Chapter 5.1 Five-membered ring systems: Thiophenes & Se, Te analogs

102

Chapter 5.1

Five-Membered Ring Systems: Thiophenes & Se, Te Analogs Erin T. Pelkey

Dartmouth College, Hanover, NH, USA [email protected]

5.1.1 I N T R O D U C T I O N Reports detailing the chemistry of thiophenes, benzothiophenes, and related fused heterocyclic derivatives continued in 1998. The development of synthetic methodology utilized for the preparation of novel thiophenes is principally due to the inherent electronic, physical, and electrochemical properties that the thiophene moiety can lend to novel material science related applications including conductors and non-linear optical materials. For clarity and simplicity, specific compounds will be depicted in schemes, rather than general structures. As always, the author apologizes in advance for all errors and omissions. 5.1.2 THIOPHENE RING FORMATION

The addition of elemental sulfur to a variety of four carbon units is one tactic utilized in thiophene ring synthesis. The addition of Lawesson's reagent (1) to 1,4-dicarbonyl compounds has been further exploited to synthesize numerous thiophenes including thieno[3,4-b]indole (2) <98JOC2909>, terthiophenes <98BMCL2695, 98JOC5324>, mixed copolymer oligothiophenes <98JA2798>, and dithiophenes <98CE2211>. A similar sulfur transfer reagent, thioacetamide, was utilized to synthesize thieno[3,4-b]furan 3 <98JHC71>. Lawesson's reagent (1) can also add to a,13-unsaturated ketones to give thiophenes. For example, treatment of diketone 4 with 1 and boron trifluoride gave a mixture of methyl ketone 5 and aryl ketone 6 depending on the reaction conditions <98TL9191>.

S /~OMe P'3 .S..p II S

MeO

1 O M e ~ 4

-Ar O

A r s

N

Ph

Ar

Me

2 Ar=p-OMeC6H4

3 Ar=p-OMeC6H4

1, BF3 Me~Ar

+ 5

Me~Ar 6

103 The addition of sulfur to 3-cyanoquinoline 7 in the presence of piperidine gave novel fused thiophene 8 <98JCR(S)294>. Finally, condensed thiophene 9 was prepared by treatment of the corresponding acrylic acid with thionyl chloride and a catalytic amount of pyridine <98SC2191 >. The industrial synthesis of 3-methylthiophene by the reaction of 2-methylbutanol with carbon disulfide in the presence of chromium catalysts has been discussed <98CC2541>. Me

•••/S 0

IL

COCI CI

NMe2

7 8 9 Condensation-cyclization reactions have been used to make the thiophene ring system. The condensation of 1,3-dicarbonyl compounds with thioaroylketene S,N-acetals in the presence of mercury acetate leads to the formation of the thiophene ring. For example, treatment of ketene S,N-acetal 10 with ethyl acetoacetate (11) and mercuric acetate gave thiophene 12 <98JOC5903>. Alkylation of thiophenol 13 with a-bromoketone 14 followed by cyclization in the presence of phosphorus pentoxide gave benzo[b]thiophene 15 <98JCR(S)172>. The novel fused thiophene derivative 17 was prepared by the treatment of carboxaldehyde 16 with sulfur in the presence of base <98JHC 1449>. The reaction presumably proceeds by the thiolation of the adjacent methyl group followed by condensation of the resulting thiol onto the aldehyde. The condensation of 1-1ithio-l-methoxyallene 18 with methyl isothiocyanate gave 3-methoxy-2aminothiophene 19 <98EJO253, 98TL2433>. The amino-imino tautomerization of 19 to 20 was studied by 1H NMR.

S NHAc Ph' ' j j ' ' ~ SMe

0 0 Me''jj''x~OEt

10

Hg(OAc)2

11

Me I

NHAc

CHO

~Me

Me

= ~ S Me

13

"

18

1. MeNCS 2. t-BuOK~

S

Me~lT'~r'Me S, Et3N,DMF ,,,X,,^,,N,. ,,,,,<,,.,,,N, CI o~S~o"Ph CI o~.S~o"Ph

I/LMe 15

Li

C02Et

12

Me

Br 14

h ~ P

16

~

~OMe

,,OMe

17

OMe

~

NHMe 19

NMe 20

The condensation of a-thioglycolate and related activated a-thiol-substituted compounds onto adjacent carbonyls is a classic method for the synthesis of a-substituted thiophenes. Cyclization

104 of a-chlorocarboxaldehyde 21 with ethyl thioglycolate gave 2,3-diarylthiophene 22, which was used to prepare novel anti-inflammatory compounds <98CPB279>. Similar reaction sequences were utilized to prepare a thieno[3,4-b][1,4]benzoxazine <98IJ39> and a 6-substituted benzo[b]thiophene .

