Oxirane rearrangements acid catalyzed

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... 

Unlike an A, A -disubstituted imidazoline-2-thione, the reaction of imidazolidine-2-thione 665 itself with 2-phenyloxirane in the presence of BF3 leads to the formation of l,3-bis[(i4)-2-phenylethen-l-yl]im>dazolidine-2-thione 666 (Scheme 161) <2005HCA3253>. This unusual mode of reaction is presumably due to a Lewis-acid-catalyzed rearrangement of oxirane to aldehyde followed by an ene-like reaction with the iminothiol. 

New results have likewise been reported on the acid-catalyzed rearrangement of steroid oxiranes. " Three different types of rearrangements are depicted in Eqs. 136-138. A new fragmentation is observed and isopropyl group migration occurs besides the oxirane -> carbonyl isomerization (Eq. 136). ... 

Ford GP, Smith CT (1987) Gas-phase hydrolysis of protonated oxirane. Ab initio and semiempirical molecular orbital calculations. JAm Chem Soc 109(5) 1325-1331 Coxon JM, Maclagan DGAR, Rauk A, Thorpe AJ, Whalen D (1997) Rearrangement of protonated propene oxide to protonated propanal. J Am Chem Soc 119(20) 4712 718 Korzan R, Upton B, Turnbull K, Seybold PG (2010) Quantum chemical study of the energetics and directionality of acid-catalyzed aromatic epoxide ring openings. Int J Quant Chem 110(15) 2931 2937... 

Skeletal rearrangement of alkynyl oxiranes and alkynyl aziridines is an attractive method to synthesize highly substituted furans and pyrroles under mild reaction conditions. Decades ago, mercury(II)/acid-catalyzed reactions of alkynyl oxiranes... 

Synthesis of Arene Oxides. Reaction of (Me2N)3P with aromatic dialdehydes provides arene oxides such as benz[a]anthracene 5,6-oxide (2a) (eq 5). These compounds, also known as oxiranes, are relatively reactive, undergoing thermal and acid-catalyzed rearrangement to phenols and facile hydrolysis to dihydrodiols. Consequently, their preparation and purification requires mild reagents and condition.s. The importance of this is underlined by successful synthesis of the reactive arene oxide (2b) in 75% yield using appropriate care, despite a previous report of failure of the method. While compound (2b) is a relatively potent mutagen, it is rapidly detoxified by mammalian cells. The principal limitation of the method is the unavailability of the dialdehyde precursors, which are obtained through oxidation of the parent hydrocarbons, e.g. by ozonolysis. 

The enhanced reactivity of oxiranes due to geometric strain is manifested in their acid- and base-catalyzed reactions, in their rearrangements in the presence of metals and metal compounds, and in their thermal and photocatalytic transformations. These reactions permit the development of new and varied methods of synthesis and the preparation of derivatives that in many cases are difficult to obtain by other routes. The main products of the isomerization reaction are unsaturated alcohols and carbonyl compounds. Great progress has been made in this area of organic chemistry during the past 20 years (The reader is referred to some relevant... 

More typically, mixtures of products are formed. Terpene epoxides that have been subjected to ZnBr2-catalyzed rearrangement include the oxiranes derived from limonene, carvomenthene, other pinenes, 2,3-carvene, pulegone and piperitone. ZnCh is rarely employed, but has been compared with AICI3 in a study involving sever epoxides. In most instances there appears to be no evidence for special advantages of zinc halides compared with other more commonly used Lewis acids. 

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