Benzoselenophenes

As foretold in the introduction, ring formation via attack on a double bond in the endo-trig mode is not well exemplified. The palladium(II) catalyzed oxidative cyclization of o-aminostyrenes to indoles has been described (78JA5800). The treatment of o-methyl-selenocinnamates with bromine in pyridine gives excellent yields of benzoselenophene-2-carboxylates (Scheme 10a) (77BSF157). The base promoted conversion of dienoic thioamides to 2-aminothiophenes is another synthetically useful example of this type (Scheme 10b) (73RTC1331). 

Benzoselenolo[2,3-ii]benzoselenophenes synthesis, 4, 116, 1078 Benzoselenolopyrylium cations oxidative ring contraction, 4, 969 [l]Benzoselenolo[3,4-ii][l,2,3]selenadiazoles synthesis, 6, 354 Benzo[h]selenophene, 2-acetyl-synthesis, 4, 966 Benzo[h]selenophene, 2-bromo-acylation, 4, 948... 

In his approach toward selenium-containing heterocycles with potential biological activity, Abdel-Hafez reacted 2-amino-3-(4,5-dihydro-17/-imidazol-2-yl)-4,5,6,7-tetrahydro-l-benzoselenophene 297 with triethyl orthoformate and benzaldehyde to generate the tricyclic systems 296 and 298, respectively (Scheme 21) <2005RJ0396>. Similarly, reaction with carbon disulfide gave the corresponding thiourea 299. 

All four isomeric selenolopyridines which can be derived from benzoselenophene (423— 426 Scheme 123) have been described. Ethyl 3-hydroxyselenolo[2,3-fe]pyridine-2-carboxy-late (429) has been prepared as shown in Scheme 124 (73BSF704). Treatment of ethyl 2-chloropyridine-3-carboxylate with methaneselenol yields (427). Nucleophilic displacement of bromine in bromoacetic acid with subsequent loss of methyl bromide yields (428), which after esterification is cyclized under Dieckmann conditions to give (429). The parent compound (423 colorless oil with b.p. 92 °C/1 mmHg) is prepared either by cyclization of compound (430) and subsequent decarboxylation of the intermediate acid (equation 57) or by reduction of 2-nitroselenophene and subsequent condensation of the amino compound with malonaldehyde bis(diethyl acetal) in the presence of zinc chloride (equation 58) (76BSF883). Selenolo[3,2-6]pyridine (426 b.p. 127-129°C/10 mmHg m.p. 35.5-37.0°C) has been obtained in an analogous manner. 

The slow addition of selenium oxychloride to a refluxing ethereal solution of a-arylstyrenes (319) led to the formation of benzoselenolo[2,3-6]benzoselenophenes (320 Scheme 110). X-Ray crystallography of the parent (320 R = H) confirmed the assigned structure (62JCS734). 

The parent 2-hydroxyselenophene possesses in principle three tautomeric forms (15, 16, 17) but the oxo form (17) predominates. In the 2-hydroxy-1-ben-zoselenophene (18) and l-hydroxy-2-benzoselenophene (20), where loss of heterocyclic ring resonance energy on tautomerism to 19 and 21 will be much less than for the non-annelated heterocycle, the latter oxo tautomers are preferred (Scheme 2) [26],... 

Hydroxy-1-benzoselenophene (22) and its 2,2-disubstituted derivatives exist preferentially as the oxo tautomer (23) (Scheme 3). A similar effect was observed for 3-hydroxyselenophene. Introduction of the acetyl group at the 2-position promotes enolization and for 2-acetyl-3-hydroxy-1-benzoselenophene the enol (24) not oxy form (25) is preferred, presumably by virtue of the extra stability by intramolecular hydrogen bonding. The broad IR absorption band oOH... Q = 3005-1930 cm 1 and the band of the exocyclic carbonyl group oc=0 = 1583 cm-1 both shifted to lower frequencies, as well as the signal for the acid proton (S = 12.86 ppm) in the H NMR spectrum provide evidence for structure 24. The same effects were observed for other 2-acyl-3-hydroxy-l-benzoselenophenes [47, 48],... 

Benzoselenophene is generally more reactive toward electrophilic reagents, although, like 1-benzothiophene, it undergoes mainly p-substitution (Scheme 5). 

The principal electrophiles to attack ring selenium are either oxidants or alkylating agents. Oxidation of substituted selenophenes and 2-benzoselenophene with an excess of 2,2-dimethyldioxirane leads to 1,1-dioxides, whereas oxidation of... 

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