh 12 CO 2 Et, Me 53 CO 2 Et 13

N+

R

NF

R

CF3F

F

CF3

F

46

NaH

3 Synthesis of Indolizines with Substituent in Pyridine Ring

3.1 Indolizines with Substituent at Position 6 and 8

It is diffi cult to obtain pure isomers containing fl uoro (or perfl uorinated group) at 6- or 8-position. Since the substituents cannot be inserted directly into the pyridine fragment of indolizine ring, they should already exist in the precursors of the cor-responding indolizines. However, this caused loss of regioselectivity (if β-substituted pyridinium salts were used) or necessity to use poorly available β-substituted α-picolines.

N+

R

RfN

R

Rf

N

R

Rf

N

Rf

N

Rf

For example, reaction between 3-fl uorosubstituted pyridinium N-bis-(methoxycarbonyl)methylide and methyl propiolate [ 33 ] proceeded in the yield 53 % giving predominant formation of 8-isomer ( 47a:47b = 65:35). CNDO2 calculations showed that the site selectivity can be rationalized by dipole-dipole interactions.

E.V. Babaev

173

N+

F

O

O

OMe

MeO

NF

N

F

CO2Me

CO2Me CO2Me

CO2Me

CO2Me+

47a 47b

Pd(0)53%

Another example was the copper(I)-catalyzed cycloaddition of ethyl isoprope-nyldiazoacetate to 3-F-pyridine [ 34 ]; reaction took place in the yield 60 % with clear predominance of 8-isomer ( 48a:48b = 3:1). The process represents the fi rst successful example of metal-catalyzed cyclization of a π-defi cient heterocyclic sys-tem with alkenyldiazo compounds.

NF

NF

N

F CO2Et CO2Et

N+N

O

OEt

CuBr

+

48a 48b

NCuBr

O

OEt

CuBr-

O

OEt

N CO2Et

N+

N+

N

O

OEt

CuBr

More one example is connected with generation of unsubstituted ylides [ 35 ]. As facile precursors for non stabilized pyridinium methylides N-(trimethylsilylmethyl)pyridinium trifl ates were synthesized. Cesium fl uoride induced desilylation of the precursors liberated the nonstabilized pyridinium methylides which were trapped as the cycloadducts to dimethyl acetylenedicarboxylate. Trapping of 3-CF 3 -pyridinium ylide gave the mixture of isomers ( 50a:50b = 1:5) in 53 % overall yield.

N+

F3Si

CF3

N

N

CF3

N+

CH2

CF3

CF3

CO2Me

CO2Me

CO2Me

CO2Me

CO2Me

CO2Me

CsF

50a

50b


174

In the patent literature there were two examples of preparation of indolizyl-1- acetic acid according to Chichibabin methodology: the fi rst one – 6- fl uoroindilinzine 51 [ 36 ] and the second one – 6-CF 3 -derivative [ 37 ].

NF

CO2Et

NF

CO2Et

NCF3

CO2Et

NCF3

CO2Et

NaHCO3

BrCH2COMe

26%

51

BrCH2COMe

5236%

Successful cycloisomerization of acyclic alkynyl imines to pyrroles caused an attempt to the cycloisomerization of the cyclic alkynyl imines; thus 2-alkynyl pyri-dine with CF 3 -group gave a product of cyclization, indolizine 53 [ 38 ].

NCF3

NCF3

CuCl/Et3N

73%53

2-(Pyridin-2-yl-methylene)malonates and arynes reacted to produce pyrido[1,2- a]indoles which in some cases ( 54 , 55 ) correspond to 6-fl uoroderivatives of benzo-indolizines [ 39 ].

NF

CO2Et CO2EtEtO2CEtO2C

NF

NF

N

NF

N PhSiMe3

SiMe3

OSO2CF3

OSO2CF3

39%

54

22%

55

A base-promoted conversion of ortho-trifl uoromethyl benzyl derivatives of NH-heterocycles into a respective fl uorinated isoquinolines (38–57 % isolated yields) was reported [ 40 ]. The reaction is general for the benzylated derivatives of the electron-rich NH heterocycles, particularly indoles. The outcome of the reaction

E.V. Babaev

175

could be explained by an intermediate formation of a highly reactive quinone methylide species.

