Cr(Co) MeO Me Scheme 3 OMe (OC)5W (OC)5W Scheme 5 MeO,C CBH13 (i)MeOH Meo. STol R2 methyl ketone corresponding to 4. In an extension of a STol method reported earlier Iwasawa and co-workers have des- Scheme 6 cribed a synthesis of substituted methyl furan-3-carboxylates uch as 6 from tungsten carbene complexes, lithium acetylide and aldehydes. Tetrasubstituted furan-3-carboxylates have synthesis of 2-substituted furan-4-methanols involves the also been synthesised in moderate yield from 3-hydroxy- intermediacy of enones 14, which are prepared by a Horner- 1, 2-dioxane-4-carboxylates (cyclic peroxides) by reaction Wadsworth-Emmons reaction then cyclised by reaction wit HCLI9 The dimerisation of terminal allenic ketones 15 le ads to 2, 4-disubstituted furans 16 in preparatively useful yields when The palladium catalysed annelation of iodo compounds with pdCL(MeCN), is used as the catalyst in acetonitrile. A poten- internal alkynes, previously used by Larock's group to syn hesse a variety of heterocycles, has now been applied to furan tially general route to 2, 4-disubstituted furans has been used by Furstner and his co-workers in a synthesis of the terpene synthesis. For example, the tetrahydrobenzofuran 8 was pro- ircinin-4, the structure of which incorporates a 2, 4-dialkylfurar duced (69%) from the vinyl iodide 7 and 4, 4-dimethylpent subunit. This makes use of essentially the same methodology 2-yne. The mercury(m) catalysed cyclisation of the allenic as was invented for 2, 4-disubstituted pyrroles in Furstner's alcohols 10, generated in situ from 3-methoxy-l-phenyl te to roseophilin and which is outli ee furans I1 ise 9 and aldehydes, leads to the 2, 3-disubstituted Scheme 26 13 14 9 11 15 An unusual route to c-fused furans is illustrated in Scheme 5 Several furans have been prepared in moderate to good yield Intramolecular cycloaddition of conjugated ynones to triple by the reaction of a-bromomethyl ketones with enol ethers bonds leads to the formation of furans such as 12 which, the the presence of the catalyst [ReCI(N)(PMe, Ph)]. It is pro- authors suggest, are formed by way of strained bicyclic allenes posed that this generates acylmethyl radicals as the reactive and carbenes 6 Two existing routes to 3. 4-disubstituted furans have been mproved. The Garst-Spencer furan annelation from 3- (butylthio )enones was modified by replacing the butyl group ith a 4-tolyl group and by using iodine as the aromatising agent (Scheme 6). The oxidation of 2-substituted and 2, 3- disubstituted but-2-ene-1, 4-diols 13 in a two phase system led to a variety of 3-substituted and 3, 4-disubstituted furans in Scheme 7 good yield; for example, 3, 4-dibromofuran was prepared ( 83%) in this way. Cyclisation reactions involving palladium catalysis are pre- There are relatively few good methods for the synthesis of ominant among recently described methods for preparing dor furans with specific substituents at the 2 and 4 positions benzofurans. Details have been published of the sequential and some useful new methods have been described. A simple palladium catalysed coupling of 2-iodophenols with alk-l-yne 2850 J. Chem. Soc. Perkin Trans. 1. 1999. 2849-28662850 J. Chem. Soc., Perkin Trans. 1, 1999, 2849–2866 methyl ketone corresponding to 4. 10 In an extension of a method reported earlier 11 Iwasawa and co-workers have des￾cribed a synthesis of substituted methyl furan-3-carboxylates such as 6 from tungsten carbene complexes, lithium acetylides and aldehydes.12 Tetrasubstituted furan-3-carboxylates have also been synthesised in moderate yield from 3-hydroxy- 1,2-dioxane-4-carboxylates (cyclic peroxides) by reaction with acids.13 The palladium catalysed annelation of iodo compounds with internal alkynes, previously used by Larock’s group to syn￾thesise a variety of heterocycles, has now been applied to furan synthesis. For example, the tetrahydrobenzofuran 8 was pro￾duced (69%) from the vinyl iodide 7 and 4,4-dimethylpent- 2-yne.14 The mercury() catalysed cyclisation of the allenic alcohols 10, generated in situ from 3-methoxy-1-phenyl￾thioprop-1-yne 9 and aldehydes, leads to the 2,3-disubstituted furans 11. 15 An unusual route to c-fused furans is illustrated in Scheme 5. Intramolecular cycloaddition of conjugated ynones to triple bonds leads to the formation of furans such as 12 which, the authors suggest, are formed by way of strained bicyclic allenes and carbenes.16 Two existing routes to 3,4-disubstituted furans have been improved. The Garst–Spencer furan annelation from 3- (butylthio)enones was modified by replacing the butyl group with a 4-tolyl group and by using iodine as the aromatising agent (Scheme 6).17 The oxidation of 2-substituted and 2,3- disubstituted but-2-ene-1,4-diols 13 in a two phase system led to a variety of 3-substituted and 3,4-disubstituted furans in good yield; for example, 3,4-dibromofuran was prepared (83%) in this way.18 There are relatively few good methods for the synthesis of furans with specific substituents at the 2 and 4 positions and some useful new methods have been described. A simple Scheme 3 Scheme 4 synthesis of 2-substituted furan-4-methanols involves the intermediacy of enones 14, which are prepared by a Horner– Wadsworth–Emmons reaction then cyclised by reaction with HCl.19 The dimerisation of terminal allenic ketones 15 leads to 2,4-disubstituted furans 16 in preparatively useful yields when PdCl2(MeCN)2 is used as the catalyst in acetonitrile.20 A poten￾tially general route to 2,4-disubstituted furans has been used by Fürstner and his co-workers in a synthesis of the terpene ircinin-4, the structure of which incorporates a 2,4-dialkylfuran subunit.21 This makes use of essentially the same methodology as was invented for 2,4-disubstituted pyrroles in Fürstner’s route to roseophilin and which is outlined in Section 4 (see Scheme 26). Several furans have been prepared in moderate to good yield by the reaction of α-bromomethyl ketones with enol ethers in the presence of the catalyst [ReCl(N2)(PMe2Ph)4]. It is pro￾posed that this generates acylmethyl radicals as the reactive intermediates (Scheme 7).22 Cyclisation reactions involving palladium catalysis are pre￾dominant among recently described methods for preparing benzofurans. Details have been published of the sequential palladium catalysed coupling of 2-iodophenols with alk-1-ynes Scheme 5 Scheme 6 Scheme 7