Furan derivatives, hydrogenation

Uses. Furfural is primarily a chemical feedstock for a number of monomeric compounds and resins. One route produces furan by decarbonylation. Tetrahydrofuran is derived from furan by hydrogenation. Polytetramethylene ether glycol [25190-06-1] is manufactured from tetrahydrofuran by a ring opening polymeri2ation reaction. Another route (hydrogenation) produces furfuryl alcohol, tetrahydrofurfuryl alcohol, 2-methylfuran, and 2-methyltetrahydrofuran. A variety of proprietary synthetic resins are manufactured from furfural and/or furfuryl alcohol. Other... 

The addition of p-quinone to enamines normally produces furan derivatives, especially when the enamine possesses a 3 hydrogen (see Section III. A). 1,2 Cycloaddition is claimed to take place to give a cyclobutane derivative when p-quinone and an enamine with no jS hydrogens are allowed to react at low temperatures (51). However, little evidence is reported to verify this structural assignment, and the actual structure probably is a benzofuranol (52). Reaction of a dienamine (formed in situ) with p-quinone in the presence... 

It should be pointed out that not all benzoin derivatives (75) are suitable for use as photoinitialors. Benzoin esters (75, R=aeyl) undergo a side reaction leading to furan derivatives. Aryl ethers (75, R=aryl) undergo (3-seission to give a phenoxy radical (an inhibitor) in competition with a-scission (Scheme 3.54). Benzoin derivatives with a-hydrogens (75 R-Il) are readily autoxidized and consequently can have poor shelf lives. 

The acid XXVI was subsequently studied by Garcia Gonz lez.6 He was at first unable to isolate the furan derivative (III) after oxidation with 30 % hydrogen peroxide under the same mild conditions as had been used for converting compound II to this substance. Subsequently, the isolation of some 20% of compound III was successfully accomplished by oxidizing compound XXVI very slowly with potassium permanganate at low tempera-... 

Furan derivatives are sometimes convenient starting materials in this group of syntheses.14 For example, catalytic hydrogenation of... 

Indolizines with a saturated six-membered ring may be obtained from pyrrole or furan derivatives.225,226 The reaction of 3,4-dimethylpyrryl-magnesium bromide with y-chlorobutyronitrile followed by dilute acid gave 5,6,7,8-tetrahydro-l,2-dimethyl-5-oxoindolizine. 5,6,7,8-Tetrahy-droindolizine can be obtained by the cyclodehydration of 3-(2-furyl)-propylamine over alumina at 400°C. The same product is obtained by the treatment of 3-(2-pyrryl)propyl cyanide with hydrogen chloride in the presence of boron trifluoride etherate catalyst, followed by Wolff-Kishner reduction of the intermediate ketone. 

The deuterium-labelled 428 has been synthesized503 via PhC=CCD20H to confirm the source of the C —H hydrogen in 429. The -NMR spectra of the furan, derived from... 

Benzo[ ]furans (coumarones) are readily hydrogenated to the 2,3-dihydro derivatives over palladium and nickel catalysts under mild conditions. Thus 5-methoxybenzo-furan was hydrogenated to 2,3-dihydro-5-benzofuran in 95.6% yield over Pd-C in ethanol at room temperature (eq. 12.110).214... 

It was proposed that either a hydrogen bond or polar interactions favor this approach. When the O-methylated furan derivative 16 was treated under the same conditions, no conversion was observed. 

Pyrylium salts are also suitable for the preparation of furans.7Z,7S Oxidation of these salts by aqueous hydrogen peroxide in the presence of perchloric acid leads to ring contraction, with the formation of furan derivatives. Nenitzescu and Balaban described the following example72 ... 

Ionic hydrogenation is also described for indole, carbazole and furan derivatives (79MI1). However, it is most important for thiophenes since the latter are well known as poisons for most hydrogenation catalysts. 

Re-aromatisation by elimination of hydrogen fluoride has been used as a route to thiophene and furan derivatives [20, 21] (Figure 9.5). 

A study of the photodeconjugation reaction of the ester (136) in the presence of a variety of optically active amines such as (IR,2S)-l-phenyl-2-isopropylamino propanol has shown that the product can be obtained with an enantiomeric excess of about 70 X. Irradiation of the alkynylketones (137) in alcohols (methanol, ethanol or propanol) affords the furan derivatives (138) by a free radical hydrogen abstraction path. Excitation of the cyclohexenone derivatives (139) yields the cyclized products (140) the structures of which were verified by X-ray crystallography. The formation of the amide products is reminiscent of a Norrish Type II process. 

Experiments showed that furan derivatives, which are frequently found to occur in the products of hydrolysis of pentose-containing polysaccharides with mineral acids, are not formed on solvolysis with hydrogen fluoride. ... 

Another indirect route to endo substituents in the cycloadduct commences with an appropriately substituted furan derivative. After cycloaddition, the ole-finic bridge is hydrogenated from the less hindered side, forcing the substituents into the endo position. This strategy was used to mimic a net dihydroxylation from the more hindered face, Eq. 67 [111]. 

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