Benzol< furans

The lability of thieno[3,4-6]thiophene (3) and other iso-annelated systems, such as benzo[c]thiophene and benzole] furan, may be due to the strain effect (Mills-Nixon effect see also Zwanenburg et alP and references therein) in the condensed five-membered ring. The stability of the iso-annelated dithienothiophenes 7—9 is noteworthy. Simple LCAO MO method calculations on benzo[c]thiophene indicate that its instability is due to low specific delocalization energy and high free valence index at position 1. 

Furans also undergo cycloadditions with o-benzoquinones. Thus furan, 2-methylfuran, 2,5-diphenylfuran and benzol ]furan yield dihydrofurobenzodioxins of type (238) with tetrachloro-l,2-benzoquinone (Scheme 83). The reaction of furan with 1,2-benzoquinone affords only a 1 % yield of adduct because most of the quinone undergoes polymerization. The reaction with 2-methylfuran produces a 25% yield of adduct, however. The reactions are thought to involve the electrophilic attack of the quinone on the furan to produce a carbonium ion. In the case of 2-methylfuran the more stable carbonium ion (239) is produced. Evidence for a two-step mechanism is the diversion of the intermediate (239) to the addition product (240) which may be isolated when the reaction is conducted in the presence of ethanol (69JCS(C)1694). 

A broadly applicable synthesis of benzol/-]furans by Warrener relies on DielsAlder/retro DielsAlder reactions with s-tetrazines. Using this methodology with 3,6-di(pyridin-2-yl)-i -tetrazine 162 the generation of an isolable 5,6-(bistri-methylsilyl)benzo[c]furan 163 has been achieved (Scheme 97) <2002T9413, CHEC-III(3.07.6)551>. 

Treatment of 2- or 4-(p-formylphenyl)-2/f-l,2,3-triazoles with p-tolyl-substituted heterocycles such as benzo[6]furans,58,63 benzol-thiophenes,56,63 benzoxazoles,63,64 benzisoxazoles,54 oxazoles,63 isoxa-zoles,63 oxadiazoles,63,65 benzotriazoles,63 and y-triazolo [ 1,5-a]-pyri-dines19 has led to the formation of a number of new stilbene derivatives some of which are shown in Table XI. 

Untersucht man homologe Reihen, die als Basisgruppen Benzol, Thiophen und Furan-Gruppen sowie Doppelbindungen im Molekiil enthalten, so zeigt sich, daB mit langer werdendem Alkyl die i>-Werte... 

Brom-l-ethoxy-ethan- -dimethylester XII/1, 444 Brommcthan- -dibromid XII/1, 398 Brommethan- -diethylester XII/1, 427 2-Brom-l-methoxy-ethan- -diethylester XII/1, 444 2-Brom-4-methyl-benzol- XII/1, 369 5-Brommethyl-4,5-dihydro-furan-3- -dichlorid E2, 329... 

Benzo[b]furan Benzo[fr]thiophene Benzojb]- selenophene Benzol [b]-tellurophene... 

The five-membered ring heterocycles possess DielsAlder reactivity of varying degree. This is most pronounced in the case of furan and benzol]-fused heterocycles such as isoindole. In this capacity they are functioning as heterocyclic analogues of cyclopentadiene, and high DielsAlder reactivity can be correlated with low aromaticity. 

Competitive metallation experiments with A-methylpyrrole and thiophene and with A-methylindole and benzol] thiophene indicate that the sulfur-containing heterocycles react more rapidly with -butyllithium in ether. The comparative reactivity of thiophene and furan with -butyllithium depends on the metallation conditions. In hexane, furan reacts more rapidly than thiophene but in ether, in the presence of tetramethylethylenediamine (TMEDA), the order of reactivity is reversed. 

The reactions of the lithio derivatives of benzo[A]-fused systems indole, benzo[ ]furan, and benzol/ ]thiophene are similarly diverse. Since indole and benzo[A]thiophene undergo electrophilic substitution mainly in the 3-position, the ready availability of 2-lithio derivatives by deprotonation with -butyllithium is particularly significant. The ready availability of 3-iodoselenophene and hence of 3-lithioselenophene provides a convenient route to 3-substituted selenophenes. 2-Lithiotellurophenes are especially important precursors of tellurophene derivatives because of the restricted range of electrophilic substitution reactions which are possible on tellurophenes. 