MeS

~

CHO

F~

CO2HCH2SH,Et3N

MeS

FEY-s-v -0o2.

-'CI 21

22

The synthesis of a-amino- and fl-amino-substituted thiophenes can be accomplished by the intramolecular cyclization of thioglycolates or a-thioketones onto nitriles. This strategy was utilized to synthesize thieno[2,3-d]pyrimidine 23 <98H(48)1157>, a-aminothiophene 24 <98IJC399>, ~-sulfonylthiophene 25 <98JHC927>, and fl-sulfonylthiophene 26 <98JHC933>.

Ph ,,~~ EtO

~CN SACO2Et 23

PhHN~3C~~2HIt

SO2CH2R NH2

=CN R= CO2Me

24

25 R 26

Intramolecular free radical cyclizations have been used to synthesize condensed thiophenes. A free radical 5-endo-trig cyclization of o - b r o m o t h i o p h e n o l 27 gave 2,3dihydrobenzo[b]thiophene 28 <98JOC3318>. Interestingly, the same reaction with obromothiopheno129 gave a mixture of the desired product 30 and a rearranged product 31. The latter was presumably formed by an initial 6-endo-trig cyclization, ~-scission releasing a thiol radical, and 5-exo-trig cyclization by the thiol radical. An intramolecular free radical cyclization was used to synthesize fused benzo[b]thiophene 33 from 2-sulfolene 32 <98JOC4645>. Treatment of 33 with NBS and AIBN then gave o-quinodimethane precursor 3-sulfolene 34.

T B D M S OMe' ~BrR ~

Me Bu3SnH,AIBN TBDMSO.v~

fMe TBDMSO Me M( Mq

Me 27 R = H 29 R = M e

Me 28 R = H 30 R = M e

Me 31

105

gr

Cso 'u'sn",'""

02

~-

I~SO2 S"

S" 32

33

34

The synthesis of fused thiophenes has also been accomplished by intramolecular acidcatalyzed cyclizations. Treatment of acetal 35 with PPA gave benzo[b]thiophene 36 <98SC3479>. Additional fused benzo[b]thiophenes <98JCS(P1)1059, 98JHC847> were synthesized by similar acid-catalyzed cyclizations.

c, eO e

c' s,

PPA,toluene,70 ~

35

36

Several unique syntheses of the thiophene ring system have appeared. Treatment of cyclopropenethione 37 with triphenylphosphine gave thieno[3,2-b]thiophene 38 <98JOC163>. An intramolecular McMurry-type coupling of diketone 39 with TIC14 gave diol 40 which was dehydrated with p-TsOH to give thiophene 41 <98JOC4912>. This reaction was utilized to prepare a variety of sterically congested 3,4-disubstituted thiophenes.

S

PPh3

ph/~Ph

Ph~ Ph" "S"

37

O

.S..ph

OHOH ' miCl4' t ' ZnB" ut'eti" ~ S'~

O

"et~ t'BU"s' ~1-

"Ph

38

39

t-B

t-Bu

p-TsOH=

40

41

The photochemical rearrangement of dithiin 42 gave 2,5-diarylthiophene 43 <98EJO2365>. Treatment of disulfide 44 with isonitrile 45 under photochemical conditions gave fused benzo[b]thiophene 46 <98T5587>. An interesting cycloaddition of a colchicine derivative led to novel fused thienocolchicine <98EJO2451>. Finally, heating sulfoxide 47 in carbon tetrachloride gave hemiacetal 48 which was converted to dihydrothiophene 49 upon dissolution in methanol <98T11603>.

hv

SH

M e 2 N ~ N M e 2 42

Me2N

NMe2 43

106

CN

CI

hv

,

~ N

NO" ~ 44

iIf

"[ "S" v

NC 45

46

C.H2OPh

0 3, ~le

~'~'/"~i;"~ S""

cc,,

=

o.

