Five-Membered Ring Systems: Thiophenes & Se, Te Analogs

87

Chapter 5.1 Five-Membered Ring Systems: Thiophenes & Se, Te Analogs Erin T. Pelkey

Stanford University, Stanford, CA, USA email: [email protected]

5.1.1 INTRODUCTION Reports detailing the chemistry and syntheses of thtophenes, benzo[b]thiophenes, and related ring systems that have appeared during the past year (Jan-Dec 2000) are the primary focus of this review. Different aspects of chemistry that involve thiophenes have been reviewed during the past year <00AM481, 00BCSJ1, 00CR2537, 00CSR109, 00PAC1645>. 5.1.2 THIOPHENE RING SYNTHESIS One general strategy for preparing the thiophene ring system is to add sulfur to activated four carbon units. For example, treatment of the zirconocene-based oligomer 1 with sulfur chloride gave the thiophene-based oligomer 2 by replacement of the zirconium moieties with sulfur <00AC2870>. The synthesis of a thieno[3,4-c]thiophene involved the addition of sulfuryl chloride to a 3,4-dicyanomethylthiophene <00TL8843>. The central thiophene ring of the structurally interesting thiahelicene 4 was prepared by dilithiation of 3 with LDA followed by double displacement of bis(phenylsulfonyl) sulfide <00AC4481>. Additional examples of thiophene ring synthesis involving the treatment of 1,4-dicarbonyl compounds with Lawesson's reagent appeared including a synthesis of a terthiophene <00JMAT107> and a novel fluorophore <00CC939>.

$2CI2 1

2

Br ~ T M S TMS~ ~ 3

s~TMS 1.LDA,ether ~ r~T 2.(PhSO2)2S Br MS 4

One of the most common strategies for the preparation of thiophenes involves the intramolecular condensation of ~-thioglycolates (and related a-substituted thiols) onto adjacent carbonyls. One prominent example involved the synthesis of naturally occurring

E.T. Pelkey

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anthrathiophene pigment 7 <00OL2351>. A novel addition-elimination of tosylate 5 with methyl thioglycolate gave 6, which was treated with sodium methoxide to effect an intramolecular cyclization giving 7. This synthesis helped ensure that the correct structure 7 had been assigned to the natuaral product. The preparation of the peri-substituted thieno[2,3b]thiophene 10 was also achieved using this type of condensation with diester 9 <00SC1695>. The latter was prepared by treating 8 with carbon disulfide and ethyl bromoacetate. Related reaction sequences have been utilized to prepare a variety of thiophenes including benzo[b]thiophene-2-carboxylates <00TIA973, 00TL5415>, a 2-acetylbenzo[b]thiophene <00H(53)1175>, 2-trifluoromethyl-thiophenes <00S1078>, and benzothiazole-substituted thiophenes <00HC94>. A related cyclization reaction of thioacetamides with activated bromides was used to prepare cyanovinyl-substituted thiophenes <00EJOC1327>, thiopheneot-carboxylates, <00EJOC3273> and a-amine-substituted thiophenes <00JCS(P1)4316>.

C.O2Me O

OTs

OH O

OH

HS~CO2Me

0 LS

OH O

5

Ar Ar KF, CS 2 j ~ v ~ O grCH2eO2Et 0 ' 8

Me02C"~----S

OH

6

7

AF~AF '/~r , ~]S~OS O NaOEt, I EtO2C CO2Et

Ar

Ar

Et02C

9

C02 10

The condensation of activated thiols onto adjacent nitriles is a common method for the preparation of amine-substituted thiophenes. A three component condensation was utilized to prepare ct-aminothiophene 11 <00TL1597>. An alternate method for preparing aminosubstituted thiophenes involved the treatment of ketene S,N-acetal 12 with an activated carbonyl compound 13 which gave thiophene 14 <00JOC3690>. This type of reaction has also been utilized to prepare building blocks for the synthesis of fused thiophenes <00JHC363>. O

CO2Etmorpholine M e \ 9 O2Et " ~ O I~CN EtOH, A/7~ MeO/ $8 MeO" "S~ NH2 11

jJ~P(O)(OEt)2 NHMe S NHMe 13 h ~ ph~.,,~SMe Hg(OAc)2" P P(O)(OEt)2 12 14

Another method of thiophene synthesis involves either acid- or base-mediated cyclization of acyclic prcecursors. An acid-catalyzed cyclization was utilized to prepare benzo[b]thiophenes that were evaluated as retinoic acid receptor (x agonists <00JMC2929>. Treatment of alkynyl sulfide 15 with potassium tert-butoxide gave 2,3-dihydrothiophene 16, perhaps via a 5-endodig cyclization of a terminal alkyne intermediate arising from a rearrangement <00TL5637>. Treatment of thiol 17 with sodium hydride gave 2-fluoro-4,5-dihydrothiophene 18 by a 5-endo-

Five-Membered Ring Systems: Thiophene& Se, Te, Analogs

89

trig cyclization of the resulting sulfide onto a gem-difluoroalkene <00CC1887>. An important new thiophene synthesis involved the palladium-mediated cyclization of enyne thiol 19 giving thiophene 20 <00OL351>. Importantly, this reaction occurs under neutral conditions, while similar thiophene ring forming reactions are usually performed under strongly basic conditions (vide infra). A similar cyclization of alkynyl thiols mediated by group VI metals (Cr, Mo, W) gave dihydrothiophenes <00S970>.

F•Ph

~___,~sfMe t-BuOK ~ CH3CN

FHS'J

15

16

Me __~~ PdI2'KI'DMA ~ Et ' " Et 19

Me Me

~ M NaH Ph e F" "S"

17

18

~-~H (R*O)2

20

21

hv'AIBN

(R*O)2

)~ 22

R*-OH--menthol

Radical cyclization reactions have been utilized to prepare tetrahydrothiophenes. For example, ultraviolet irradiation of thiol 21 in the presence of AIBN gave tetrahydrothiophene 22, importantly with no epimerization <00OL3757>. Radical cyclizations of 13-thioacrylates were utilized to prepare a variety of 5- and 6-membered ring sulfur heterocycles including tetrahydrothiophenes <00T3425>. A novel radical cascade reaction approach was utilized to prepared fused thiophenes. Treatment of 23 and diazonium salt 24 with base gave 26 presumably via intermediate 25 <00JOC8669>. Finally, a novel cyclization of sulfur-tethered bis-allenes gave thiophenes via a diradical intermediate <00TL2675>.

NCS+B 23

N 24

L

25

S J

26

Syntheses of thiophenes using 1,3-dipolar cycloadditions have been studied. The cycloaddition between 2-aminothioisomiinchnone 28 and arabinose-derived alkene 27 gave a mixture of dihydrothiophene diastereomers 29 and 30 <00JOC5089>. A computational-based rationale for the facial selectivity was offered. Additional research on the same reaction with 2-aminothioisomiinchnones and achiral alkynes gave either thiophenes or pyridones depending on the substituent on the thiazolium nitrogen <00T1247>. A thiophene product was obtained with a 1,3-dipolar cycloadditon of a 1,3-dithiole and alkynes <00HC434>. Theoretical calculations for [5+2] cycloadditions giving products containing tethered tetrahydrothiophenes were reported <00JOC5480>.