N

F

R

R'N

F

F

R

R'

NFF

F

R

R'

LDA

N

FF

F

R"

LDAN

F

R"

- HF - HF

R R’ R” Yield, % R R’ R” Yield, %

H H – 38 – – H 44 Me H – 39 – – Cl 52 Me Me – 48 – – OMe 43

In several cases the reported structures of 8-fl uoro indolizine were in fact mix-tures; however, no analysis of traces of the 6-fl uoro isomer was performed. Thus, to the product of reaction of 3-fl uoropyridine, CH-acid and active quinone compound the structure of 8-F-indolizine was assigned [ 41 , 42 , 43 ]. Similarly, cycloaddition to β-CF 3 -pyridinium ylide is claimed to result in 8-CF 3 -derivative of indolizine [ 25 ].

NF

O

X

O

O

O

Cl

Cl

N N

O

O

F COX

N

+ +

N+

R

CF3

OiPr

N

CF3

CO2Me

OiPr

R

N R=C6H4CO2Me

baseMnO2

19%

A novel and effi cient preparation of 4-polyfl uoroaryl substituted pyrrolo[1,2-a]quinolines via a palladium-catalyzed reaction of 1-[2-(2,2-dibromo-ethenyl)phenyl]-1H-pyrrole with polyfl uorinated arenes was described [ 44 ]. The reaction is also applicable to obtain indoloquinolines with the same substitution pattern.


176

N

F

F

X

F

FN

BrBr

FF

F

F

X

Pd(OAc)2

Ru-Phos

X Yield, % X Yield, %

F 90 Me 68 MeO 80 CF 3 82


7-F or 7-CF 3 indolizines can be found only in the patent literature, namely com-pounds 56a [ 45 ], 56b [ 25 ], 56c , 56d [ 46 ] and 56e [ 37 ].

N

F

Op-FC6H4

N

CF3

N

CF3

CO2EtN

CF3

CN

N

CF3

CO2Et

56a 56b

56c 56d

56e

A water-accelerated palladium-catalyzed reaction of gem-dibromoolefi ns with a boronic acid via a tandem Suzuki-Miyaura coupling and direct arylation was reported [ 47 ]. One of the products, 57 , corresponded to 7-CF 3 derivative of benzoindolizine.

N

CF3

Ph

N

BrBr

CF3

Pd(OAc)2

S-Phos

B(OH)2

85%

+

57

E.V. Babaev

177


The only reaction which allowed introducing fl uorine atom at position 5 of indoli-zine ring was the Ugi-type reaction [ 48 ]. The resulting 5-fl uoroinolizine 58 was sensitive to nucleophiles giving rise to mini-library of 5-subsituted derivatives.

N

F

CN

N

F

CN

NH

NC

CHON

Nu

CN

NH

Nu

58

Indolizine (Nu) Yield, % Indolizine (Nu) Yield, %

F ( 58 ) 65 O(CH 2 ) 3 NMe 2 17 SCH 2 CO 2 Me 98 1-Morpholinyl 61 SCH 2 CO 2 H 95 NH(CH 2 ) 2 NMe 2 48 SMe 49

Finally, a palladium- and copper-catalyzed tandem N-H/C-H bond functionaliza-tion reaction of ortho-(2-chlorovinyl)bromobenzenes with indoles and pyrroles has been developed [ 49 ].

N

CF3

Cl

CF3

Br

N

N

CF3N

Cl

CF3

Br

RR'

RR"

R

R'

R

R"

+

+

50-73%

51-77%

A variety of CF 3 -containing indolo- and pyrrolo[2,1-a]isoquinolines were pre-pared in moderate to good yields via the cyclization of 1-bromo-2-(2-chloro-3,3,3-trifl uoroprop- 1-enyl)benzenes with indoles and pyrroles.

4 Conclusion

Fluorinated indolizines remained relatively rare class of compounds. In contrast to indolizines substituted at pyrrole fragment the structures having a perfl uoro- substituent in the pyridine ring are less available. This is caused by the fact that 3


178

and 1 substituted indolizines are easily available by direct insertion of fl uorine containing group. It should be mentioned that 1-, 2- and 3-substituted indolizines could be easily obtained by 1,3-dipolar cycloaddition reactions, whereas there is lack of general methods to the structures substituted in pyridine ring.

Acknowledgments This work was funded by RFBR (grant 12-03-00644-a).

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E.V. Babaev

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Fluorine in Heterocyclic Chemistry Volume 1 · Fluorine in Heterocyclic Chemistry Volume 1 5-Membered Heterocycles and Macrocycles . ... The reaction of N-CH 2 CF 3 pyridinium salt