~le

~e ~

~

O

S,,~OHH2oPh MeOl--I=

CH2OPh

II

o 47

48

49

5.1.3 T H I O P H E N E RING SUBSTITUTION The thiophene ring system can be elaborated utilizing standard electrophilic aromatic substitution reactions. Monobromination of unsubstituted a-positions of the thiophene ring readily occurs with one equivalent of NBS <98AM541, 98CC2657, 98CPB279, 98JOC5497, 98JOC6132, 98JOC7413, 98JA7643, 98JA10372, 98JA10990, 98JA13453>. The dibromination of S-oxide 50 with bromine occurs stereospecifically to give 3-sulfolene 51 as confirmed by x-ray crystallography <98H(48)227>. Reductive alkylation of electron-rich thiophenes (by treatment with aldehydes followed by PhSeH) has been utilized to prepared aalkylated thiophenes <98T9055>. Nitration of a substituted thiophene with nitric acid gave a 2nitrothiophene <98CPB279>. The Friedel-Crafts reactions of different thiophenes proceeded to give a-acylated products <98BMCL2763, 98CPB279, 98CE2211, 98JA13543, 98T5599>, while the related Friedel-Crafts reactions of benzo[b]thiophenes gave ~-acylated benzo[b]thiophenes <98JHC649, 98BMCL2527>. The novel fused thiophene, cyclohepta[cd]benzo[b]thiophene (52), was synthesized from benzo[b]thiophene using FriedelCrafts acylation chemistry, while a thrombin inhibitor was synthesized using a Friedel-Crafts acylation of a 2-arylbenzo[b]thiophene <98BMCL2527>.

,, TMS

Br2 TMS

TMS ,TMS,,

O

Br"; ",S," "Br O

50

51

52

The chemistry of thiophene 1-oxides and thiophene 1,1-dioxides continues to be explored. Oxidation of thiophenes with m-CPBA proceeds to give thiophene 1,1-dioxides <98ACS533, 98AM551, 98JOC4912, 98JOC5497>, while the reaction can be modified to give thiophene 1oxides by adding BF3-etherate <98H(47)793, 98EJO1841>. For example, treatment of thiophene 54 with m-CPBA gave thiophene 1,1-dioxide 56 <98JOC5497>, while a similar reaction of 55 with m-CPBA in the presence of BF3-etherate gave thiophene 53 <98H(47)793>. The oxidation of a 2-benzoylthiophene with dimethyldioxirane gave a 3-hydroxythiophene-1,1-

107 dioxide by addition of water to the initially formed electron-deficient thiophene 1,1-dioxide <98TL5049>.

m-CPBA T./~-~ ~BF3-ether T/,~ SMDB TBDMS SMDB R 53

m-CPBA

T~ SMDB

54 R = Br 55

Br 56

R=TBDMS

Nucleophilic addition to electron-deficient thiophenes has been reported including additionelimination reactions of nitrothiophenes with both thiols <98H(48)259, 98JHC1521> and amines <98H(48)1555, 98JCS(P2)325>. Treatment of fl-nitrothiophene 57 with benzyl chloride 58 and sodium sulfide gave thiophene 59 which was reduced with triethylsilane in TFA to give 2-ethylthiophene 60 . Addition-elimination of ~-nitrothiophene 61 with benzylamine gave tx-aminothiophene 62 which was utilized to prepare benzodiazepine 63 <98H(48)1555>. Addition-elimination of 5-bromothiophene-2-carboxaldehyde with a variety of primary and secondary amines in water has been studied <98SL383>. The oxidative nucleophilic substitution of hydrogen of 2-nitrothiophene with the anion derived from 2phenylpropanenitrile in the presence of KMnO4 was reported <98S 1631>. Finally, nucleophilic substitution of 3,4-methoxythiophene with alkylthiols was utilized to prepare 3,4bis(alkylthio)thiophenes <98SC2237>.

