Tetrahydrofuran derivatives metalation

Recently, the transition-metal-catalyzed enanti-oselective enyne cycloisomerization has been reported.99 Cao and Zhang reported that, in the presence of catalytic amounts of [Rh(bicpo)Cl]2 and AgSbFe, the enantioselective cycloisomerization of the 1,6-enynes 50 gave the functionalized lactams 51 in good yields with high ee values (Scheme 17).99a Hatano et al. reported that, in the presence of catalytic amounts of palladium(II) and (i )-SEG-PHOS, the asymmetric cycloisomerization of the 1,6-enyne 52 gave the tetrahydrofuran derivative 53 in 99% yield with >99% ee (Scheme 18).99b It should be noted that the quaternary chiral center is constructed with extremely high ee in almost quantitative yield. 

In 2000, Fiirstner et al. reported platinum-catalyzed reaction of alkynyl allyl ethers 1 to tetrahydrofuran derivatives 2 (Scheme 27.1) [2]. After this pioneering work, the TT-acidic metal-catalyzed tandem carbon-heteroatom bond formation/1,3-migration reaction became a powerful synthetic method of polysubstituted heteroarenes, since the aromatization process often drives the migration of various carbon functional groups involving cleavage of a carbon-heteroatom bond. 

Compared with calixfurans, their saturated derivatives, i.e., calixtetra-hydrofurans, such as 89 were found to show better binding abihties for metal cations [18,20,32]. Some calix[4]tetrahydrofuran derivatives have been employed as ionophores of lithiiun ion-selective membrane electrodes [34,59]. 

Application of the reaction to the 2-azidobenzoyl derivative of diethylene glycol monomethyl ether 92, in a mixture of tetrahydrofuran and diethylene glycol monomethyl ether as the nucleophile, affords 2-(2-methoxyethoxy)ethyl 2-[2-(2-methoxyethoxy)ethoxy]-37/-azepine-3-carbo-xylate (93), which displays metal cation complexing properties towards lithium, potassium, and. to a lesser extent, barium and calcium cations.198... 

Metallic tin reacts with methyl halides and 2-halogenopropanoates at 135°, catalyzed by magnesium and butyl iodide in tetrahydrofuran, to give compounds MeSnXjCHgCHgCOjR, from which various other derivatives were prepared (42). 

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. 

Transition metal catalysis on solid supports can also be applied to indole formation, as shown by Dai and coworkers [41]. These authors reported a palladium- or copper-catalyzed procedure for the generation of a small indole library (Scheme 7.23), representing the first example of a solid-phase synthesis of 5-arylsulfamoyl-substituted indole derivatives. The most crucial step was the cydization of the key polymer-bound sulfonamide intermediates. Whereas the best results for the copper-mediated cydization were achieved using l-methyl-2-pyrrolidinone (NMP) as solvent, the palladium-catalyzed variant required the use of tetrahydrofuran in order to achieve comparable results. Both procedures afforded the desired indoles in good yields and excellent purities [41]. 

While the notion that the alkoxides derived from aliphatic alcohols are poor nucleophiles toward 7r-allylmetal complexes has prevailed over the years, much progress made in the recent past has rendered the transition metal-catalyzed allylic alkylation a powerful method for the O-allylation of aliphatic alcohols. In particular, owing to the facility of five- and six-membered ring formation, this process has found extensive utility in the synthesis of tetrahydrofurans (THFs) (Equation (29))150-156 and tetrahydropyrans (THPs).157-159 Of note was the simultaneous formation of two THP rings with high diastereoselectivity via a Pd-catalyzed double allylic etherification using 35 in a bidirectional synthetic approach to halichondrin B (Equation (30)).157 The related ligand 36 was used in the enantioselective cyclization of a Baylis-Hillman adduct with a primary alcohol (Equation (31)).159... 

DMA in 500 ml ether mix rapidly with 270 ml 0.9 M phenyl-Li, boil fifteen hours and extract as for (VI) or as described previously to get 8 g oily 4-methoxy-indoline (or its 1-methyl derivative) (VII). Alternatively, add 36 g naphthalene to 300 ml tetrahydrofuran and add 11 g Na metal cut in small pieces. Reflux and stir three hours and add 18 g (VI) and 8 g DEA in 200 ml tetrahydrofuran rapidly and boil twelve hours. Evaporate in vacuum, dissolve the oily residue in 2N HCI and extract with ether. Proceed as described to get (VII). 4 g (VII) in 200 ml dry pyridine add to 6 g Cu chloride in 400 ml pyridine and reflux 1 xh hours. Pour on water and extract with ether. Wash extract with 4N HCI and then water and dry and evaporate in vacuum the ether to get 2 g of the indole (VIII). Alternatively, dissolve 4 g (VII) and 9.5 g cinnamic acid in 700 ml mesitylene, add 1 g 5% palladium-carbon and reflux five hours. Filter, wash with HCI and NaHC03 and dry and evaporate in vacuum the mesitylene to get the red, oily (VIII) (can chromatograph on alumina and elute with benzene-petroleum ether). 

The explosive carbonylalkali-metals , previously formulated as monomeric compounds, are either dimeric acetylene derivatives of the general formula MOC=COM. or are trimers of the latter and formulated as salts of hexahydroxybenzene. Many true carbonylmetal complexes are air-sensitive and pyrophoric, not always immediately. Individually indexed compounds are Bis(dicarbonylcyclopentadienyliron)-bis(tetrahydrofuran)magnesium, 3835... 

As shown in Scheme 4, alkynyl metallates derived from propynoic acid amides can also be used as source of the allenylidene C3 unit, their reactions with the tetrahydrofuran solvates [M(CO)5(THF)] affording the N/O-substimted allenylidene complexes 6 after selective O-alkylation with [R30][BF4] [29]. 

The formation of benzene (or substituted benzene derivatives) is a common transformation catalyzed by numerous homogeneous and heterogeneous metal catalysts, mainly Co, Rh, Pd, and Ni.63-69 Even highly crowded molecules, such as hexaisopropylbenzene, could be synthesized in the presence of metal carbonyls such as [Co(CO)4]2.70 A very simple catalyst system, Me3SiCl and Pd on carbon in refluxing tetrahydrofuran, has been shown to transform symmetrical alkynes into hexaalkylbenzenes in excellent yield.71... 

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