E.T. Pelkey

90

Ph. N

Ph "N

NII

Ph

I~

OE)

eh-(,

R*

I~

"

"[[,R.§ BO,N.. ,sNp , R* = chiral sugar M~ 27

Ph. N R* :

H

Bn ~ C O N H P h "N~ ~ " ~Ph Md

+ Bn ~ l , ph "N~S~CONHPh Mc~

29

30

28

Another approach to synthesizing thiophenes and additional heterocycles involves the extrusion of sulfur from the corresponding sulfur heterocycles. Treatment of highly fluorinated 31 with sulfur and iodine gave a small yield of thiophene 34 via intermediates dithietene 32 and 1,4-dithiin 33 <00JFC323>. Interestingly, 34 was irradiated to give Dewar thiophene 35 en route to an attempted preparation of a fluorinated tetrahedrane. Finally, an extensive study was recently reported on the synthesis and chemistry of 1,2-dithiins, useful thiophene precursors via sulfur extrusion <00JA5052>.

Rf. Rf - - .,Rf Rf= CF2CF2CI 31

Rf

2

R 32

S''Rf

---~R

Rf

33

34

' Rf~~; 35

5.1.3 THIOPHENE RING SUBSTITUTION The unsubstituted a-positions of the thiophene ring system continue to be elaborated using standard electrophilic aromatic substitution reactions including bromination (NBS) <00CC877, 00CC1631, 00CC2487, 00JA1820, 00JA6746, 00JMAT1777, 00JMC1293, 00P5681>, iodination (12, Hg II) <00AC3481, 00CC877, 00JCS(P1)1211>, and Friedel-Crafts acylation (trifluoroacetaldehyde imine, BF3) <00SL1058>. The fluorination of thiophene with gaseous SF3 has been studied using MS experiments <00JOC3920>. The treatment of thiophene 36 with chlorosulfonic acid gave thiophene-4-sulfonyl chloride 37 which was utilized to prepare biotin conjugate 38 <00CCl199>. The regioselectivity of the formylation of 3methylthiophene (39) has been studied and the highest selectivity (41/40; 46:1) was achieved using Rieche conditions (MeOCHC12, TiCI4) <00TL2749>. The hydroxymethylation of bis(thiophene) 42 with formaldehyde in the presence of diamine 43 (double Mannich reaction) gave macrocycle 44 <00JCS(P1)1877>. Finally, the synthesis and chemistry of 2silyloxythiophenes continues to be studied <00JOC2048> and has been reviewed <00CSR109, 00PAC1645>. o O

o

36

o

O ,,

O

~

O

SO2C'

37

~CF3~S-N o

H

~

HN-J( .[ ,,NH

N ~ I I ~ . / ~ . . |....k / ~

38

Five-Membered Ring Systems: Thiophene& Se, Te, Analogs

Me

Me

o42

Me

91

c.o Q o

AcOH

+

m

HH 39

40

N N

41

Ph 43

44

The synthesis and chemistry of iodonium thiophene derivatives have been studied <00AM133, 00TL5393>, for example, the preparation of 46 involved the ipso substitution of 2-stannylthiophene 45 <00CC649>. A similar ipso substitution of 2-stannylbenzo[b]thiophene 47 with tetranitromethane gave 2-nitrobenzo[b]thiophene (48) <00JOMC187>.

G 45

PhI(OH)OTs SnBu3 ' ~ 1 ~ Q @OTs

C(NO2)4, DMSO ~ [~SLSnMe3

46

47

NO2 48

Oxidation of the sulfur of thiophene to either thiophene-l-oxide (49) or thiophene-l,1dioxide (50) modifies the electronic structure (aromaticity, polarizability) and these effects have been studied using theoretical methods <00CC439, 00JMS203>. Sulfur oxidized thiophenes are susceptible to nucleophilic addition, for example, the addition of amines to 2,5disilylthiophene-l,l-dioxides has been studied <00EJOC3139>. The conversion of thiophene1-oxide 51 to the corresponding thiophene-l-imide 52 was achieved by activation with trifluoroacetic anhydride followed by the addition of tosylamide <00TL8461>. Nucleophilic addition of sodium ethoxide to 52 gave 2-ethoxythiophene 53 via a Michael addition and subsequent loss of tosylamide (Pummerer-like reaction) <00CL744>. Cycloadditions of thiophene-l-oxides with methylenecyclopropanes have been studied <00JCS(P1)2968>.

t-Bu

y

o 49

50

t-Bu 1.(0F300)20 t-Bu

t-Bu NaOEt t- Bu

p

t-

Bu

>

0

NTs

51

52

53

Nvcleophilic substitution of thiophene can also be enabled by the presence of electron withdrawing groups (e.g.,-CHO <00SC1359>,-COMe <00T7573>,-NO2 <00JCS(P1)1811>) on carbon. The regioselectivity of the addition of amine nucleophiles onto 3,5dibromothiophene-2-carboxaldehyde (54) has been studied and found to be independent of reaction conditions (para product 55 favored over ortho product 56) <00SIA59>.

. ~ Br 54

Br

Br

N3morpholine tE ~ CHO ..D, ~/'N O"v')

. ~ CHO + Br

55

56

~'---~ CHO

E.T. Pelkey

92

One of the more common methods for functionalizing the thiophene ring involves (xlithiation <00AC4481, 00CC1631, 00CM1508, 00JA1820, 00JA6746, 00JMAT1777, 00SM89, 00T3255>. The cc-cuprate formed by a-lithiation of 57 followed by treatment with copper iodide was treated with iodide 58 to give phosphonate 59 <00TL617>. Treatment of polycyclic thiophene 60 with n-butyllithium and TMEDA followed by iodomethane gave the cz-lithiation product 61 rather than the product resulting from directed ortho metalation (ortho to the methoxy group) <00SC3569>. The preparation of the novel azulene-fused thiophene 64 involved the cx-lithiation of benzo[b]thiophene 62 to prepare 2-cycloheptatrienylthiophene 63 <00JHC1363>. C6H.13

1. n-BuLi

3.

C6H.13

1. n-BuLi

O

II I~P(OEt)2

06H13

O~

57

58

S"

C6H13

MeO"

~

2. Mel

S

Me

MeO"

59

60

61 COMe

~s,f

Br

1. LDA 2. CFHF+BF,,

~

jBr

63

62

~

64

Directed ortho metalation can be utilized to regiospecifically lithiate the thiophene ring. For example, the directed lithiation of 2-amidothiophene 65 with tert-butyllithium followed by treatment with acetaldehyde gave 3-(cz-hydroxyethyl)thiophene 66 <00SL1788>. The lithiation of dicarbamate 67 has been studied <00T2985>. Interestingly, treatment of 67 with three equivalents of n-butyllithium followed by quenching with allyl bromide gave thieno[3,4d]imidazolone 68 after an unexpected, regiospecific intramolecular cyclization. O 1. tert-BuLi

OH

2. CH3CHO ,.NEt 2 ,

NHBoc 1. n-BuLi (3 equiv)

NEt 2

O 65

BocHN

O 66

L.