NO2 CI/~~CI M

CI

02

.. M

TFA,SiEt3H

S

Na2S I 57

59

60

O

NO2 CI 61

PhCH2NH2 ~ K2CO3 62

NO2 I~~.~

_=

H ~ N~ L'pho 63

Treatment of thiophenes with organolithium bases leads to ~t-deprotonation which can be utilized to prepare a-substituted thiophenes <98AC(E)652, 98AM541, 98BMCL2695, 98CC2225, 98CE1235, 98CE2211, 98JCS(P1)853, 98JCS(P1)3737, 98JOC5324>. The solidphase lithiation of a resin-bound thiophene has been reported <98SIA05>. Treatment of resinbound thiophene 64 with n-BuLi followed by TMSBr gave 65 which was lithiated and treated with dimethylacetamide to give tx-acetylthiophene 66. Cleavage from the trityl resin with TFA gave trisubstituted thiophene 67. Dilithiation of naphtho[2,3-c]thiophene (68) followed by quenching with ethyl chloroformate gave diester 69 <98T7075>. Finally, lithiation of benzo[b]thiophene 70 followed by quenching with TBDMSC1 gave 2-silylated

108 benzo[b]thiophene 71 <98JCS(P1)2559>. Directed ortho lithiation of 71 and quenching with iodomethane then led to 2,4,5-substituted benzo[b]thiophene 72.

polystyrene resin ~OIO

1. n-BuLi 2. TMSBr

. ~

O/O

SMT

64

1. n-BuLi 2. NMe2COMe

65

SMTJ~.S ~

Me

"

SM

Me

0

0

66

67

1. n-BuLi 2. CICO2Et

EtO2C

68

OCONEt2 [~ ~j S

CO2Et 69

OCONEt2 OCONEt2 1. n-BuLi [~ L 1. n-BuLi__ M e ' ~ L 2. TBDMSCI S TBDMS 2. Mel S TBDMS

70

71

72

The preparation of o~- and ~-substituted thiophenes can achieved by halogen-metal exchange with o~-halogens reacting preferentially in the presence of ~-halogens <98ACS533>. A halogenmetal exchange reaction was used to synthesize a thiophenesulfonyl chloride <98BMCL2301>. Treatment of 2,4-dibromothiophene 73 with n-BuLi followed by tributylborate gave boronic acid 74 <98JOC6643>. Palladium-catalyzed cross-coupling of 74 with a crown ether-substituted aryl iodide gave novel crown ether 75.

Me M

Br 73

1. n-BuLi Br Me 2. (n-BuO)3B ~ " Pd(0),Arl 3. HCI M~S';"" B(OH)2 74

109 Organometallic cross-coupling reactions of metallated thiophenes is a powerful method for the preparation of highly functionalized thiophenes <98AM541, 98AM551, 98CE1235, 98CE2211, 98JCS(P1)3737, 98JA7643, 98JA10990, 98JCS(P2)493, 98JO5497, 98JO6643, 98JO7413>. Highly functionalized oligomers of mixed thiophene heterocycles have been synthesized using bis-cross-coupling reactions. For example, treatment of thiophene 76 with 2,6-dibromopyridine (77) gave pyridine 78 . Cross-coupling of brominated thiophene 1,1-dioxides has been reported to be more efficient then cross-coupling the corresponding brominated thiophenes <98AM551, 98JOC5497>. Treatment of stannylated dithiophene 79 with thiophene 1,1-dioxide 81) gave terthiophene 81 <98JOC5497>.

Br'~Br ....

B(OH)2

~

Pd(PPh3)4

76

78

Br~C6H13 80

06H13~

SnBu3

Pd(AsPh3)4

06H13~ C 6 H 1 3 81

79

Finally, as seen previously, highly functionalized thiophenes can be prepared by the organometallic cross-coupling of halogenated thiophenes <98AM593, 98EJO525, 98H(48)1537, 98JA7643, 98JA10990, 98JOC5497, 98JOC7413, 98JOC8632, 98T6999>. Cross-coupling of 2-bromothiophene 84 with alkene 85 gave ketone 86 <98H(48)1537>. Peralkynylation of novel thiophene macrocycle 82 with phenylacetylene gave macrocyclic thiophene 83 <98EJO525>. Finally, the ruthenium-catalyzed coupling of thiophene with pentafluorophenylsulfonyl chloride gave a 22:6 ratio of 2-pentafluorophenylthiophene and 3-pentafluorophenylthiophene <98JFC91>.

Ph

Ph

Br Ph ---~ Pd(PhCN)2Cl2 Br

Br 82

hS p

% 83

Ph

110

OH 85

Br~-~CO2Me

~.~,,At~CO2Me

Pd(OAc)2

84

86

5.1.4 RING ANNELATION ON THIOPHENE The electron rich thiophene ring can easily be annelated by intramolecular Friedel-Crafts acylation chemistry to give fused thiophenes including <98JOC7107, 98JHC1429>. Treatment of dibenzo[b]thiophene 87 with triflate 88 gave fused dibenzo[b]thiophene 89 <98JOC6132>. An intramolecular annelation was utilized to synthesize a thiophene-substituted analogue of podophyllotoxin. Thus, treatment of thiophene 90 with SnC14 gave thienolignanolide precursor 91 <98TL2001 >.

