Epoxidation Lewis acid catalysation

The first enantioselective synthesis of cis- and trans- 3-hydroxyflavanones is based on the Lewis-acid-catalysed reaction of phenylmethanethiol with chalcone epoxides <96CC2747>. Further support for the intermediacy of epoxides in the Algar-Flynn-Oyamada flavone synthesis has been provided by the isolation of epoxides in the corresponding preparation of 3-hydroxy-2-phenylquinol-4-ones <96JCS(P2)269>. 

The baccharane derivative (131) is formed by Lewis acid-catalysed rearrangement of both the a- and /3-epoxides (132) and (133) of the A -isomer of betulin diacetate. The rearranged product has been synthesized by an independent (131) route. The stereochemistry of the /S-epoxide (133) has been confirmed by X-ray analysis. 

Lewis Acid Catalysed Rearrangement of Humulene 8,9-Epoxide... 

Stereo- and regio-selective epoxidation of tricyclic trienes, such as (36), followed by Lewis acid-catalysed opening of the resulting monoepoxide with alcohols, has been reported to occur exclusively at the vinyl terminus of unsaturated system through a typical S 2 process, affording 1,6-dioxygenated derivatives (e.g. 37).49... 

The chemistry of epoxides is dominated by the basicity of the oxygen lone pairs and the release of ring strain as the three-membered ring opens. Many of the reactions of epoxides are acid or Lewis acid catalysed. The catalyst may coordinate with the oxygen, increasing the polarity of the C -O bond and the sensitivity of the carbon atom to reaction with a nucleophile. The reactivity of epoxides are summarized in Box 2.5. 

Whereas the foregoing Diels-Alder based strategies involve vertical disconnections a recent approach to 11-deoxyanthracyclinones employs a diagonal disconnection [125]. Lewis acid catalysed reaction of the acryloyl quinone 164 with various pentadienyl tins resulted in a tandem Michael/Diels-Alder sequence to give the tetracyclic products 166 in one step. These were converted to ( )-11-deoxyanthracyclinones via the epoxides 167. Another diagonal ... 

New preparative syntheses of 6-0-(2-amino-2-deoxy-a-D-glucopyranosyl)-D-chiro-inositol 1-phosphate and a 1,2-cyclic phosphate derivative have been reported. Differential protection was achieved by initially locking the four equatorial hydroxyls of D-c/iiro-inositol as cyclic Tipds ethers and then generating a ci5-epoxide from the two remaining trans-diaxial hydroxyls under Mitsunobu conditions. Lewis acid catalysed trans-diaxial opening with allyl alcohol allowed for selective protection at 0-1 and 0-6. ... 

Oxoisophorone 68 can be converted into trimethylcyclohexanone (74), in a series of reduction and elimination steps [82], opening up a new route to p,p-carotene (3) and vitamin A [85,86]. The Cio-epoxide 75 can be obtained by Wittig olefination of 73 with methylene-triphenylphosphorane, isomerization and reaction with peracids. Lewis acid-catalysed rearrangement to give the five-membered ring yields the capsorubin synthon 76 [9]. 

The acetal for the second building block is obtained in two stages from iso-vanUlin in 96 % yield. After the introduction of iodine, the hydroxy-group is reprotected and the aldehyde converted into its homologue by reaction with a sulfur ylide, and then by Lewis acid-catalysed opening of the epoxide. In this way, there results an appropriate building block for ring A of the morphine skeleton. 

Two Lewis acid catalysed rearrangements of epoxides have been described which result in the formation of spiro-ring compounds. 