2. allyI-Br 67

68

Another method that is often utilized to regiospecifically functionalized the thiophene ring involves the halogen-metal exchange of halogenated thiophenes <00H(52)761, 00JA6746, 00JCS(P 1) 1211, 00JCS(P2) 1453, 00T7205>. The lithiation of 2-bromothiophene 69 followed by treatment with dichloro-diisopropylsilane gave 2-silylthiophene 70, a building block for the synthesis of oligothiophenes <00JOC352>. The lithiation of 3-bromothiophene (71) followed by treatment with bis-electrophile, N,N-dimethylcarbamyl chloride (72), gave ketone 73 <00S1253>.

Five-Membered Ring Systems: Thiophene & Se, Te, Analogs

93

0 1 n-BuLi

9

9

Br

~

2. CI2Si(i-Pr)2

1. n-B,,.~Li

2.

69

71

0

CI

72

NMe2

73

The organometallic cross-coupling of metallated thiophenes continues to be an effective technique for the preparation of highly functionalized thiophenes. Metallated thiophenes that have been utilized in cross coupling reactions include thiophene-2-borates <00CC1649, 00CC2487, 00JOC3883, 00S 1229, 00TL3197>, thiophene-3-borates <00OL3417, 00TL2185 >, thiophene-2-magnesium bromides <00P5681>, thiophene-2-stannanes <00AM668, 00AC4547, 00JA1820, 00JMAT1777, 00SL963, 00TL5521>, and thiophene-2-zincates <00CC1631, 00SM33>. A novel solid-phase synthesis of regioregular oligothiophenes (tetrathiophenes to dodocathiophenes) has been reported <00JCS(P1)1211> utilizing cross-coupling reactions. For example, the palladium-catalyzed cross coupling of borate 75 with iododithiophene 74 gave tetrathiophene 76. A novel one step synthesis of piperazinone 80 involved the multicomponent reaction of thiophene 77, diamine 78, and glyoxylic acid (79) <00TL9607>. C6H13 061"1,13

C6H13 7s

06H13

06]"1,13

.

CsF, Pd (PPh3)4

0

C61"~13 7'6

74

B(OH)2 + MeHN 77

C61"~13

NHMe + H 78

o 79

oMi

0 80

N tvlle

The organometallic cross-coupling reaction of halogenated thiophenes is another related method for the preparation of highly functionalized thiophenes. Halogenated thiophenes that have been utilized in palladium-catalyzed cross coupling reactions include ct-bromothiophenes <00BMCIA15, 00CC2487, 00JA6746, 00JHC281, 00JMAT1303, 00SC2281, 00SM47, 00T2985>, 13-bromothiophenes <00BMCIA15, 00EJOC2357, 00JMC1293>, and cxiodothiophenes <00JOC352>. The synthesis of the novel bis-cyclobutene 83 was achieved by a Stille coupling of 2,5-diiodothiophene (81) and stannane 82 <00T4249>. Palladiumcatalyzed Sonogashira coupling reactions and related variants have been utilized to prepare alkyne-substituted thiophenes <00AC3481, 00HCA3043, 00T2985, 00TL3607, 00TL5151>. Palladium-catalyzed aminations of halothiophenes have been utilized to synthesize aminothiophenes <00CC133>. For example, 13-aminothiophene 85 was prepared from 13-

E.T. Pelkey

94

bromothiophene 84 by a palladium-catalyzed amination with n-butylamine <00TL7731>. Treatment of 81 with sulfide 86 in the presence of palladium and copper gave the S-arylation product, o~-thiothiophene 87, via a novel depropargylation reaction <00TL7259>. The perfluorohexyl-dithiophene 89 was prepared utilizing a copper-catalyzed coupling reaction of dithiophene 88 with perfluorohexyl iodide <00AC4547>. Other metals that have been reported in coupling reactions with halothiophenes include nickel <00P423, 00TL5039> and indium, the latter (with palladium) was utilized in a novel Barbier-type allylation reaction <00CC645>. Bu3Sn\ l

Br

BuNH2

NHBu

cs co I

I

Pd(PPh3)4

C02Me

81

83

[~ 81

NHTs

84

~NHTs

86 si~ ~,

PdNIba3

02Me

85

1. Cu-bronze

S,~ff~ i

Pd (PPh3)2CI2 Cu, Et3N

5 mol%

#~_/S~B

r 2. C6F13I

88

87

89

Photochemical substitution reactions of thiophene <00T9383> and electron-deficient (-NO2) diiodo-thiophenes <00JCS(P1)3513> have been reported during the previous year. Irradiation of a-iodothiophene 90 in the presence of methyl acrylate gave a mixture of the addition product 91 and the substitution product 92 <00EJOC1653>. Photodecarboxylation of thiophene-2-acetic acid in the presence of nitrogen aromatic compounds (e.g., acridine) gave the corresponding o~-(arylmethyl)thiophenes <00T6845>. ~C02Me hv, CH3CN 9O Me

"S" "CHO

+ MeO2C'~S~CHO

91 OMe

H 93

I" v

92 OMe

OMe 0

CI3CC02H . . 94

S 95

The side-chain functionalization of thiophenes have been reported including Wittig olefinations <00CCl139, 00EJOC1703, 00EPJ2005, 00TL5521>, the nucleophilic addition of n-butyllithium to the tosylhydrazone of thiophene-2-carboxaldehyde <00TL2667>, and the nucleophilic addition of amines to 2-ethynyl-5-nitrothiophene <00OL2419>. The preparation of chiral thiophene alcohols from thiophene-2-carboxaldehydes has been accomplished by the asymmetric addition of diethylzinc performed in the presence of chiral tryptophan-derived

95

l~ive-Membered Ring Systems: Thiophene & Se, Te, Analogs

ligands <00TA2315> and by asymmetric reduction mediated by a chiral ruthenium complex <00OL1749>. Finally, the attempted reduction of ethyl 3-methoxythiophene-2-carboxylate to thiophene-2-methanol 93 with lithium aluminum hydride led primarily to a by-product, dithenylmethane 94 <00JCS(P1)3020> which inspired further investigation. Interestingly, treatment of 93 with trichloroacetic acid gave 94 and the spiro product 95, the latter by acidcatalyzed condensation of additional 93 with 94. 5.1.4 RING ANNELATION ON THIOPHENE The electron-rich thiophene ring system can be elaborated into complex, fused thiophenes by acid-mediated intramolecular annelation reactions. For example, treatment of alcohol 96 with trimethylsilyl triflate promoted a Friedel-Crafts acylation and subsequent dehydration giving benzo[b]thiophene 97, a potential analgesic <00JMC765>. Treatment of ketone 98 with p-toluenesulfonic acid resulted in the formation of fused benzo[b]thiophene 99 <00T8153>. Another variant involved the cyclization of epoxide 100 to fused benzo[b]thiophene 101 mediated by boron trifluoride-etherate <00JOC3883>.