..~_~OTf NMe2

.o

o

87

89

~S S~~C) HO I

SnCI4

S S ~ ,\

0

-

0

MeO'~OMe OMe

MeO"~OMe OMe

90

91

An intramolecular palladium-catalyzed cross coupling reaction for the synthesis of a fused thiophene was reported. Treatment of vinylogous enamide 92 with palladium acetate proceeds to give fused thiophene 93 by an intramolecular Heck reaction <98SC 1839>.

0 ~~

0 Pd(OAc)2

I1~.S

COPh 92

COPh 93

111 A reductive ring expansion of oxime 94 with DIBAL gave azepine 95 <98H(48)919>. Treatment of carbazonate 96 with thionyl chloride gave thieno[2,3-d][1,2,3]thiadiazole 97 <98JCS (P 1)853>.

N(OH)

H DIBAL

94

95

MeO2C~ SOCI2 ~,S~"~N- NHCO2Et 96

97

A variety of cycloaddition reactions onto the thiophene ring system have been reported. The 1,3-dipolar cycloaddition between 3-nitrobenzo[b]thiophene 98 with mtinchnone 99 gave benzothieno[2,3-c]pyrrole 101 by the loss of HNO2 and CO2 from the initial dipolar cycloadduct 100 <98SL1061>. Treatment of hydrazonoyl chloride 102 with silver carbonate generated intermediate nitrilimine 103 which underwent an intramolecular 1,3-dipolar cycloaddition to give pyrazoline 104 <98JCS(P1)4103>. The Diels-Alder cycloaddition of vinylthiophene 105 with DMAD gave benzo[b]thiophene 106 after loss of acetylene from the initial cycloadduct <98SC2531>.

P

=JJ

~NO2

Ph CH2Ph

N~CH2Phl

99

-CO2

S 101

100

98

PhHN,,. /(31

N-N o

Ph~ e

Ag2CO3,_

Ph N. \0

102

103

Ph

104

0

112

I~h [~

+P.hI DMAD toluene,~ MeO2C S"~ CO2Me

S~1 105

106

Finally, an interesting electrocyclization was used in the synthesis of a thieno[2,3f]benzo[cd]indol-4-one . Treatment of thiophene-3-acetonitrile 108 and benzoic acid 107 with LDA led to intermediate cz-lithiated acetonitrile 110 and benzyne 109 which underwent an intermolecular electrocyclization to give 111. Intramolecular cyclization of aryllithium 111 onto the pendant nitrile gave imine 112 which cyclized to lactam 113 after workup.

C02H

?N

C02Li LiN~C

OMe

OMe

107

108

CO2Li ~N

109

110

Li OMe S

111

OMe S

112

113

5.1.5 THIOPHENES AS INTERMEDIATES The thiophene ring is often used to prepare more complex molecules taking advantage of the special reactivity of thiophenes and the ease of removing sulfur and sulfur dioxide. Ring opening reactions of thiophene-1,1-dioxides with amines has been utilized to prepare 1,3-dienes <98ACS533, 98T1817, 98T9529>. For example, treatment of thiophene-l,l-dioxide 114 with pyrrolidine 115 gave 1,3-diene 116 <98T1817>. Novel ring opening reactions of oxosubstituted thiophenes were used to prepare 2,1-benzisothiazole 117 <98TL3311> and pyrrolo[3,2-c]pyridine 118 <98TL4511>. Finally, addition of n-BuLi to thieno[3,2-d]thiazole 119 gave the ring-opened adduct 12t) which was converted into thiazole 121 by treatment with N-chlorosuccinimide followed by oxidation with m-CPBA <98SL407>.

Br

114

~ CH20H 115 =

X

.

