Acid-catalyzed ring opening

Epoxides react very readily with nucleophiles in acid-catalyzed reactions. For example, ethylene oxide reacts in dilute aqueous HCl to form ethylene glycol. 

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How do the bond distances of 1 2 epoxypropane change on protonation of the nng oxygen" Assum ing that the longer C—O bond is the weaker of the two do the bond distances in the protonated form correlate with the regioselectivity of acid catalyzed ring opening ... 

Predict which carbon undergoes nucleophilic attack on acid catalyzed ring opening of cis 3 3 3 tnfluoro 2 3 epoxybutane Examine the C—O bond distances of the protonated form of the epoxide on Learning By Modeling How do these bond distances compare with your prediction" ... 

When acid-catalyzed ring opening is not synchronous with nucleophilic attack, the intermediate carbonium ion can undergo rearrangement (193,195) (66JOC3941, 73CJC1448). 

Thiirane 1-oxide undergoes acid-catalyzed ring opening by ethanethiol to give ethyl 2-ethylthioethyl disulfide. Treatment of thiirane 1,1-dioxide with thiolate anions, sodium sulfide or thiourea gives /3-mercaptosulfinic acid derivatives (75S55). Thiiranium ions are attacked at carbon by most sulfur nucleophiles (79ACR282), but see Section 5.06.3.4.3 for exceptions. 

Whereas the acid-catalyzed ring opening of 4,5-epoxy-3-ketones in the a-and -series normally gives rise to the type of products discussed above, in some circumstances 4, 5jS-epoxy-3-ketones give abnormal, rearranged products. These products are 2a-hydroxy-A" -3-ketones (or esters thereof) (166). 

Acid-catalyzed ring opening of epoxides is discussed in Section 16.13. 

Ring cleavage of l-oxo-l,2,3,4-tetrahydro-jS-carboline derivatives (374) may be accomplished by base-catalyzed hydrolysis to yield tryptamine-2-carboxylic acids (375). In the case of the 1,9-dimethyl derivative decarboxylation accompanied acid-catalyzed ring-opening, and the corresponding tryptamine (376) was obtained directly. 

The acid-catalyzed ring opening of the four-memhered ring lactone in... 

Diels-Alder cycloaddition of 3,4-bis(trifluoromethyl)furan with ethyl propynoate involved addition of two a,/3-unsaturated esters followed by acid-catalyzed ring opening, rearrangement, and elimination of ethanol to give a 6,7-bis(trifluoromethyl)isocoumarin-3-carboxylate [92JFC(56)359]. 

Scheme 2.34 Acetic acid-catalyzed ring-opening.

The mechanistic interpretation of the acid-catalyzed ring opening reaction of thiirane oxides125 is based on the push-pull mechanism162 with a transition state in which the bonded hydrogen atom plays a major role (see equation 59). 

Poly(tetramethylene oxide) polyols (see Scheme 4.4) are a special class of polyethers syndiesized via acid-catalyzed ring-opening polymerization of tetrahy-drofuran. Although less susceptible to side reactions, the synthesis of these C4 ethers is less flexible in terms of product composition and structure. Thus, because of diis syndietic route, only two-functional glycols are available and copolymers are not readily available. Molecular weights of commercial C4 glycols range up to about 3000 g/m. 

The majority of reactions of aziridines deal with acid-catalyzed ring opening with various nucleophiles. In this review only reactions with aziridine-2-car-boxylic esters are summarized (for other types of aziridines, see ref. [3]). 

A mechanism analogous in many ways to that of the acid-catalyzed ring opening reaction was advanced for the reaction of the thiirane oxide with alkyl chloromethyl ethers S . The first step is the displacement of the chloride by the sulfoxy oxygen (equation 24). In view of the above mechanistic interpretation, it is quite surprising that the parent thiirane oxide (16a) was found to be protonated on sulfur and not at oxygen in FSOjH-SbFfi at — 78 °C, according to NMR studies s . 

Under some circumstances, acid-catalyzed ring opening of 2,2-disubstituted epoxides by sulfuric acid in dioxane goes with high inversion at the tertiary center.116... 

Base and acid catalyzed ring opening polymerization of cyclotri-siloxane, -[Me2SiO]3- [e.g. reaction (10)], is a well-known method of generating high molecular weight polysiloxanes. 

Rochow et al. (27-28) have examined acid catalyzed ring opening polymerization as an approach to the formation of polysilazanes. 

If the epoxide is unsymmetrical, the nucleophile attacks primarily at the more substituted carbon atom in acid-catalyzed ring opening. 

The susceptibility of the sydnone ring to acid-catalyzed hydrolysis at elevated temperature is exploited for the preparation of hydrazines <2000MI227>. Primary amines are first converted to sydnones, which are then hydrolyzed to give hydrazines that may be isolated or reacted further without isolation <1996CHEC-II(4)165>. As masked hydrazines, 3-arylsydnones are firstly subjected to acid-catalyzed ring opening and then allowed to react further in situ with 2-(4-chlorophenyl)hydrazono-3-oxo-butyric acid to form substituted pyrazolidinones (Equation 2) <2005SC2169>. 

The formation of triazoles by acid-catalyzed ring opening and isomerization of various other C4-substituted sydnones has also been explored <2001MI769>. When applied to 3-(2-acetylphenyl)sydnone, conditions that are successful for C4 acylation of other 3-arylsydnones result unexpectedly in the formation of the jV-acctyl-3-methylindazole 72. Initial protonation of the carbonyl in the ortho-acetyl group triggers attack by the N2 position of the sydnone ring and concludes with hydrolysis and loss of carbon dioxide (Equation 3) <1996SC2757>. 

Reactions of [Pt(dipic)Cl] , dipic = dipicolinate, with 1-methylimida-zole or with 1,2-diaminoethane, monitored in DMF solution, involve replacement of chloride followed by opening of the dipic chelate ring (216). Kinetic data for acid-catalyzed ring opening in hydrolysis of [Pt(dipic)Cl] and of [Pt(glygly)Cl] (217) are compared with those for carboplatin (218) in Table VIII. 

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