N~X'N~Tr

~H

N

MeS TMSOTf 0

CN p-TsOH, M e S ~

97

L-~

jL ~-~ ,BF3-ether 100

~

~OMe

~1

S , ~ 0~1 "(3Me

The synthesis of complex thiophene-containing polycyclic hydrocarbons has been achieved utilizing intramolecular photocyclization reactions of f~-chlorobenzo[b]thiophenes, and recent examples include dibenzo[f,h]benzothieno[2,3-c]quinolin-lO(9H)-one 102 <00JHC997> and naphtho[2',l':4,5]thieno[2,3-c]naphtho[1,2-f]quinolin-6(5H)-one 103 <00JHC171>. The photocyclization of 3-styrylthiophenes to fused thiophenes has been studied <00JHC959, 00TL1951>. An interesting photorearrangement involving a [1,9] hydrogen shift occurs upon irradiation of electron-rich stilbenes (e.g., 104 --->105) <00JA8575>. o

S 102

s 0

104

Me

105

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96

A copper-mediated cyclization of metallated thiophenes has been utilized to prepare polycyclic thiophenes and thiophene cyclophanes. Treatment of dibromide 106 in succession with n-butyllithium (halogen-metal exchange), zinc chloride (transmetallation), and copper chloride gave 7H-cyclopenta[1,2-b;4,3-b']dithiophene (107) <00H(52)761>. This conversion has also been achieved using a palladium-mediated cyclization performed in the presence of hexamethylditin <00H(52)761>. Copper-mediated cyclizations hhve also been applied to the syntheses of cyclopenta[2,1-b;3,4-b']dithiophen-4-one (108) (three steps from 73) <00S1253> and cyclophane 109 <00CC2329>.

S

1. n-BuLi 2. ZnCI2

Br Br

S

O

106

107

108

109

Cyclometallation of thiophene imine 110 with platinum complex 111 proceeded to give metallacycle 112 via an intramolecular C-H insertion on the thiophene ring <00JOMC22>. BnN

~

C)SbCI6

Pt2Me4(SMe2)2 M ~

N---Bn

"S~" Pt-L..SMe2 Me" 112

110 ,~~OH MeO

pyr

S + ~Me NHPhth 116 113

Me ~ ~ 1 ~ ~ ~ s

Me S =0

Md

Me

118

+

~--Ph

Ph

9 M(

Me M 113

C)SbCI6 114

115

~O~../S,,~~OMe -'-MeO- ''r

"S"~S M~e " 117

PhOC __ COPh i,

Me Me S ~[ ~ ~ "~~/ [~/ ~ C O P' hr " ~N"~'~ / -"i~/-"L'co Ph Me

119

I~le

Treatment of 2-methylthiophene (113) with nitrilium ion 114 gave the novel heterobicyclic ring system, 3-azo-6-thiabicyclo[3.2.1]octa-3-ene 115, via an ene-like reaction <00JOC3569>. The cycloaddition between the o-thioquinone derivative 116 and 113 gave the 2:1 cycloadduct, [1,4]oxathiin 117 <00SL61>. Cycloadditions of thiophenes <00TL5005>, thiophene-l-oxides <00H(52)1215>, and thiophene-l,l-dioxides <00H(52)365> with dienophiles give benzenoid products after extrusion of the sulfur moieties from the bicyclic cycloadducts. For example, treatment of thiophene-l-oxide 118 with dibenzoylacetylene gave fused benzene 119 <00H(52)1215>. Finally, thiophene-based and other heterocyclic quinodimethanes have been investigated using theoretical methods <00JOC7971>.

Five-Membered Ring Systems: Thiophene& Se, Te, Analogs

97

5.1.5 THIOPHENE INTERMEDIATES IN SYNTHESIS The thiophene ring system can be utilized as a synthetic scaffold for the preparation of nonthiophene materials as the sulfur moiety can be removed by reduction (desulfurization) or extrusion (loss of SO2). The extrusion of sulfur dioxide from 3-sulfolenes (2,5dihydrothiophene 1,1-dioxides) give dienes (butadienes or o-quinodimethanes) that can be utilized to prepare six-membered rings by cycloaddition chemistry. For example, thermolysis of 3-sulfolene 120 provided tricyclic pyrazole 122 via an intramolecular cycloaddition of the oquinodimethane 121 that results by extrusion of sulfur dioxide <00JOC5760>. Syntheses of 3sulfolenes 123 <00JCCS83> and 124 <00S507> have recently been reported.

N.-...,

A _ ~ ' ~ "-"

SOPh

9

Oi

CO2Me

123

122

124

The reductive extrusion of sulfur from thiophene derivatives using Raney nickel has been utilized to prepare a variety of materials. For example, treatment of 125 with Raney nickel gave ester 126 <00OL3719>. Hydrolysis of 126 with aqueous lithium hydroxide gave 8hydroxyhexadecanoic acid (127), an inhibitor agent of spore germination. Treatment of 128 with excess Raney nickel proceeded to give alcohol 129 by reductive desulfurization the dithiane and tetrahydrothiophene with concomittant removal of the benzyl protecting group <00S 1863>. Finally, the thiophene ring can be considered an "n-butyl synthon" as reduction of thiophene 130 gave C-butyl glycoside 131 <00TA4463>.

1. RaneyNi 2. aq. LiOH

CH3(CH2 ) 7 ~ C O 2 M e OH 125

CH3(CH2)7~CO2 OH

R

126 R = Me 127

R=H

OBn

D

S

Me Me 129

128

MeO

O """O

M'~~'*OM e 130

OH

Raney Ni

Raney

Ni

MeO'~~~"l~ Me(~ ~Vle 131

Me

E.T. Pelkey

98

The addition of cuprate 133 to benzo[b]thiophene-l-oxide 132 unexpectedly led to the ringopened product 134 <00JOC8811>. The reaction proceeded via an unprecedented 1,2-addition onto the sulfur moiety rather than via the expected 1,4-addition pathway giving the 3substituted benzo[b]thiophene. The corresponding thiol of 134 (prepared by reduction of the sulfoxide) was found to be an inhibitor of tubulin polymerization. Finally treatment of sulfur ylide 135 with cesium fluoride led to a mixture of ring enlarged benzothiocine 136 ([2,3]sigmatropic rearrangement product) and thiol 137 (Hoffman elimination product) <00JOC7055>. A mechanistic model involving the cis-trans isomerization of 135 is proposed to explain the formation of both products. When this reaction was performed in DMSO, the aromatized product derived from 136 was also obtained.

/ Br

t MeO ~ Me~e ~ CuU

MeO J Jtt'S (~

133

~ O M e

2

MeO"":11 "OMe OMe 134

L

v "OMe

1

132

_..,.,.TMS

CsF.