116

I

N

~

r

113

I~lMe2

Ph

CN Ph 118

117

CI

3~C, Me

n-BuU=

/~'~i' Me

119

II\\N'-~~--~C'

2."NCSm-cPBA= Me/%s/~ Sn'Bu

u

120

121

The extrusion of sulfur dioxide from 3-sulfolenes has been utilized to generate either butadienes or o-quinodimethanes which are useful intermediates for cycloaddition chemistry. Thiophene 1,1-dioxide 122 was used to prepare fused indole 127 <98JO4645>. Treatment of 122 with aniline 123 gave 2-sulfolene 124. Intramolecular free-radical cyclization of 124 with Bu3SnH and AIBN gave indoline 125. Protection of 126 with tosyl chloride gave 126 which was brominated with NBS followed by dehydrobromination to give fused indole 127. Treatment of dithiane 128 with phosphorus ylide 129 gave tetrathiafulvene 130 <98AM330>. The preparation of terthiophene 131 was accomplished utilizing cross-coupling chemistry <98S 1372>. The o-quinodimethane derived from 131 was treated with C60 to give fullerene cycloadducts.

Br.

Br

I

QO I

9 H

122

123

&

124

,.~SO2 N" H 125

NBS,AIBN

TsCI,Et3N ~N

SO2

"~SO2 Ts

TS 127

126

(~) e H ~CO2Me + Ph3P'-~SLCO2M e 128

02 Bu3SnH" AIBN

129

_...S ;o

s

~S 130

S~._/C02Me S

CO2Me

114 Heating pyrazole sulfolene 132 with N-phenylmaleimide in 1,2,4-trichlorobenzene led to cycloadduct 133 <98S1331>. An intramolecular cycloaddition of the o-quinodimethane derived from 3-sulfolene 134 gave isoquinoline 135 <98T12609>. Finally, lithiation of benzo[b]furan 136 with LiHMDS followed by quenching with iodomethane gave predominantly monomethylated product 137 <98JOC4645>. Heating 137 with DMAD in toluene (sealed tube) led to dibenzofuran 138 after extrusion of sulfur dioxide, cycloaddition of the resulting oquinodimethane intermediate, and oxidation of the adduct with DDQ.

.~

NHAc

N"NNI NHAc

s02

0

x /,

1,2,4-trichlorobenzene, A 131

02 O 132

Ph

O

133

210 ~

134

O 136

2. Mel

135

O 137

2. DDQ

O

CO2Me

138

Finally, cycloadditions of thiophene 1-oxide and thiophene-1,1-dioxides has been utilized to prepare aromatic compounds by the extrusion of sulfur monoxide or sulfur dioxide from the initially formed bicyclic cycloadducts. Oxidation of 2,3,4,5-tetrabromothiophene (139) with mCPBA in the presence of BF3-etherate and N-phenylmaleimide gave bicyclic intermediate sulfoxide 140 . Electrochemical extrusion of sulfur monoxide from 140 gave phthalimide 141. Oxidation of [8](2,4)metathiophenophane 142 to the corresponding sulfoxide followed by a cycloaddition with DMAD gave [8]metacyclophane 143 after extrusion of sulfur monoxide <98EJO 1841>. An additional report of the cycloaddition of thiophene 1-oxides to bicyclic sulfone cycloadducts has appeared <98H(47)793>. Finally, the sterically congested bisneopentylbenzene (147) was prepared by a cycloaddition of a thiophene-l,l-dioxide <98JO4912>. Thus, treatment of thiophene-l,l-dioxide (144) with vinyl sulfone (145) gave cycloadduct 146 which readily loses sulfinic acid and sulfur dioxide to give 147.

115

O NPh Br

Br

Br B r ~

O

Br~Br

m-CPBA BF3-ether

O

electrochemical extrusion of SO [ ~ NPh ~

NPh O

Br" "1~ ~ Br "

139

140

141

gr

BF

~/-C::/:

m-CPBA BF3-ether 142

e

143

~'~'SO2Ph

02 -PhSO2H

145

9

R"

144

5.1.6

O

-SO2

SO2Ph 146

147

INTERESTING THIOPHENE DERIVATIVES

A large number of biologically active thiophene derivatives have been synthesized and evaluated <98JMC699, 98CPB279, 98BMCL2527>. A new synthesis of kahweofuran (148), one of the flavor components in coffee, was recently reported <98JCR(S)74>. Spermine-related derivative 149 is a polyamine inverse agonist <98EJM3>. Interestingly, terthiophene 150 and related derivatives show PKC inhibitory activity <98BMCL2695>. Naphtho[2,3-b]thiophene4,9-dione 151 showed significant cytotoxicity in leukemia cells <98BMCL2763>. Finally, thieno[2,3-d][1,3]oxazin-4-one 152 has been evaluated as human leukocyte inhibitor <98JMC1729>. ~

