A-ring opening

Angle strain is the main source of strain in epoxides but torsional strain that re suits from the eclipsing of bonds on adjacent carbons is also present Both kinds of strain are relieved when a ring opening reaction occurs... 

Lactam polymerization represented by reaction 5 in Table 5.4 is another example of a ring-opening reaction, the reverse of which is a possible competitor with polymer for reactants. We shall discuss this situation in Sec. 5.10. 

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... 

Under acidic conditions, furfuryl alcohol polymerizes to black polymers, which eventually become crosslinked and insoluble in the reaction medium. The reaction can be very violent and extreme care must be taken when furfuryl alcohol is mixed with any strong Lewis acid or Brn nstad acid. Copolymer resins are formed with phenoHc compounds, formaldehyde and/or other aldehydes. In dilute aqueous acid, the predominant reaction is a ring opening hydrolysis to form levulinic acid [123-76-2] (52). In acidic alcohoHc media, levulinic esters are formed. The mechanism for this unusual reaction in which the hydroxymethyl group of furfuryl alcohol is converted to the terminal methyl group of levulinic acid has recendy been elucidated (53). 

HO—R—COOH, or an amino acid, H2N—R—COOH. In some cases, such monomers self-condense to a cycHc stmcture, which is what actually polymerizes. For example, S-caprolactam (1) can be thought of as the self-condensation product of an amino acid. Caprolactam undergoes a ring-opening polymerization to form another... 

From Multiring Systems Containing Pyrazoles. The pyrazolopyrimidine (65) on treatment with diazomethane forms the cyclopropane (66), which undergoes a ring-opening reaction with potassium hydroxide to yield the pyrazole (67) (eq. 16) (44). 

Sahcyhc acid, upon reaction with amyl alcohol and sodium, reduces to a ring-opened ahphatic dicarboxyhc acid, ie, pimelic acid (eq. 5). The reaction proceeds through the intermediate cyclohexanone-2-carboxyhc acid. This novel reaction involves the fission of the aromatic ring to i j -hexahydrosahcyhc acid when sahcyhc acid is heated to 310°C in an autoclave with strong alkah. Pimelic acid is formed in 35—38% yield and is isolated as the diethyl ester. 

Other classes of yellow couplers reported ia the Hterature include the iadazolones (71) and the benzisoxazolones (72). Neither of these stmctures contains an active methylene group dye formation is beUeved to occur through a ring-opening process. 

Polymerization. The polymer chain is formed by a ring-opening reaction caused by the action of alkaUes on the monomer, a cycHc siloxane ... 

One or two cases are reported of a ring opening involving attack on a pyridinone ring in a 7-oxotetrahydropyrido[2,3-[Pg.209]

An unusual reaction involving s-triazine (247) and ethyl acetoacetate with sodium ethoxide leads eventually to the pyrido[4,3-ring opening and Dimroth-type rearrangement of the intermediate (248) (80JHC389>. 

Base treatment of isoxazolin-5-ones produced the cyano aeids (70JOC3130), while ethoxide treatment of iV-alkyl-A -5-ones gave, after H2O treatment, a malonyl monoamide monoester (69JOC2981) and not an oximoacrylate as was reported earlier (Scheme 61) (23HCA102). The base treatment of a 4-bromoisoxazolin-5-one generated a ring-opened oxime (Scheme 62) (79JOC873). 

The treatment of 4-benzoyl-3-phenylisoxazolin-5-one with KOH generated 3,5-diphenyl-isoxazole-3-carboxylic acid via a ring-opened intermediate as shown in Scheme 69 (61CB1956). 

Reaction of 2,5-dimethylisoxazoline-3-thione with thiophenol produced a ring-opened thioamide, while pyrolysis gave two products via a thiocyclopropene intermediate (Scheme 76) (80CPB103). 

In a number of cases the intermediate oxime has been isolated in the reaction of hydroxylamine and /3-keto esters. The reaction of ethyl acetoacetate with hydroxylamine generated an oxime which cyclized on base treatment (Scheme 144) (70MI41600). Likewise, treatment of an analogous amide with hydroxylamine generated a ring opened material which cyclized on treatment with HCl (Scheme 144) (67T831). The presence of a minor contaminant in the standard reaction of ethyl acetoacetate with hydroxylamine was discovered and identified as an isomeric isoxazolin-3-one. The mechanism of product formation has been discussed (70BSF2685). 

Finally, the bimolecular cycloaddition of alkynes with 2-phenylazirines in the presence of molybdenum hexacarbonyl has been studied (79TL2983). The pyrrole derivatives (294) obtained appear to arise from an initial [2 + 2] cycloaddition followed by a ring opening reaction. 

The bis(silyloxy)cyclobutenes are also subject to a variety of special reactions. Probably the most interesting is the observation that they readily undergo a ring-opening reaction leading to a butadiene derivative. This reaction has already been used to prepare large-ring diketones from cyclic 1,2-diesters. 

The monomer, norbomene (or bicyclo[2.2.l]hept-2-ene), is produced by the Diels-Alder addition of ethylene to cyclopentadiene. The monomer is polymerised by a ring-opening mechanism to give a linear polymer with a repeat unit containing both an in-chain five-membered ring and a double bond. Both cis-and trans- structures are obtainable according to the choice of catalyst used ... 

One variation in polyester intermediates that has roused some interest are those prepared by a ring-opening polymerisation of e-caprolactone and methyl-e-caprolactones with titanium catalysts and diol and triol initiators Figure 27.6). 

Pyridone also catalyzes epimerization of the anomeric position of the tetramethyl ether of glucose. The mechanism involves two double proton transfers. The first leads to a ring-opened intermediate, and the second results in ring closure to the isomerized product ... 

The ring system 13.18 (R = Cl) is a source of hybrid PN/SN polymers containing three-coordinate sulfur via a ring-opening polymerization process. This polymerization occurs upon mild thermolysis at 90°C (Section 14.4)." ... 

The oxazolone 43, prepared by reaction of 0-methylcaprolactim (42) with compound 1, undergoes a ring-opening reaction with methanolic HCl and cyclizes in alkaline medium to 1,5-pentamethyl-ene-2-phenylimidazole-4-carboxylic acid (44), which can be decar-boxylated easily. 

Arylidene-5-oxazolones undergo a ring-opening reaction with aromatic thiols and a second mole of thiol is then incorporated to give products such as 48. ... 

Amino-3-alkyl derivatives of474 have been prepared and found to undergo facile nucleophilic 5-substitution with methanolic hydrazine (65°, 4 hr, 75% yield), hydroxide ion (20°, 3 hr, 75% yield), or pro-panolic benzylamine (100°, 5 hr, 80% yield). The first two reactions may proceed by a ring-opening and reclosure sequence. ... 

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