+

| cl04@ 135

136

137

5.1.6 BIOLOGICALLY IMPORTANT THIOPHENE DERIVATIVES A number of biologically active thiophene-containing compounds have been designed, synthesized, and evaluated. One of the more common scaffolds utilized in medicinal chemistry is the benzo[b]thiophene moiety, and examples of which include thrombin inhibitor 138 <00BMCLl199, 00BMCL2347, 00JMC649> and protein tyrosine phosphatase 1B inhibitor 139 <00JMC1293>. Additional biologically active benzo[b]thiophenes that been synthesized and/or evaluated include an inhibitor of urokinase-type plasminogen activator (structural study) <00CB299>, dual inhibitors of thromboxane A2 synthase and aromatase <00JMC1841>, adrenoceptor agonists <00JMC765>, analgesics <00CCC280>, and anti-inflammatory agents <00CCC1082>. OH

138

99

Five-Membered Ring Systems: Thiophene& Se, Te, Analogs

The biological activity of a variety of fused thiophene analogues have been synthesized and evaluated, and examples of which include phosphodiesterase 7 inhibitors (e.g., benzo[b]thieno[3,2-a]thiadiazine 140) <00JMC683> and antioxidants (e.g. thieno[3,2-c] carbazole 141)<00JMC1762>. Additional examples of biologically active fused thiophenes include antimicrobial agents (benzo[b]thieno [2,3:6,5 ]pyrimidino [6,1 -f]pydridazines) <00HC403>, antidepressant agents (pyrazolo[2,1]benzothiazepines) <00JHC389>, adrenoceptor antagonists (benzo[b]thieno[3,2-d]pyrimidines) <00JMC1586>, and DNAbinding agents (imidazo-[4',5':4,5]thieno[3,2-d]pyrimidin-5(6H)-ones) <00JMC4877>. The synthesis and evaluation (mutagenicity) of the carcinogenic metabolite, acenaphtho[1,2-b] benzo[d]thiophene 142, was reported <00JOC8134>. O

~ S

Me 140

Me...N ~

S

CO2Et ~

o

[

141

~ 142

Finally, the biologically activity of non-fused thiophenes have been synthesized and evaluated, and examples of which include progesterone receptor antagonist 143 <00BMCL415>, anti-trypanosomal agent 144 <00CB733>, and mercaptocarboxylate inhibitor 145 (structural study) <00B4288>. Additional examples of biologically active non-fused thiophenes include carbonic anhydrase inhibitors <00BMC2145>, adenosine A1 receptor antagonists <00DDR227> and agonists <00JOC8114>, cyclooxygenase (COX) inhibitors <00EJMC499>, dopamine receptor agonists <00EJP255>, antibacterial agents <00JAB546>, and antitumor agents <00JMC167>. Glycopolythiophenes have been synthesized using solidphase methods and evaluated as novel detectors of Influenza Virus <00BCC777>. Finally, the synthesis and biological evaluation of tetrahydrothiophene derivatives (bioisosteric replacements ofpyrans) has been reported <00JMC1264> and reviewed <00S1637>.

CF3 Me

NC

P,h

MeO

,., e

143

"N H " "Me

144

H ff-~

' N, N ,, ,.

CF3

N SH O

I~,'N

,~,/N.. N" CO20 145

5.1.7 NOVEL THIOPHENE DERIVATIVES The unique electronic and physical properties of thiophenes make it a useful building block for a variety of novel materials. The preparation of thiophene-containing porphyrins and related higher order macrocyclic materials have been reported including water-soluble thiophene-modified porphyrin 146 (photodynamic therapy) <00JMC2403>, heptaphyrin 147 (a novel 30n aromatic system) <00OL3829>, calix[n]thieno[n]pyrroles <00TL2919>, thiophenemodified porphyrin pentamers <00TL3709>, dioxadithiaporphycenes <00TL10277>, and thiophene-substituted phthalocyanines <00CC 1649, 00T4085>.

100

E.T. Pelkey

R

R

R 146 R = S03Na

147 R = mesityl

In addition to modified porphyrin derivatives, a variety of novel thiophene-containing macrocycles have been prepared including dehydrothieno[18]annulene 148 <00CC1733>, mixed cyclooligothiophenediacetylenes (structurally related to 148) and cyclo[n]thiophenes <00AC3481>, thiophene-containing cyclophanes (e.g., 109) <00CC2329>, and a pyridinethiophene cyclophane <00CC2465>. The novel oligothiophene cyclophane, [2.2]quinquethiophenophane 149, was synthesized and evaluated as a 7t-dimer model <00OIA197>. S

148

149

The unique electronic properties of the thiophene ring system are often utilized to manipulate the electronic and optical properties of various materials including dyes, light emitting diodes, and molecular devices. A variety of thiophenes conjugates to other interesting organic materials have been prepared including ferrocene complexes (e.g., 150 <00SC2281>) <00AM599, 00OM1008, 00OM1035, 00PH291>, chromium complex 151 <00TL3607>, and derivatives of C6o (e.g., 152 <00CC2487>) <00AM908, 00JPC5974>. Syntheses of complex thiophene-containing helicenes (e.g., 4 <00AC4481>), potentially important materials in the field of molecular recognition, have been reported <00CC97, 00CCl139, 00JCS(P2)2492, 00JHC1009>. Thiophenes with push-pull substitution have been synthesized and/or evaluated <00AC556, 00JAIl54, 00SM213> including dithiophene 153 <00S1229>, ~-nitrothiophene 154 <00OL2419>, and thioindigo 155 <00TL2983>. A number of thiophenes with interesting optical <00AM978, 00AM1587, 00CC939, 00CM284, 00CM1508, 00HCA3043, 00JAIl021, 00JOC2900, 00OL2979> and/or structural properties (e.g., liquid crystals) <00AM1336, 00JA585, 00JMAT1303> have been prepared and evaluated. Finally, the transformation of open cyclopentene 156 to closed cyclopentene 157 by irradiation of a single crystal has been studied using x-ray crystallography <00BCSJ2179>. The photochromic properties of closely related systems <00AM1597, 00CLl188, 00CL1340, 00JA3037, 00JA8309, 00OL2749> have been widely studied during the past year.

101

Five-Membered Ring Systems: Thiophene& Se, Te, Analogs

Me

s

o

N-Me

N02

SMe

154

SMe

:.~

EE F F F F

F ~M Me

~e eM 156

Me

360nm 650nm (singlecrystal)

Me

Me 157

Novel C2-symmetric thiophene-containing ligands have recently been prepared and utilized in asymmetric synthesis. Dithiophene 158 was utilized as a ligand in the asymmetric reduction of 13-ketoesters (prostereogenic carbonyl) and acrylic acids (carbon-carbon double bond) <00JOC2043>. Dibenzo[b]thiophene 159 was utilized as a ligand in enantioselective Heck reactions of 2-pyrrolines <00SL1470>.