H

2

N

~

N

H

2

Me 148

149

0 O H C ~ C H O 150

H3COC

0

sLCOCH3 Me. 9"0 I I I MeAS L N~ S O 151

H 152

116 A large body of literature has been devoted to synthesizing fused thiophene derivatives including notably the novel boron-substituted uracil derivative, thieno[3,2-c][ 1,5,2]diazoborin3(2H)-one 153 <98JHC887>. Synthetic details of the preparation of fused thiophenes is beyond the scope of this review, but a sample of the fused thiophene compounds that have been synthesized during the last year are shown below including: thieno[3,2-d][ 1,2,3]thiadiazole 154 <98(48)259>, thiadiazine 155 <98JHC933>, thieno[b]quinolizidine 156 <98SC945>, thiaisatoic anhydride 157 <98T10789>, dithienopyridine 158 <98T9055>, thienoimidazothiazole 159 <98JHC923>, a thieno[3,4-c][1,2]thiazole <98JHC1449>, diazepines <98JCR(S)688, 98T5369>, a thieno[2,3-e]pyrimidinone <98JHC1269>, a thieno[2,3-d]pyrimdine <98JCR(S)290>, and a dihydropyrimidothienopyridazine <98T8107>.

OH

13-N-CH2Ph

I~',SI~m,'~ O H

N.-N MeO2C~

153

s'

II~.SL m,.m H

154

H N

,Ls, O

O

O~S~I~CN 155

,Ls 156

N

Ph

0 157

158

159

A variety of macrocylic thiophenes have been prepared with interesting properties including the previously mentioned [20]annulene 83 <98EJO525> and thiophene-substituted crown ethers <98JOC6643, 98CM2167>. The novel boron-bridged tetrathiaporphryrinogen 160 was prepared and might possess interesting properties due to the electron-donor/electron-acceptor characteristics <98AC(E)652>. Thiophene-substituted expanded porphyrin 161 was prepared using an acid-catalyzed cyclization <98TL1961>. Finally, a synthesis of the impressive thiophene-containing porphyrin 162 appeared <98JOC5080>.

I~(i-Pr)2 ~'a tr-a ~,'%.~,,~ p h,c,x'~')--t'~Y~,,ph (I-Pr)2NBS S.. B.N(/-Pr)2 ?IN" " / ~

160

161

AI / # - ~ ~ S

- ~

A ~,"~",~,,~/ :-/.~k__

162

Finally, novel thiophene-containing compounds with a variety of different structures have been studied. A variety of tetrahydrothiophenes have been prepared <98JOC1368, 98JOC4532, 98TL6877> including those to be used as base replacements for the tetrahydrofuran rings in nucleosides <98JOC4821>. The photochemical ring closure of 163 to 164 <98BSCJ1101>

117 and a variety of related ring closures <98JOC6643, 98JOC9306, 98BCSJ985, 98EJO2333, 98TL4445> have been studied.

CN

CN

o- c"o

c"

~ S ' ~ M e MdSIJ~L ~Me Ph

-" ,.._

.~---~. ,Me ~S,~;Me Mi/L,,SL ph

163

5.1.7

164

OLIGOMERS, POLYMERS, AND SUPERCONDUCTORS

The unique electrochemical properties and relative ease associated with its chemical manipulation make the thiophene ring an ideal building block for novel oligomeric and polymeric materials. A number of syntheses of thiophene-based oligomers appeared during the last year including octithiophene 165 <98CC2743>, dodecithiophene 166 <98JA2798>, hexadecithiophene 167 , hexithiophene 168 <98JOC6832>, oligomeric vinylthiophene 169 <98JA8150>, star-shaped hexithiophene 170 <98CC2225>, pentithiophene 171 <98CC2657>, quaterthiophene S,S-dioxide 172 <98AM551, 98JOC1742>, anthradithiophene 173 <98JA664>, dithiophene 174 <98JOC5497>, additional quaterthiophenes <98CM1459, 98JOC5497>, terthiophenes <98JA10990, 98JA13453, 98JOC5324>, and a mixed thiophene-pyridine oligomer <98TL8821>. Interestingly, the asymmetrically substituted pentithiophene 171 was synthesized using solid-phase chemistry on macroporous Merrifield-type resin <98CC2657>. The use of solid-phase chemistry greatly aided in the purification of the oligomeric thiophene products obtained.

