Alkyl furans

Compounds which are rather unstable. Typical members of this class are 2-furaidehyde, 2-furfuryl alcohol and 2-alkyl furans, the latter being more resistant than the former. The action of acids or oxygen on these derivatives produces appreciable resinification, but, if properly purified and stored in vacuo, they are indefinitely stable25 16s. ... 

Fig. 13. Insecticidal compounds from the Piperaceae and the general structure of natural and synthetic insecticidal alkyl furans.

Alkyl furans are another class of naturally occurring compounds with insecticidal activity (Fig. 13)- This group of compounds, commonly referred to as avocado furans is present in avocados, Persea americana (Lau-raceae) and related plants. When tested against beet armyworm, Spodoptera... 

Cationic polymerization of furan (53) and alkylated furans is often complicated by the formation of crosslinked, insoluble materials (77MI11102). Early reports postulated 1,2-polyaddition as a primary reaction followed by crosslinking through the resultant dihydrofuran moieties. More recent results (80MI11106) shed considerable doubt on the 1,2-addition sequence, at least beyond the trimer stage. The ultimately formed poly(furans) are believed to be composed mainly of units with structural features similar to those of tetramers (54 Scheme 14). 

Mono- and di-alkylated furans were synthesized in a one-pot preparation from 2-propynyl-2-tetrahydropyranyl ether (106), butyllithium and formaldehyde. The intermediate allenyl ether (107) presumably cyclizes via a 2-(2-tetrapyranyloxy)-2,5-dihydrofuran (108) to afford the heterocycle (109) (79AG(E)875). In a similar manner, singly and doubly branched tetrahydropyranyloxybutynolates afforded the substituted furans (110) (Scheme 20). The thermocatalytic isomerization of ethyl l-methyl-2-phenylcyclopropene-3-carboxylate yielded the furan, possibly by a 1,3-sigmatropic displacement step or by a non-concerted biradical intermediate (75T2495). 

Furan and the simple alkyl-furans are relatively stable to aqueous mineral acids, though furan is instantly decomposed by concentrated sulfuric acid or by Lewis acids, such as aluminium chloride. Furan reacts only slowly with hydrogen chloride, either as the concentrated aqueous acid or in a non-hydroxylic organic solvent. Hot, dilute aqueous mineral acids cause hydrolytic ring opening. 

The hydrolysis (or alcoholysis) of furans involves nucleophilic addition of water (or an alcohol) to an initially formed cation, giving rise to open-chain 1,4-dicarbonyl-compounds or derivatives thereof. This is in effect the reverse of one of the general methods for the construction of furan rings (18.13.1.1). Suc-cindialdehyde cannot be obtained from furan itself, presumably because this dialdehyde is too reactive under conditions for hydrolysis, but some alkyl-furans can be converted into 1,4-dicarbonyl products quite efficiently, and this can be viewed as a good method for their synthesis, and of cyclopentenones derived from them. ° Other routes from furans to 1,4-dicarbonyl compounds are the hydrolysis of 2,5-dialkoxy-tetrahydro-furans (18.1.1.4) and by various oxidative procedures (18.2). 

Carboxylic acid anhydrides or halides normally require the presence of a Lewis acid (often boron trifluoride) for Friedel-Crafts acylation of furans, though trifluoroacetic anhydride will react alone. Aluminium-chloride-catalysed acetylation of furan proceeds 7 x lO times faster at the a-position than at the P-position. 3-Alkyl-furans substitute mainly at C-2 2,5-dialkyl-furans can be acylated at a P-position, but generally with more difficulty. 3-Bromofuran is efficiently acetylated at C-2 using aluminium chloride catalysis. ... 

Bei Normaldruck und 200-300° werden 5-Acyl- bzw. 5-(l-Hydroxy-alkyl)-furane an Palladium/Kohle Iediglich in der Seitenkette hydriert4,5. 

Furan and the simple alkyl furans are relatively stable to aqueous mineral acids, though furan is instantly decomposed by concentrated sulfuric acid or by Lewis acids... 

Electrophihc fluorination of the 2-alkyl-furan lithium derivative with V-fluoro-... 

The furan ring is found in furoic acid (3.38). Some alkyl furan derivatives have value as odorants. Thus, rose furan (3.39) gives the characteristic odor of rose oil. Other derivatives have potent biological activity, such as beta-furyl isoamyl ketone (perilla ketone, 3.40), which causes pulmonary edema in animals grazing on plants containing this substance (such as Perilla Frutescens Britton). Ascorbic acid (the well-known Vitamin C), which when written as the enoUc form (3.42) is... 

Nicotine has been prepared in one step by ring expansion of cyclopropyl 3-pyridyl ketone Phthalimidines can be obtained by intramolecular Friedel-Crafts ring closure of carbamyl chlorides . Easy preparations of 2-methylindoles from ar. amines and 2,3-dibromopro-pene through N-(2-bromallyl)anilines and of 1-azaadamantanes from N-tosylpiperidone enamines through 3-azabicyclo[3.3.1]non-9-ones have been reported. 3-Hydroxypyridinium salts have been prepared by a new method, the chlorine oxidation of N-monosubst. 2-(a-amino-alkyl)furans . 

The aldehyde group stabilizes the furfural stmcture against the acid-catalyzed fission of the furan ring that readily occurs in furan and alkyl furan derivatives. Basic-type stabilizers, such as tertiary amines, can be used to inhibit oxidation of the furan structure [3]. Furfural can be oxidized to furoic acid or reduced to furfuryl alcohol which can be converted to furan by a catalyzed decarbonylation reaction. On an industrial scale furfural is the source for furfuryl alcohol, tetrahydrofurfuryl alcohol, furan, and tetrahydrofuran. Nitro-furan compounds derived from furfural are used as bactericides in humans. 

The synthesis of (polyhydroxy)alkyl-furan, -pyrrole, and -indole derivatives, etc., by condensation of aldoses and ketoses with P-dicarbonyl compounds has been reviewed. ... 

In a sequence that is the equivalent of an oxocarbenium-induced C-C bondforming reactirai, McQuade and coworkers have used organic catalysts to carry out the coupling of 2-hydroxy tetrahydrofuran 48 with methyl ketones 49 to give 2-alkyl furans (Scheme 15) [13]. Mechanistically the reaction is proposed to proceed through the addition of a thiourea-stabiUzed enolate from the reaction of 49 with 51 to an iminium intermediate that comes from the condensation of 48 with proline catalyst 50. Hydrolysis and Michael cyclization or displacement of the ammonium ion subsequent to the Mannich reaction gives the observed product 52. 

Shimizu et al. also demonstrated that, although the [4+3] cycloaddition proceeded through the same 2-siloxyallyl cation as intermediate, the reaction mechanism and outcome could still be strongly influenced by solvent (Scheme 18.10). While the reactions performed in nitromethane were concerted, in the less polar solvent system of THF/ether, the reaction of (Z)-39 was more stepwise, class B/C-like, resulting in the formation of alkylated furan 43 as the major reaction product. The reaction of 44 in THF/ether yielded cycloadducts 46 and 47 in a ratio that did not correspond to the geometry of the enol ether, in addition to yielding alkylated furans 49 and 50 as the major products. 

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