Ph2N ,-~

Me Me~PPh

2

Me~s~/~Ph2 Me 158

~~/~PPh2

P

h

~L~~ ~---PPh2

~

Ph2N 159

2

N

NPh2 ~

Ph2 ~.(" _ NPh2 160

Finally, novel thiophene-containing dendrimeric materials have been prepared <00CC507, 00CM2372, 00JCS(P2)1976> including C6-symmetric dendrimer 160 <00AM668>. 5.1.8 THIOPHENE OLIGOMERS AND POLYMERS The thiophene ring is a common building block for novel oligomeric and polymeric materials. The synthesis of monodisperse thiophene oligomers conti,lues to be widely studied and the preparation of one class of oligomers, oligothienylenevinylenes <00CEJ1698,

E.T. Pelkey

102

00CM2581, 00TL5521>, was reviewed <00ACR147>. The conformational properties of simple thiophene oligomers have been studied utilizing theoretical methods <00JHC847>. The synthesis and/or evaluation of monodisperse thiophene oligomers that have appeared include: dithiophenes <00JA6746> (e.g., 161 <00AM563>), trithiophenes <00CC1005, 00JA6746, 00MAC4628> (e.g., palladium-incorporated dimer 162 <00JA10456>), tetrathiophenes <00AC2680, 00JA6746, 00JCS(P 1) 1211, 00TL5521> (e.g., 163 <00JOMC8>), sexithiophenes <00AC4547, 00CC383, 00JA1820>, octathiophenes <00CC81, 00JCS(P1)1211>, dodecathiophenes (from 76) <00JCS(P1)1211>, and heptadecathiophenes (!) <00JA7042>. Syntheses of thiophene oligomers containing thiophene-l,l-dioxide moieties have also been reported <00JA9006, 00SM47, 00SM83, 00SM235>. Various mixed thiophene co-oligomers have been prepared including silole/thiophene 164 <00AC1695>, phosphole/thiophene 165 <00AC1812>, and phenylene/thiophene 2 <00AC2870>. Additional examples of thiophene co-oligomers include furan/vinylene/thiophenes <00EPJ2005>, phenylene/thiophenes <00JHC25, 00JHC281>, and pyrrole/thiophenes <00JMAT107>.

S

H H

v

~

,J ' I-'N-CI2H25

o

~d" 15d" 2

161

Me a'-a

~'~s

Ph

S

C6H13 "C6H13 163

~S

162

Ph S

S

ph 2

164

Ph 165

The synthesis and evaluation of thiophene polymers continues to be widely studied. Monolayers of electrostatically charged thiophene polymer 166 complexed with a biotinylated material have been studied in avidin-based assays <00CC1847>. The self-assembly properties of amphilic polymer 167 has been studied using Langrnuir-Blodgett films <00JA5788>. The preparation of green electroluminescent <00CC1631>, copper-entwined <00CEJ1663>, and radical-containing <00MAC8211> thiophene polymers have been reported. Many additional studies of thiophene polymers have been appeared during the last year <00AM567, 00AM589, 00AM1594, 00CC877, 00CM2996, 00JA5788, 00MAC5481, 00P423, 00P3147, 00P5681, 00P9147, 00SM133, 00SM305, 00SM433>. Examples of co-polymers of thiophenes with other materials that have been studied include 1,3,4-oxadiazole/thiophene co-polymer 168 <00JOC3894>, fluorophenylene/thiophene co-polymer 169 <00SM33, 00SM151>, and cobaltsalen/thiophene co-polymer 170 <00CM872>. Additional examples of thiophene-containing co-polymers include bipyridine/thiophenes <00CM1611>, fluorene/thiophenes <00CM1931>, crown ether/thiophenes <00JMAT263>, phenylene/thiophenes <00JMAT927, 00MAC2462>, ethylene glycol/thiophenes <00JMAT 1777>, pyrrole/thiophenes <00JMAT 1785>, quinoxaline/thiophenes <00PB231 >, and divinylphenylene/thiophenes <00SM437>.

Five-Membered Ring Systems: Thiophene& Se, Te, Analogs

.08H17

003~

__~oH17

Me S 166

103

_

)

167

(~03S

n

L

"

N--N

""

N-N J n

168

~O__/__OM e

F F

0/'-"'~0

~N~N__ ~o

"~ F

o

~ I~

1

170

169

5.1.9 SELENOPHENES AND TELLUROPHENES A modest number of reports on the chemistry of selenophenes and tellurophenes appeared during the past year. The preparation of selenophene 172 was accomplished by treatment of titanocycle 171 with selenium diselenocyanate <00JA5052>. The formation of 172 could also be achieved by irradiation of 1,2-diselenin 1 7 3 . An interesting synthesis of dihydroselenophenes involved the thermolysis of 1,2,3-diselenadiazole (denitrogenation) in the presence of ethyl acrylate <00JOMC488>. This reaction was accompanied by the formation of 1,4-diselenins. A Pummerer-related synthesis of benzo[b]selenophenes involved the oxidation of 2-benzo[b]selenopyrans <00H(52)1021>. The preparation of selenophene-containing porphyrin 176 (note inverted pyrrole ring) was achieved by treatment of diol 174 and tripyrrole 175 with boron trifluoride-etherate <00JOC8188>. Additional syntheses of selenophene-modified porphyrin materials have also appeared <00EJOC1353, 00JCS(P2)1788>. The laser photolysis of selenophene and tellurophene has been reported <00AOC715, 00JOC2759>.

t_BUT.~it_BuSe(SeCN)2 t_Bu.~~et_Buhv

t-Bu~-Bu

/-PRO~ "Oi-Pr 171

172

173

104

E.T. Pelkey

OH

,~ H

~

.,Me

,~

Me

BF3-ether +

,

h

chloranil ~~-OH

~

M

174

e

Me

176

175

The synthesis of ditellurophenes and mixed selenophene/tellurophene trimers has been reported <00H(52)159>. The structul'e of 1,1-diiodotetrahydrotellurophenes and related compounds has been studied <00JOMC96>. Finally, the synthesis of the novel benzo[c]tellurophene (179) has been achieved <00JOC5413>. Treatment of 177 with tellurium and sodium iodide followed by silver trifluoroacetate gave 178. The base-mediated double elimination of the trifluoroacetates of 178 proceeded smoothly to give 179. The cycloaddition and lithiation chemistry of 179 was investigated, for example, the double a-lithiation with nbutyllithium followed by quenching with ethyl chloroformate gave diester 180.

1. Te, Nal CI 2. CF3CO2Ag ~ II Cl ~

~~I~

9

.I 178

177

/OCOCF3 Et3N- [ ~ ~ ~ T e me, OCOCF3 179

1. n-BuLi C02 Et 2"ClCO2Et, ~ ~ / T e 180

CO2Et

5.1.10 R E F E R E N C E S 00ACR147 00AM133 00AM481 00AM563 00AM567 00AM589 00AM599 00AM668 00AM908 00AM978 00AM1336 00AM1587 00AM1594 00AM1597 00AC556 00AC1695 00AC1812