~

TMS SPO(H2C)I~~

SMT

(CH2)I~

SMT

9 " 10

165

166

~CeH17 B

n

O

2

C

C8H17 ~

?

O

. 2

Bn

.,C8H17 S

C8H17,~_ S C8H17 168

167

C6H13., C6H13

C6H13 C6H13

~6H13C6H13 169

170

CsH17

S

118

MeO2C

~C8H17 S/~

S

_j,C8H17 T B D M S ~ T B D M S

171

172

173

174

Oligomeric thiophenes with push-pull substitution have been prepared including porphyrin 162 <98JOC5080>, dithiophene 175 <98T8469>, and dithiophene 176 <98CC1739>. Thiophenes are often useful building blocks for novel conducting materials <98CE1235, 98AM541> including fused thiophene 177 <98JA2206>.

S N

S

CH2OH

NO2 Bu2N~ ~ ~ ~ ~ T ~ C N CN

175

176

177

Finally, methods for the preparation of interesting thiophene-based polymers have been described during the past year including blue-light emitting polythiophene 178 <98AM593>, boron-substituted polymer 179 <98CC963>, nucleobase-substituted polythiophene 180 <98AM324>, and amphiphilic polythiophene 181 <98JA7643>.

o ~ 178

(

~

1

" n

179

0

NI~~~-.,.~ O

| | CO2NMe4

0 ~

S

S 012H25

180

181

119 5.1.8

SELENOPHENES AND TELLUROPHENES

A handful reports on selenophene related chemistry appeared during the past year. The synthesis of selenophene-l-oxide 182 involved oxidizing the corresponding selenophene with dimethyl dioxirane <98JA12351>. The thermolysis of selenophene-l,l-dioxide 183 was investigated. The preparation of selenophene-3-carboxaldehyde 184 involved an interesting cycloaddition-cycloreversion sequence of a vinylogous selenium amide <98SC301, 98T2545>. The novel tetrathiafulvene 185 was prepared <98JOC8865>. Finally, treatment of furan 186 with selenium transfer reagent 187 gave selenolo[3,4-b]furan 188 <98JHC71>.

t-B~.~ ~ O

Phk.~~Ph t-Bu

Me2N/~CHO

P,, ..ou%.Ph

182

"Se -CH(OEt)2

183

185

Se EtA NEt2

O

187

h.~Ar p

184

.....

Br

Ar h~~e

--

P

186

188

Finally, there were a couple of reports involving tellurophene chemistry during the past year. Treatment of alkyne 189 with t-BuLi followed by quenching with tellurium powder gave benzotelluro[3,2-b]benzotellurophene 190 <98JHC725>. In general, ortho-lithiated arylalkynes are good sources for benzo[b]tellurophenes (tellurium powder), benzo[b]selenophenes (selenium powder), and benzo[b]thiophenes (sulfur powder) <98JHC725>. Lastly, treatment of acid chloride 191 with sodium hydrotelluride gave tellurophthalide 192 <98JHC 165>.

/Te~

1. t-BuLi 2. Tepowder

189

190 _Ph

NaHTe

P~T

h I e

0

191

192

120

5.1.9

REFERENCES

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98JMC1729 98JO163 98JO1368 98JO1742 98JO2909 98JO3105 98JO3318 98JO3346 98JO4532 98JO4645 98JO4821 98JO4912 98JO5059 98JO5080 98JO5324 98JO5497 98JO5903 98JO6086 98JO6132 98JO6643 98JO6891 98JO7107 98JO7413 98JO8310 98JO8632 98JO8865 98JO9306 98JO9828 98S1331 98S1372 98S1476 98S1631 98SC301 98SC713 98SC949 98SC1839 98SC2191 98SC2237 98SC2531 98SC3479 98SL383 98SL405 98SL407 98SL1061 98T1817 98T2545 98T5587 98T5599

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123

98T6999 98T7075 98T7431 98T8107 98T8469 98T9055 98T9401 98T9529 98T10789 98T11603 98T12609 98TA2563 98TA2567 98TL1961 98TL2001 98TL2433 98TL3311 98TL4445 98TL4581 98TL5049 98TL5369 98TL6877 98TL8821 98TL9191

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