Furans transition metal-catalyzed

The last method for the preparation of 2-quinolones described in this chapter relies on a intramolecular Heck cyclization starting from heteroaryl-amides (Table 2) [57]. These are synthesized either from commercially available pyrrole- and thiophene-2-carboxylic acids (a, Table 2) or thiophene-and furan-3-carboxylic acids (b, Table 2) in three steps. The Heck cyclization is conventionally performed with W,Ar-dimethylacetamide (DMA) as solvent, KOAc as base and Pd(PPh3)4 as catalyst for 24 h at 120 °C resulting in the coupled products in 56-89% yields. As discussed in Sect. 3.4, transition metal-catalyzed reactions often benefit from microwave irradiation [58-61], and so is the case also for this intramolecular reaction. In fact, derivatives with an aryl iodide were successfully coupled by conventional methods, whereas the heteroarylbromides 18 and 19, shown in Table 2, could only be coupled in satisfying yields by using MAOS (Table 2). 

This article aims to review papers that were published in 2006 on reactions and syntheses of furans, benzofurans and their derivatives. Two reviews have summarized the syntheses of furans <06OBC1627> and tetrahydrofurans <06EJO1627>. Another review <06001613> records the progress of transition metal-catalyzed asymmetric ring opening of oxabenzonorbornadienes. 

Since halogen-substituted benzo[. ]furans play an important role in the transition metal-catalyzed coupling of benzo[. ]furans with other substrates, synthetic methods to regioselectively synthesize substituted benzo[ ]furan halides have become very critical routes. Several syntheses of benzo[ ]furan based aryl halides are described here. 

Transition metal-catalyzed decomposition of a-diazoesters of type 60 result in the formation of a benzo[intramolecular Diels-Alder reaction with a tethered vinyl group followed by spontaneous N-assisted opening of the endoxide bridge to yield 11-azasteroid analogs (Scheme 92) <1999J(P1)59>. 

Transition metal-catalyzed cycloaddition reactions of alkynes in syntheses of furans 88CRV1081. 

Syntheses of y-lactams, epoxides, furans, etc. by transition metal-catalyzed/me-diated reactions of allenes with a nucleophilic functionality connected to the a-carbon atom 03ACR701. 

Transition metal-catalyzed annulations (cycloisomerizations of alkynylimines to pyrroles, alkynylketones to furans, and syntheses of multisubstituted heterocycles, particularly, alkaloids) 03SL2265. 

There are no novel results concerning acylcarbene transfer to C-C double bonds of heteroaromatic compounds such as thiophene and furan under photochemical conditions (see Houben-Weyl, Vol. 4/3, p288). More recent studies have focussed completely on the transition metal catalyzed version which, especially for a-diazo esters as Cj building blocks, displays higher efficiency and broader scope, whereas it often yields products other than cyclopropanes when a-diazo ketones are employed, see Section 1.2.1.2.4.2.6.3. and Houben-Weyl, Vol. E19b, ppll56, 1283, and 1332. 

Halogenated furans are important intermediates for the synthesis of a number of functionalized furan derivatives through metal-halogen exchange and transition metal-catalyzed cross-coupling reactions, as will be demonstrated throughout this chapter. 

Transition metal-catalyzed cross-coupling and C-H activation have become an important method for the formation of carbon-carbon bonds in the synthesis of furan and benzofuran containing pharmaceutical compounds. The reaction conditions required for these transformations are generally mild and tolerant of a number of different functional groups including ethers, esters, and amines to name a few. 

The Feist-Benary and Paal-Knorr syntheses are commonly employed in the preparation of furan ring systems. In special cases where furan derivatives are difficult to prepare by other methods, Diels-Alder and retro-Diels-Alder reactions have become important methods for their synthesis. Finally, transition metal-catalyzed cyclization and cycloisomerization reactions have recently gained significant attention for their utility in the synthesis of highly functionalized furans. Key examples of these syntheses are highlighted in the sections below. 

A regioselective transition metal-catalyzed cycloisomerization reaction of boron-containing alkynyl epoxides toward C-2- and C-3-borylated furans was developed. It was found that the copper catalyst as well as the gold catalyst with a relatively more basic triflate counterion favored boryl migration toward C-3-borylated furans, whereas employment of the cationic gold hexafluoroantimonate afforded C-2-borylated furans via a formal 1,2-hydrogen shift (14JA13146). 