Roncali, J. Acc. Chem. Res. 2000, 33, 147. Tykwinski, R. R.; Kamada, K.; Bykowski, D.; Ohta, K.; McDonald, R. Adv. Mater. 2000, 12, 133. Groenendaal, L. B.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Adv. Mater. 2000, 12, 481. Rep, D. B. A.; Roelfsema, R.; van Esch, J. H.; Schoonbeek, F. S.; Kellogg, R. M.; Feringa, B. L.; Palstra, T. T. M.; Klapwijk, T. M. Adv. Mater. 2000, 12, 563. Storsberg, J.; Ritter, H.; Pielartzik, H.; Groenendaal,L. Adv. Mater. 2000, 12, 567. Meskers, S. C. J.; Peeters, E.; Langeveld-Voss, B. M. W.; Janssen, R. A. J. Adv. Mater. 2000, 12, 589. Wolf, M. O.; Zhu, Y. Adv. Mater. 2000, 12, 599. Wu, I.-Y.; Lin, J. T.; Tao, Y.-T.; Balasubramaniam, E. Adv. Mater. 2000, 12, 668. Apperloo, J. J.; Langeveld-Voss, B. M. W.; Knol, J.; Hummelen, J. C.; Janssen, R. A. Adv. Mater. 2000, 12, 908. Cornil, J.; Calbert, J.-P.; Beljonne, D.; Silbey, R.; Br6das, J.-L. Adv. Mater. 2000, 12, 978. Zhang, H.; Shiino, S.; Shishido, A.; Kanazawa, A.; Tsutsumi, O.; Shiono, T.; Ikeda, T. Adv. Mater. 2000, 12, 1336. Yoshida, Y.; Tanigaki, N.; Yase, K.; Hotta, S. Adv. Mater. 2000, 12, 1587. Apperloo, J. J.; Janssen, R. A.; Nielsen, M. M.; Bechgaard, K. Adv. Mater. 2000, 12, 1594. Tian, H.; Tu, H.-Y. Adv. Mater. 2000,12, 1597. Hartmann, H.; Eckert, K.; Schr~Sder,A. Angew. Chem., Int. Ed. 2000, 39, 556. Yamaguchi, S.; Goto, T.; Tamao, K. Angew. Chem., Int. Ed. 2000, 39, 1695. Hay, C.; Fischmeister, C.; Hissler, M.; Toupet, L.; R6au, R. Angew. Chem., Int. Ed. 2000, 39, 1812.

F i v e - M e m b e r e d Ring Systems: Thiophene & Se, Te, Analogs

00AC2680 00AC2870 00AC3481 00AC4481 00AC4547 00AOC715 00B4288

00BCC777 00BMC2145 00BMCIA15 00BMCLll99 00BMCL2347 00BCSJ1 00BCSJ2179 00CB299 00CB733 00CC81 00CC97 00CC133 00CC383 00CC439 00CC507 00CC645 00CC649 00CC877 00CC939 00CC1005 00CCl139 00CCl199 00CC1631 00CC1649 00CC1733 00CC1847 00CC1887 00CC2329 00CC2465 00CC2487 00CCC280

105

Mena-Osteritz, E.; Meyer, A.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W.; B/iuefle, P. Angew. Chem., Int. Ed. 2000, 39, 2680. Suh, M. C.; Jiang, B.; Tilley, T. D. Angew. Chem., Int. Ed. 2000, 39, 2870. Kr6mer, J.; Rios-Carreras, I.; Fuhrmann, G.; Musch, C.; Wunderlin, M.; Debaerdemaeker, T.; Mena-Osteritz, E.; B~iuefle, P. Angew. Chem., Int. Ed. 2000, 39, 3481. Rajca, A.; Wang, H.; Pink, M.; Rajca, S. Angew. Chem., Int. Ed. 2000, 39, 4481. Facchetti, A.; Deng, Y.; Wang, A.; Koide, Y.; Sirringhaus, H.; Marks, T. J.; Friend, R. H. Angew. Chem., Int. Ed. 2000, 39, 4547. Pola, J.; Bastl, Z.; Subrt, J.; Ouchi, A. Appl. Organomet. Chem. 2000,14,715. Concha, N. O.; Janson, C. A.; Rowling, P.; Pearson, S.; Cheever, C. A.; Clarke, B. P.; Lewis, C.; Galleni, M.; Fr6re, J.-M.; Payne, D. J.; Bateson, J. H., Abdel-Meguid, S., S. Biochemistry 2000, 39, 4288. Baek, M.-G.; Stevens, R. C.; Charych, D. H. Bioconjugate Chem. 2000,11,777. lilies, M.; Supuran, C. T.; Scozzafava, A.; Casini, A.; Mincione, F.; Menabuoni, L.; Caproiu, M. T.; Maganu, M.; Banciu, M. D. Bioorg. Med. Chem. 2000, 8, 2145. Zhi, L.; Tegley, C. M.; Pio, B.; West, S. J.; Marschke, K. B.; Mais, D. E.; Jones, T. K. Bioorg. Med. Chem. Lett. 2000,10,415. Takeuchi, K.; Kohn, T. J.; Harper, R. W.; Lin, H.-S.; Gifford-Moore, D. S.; Richett, M. E.; Sail, D. J.; Smith, G. F.; Zhang, M. Bioorg. Med. Chem. Lett. 2000,10,1199. Takeuchi, K.; Bastian, J. A.; Gifford-Moore, D. S.; Harper, R. W.; Miller, S. C.; Mullaney, J. T.; Sail, D. J.; Smith, G. F.; Zhang, M.; Fisher, M. J. Bioorg. Med. Chem. Lett. 2000,10, 2347. Nakayama, J. Bull. Chem. Soc. Jpn. 2000, 73,1. Yamada, T.; Kobatake, S.; Irie, M. Bull. Chem. Soc. Jpn. 2000, 73, 2179. Katz, B. A.; Mackman, R.; Luong, C.; Radika, K.; Martelli, A.; Sprengeler, P. A.; Wang, J.; Chan, H.; Wong, L. Chem. Biol. 2000, 7, 299. Du, X.; Hansell, E.; Engel, J. C.; Caffrey, C. R.; Cohen, F. E.; McKerrow, J. H. Chem. Biol. 2000, 7, 733. Langvelde-Voss, B. M. W.; Janssen, R. A. J.; Spierin, A. J. H.; van Dongen, J. L. J.; Vonk, E. C.; Claessens, H. A. Chem. Commun. 2000, 81. Yamada, K.-i.; Kobori, Y.; Nakagawa, H. Chem. Commun. 2000, 97. Watanabe, M.; Yamamoto, T.; Nishiyama, M. Chem. Commun. 2000, 133. Kilbinger, A. F. M.; Cooper, H. J.; McDonnell, L. A.; Feast, W. J.; Derrick, P. J.; Schenning, A. P. H. J.; Meijer, E. W. Chem. Commun. 2000, 383. Bongini, A.; Barbarella, G.; Zambianchi, M.; Abrizzani, C.; Mastragostino, M. Chem. Commun. 2000, 439. Sebastian, R.-M.; Caminade, A.-M.; Majoral, J.-P.; Levillain, E.; Huchet, L.; Roncali, J. Chem. Commun. 2000, 507. Anwar, U.; Grigg, R.; Rasparini, M.; Savic, V.; Sridharan, V. Chem. Commun. 2000, 645. Martin-Santamafia, S 9Carroll, M. A.; Carroll, C. M.; Carter, C. D.; Pike, V. W.; Rzepa, H. S.; Widdowson, D. A. Chem. Commun. 2000, 649. Kowalik, J.; Tolbert, L. M. Chem. Commun. 2000, 877. Raimundo, J.-M.; Blanchard, P.; Brisset, H.; Akoudad, S.; Roncali, J. Chem. Commun. 2000, 939. Skabara, P. J.; Roberts, D. M.; Serebryakov, I. M.; Pozo-Gonzalo, C. Chem. Commun. 2000, 1005. Caronna, T.; Sinisi, R.; Catellani, M.; Malpezzi, L.; Meille, S. V.; Mele, A. Chem. Commun. 2000,1139. Taki, M.; Murakami, H.; Sisido, M. Chem. Commun. 2000,1199. Pei, J.; Yu, W.-L.; Huang, W.; Heeger, A. J. Chem. Commun. 2000,1631. Muto, T.; Temma, T.; Kimura, M.; Hanabusa, K.; Shirai, H. Chem. Commun. 2000,1649. Sarker, A.; Haley, M. M. Chem. Commun. 2000,1733. Kumpumbu-Kalemba, L.; Leclerc, M. Chem. Commun. 2000, 1847. Ichikawa, J.; Fujiwara, M.; Wada, Y.; Okauchi, T., Minami, T. Chem. Commun. 2000,1887. Kabir, S. M. H.; Iyoda, M. Chem. Commun. 2000, 2329. Hanton, L. R.; Richardson, C.; Robinson, W. T.; Tumbull, J. M. Chem. Commun. 2000, 2465. Cravino, A.; Zerza, G.; Maggini, M.; Bucella, S.; Svensson, M.; Andersson, M. R.; Neuebauer, H.; Sariciftci, N. S. Chem. Commun. 2000, 2487. Rfidl, S.; Hezky, P.; Urb~inkov~i,J.; V~ichal, P.; Krejci, I. Coil. Czech. Chem. Commun. 2000, 65, 280.

