Acid-catalyzed rearrangement reactions

One of the mdustnal processes for the preparation of phenol discussed in Section 24 6 includes an acid catalyzed rearrangement of cumene hydroperoxide as a key step This reaction proceeds by way of an intermediate hemiacetal... 

Garyophyllene. (-)-CaryophyUene can be isolated from Indian turpentine and has been used to prepare a number of woody aroma products. The epoxides are produced by reaction with peracids. Acetylation of caryophyUene also gives a usehil methyl ketone (180) (Fig. 8). Acid-catalyzed rearrangement of caryophyUene in the presence of acetic acid gives a mixture of esters, which are related to caryolan-l-ol and clovan-2-ol (181). 

Maltol. Otsuka Chemical Co. in Japan has operated several electroorganic processes on a small commercial scale. It has used plate and frame and aimular cells at currents in the range of 4500—6000 A (133). The process for the synthesis of maltol [118-71 -8], a food additive and flavor enhancer, starts from furfural [98-01-1] (see Food additives Flavors and spices). The electrochemical step is the oxidation of a-methylfurfural to give a cycHc acetal. The remaining reaction sequence is acid-catalyzed ring expansion, epoxidation with hydrogen peroxide, and then acid-catalyzed rearrangement to yield maltol, ie ... 

Primary and secondary alcohols are best converted into alkyl halides by treatment with either thionyl chloride (SOCl2) or phosphorus tribromicle (PB ). These reactions, which normally take place readily under mild conditions, are less acidic and less likely to cause acid-catalyzed rearrangements than the HX method. 

Curran s synthesis of ( )-A9(l2)-capnellene [( )-2] is detailed in Schemes 30 and 31. This synthesis commences with the preparation of racemic bicyclic vinyl lactone 147 from ( )-norbomenone [( )-145] by a well-known route.61 Thus, Baeyer-Villiger oxidation of (+)-145 provides unsaturated bicyclic lactone 146, a compound that can be converted to the isomeric fused bicyclic lactone 147 by acid-catalyzed rearrangement. Reaction of 147 with methylmagne-sium bromide/CuBr SMe2 in THF at -20 °C takes the desired course and affords unsaturated carboxylic acid 148 in nearly quantitative yield. Iodolactonization of 148 to 149, followed by base-induced elimination, then provides the methyl-substituted bicyclic vinyl lactone 150 as a single regioisomer in 66% overall yield from 147. 

Sulfur ylides can also transfer substituted methylene units, such as isopropylidene (Entries 10 and 11) or cyclopropylidene (Entries 12 and 13). The oxaspiropentanes formed by reaction of aldehydes and ketones with diphenylsulfonium cyclopropylide are useful intermediates in a number of transformations such as acid-catalyzed rearrangement to cyclobutanones.285... 

An additional example of an oxonium ion generated via the acid catalyzed rearrangement has been used to prepare a dihydropyran <06TL6149>. The oxonium ion 54 generated by the reaction of an epoxide with ZrCl4 can be trapped by a nucleophile such as butynol to prepare dihydropyran 55. A variety of mono- and disubstituted epoxides have been used in this reaction. 

Regarding the first problem, the most elemental treatment consists of focusing on a few points on the gas-phase potential energy hypersurface, namely, the reactants, transition state structures and products. As an example, we will mention the work [35,36] that was done on the Meyer-Schuster reaction, an acid catalyzed rearrangement of a-acetylenic secondary and tertiary alcohols to a.p-unsaturatcd carbonyl compounds, in which the solvent plays an active role. This reaction comprises four steps. In the first, a rapid protonation takes place at the hydroxyl group. The second, which is the rate limiting step, is an apparent 1, 3-shift of the protonated hydroxyl group from carbon Ci to carbon C3. The third step is presumably a rapid allenol deprotonation, followed by a keto-enol equilibrium that leads to the final product. 

Interesting direct evidence for the reaction pathway of these metal-catalyzed reactions was obtained from stoichiometric reactions of allenylcarbinols such as 61 with the [j7S-CsHsFe(CO)2]+ (Fp+) complex. From the isobutene adduct of Fp+ and 61 the jt-complex 65 was obtained. The complex 65 was suspected to be the product of an acid-catalyzed rearrangement indeed, in the presence of N,N-dimethylaniline as a base, 63 was isolated (Scheme 15.14) [30],... 

These intrazeolite ene reactions are susceptible to many of the problems associated with photo-oxidations in solution. In particular, sensitizer bleaching, competition between type I vOi, and both type IIavRH and type 11 02 reactions, and reduced photostability of the products can complicate the synthetic use of these reactions [19]. In addition, many substrates are susceptible to acid-catalyzed rearrangements that can lead to complicated reaction mixtures. 

When a 4-methyl group is present, the reaction follows a different course (Section IV,D,l,c) earlier claims of an acid-catalyzed rearrangement to the isomeric 4-hydroxymethyl-l//-imidazole were later corrected. ... 

Some unusual reactions have been described for 2-(4-chlorophenyl)-2-(3,3-dimethylallyl)-4-phenyl-5(277)-oxazolone 84. This compound undergoes a Lewis acid-catalyzed rearrangement to give a tetrahydrofuropyrrole 85. On the other hand, depending on the reaction conditions, thermolysis of 84 produces the azabicyclohexene 86 or a substituted 2,3-dihydropyridine 87 together with the caged compound 88 formed by dimerization of the 2,3-dihydropyridine and the azabicyclohexene (Scheme 7.21). " ... 

Another group of acid-catalyzed rearrangements involves cleavage of the 3,4-bond followed by cyclization between positions 2 and 4, for example the conversion of (218) to the quinazoline carboxaldehyde (219). Similar reactions are given by (220) and (221). 

In the case of tricyclo[4.4.2.01-6]dodecan-2-one, acid-catalyzed rearrangement gave tricy-clo[4.3.3.01,6]dodecan-7-one (8), the propellane-like skeleton of which is otherwise difficult to construct synthetically. The reaction mechanism requires two consecutive Wagner-Meerwein... 

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