Amino-4-(2-furyl)-l,3-tliiazol (R1 = 2-Furyl) S0 0,38 g (0,005 mol) Thioharnstolf, 0,722 g (0,0051110I) 2-(Chlor-acetyl)-furan und 0,026 g (0,025 mol) wasserfreies Natriumcarbonat werden in 5 ml heiBem Ethanol gelost. Es wird 5 min unter RfickfluB erhitzt. Nach dem Abkfihlen wird das Endprodukt durch Zusatz von Wasser ausgefallt, abgesaugt und aus Benzol/Leichtbenzin umkristallisiert Ausbeute 0,788 g (95%) Schmp. 124,5°. 

Photodimerisierung wurde durch 4ostdge. Bestrahlung (125 W Philips HPK.) von Benzo-[b]-furan in Benzol unter Argon mit Acetophenon als Sensibilisator herbeigefuhrt2 ... 

Phenyl-benzo- [b]-furan ergibt ein Dimeres noch unbekannter Konstitution (46 % d. Th. F 279-280°)2.1,3-T)ipheny]-<(benzo-[e]-furan)> gehtin Benzol mit guter Ausbeutein 1,2,7,8-Tetraj>henyl-(dibenzo-9,10-dioxa-tricyclo [4.2.1, li[Pg.554]

Auch eine C (J-Droifaclibitidung kann angegriffen werden. Bestrahlung von 2-Hydroxy-1-athinyl-benzol in 10 2 molarem waCrigen Natriumhydroxid (Rayonet-Reaktor X = 254nm 13 Stdn.) ergibt Bmzo- b -furan (60%d.Th.) und 2-Hydroxy-ace.tophenon (20%d.Th.)1 ... 

Wahrend 2.5-Dihydro-furan in Benzol durch ein Vycor-Filter bestrahltzuFuran und Tetra-hydrofuran disproportioniert, bildet sich in Ather, Methanol oder Cyclohexan zusatzlich Vinyl-oxiran ( 10% bez. auf umgesetzen Ather)6 ... 

Co2(CO)s stark subst. Alkene zu Alkanen oc, /J-unges.-Carbonyl-Verbindungen, Furane, Thiophene polycyclische Aroma ten (partiell) vgl. Tab. 10 (S. 55) 200 (H, + CO) 200-300 (H2+CO) 150-180 (H2 + CO) 300 120-180 60-250 Benzol, Hexan Benzol Unter anderen Bedin-gungen Hydroformy-lierung bei schwach substituierten Alkenen. Aktive Form bei hohem H,-Druck CoH(CO)4 10.11... 

Die katalytische Hydrierung des Furan-Ringes zum Tetrahydrofuran erfolgt leichter als die von Benzol, trotzdem lassen sich olefinische Seitenketten am Furan im allgemeinen ohne Schwierigkeiten selektiv hydrieren. Die bei der Furan-Hydrierung als Zwischenstufe auftretenden Dihydrofurane konnen dann isoliert werden, wenn sie durch Kondensation... 

Unter Erhalt der Keto-Funktion werden aromatische Amino-7 oder Nitro-ketone eben-falls dimethyliert. So erhalt man z, B. aus einem Mol 5-Nitro-3-oxo-2-methyl-2,3-dihydro-(benzo-[b]-furan) und zwei Molen Formaldehyd in Benzol (10% Palladium/Kohle) iiber die 5-Amino-Verbindung das 5-Dimethylamino-3 -oxo -2-methyl-2,3-dihydro- (ben-zo-[b]-furan)s (in polaren Losungsmitteln treten Folgereaktionen ein mit iiberschiissi-gem Formaldehyd Aldolreaktion in 2-Stellung) ... 

Base-catalyzed proton-deuterium exchange at H-2 and H-3 in benzol-thiophene has been extensively studied,255"258 but the results are contradictory. H-2 is replaced faster than H-3 in benzo[h] thiophene, and faster than H-2 in benzo[i>]furan. The marked ability of the sulfur atom to stabilize the 2-carbanion has been discussed in terms of d-orbital participation, electronegativity, and polarization phenomena, but no convincing conclusions have yet been reached. 

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