106

00CCC1082 00CEJ1663 00CEJ1698 00CL744 00CLl188 00CL1340 00CM284 00CM872 00CM1508 00CM1611 00CM1931 00CM2372 00CM2581 00CM2996 00CR2537 00CSR109 00DDR227 00EJMC499 00EJOC1327 00EJOC1353 00EJOC1653 00EJOC1703 00EJOC2357 00EJOC3139 00EJOC3273 00EJP255 00EPJ2005 00HCA3043 00HC94 00HC403 00HC434 00H(52)159 00H(52)365 00H(52)761 00H(52)1021 00H(53)1175 00H(52)1215 00JA585 00JA1154 00JA1820 00JA3037 00JA5052

E.T. P e l k e y

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F i v e - M e m b e r e d R i n g Systems: Thiophene & Se, Te, Analogs

00JA5788 00JA6746 00JA7042 00JA8309 00JA8575 00JA9006 00JA10456 00JA11021 00JAB546 OOJCS(P1)1211 OOJCS(P1)1811 OOJCS(P1)1877 OOJCS(P1)2968 00JCS(P1)3020 00JCS(P1)3513 00JCS(P1)4316 00JCS(P2)1453 00JCS(P2)1788 00JCS(P2)1976 00JCS(P2)2492 00JCCS83 00JFC323 00JHC25 00JHC171 00JHC281 00JHC363 00JHC389 00JHC847 00JHC959 00JHC997 00JHC1009 00JHC1363 00JMAT107

107

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00JMAT263 00JMAT927 00JMAT1303 00JMAT1777 00JMAT1785 00JMC167 00JMC649

Bouachrine, M.; I_Are-Porte, J.-P.; Moreau, J. J. E.; Serein-Spirau, F.; Torreilles, C. J. Mater. Chem. 2000,10,263. L6re-Porte, J.-P.; Moreau, J. J. E.; Serein-Spirau, F.; Torreilles, C.; Righi, A.; Sauvajol, J.-L.; Brunet, M.J. Mater. Chem. 2000,10,927. Matharu, A. S.; Grover, C.; Komitov, L.; Andersson, G.J. Mater. Chem. 2000,10,1303. Kilbinger, A. F. M.; Feast, W. J.J. Mater. Chem. 2000,10, 1777. Ryder, K. S.; Schweiger, L. F.; Glidle, A.; Cooper, J. M.J. Mater. Chem. 2000,10,1785. Zhang, S.-X.; Feng, J.; Kuo, S.-C.; Brossi, A.; Hamel, E.; Tropsha, A.; Lee, K.-H. J. Med. Chem. 2000, 43,167. Sall, D. J.; Bailey, D. L.; Bastian, J. A.; Buben, J. A.; Chirgadze, N. Y.; Clemens-Smith, A. C.; Denney, M. L.; Fisher, M. J.; Giera, D. D.; Gifford-Moore, D. S.; Harper, R. W.; Johnson, L. M.; Klimkowski, V. J.; Kohn, T. J.; Lin, H.-S.; McCowan, J. R.; Palkowitz, A. D.; Richett, M. E.; Smith, G. F.; Snyder, D. W.; Takeuchi, K.; Toth, J. E.; Zhang, M. J. Med. Chem. 2000, 43, 649.

108

00JMC683 00JMC765 00JMC1264 00JMC1293 00JMC1586 00JMC1762 00JMC1841 00JMC2403 00JMC2929 00JMC4877 00JMS203 00JOC352 00JOC2043 00JOC2048 00JOC2759 00JOC2900 00JOC3569 00JOC3690 00JOC3883 00JOC3894 00JOC3920 00JOC5089 00JOC5413 00JOC5480 00JOC5760 00JOC7055 00JOC7971 00JOC8114 00JOC8134 00JOC8188 00JOC8669 00JOC8811 00JOMC8 00JOMC22 00JOMC96 00JOMC187 00JOMC488 00JPC5974

E.T. Pelkey

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Five-Membered Ring Systems: Thiophene & Se, Te, Analogs 00MAC2462 00MAC4628 00MAC5481 00MAC8211 00OL351 00OL1749 00OL2351 00OL2419 00OL2749 00OL2979 00OL3417 00OL3719 00OL3757 00OL3829 00OL4197 00OM1008 00OM1035 00PH291 00P423 00P3147 00P5681 00P9147 00PB231 00PAC1645 00SL61 00SL459 00SL963 00SL1058 00SL1470 00SL1788 00SC1359 00SC1695 00SC2281 00SC3569 00SM33 00SM47 00SM83 00SM89 00SM133 00SM151 00SM213 00SM235 00SM305 00SM433 00SM437 00S507 00S970 00S1078 00S1229

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110

00S1253 00S1637 00S1863 00T1247 00T2985 00T3255 00T3425 00T4085 00T4249 00T6845 00T7205 00T7573 00T8153 00T9383 00TA2315 00TA4463 00TL617 00TL1597 00TL1951 00TL2185 00TL2667 00TL2675 00TL2749 00TL2919 00TL2983 00TL3197 00TL3607 00TL3709 00TL4973 00TL5005 00TL5039 00TL5151 00TL5393 00TL5415 00TL5521 00TL5637 00TL7259 00TL7731 00TL8461 00TL8843 00TL9607 00TL10277

E.T. Pelkey

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