Cao, H., Jiang, H. F., and Huang, H. W. 2011. Transition-metal-catalyzed domino reactions Efficient one-pot regiospecific synthesis of highly functionahzed polysubstituted furans from electron-deficient alkynes and 2-yn-l-ols. Synthesis. (7) 1019—1036. 

Transition-metal-catalyzed domino reactions have been used as powerful tools for the preparation of polysubstituted furans in a one-pot manner. An efficient synthetic... 

Transition metal-catalyzed cydoisomerization of allenyl ketones into furan products was first introduced by Marshall. In the first report from this group, it was demonstrated that various alkyl-substituted furans 2 could be prepared in high yields via the Ag-[50] or the Rh(I)-catalyzed [85, 86] cydoisomerization of allenyl ketones 1 (Scheme 8.1) [87]. This type of transformation of allenyl ketones [88-90] proved to be highly effident for the synthesis of up-to-trisubstituted furans. Furthermore, Marshall applied this strategy as the key step in the construction of the 2,5-disubstituted furanocycle 2a (Scheme 8.2) [87]. 

I 8 Transition Metal-Catalyzed Synthesis of Monocyclic FIve-Membered Aromatic Heterocycles Table 8.1 Au(i)-catalysed synthesis of furans 38. 

Next, Gevorgyan et al. disclosed an alternative route to tetrasubstituted and even fused furans via a transition metal-catalyzed migratory cydoisomerization of skipped propargylic systems. Thus, syntheses of 3-acyloxyfurans 69 from alkynyl acetates 68 occurred with a formal 1,2-migration in the presence of 5 mol% of Ag-catalyst at room temperature (Scheme 8.28) [139,140[. Several other transition metals such as Cu(II),... 

Recently, a novel approach for the synthesis of highly substituted furans vas developed by Larock and covrorkers (Scheme 8.37). It vas reported that 2-alkynyle-nones [151-153] 93 undervrent a facile transition metal-catalyzed cycloisomerization affording highly substituted furans 94 in the presence of the external O- or electron... 

The first example of a transition metal-catalyzed cycloisomerization of cyclopropenes [184,185] into furans was demonstrated by Nefedov (Scheme 8.61) [186]. It was proposed that this rearrangement proceeds via a carbenoid intermediate [187]. Formation of the furan products via the cycloisomerization of potentially involved cyclopropene intermediates in the Rh(II)-catalyzed cyclopropenation reaction of alkynes was later reported by several research groups, including Liebeskind, Davies, and Muller [188-192]. 

Analogously to the precedent transition metal-catalyzed cycloisomerization of (Z)-pent-2-en-4-yn-l-ols like 113 into furan compounds (Scheme 8.45), Gabriele disclosed a general and a very convenient route leading to pyrroles 218 from nitrogen analogs of these reactive precursors (Scheme 8.81) [269]. Thus, di-, tri-, and tetra-substituted pyrroles 218 with different substitution patterns could be readily synthesized via the Cu(I)- or Cu(II)-catalyzed [270] cycloisomerization of (Z)-(2-en-4-ynyl)amines 217. The mechanism of this transformation is similar to that proposed for the transition metal-catalyzed synthesis of furans (Scheme 8.46). 

Kondo and Mitsudo are the pioneers of transition-metal-catalyzed retro-allylation [7]. They described this phenomenon as deallylation and fi-allyl elimination . Ruthenium-catalyzed reaction of tertiary homoallylic alcohols proceeds in the presence of allyl acetate under an atmosphere of carbon monoxide in tetrahydro-furan (THF) at a relatively high temperature of 180 C to afford the corresponding ketones (Scheme 5.3). Although the exact roles of allyl acetate and carbon monoxide are not clear, they propose a mechanism involving oxidative addition of the... 

The furan ring is a common structural motif present in many biologically active molecules and pharmaceutical substances. Accordingly, the development of efficient synthetic routes to produce furans has been a major research field for over a century. Transition-metal-catalyzed transformations are probably the most powerful tools presently available for preparing furans from readily accessible starting materials [47-56]. Alkynes have been widely employed as building blocks for this end. In this context, the first metal-mediated synthetic approach to furocoumarins involving... 

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