and endo cyclisation to 2-substituted benzofurans. With silyl protected alkynols the method provides a route to benzofuran 3-methanol (Scheme 8)and to other alkan-3-ols. A variant which leads to 2, 3-disubstituted benzofurans is to carry out the reaction with allyl 2-alkynylphenyl ethers; for example, the ether 17 gave 3-allyl-2-methylbenzofuran 18(76%)on pal adium(o)catalysed cyclisation. A T-allylpalladium complex is reaction probably involves the generation and cyclisation of a suggested as an intermediate.3-Allenylbenzofurans have als phenoxy radical. been prepared by a similar method. a different approach to Two base-induced cyclisation reactions that lead to benzo- 3-allylbenzofurans has been described that is based on salicyl- furans are illustrated in Scheme 1033and in Scheme 11. The aldehyde derivatives; the aldehyde function is converted into an base induced fragmentation of 1, 2, 3-thiadiazoles is a prece- allyl vinyl ether (such as 19) by Wittig olefination and this is dented reaction and results in the generation of the anions 26 then subjected to Claisen rearrangement. The aldehydes(such intermediates in the route to 2-alkylthiobenzofurans as 20)so formed are then converted into 3-allylbenzofurans by acid catalysed cyclisation and dehydration NaH. DMF 80% TBDMS Scheme 10 OTBDMS 80 Scheme 8 9197% Schen 3 Thiophenes and benzothiophenes Several 2-alkylaminothiophenes have been prepared from terminal alkynes and alkyl or aryl isothiocyanates by the rout shown in Scheme 12.Similar syntheses of 2-alkylaminothio OMOM phenes bearing dialkylamino substituents at C-56 and hetero atom substituents at C-3 have also been described Palladium catalysed cyclisation reactions involving allene have provided another route to 3-substituted benzofurans. The B2NCS allene 21 gave 3-azidomethylbenzofuran 22(71%)with sodium azide and a palladium(o) catalyst. 2 Other nucleophiles can be O Bu'OK, DMSO used to capture the intermediate organopalladium species; thus, n)Ho with sodium benzenesulfinate, 3-(phenylsulfonylmethy l)benzo- NHR uran was isolated. Phenyl allyl ethers such as 23 have been 4387% cyclised to benzofurans by heating with caesium carbonate and Scheme 12 a palladium catalyst(Scheme 9).It is suggested that the reac- tion is promoted by the formation of phenolate anions, which The tertiary amide 27 gave the 2-aminot re more reactive than free phenols in the cyclisation step. on reaction with Lawesson's reagent 3 When secondary amides Intramolecular Heck reactions of allyl 2-iodophenyl ethers were used mixtures of aminothiophene and pyrroles were mides知 yield from reactions of ketene N, S-acetals such as 29 with 1, 3- dicarbonyl compounds and mercury(un)acetate, an hown in Scheme 13 CS (CO)3. Pd(O) 27 a standard route to benzofurans is the acid catal cyclodehydration of phenoxymethyl ketones 24. A versatile Ph NAMe route to these ketones based on the reaction of anions of Hg(OAc)2 I-phenoxymethylbenzotriazoles with aldehydes, has been des- s NAMe cribed; the complete sequence leading to the benzofurans can Me 91 PhS be carried out in one pot. Benzofuran has been isolated in 60% yield from the flash pyrolysis of the cinnamyl ester 25: the Scheme 13 Chem Soc. perkin Trans 1999. 2849-2866 2851J. Chem. Soc., Perkin Trans. 1, 1999, 2849–2866 2851 and endo cyclisation to 2-substituted benzofurans.23 With silyl protected alkynols the method provides a route to benzofuran- 3-methanol (Scheme 8) and to other alkan-3-ols.24 A variant which leads to 2,3-disubstituted benzofurans is to carry out the reaction with allyl 2-alkynylphenyl ethers; for example, the ether 17 gave 3-allyl-2-methylbenzofuran 18 (76%) on pal￾ladium(0) catalysed cyclisation. A π-allylpalladium complex is suggested as an intermediate.25 3-Allenylbenzofurans have also been prepared by a similar method.26 A different approach to 3-allylbenzofurans has been described that is based on salicyl￾aldehyde derivatives; the aldehyde function is converted into an allyl vinyl ether (such as 19) by Wittig olefination and this is then subjected to Claisen rearrangement. The aldehydes (such as 20) so formed are then converted into 3-allylbenzofurans by acid catalysed cyclisation and dehydration.27 Palladium catalysed cyclisation reactions involving allenes have provided another route to 3-substituted benzofurans. The allene 21 gave 3-azidomethylbenzofuran 22 (71%) with sodium azide and a palladium(0) catalyst.28 Other nucleophiles can be used to capture the intermediate organopalladium species; thus, with sodium benzenesulfinate, 3-(phenylsulfonylmethyl)benzo￾furan was isolated. Phenyl allyl ethers such as 23 have been cyclised to benzofurans by heating with caesium carbonate and a palladium catalyst (Scheme 9).29 It is suggested that the reac￾tion is promoted by the formation of phenolate anions, which are more reactive than free phenols in the cyclisation step. Intramolecular Heck reactions of allyl 2-iodophenyl ethers have been applied to a solid phase synthesis of benzofuran-3- ylacetamides.30 A standard route to benzofurans is the acid catalysed cyclodehydration of phenoxymethyl ketones 24. A versatile route to these ketones, based on the reaction of anions of 1-phenoxymethylbenzotriazoles with aldehydes, has been des￾cribed; the complete sequence leading to the benzofurans can be carried out in one pot.31 Benzofuran has been isolated in 60% yield from the flash pyrolysis of the cinnamyl ester 25; the Scheme 8 Scheme 9 reaction probably involves the generation and cyclisation of a phenoxy radical.32 Two base-induced cyclisation reactions that lead to benzo￾furans are illustrated in Scheme 10 33 and in Scheme 11.34 The base induced fragmentation of 1,2,3-thiadiazoles is a prece￾dented reaction and results in the generation of the anions 26 as intermediates in the route to 2-alkylthiobenzofurans. 3 Thiophenes and benzothiophenes Several 2-alkylaminothiophenes have been prepared from terminal alkynes and alkyl or aryl isothiocyanates by the route shown in Scheme 12.35 Similar syntheses of 2-alkylaminothio￾phenes bearing dialkylamino substituents at C-5 36 and hetero￾atom substituents at C-3 37 have also been described. The tertiary amide 27 gave the 2-aminothiophene 28 (57%) on reaction with Lawesson’s reagent.38 When secondary amides were used mixtures of aminothiophenes and pyrroles were produced. 3-Alkylaminothiophenes were obtained in high yield from reactions of ketene N,S-acetals such as 29 with 1,3- dicarbonyl compounds and mercury() acetate; an example is shown in Scheme 13.39 Scheme 10 Scheme 11 Scheme 12 Scheme 13