2-Alkylfurans

In conclusion, furan and 2-alkylfurans can be polymerized only by acidic initiators or by y-radiation because the other standard methods of polyaddition fail to induce a chain-propagation reaction. 

A comparison of the cationic polymerization of 2,3-dihydrofurans with that of furan and 2-alkylfurans shows that the complications of the latters two, arising from the dienic character of the monomers, obviously vanish when the monomer is a simple cyclic vinyl ether with just one reactive site, viz. the carbon-carbon double bond. However, it also points out that ring opening in the polymerization of furans by acidic catalysts in the absence of water is unlikely, because otherwise it would also occur to some degree in the polymerization of dihydrofurans. 

The effect of acids on 2-alkylfurans was discussed in Section III-A-l-c). It was remarked that 2,5-disubstituted furans are less reactive in this context than mono-substituted derivatives. This observation again points to the important role played by a free C-5 position in resinification processes. 

Furyllithium reacts rapidly with trialkylborons though the furylborons 91 believed to be formed are not actually isolated. Treated with an electrophile (iodine or A-chlorosuccinimide), they transfer one alkyl residue to the furan ring and eject the boron residue in a reaction well known in other series (Scheme 46). The 2-alkylfurans are indeed produced in excellent yields and the method is better than many other syntheses.236... 

Butenolides.1 2-Alkylfurans can be converted into 5-alkyl-2(3//)-butenolides by lithiation followed by alkylation with C1B(0CH,)2. The products are unstable, but are oxidized in situ by m-chloroperbenzoic acid exclusively to the butenolides 2. 

The diastereoselective synthesis of enantiopure 5-[2-(alkoxyalkyl)-l-(hydropetoxypropyl)]-3-alkoxycarbonyl-2-alkylfurans has been reported <99TA2023>. 

Substituted2-alkylfuranes. Electrophilic substitution of 2-alkylfuranes occurs at Cs. Introduction of a phenylthio group at C5 blocks the position and also activates C4 to electrophilic attack. 

Scheme 5 Capping reaction of living PIB with 2-alkylfurans...

Also identified in this system are 2-alkylfurans. 2-Alkylfur-ans are known thermal and autoxidation products of unsaturated fatty acids (23). 2-Alkylfurans identified in this study are presumably derived by the thermal oxidation of 2,4-decadienal. 

Since the a-directing power is more pronounced in furan than in thiophene (see Section III,A, 2), the preference for substitution at C-5 is here reinforced. Substitution in 2-alkylfurans actually occurs at this position in all cases and no detectable formation of other isomers is observed.1... 

Tetraisopropylthiuram disulfide 447 is a reagent of choice for preparing thiols from the corresponding lithio derivatives (Scheme 142). 2,4-Disubstituted furans, difficult to prepare by classical methods, have been prepared from 2-phenylthio-5-alkylfurans as shown in Scheme 143. The starting material is obtained by treatment of 2-alkylfurans with -butyllithium followed by diphenyl disulfide. The practicality of this approach thus illustrates the potential of the phenylthio group as a protecting group. 

Birch and Slobbe have found that the intermediate anion from the reduction of 2-furoic acid can be alkylated in situ (equation 17). These dihydrofuroic acids can be oxidatively decarboxylated with LTA to afford the corresponding 2-alkylfurans in what clearly represents an attractive alternative to the standard two-step metallation-alkylation of the furan ring. 

When equimolar amounts of 2-lithiofurans (55a,b) and R3B are treated with iodine, 2-alkylfurans (56) are formed. Bulky alkyl groups give higher yields of 56 (79S146) (Scheme 22). 

The cycloaddition of 3-fluorobenzyne to 2-alkylfurans shows considerable regioselectivity in favor of the syn isomer 244 The range is from 2 1 (R = Me) to 10 1 (R = r-Bu) and is rationalized in terms of a nonsynchronous Diels-Alder transition state that reflects the considerable polarization of 3-fluorobenzyne and the slight polarization of the 2-alkylfuran, coupled with a steric effect of the alkyl substituent,... 

Magnesium bromide catalyzed reaction of 2,5-dimethoxy-2,5-dihydrofuran with vinyl ether derivatives afforded the corresponding functionalized 2-alkylfurans in a good yield. The reaction might proceed through the concerted mechanism, in which the MgBrg-activated dihydrofuran reacted with the vinyl ether via a cyclic intermediate <01TL2023>. 

Alkyl-4-hydroxycyclopent-2-enones may be prepared in ca. 40% yield from the appropriate 2-alkylfuran in three steps, viz. anodic methoxylation, acid-catalysed hydrolysis, and base-induced cyclization and rearrangement (Scheme 65). ... 

Furan [66, 67] and its 2-formyl derivative (Equation 10.43) [68], as well as 2-alkylfuran [69], are arylated at the 2- and 5- positions. The arylation of 3-ethoxycar-bonylfuran as well as its thiophene analogue occurs selectively at the 2-position in the presence of Pd(PPh3)4 in toluene, whereas the 5-position is attacked preferably using Pd/C in NMP (Equation 10.44) [70]. It has been proposed that carbopallada-tion and electrophihc attack predorrtinantly participate in the 2- and 5-arylations, respectively. 

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