Acid Catalyzed Rearrangements

Proton donors or Lewis acids result in the transformation of oxiranes to carbonyl compounds. The first step is the binding of the electrophilic agent followed by the splitting of the Cp-0 bond (Eq. 119). 

Much attention has been paid to the mechanism via the carbonium ion intermediate. Detailed studies have been performed on the dependence on the migrating group. In the BFs-catalyzed rearrangement of the 1,1-disubstituted oxirane 100 (Eq. 120), ° it has been proved that the H atom cis to the methyl group migrates as a consequence of the interaction between the bulky tert-butyl and the OBF3 group. In this case, 102 is the favored conformer. 

Rearrangement of deuterated derivatives of n-hexyloxirane has been investigated in order to clarify the mechanism for oxiranes not containing a tertiary carbon atom. Selective migration of the H atom trans to the alkyl group was observed. 

A fair number of publications have appeared on the isomerization of alkyl- and aryl-substituted oxiranes also containing other functional groups (e.g. Refs. ). 

BF3 also causes the very stable allenemonooxirane derivative to undergo rearrangement (Eq. 121)7  

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Synthesis of (A) started with the combination of 2,4,6-trimethylphenol and allyl bromide to give the or/Ao-allyl dienone. Acid-catalyzed rearrangement and oxidative bydroboration yielded the dienone with a propanol group in porlactone ring were irons in the product as expected (see p. 275). Treatment with aqueous potassium hydroxide gave the epoxy acid, which formed a crystalline salt with (R)-l-(or-naphthyl)ethylamine. This was recrystallized to constant rotation. 

An interesting class of 2-imino-3-amino-4-thiazolines (408) has been described (578, 701, 726). These 3-amino derivatives of 4-thiazoiine may also be prepared from 2,3-diaminothiazolium salts (406) in basic medium (101) or through the acid-catalyzed rearrangement of 2-acylaminoimino-3-phenyl-4-phenyl-4-thiazolines (407) (Scheme 233) (99, 724). 

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... 

Fig. 5. Alkaline hydrolysis of piperityl chloride to cis- and /n j -2-menthen-l-ol and acid-catalyzed rearrangement to piperitols (78) and (79).

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 ... 

The acid-catalyzed rearrangements of substituted pyrroles and thiophenes consequent on ipso protonation have been referred to previously (Section 3.02.2.4.2). There is some evidence that these rearrangements are intramolecular in nature since in the case of acid-induced rearrangement of 2-acylpyrroles to 3-acylpyrroles no intermolecular acylation of suitable substrates could be demonstrated (Scheme 10) (8UOC839). 

BECKMANN Rearrangement or fragmentation Acid catalyzed rearrangement of oximes to amides or cleavage of oximes to nitnies... 

Thermal or Lewis acid catalyzed rearrangement of N alkylanilines to o (p-) akylated anilines... 

RUZICKA - FUKUSHIMA Rearrangement Base or acid catalyzed rearrangement of I7a-hydroxy-20-keto steroids to 0-tiomo steroids. 

Acid catalyzed rearrangement of azoxybenzenes to p-hydroxyazt enzsnes... 

WESSELY - MOSER Rearrangement Acid catalyzed rearrangement d dihidroxyxanthone... 

CF3OSO2F or CH3OSO2F, PhSCH3, CF3CO2H, 0°, 30 min, 100% yield. Thioanisole suppresses acid-catalyzed rearrangement of the benzyl group to form 3-benzyltyrosine. The more acid-stable 2,6-dichlorobenzyl ether is cleaved in a similar manner. 

The preparation of neopentyl alcohol from diisobutylene herein described represents an example of acid-catalyzed addition of hydrogen peroxide to a branched olefin, followed by an acid-catalyzed rearrangement of the tertiary hydroperoxide formed. In addition to neopentyl alcohol, there are formed acetone and also small amounts of methanol and methyl neopentyl ketone by an alternative rearrangement of the hydroperoxide. 

Acid-catalyzed rearrangement of a-phenylethyl chlorocarbonates in dioxane 80 -3.01... 

Therefore, it may also be possible to aromatize the dihydrothiophene derivatives (93) obtained through the acid catalyzed rearrangement of a-hydroxymethylene-y-thiobutyrolactone derivatives (92) in ethanold °... 

Quantitative measurements of the rate of acid catalyzed rearrangement of 3-methyl-l-(2-thienyl) allyl alcohol (178) to 1-methyl-3-(2-thienyl)allyl alcohol (179) showed that (178) rearranged forty times faster than the phenyl analog but about three times slower than the... 

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. 

Mechanism of the formation of phenol by acid-catalyzed rearrangement of cumene hydroperoxide. 

Refer to Figure 26.2 and propose a mechanism for the final step in the Edrnan degradation—the acid-catalyzed rearrangement of the ATZ derivative to the PTH derivative. 

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. 

In a similar fashion, the acid-catalyzed rearrangement of 3-benzoxepin gives indenecarbalde-hyde, but no experimental details have been reported.93... 

In order to avoid acid-catalyzed rearrangements of 1,6-oxido-[10]annulene it is recommended that the distillation flask be treated with a base before use. 

For instance morphine (1) can be detected by the formation of various quinones via apomorphine as intermediate [4, 12, 13]. All morphines with an OH group in the 6-position and a double bond (codeine, ethylmorphine etc.) first undergo an acid-catalyzed rearrangement according to the following scheme [12] ... 

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