Epoxides catalyzed rearrangement

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 ... 

A modified Payne rearrangement of amino epoxides catalyzed by Lewis acid or induced by base, represents an interesting but a limited method for the synthesis of fonctionalized aziridines of high enantiopurity. The limitations are primarily due to the accessibility of the starting materials (Scheme 6) [15]. 

Predict the structure and stereochemistry of the Lewis acid-catalyzed rearrangement of the following epoxides. 

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. 

Scheme 4-21 shows the preparation of L-threitol and L-erythritol.38 Epoxy alcohols (2J ,3iS)-61 and (2S,3/ )-61. generated by asymmetric epoxidation, are exposed to sodium benzenethiolate and sodium hydroxide in a protonic solvent to undergo base-catalyzed rearrangement, yielding the threo-diol 62 and erythro-diol 63, which can then be converted to the corresponding tetraacetate of l-threitol 67 and L-erythritol 69 through subsequent transformations. 

The chemical versatility of the oxaspiropentanes makes these compounds exceedingly useful building blocks. Being a strained epoxide, they are very labile towards acid catalyzed rearrangements accompanied by carbon bond migration leading to... 

The key intermediate in Tobe et al. s synthesis of (+)-marasmic acid (27), 1-oxa-spirohexane (26), was accessed via a photocycloaddition between enone 24 and 1 (Scheme 19.6) [8], The photocydoadduct 25 was obtained in 73% yield with the desired isomer consisting of 91% of the material. The structure of the minor product obtained from this cycloaddition was not confirmed. Reduction of the carbonyl group of 25 and epoxidation of the exocyclic double bond gave 26. An acid-catalyzed rearrangement of 26 afforded the core structure of marasmic acid and was subsequently taken on to complete the synthesis of this natural product. 

An early report by Crandall,42 that epoxidation of 2,3-diisopropylidene-l-l-dimethylcyclo-propane followed by lithium iodide catalyzed rearrangement of the resulting oxaspiropentane yields 4-isopropylidene-2,2,3,3-tetramethylcyclobutanone (1), marks the beginning of an intense use of alkylidenecyclopropanes for the construction of cyclobutanones. 

Nearly all examples of epoxidation of alkylidenecyclobutanes involve 3-chloroperoxybenzoic acid.15 58-70 This is because the conditions are mild, the workup is easy and few byproducts are formed. Generally, dichloromethane or chloroform is used as solvent. Solid sodium hydrogen carbonate is occasionally added to avoid acid-catalyzed rearrangement of the spiro compound. For example, 6-isopropylidene-l,4,4-trimethylbicyclo[3.2.0]heptan-3-one reacted with 3-chloroperoxybenzoic acid and sodium hydrogen carbonate to give 2,2,3, 3, 6-pentamethyl-spiro[3-oxabicyclo[4.2.0]octane-8,2 -oxirane]-4-one (4) in quantitative yield. However, without the use of sodium hydrogen carbonate, substantial amounts of 2,2,6,9,9-pentamethyl-3-oxa-bicyclo[4.3.0]nonane-4,8-dione (5) and 2,2,6,8,8-pentamethyl-3-oxabicyclo[4.3.0]nonane-4,9-dione (6) were also formed.15-64... 

Campholenic Aldehyde Manufacture. Campholenic aldehyde is readily obtained by the Lewis-acid-catalyzed rearrangement of a-pinene oxide. It has become an important intermediate for the synthesis of a wide range of sandalwood fragrance compounds. Epoxidation of (+)- Ct-pinene (8) also gives the (+)-o -a-pinene epoxide [1686-14-2] (80) and rearrangement with zinc bromide is highly stereospecific and gives (-)-campholenic aldehyde... 

In conclusion, the pulmonary toxicity of 4-ipomeanol seems to be due to metabolic activation probably by formation of an unstable epoxide on the furan ring catalyzed by CYP4B1 in the Clara cell. The epoxide can rearrange to an a, 3 unsaturated dialdehyde (Fig. 7.37), which can interact with macromolecules and covalently bind, leading to cellular necrosis and edema. 

Direct oxidation of aromatics with m-CPBA in a two-phase system affords the most stable so-called K-region arene oxides in moderate yields.805 Careful control of pH is necessary to avoid acid-catalyzed rearrangement of the acid-sensitive product epoxides. The new powerful oxidants, dimethyldioxiranes,81,82 have also been used to convert arenes to arene oxides.806... 

Base-catalyzed rearrangement of ethylene oxides is a topic that baa, until now, received only limited attention in the literature, chiefly because epoxides undergo simple nudeophilio attack rather than isomerisation with most bases. Strictly speaking, a base-catalysed epoxide isomerization is one in which the initial event is direct proton abstraction from the oxide ring. This may bo followed by redistribution of bonding electrons in any of several possible ways, to give ultimately one or more carbonyl compounds. For the general case the course i>f such a reaction may be depicted a in Eq. (480),... 

The ability of ethylene oxide to undergo rearrangement to acetaldehyde was mentioned (see section. L2.) in connexion with the thermal decomposition and photolysis of ethylene oxide, and also (see section m.l.C.) in connexion with catalytic ethylene oxidation at elevated temperatures. This characteristic property is discussed, again below with regard, to reactions of epoxides with Qrignard reagents (see section IV.4.F,). For the purposes of this section the subject of epoxide isomerization can be divided into two parts. The first, and most extensive, is concerned with thermal and acid-catalyzed ethylene oxide isomerisation the second involves base-catalyzed rearrangement. 

The epoxidation of dehydrolinalyl acetate 35 provides the corresponding epoxide 36 (75 %) which can lead to karahanaenone 38, a key odorous component of hop oil621. The epoxide 36 can be converted to the keto acetate 37 (82%) by an electrogenerated acid (EG Acid)-catalyzed rearrangement. Hydrogenation followed by alkaline hydrolysis of 37 gives 6-hydroxy-2,6-dimethyl-7-octen-3-one (86%) and subsequent thermal dehydration at 200 °C affords 38 (Scheme 3-12). 

Scheme 3 Oxidation of carotol to its /rani-epoxide and the acid-catalyzed rearrangement product daucol [51]...

A stereospecific synthesis of dihydropyran-4-ones 937 can been achieved by a mercury(ll)-catalyzed rearrangement of l-alkynyl-2,3-epoxy alcohols 936. The configuration of the epoxide is transferred unchanged to the product (Equation 367, Table 44) <1998JOC3798>. 

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