A one-step Lewis acid-catalysed intermolecular 4- -3-cycloaddition of aromatic a,)3-unsaturated aldehyde and ketones (105) with epoxides (106) formed seven-membered oxacycles (107) under mild conditions (Scheme 34).The effect of oxygen-, sulfur-, and halogen-substituents on the reactivity of nitrogen-stabilized oxyallyl cations in 4- -3-cycloaddition reactions has been extensively investigated. Aza-oxyallyl cationic intermediates react with cyclopentadiene and furan via an aza-4 -I- 3-cycloaddition reaction to form bicyclic cycloadducts in moderate yields. The intramolecular formal 4- -4-cycloaddition of conjugated enynes with an e-deflcient cyclobutene (108) yielded a strained six-membered cyclic allene (109) that isomerized to a 1,3-cyclohexadiene (110). This intermediate underwent a thermal or acid-promoted six-electron electrocyclic ring opening to yield a 2,4,6-cyclooctatrienone (111) (Scheme 35). ... 

Lewis acid-catalysed condensations of monosubstituted epoxides with keten acetals lead to butyrolactones (Scheme 12). When = Cl, the initial adducts can be dehydrochlorinated to butenolides whereas condensations using 1-chloro-2,3-epoxypropane eventually give 4-methylenebutyrolactones. 

Allylic fra i-l,2-azidohydrins prepared by Lewis-acid-catalysed ring opening of cyclic vinyl epoxides with sodium azide via an 5 2 mechanism have been found to isomerize to trans-l,4-azidohydrins via a suprafacial allyl azide [3,3]-sigmatropic rearrangement (Scheme 16). ... 

In studies of the rearrangements of epoxides of humulene, Bryson et have shown that the 8,9-epoxide (155) undergoes Lewis acid-catalysed rearrangement producing one major alcohol (156), which is related to the bicyclodiene (157) obtained from acid-catalysed rearrangement of humulene itself. 

The dianion from a-phenylselenopropionic acid has been utilized to prepare both trans- and cis-fused a-methylenebutyrolactones. Coupling with cyclohexane-1,2-epoxide leads to mainly the trans-isomer trans. cis = 12 1) whereas alkylation with 3-bromocyclohexene followed by either acid-induced cyclization or iodolactonization gives only the c/s-isomer (Scheme 20). cis-Fused a-methylenebutyrolactones can also be prepared by Lewis acid-catalysed condensation of cyclic ketones with Meldrum s acid followed by lactonization... 

Two reviews, one discussing the mechanisms of the Lewis acid-catalysed ring-opening reactions of fiillerene epoxides by nucleophiles and the other covering the ring-opening reactions of oxiranes and aziridines in water, have been published. 

Next to iodine there is also another class of neutral Lewis acids known. Tetracyanoethylene, dicyanoketene acetals and derivatives can catalyse reaction due to their tt-Lewis acid properties. They promoted the alcoholysis of epoxides [238], tetrahydropyranylation of alcohols [239], monothioacetahzation of acetals [240], and carbon-carbon bond formation of acetals [241,242] and imines [243] with silylated carbon nucleophiles. 

These reactions are catalysed by acids such as Lewis acids, phenols, and alcohols. The hydroxyl groups formed by the amine epoxide addition are active catalysts, so that the curing reaction usually shows an accelerating rate in its early stages, typical of auto catalysis. In some cases when the amine is present in less than stoichiometric concentrations, reaction of epoxide and hydroxyl may occur to produce an ether group ... 

Treatment of the 7a,8a-epoxy-5-oxo-5,6-secocholestane aldehyde (147) with mineral acid or Lewis acid gave the acetal (148) which with BF3-Et20 rearranged to the acetal (149). The olefin (154), which was formed in the BF3-catalysed fragmentation of the exocyclic epoxides (150) and (151) and, as previously reported, in the similar fragmentation of the endocyclic epoxide (152), was shown to arise from the oxetan (153) in all cases. 

Brill and co-workers used levoglucosane as a rigid scaffold to synthesise libraries both in solution [74] and solid phase [75]. The opening of epoxide 125 by different hydroxycarboxylic acid methyl esters catalysed by Lewis acid, followed by simultaneous new epoxide formation and ester hydrolysis gave the resulting free acid ready for attachment to the solid support. Epoxide opening reactions with alkoxides, amines and thiols were carried out on solid phase... 

Indoles react with epoxides and aziridines in the presence of Lewis acids (see 20.4.1 for reaction of indolyl anions with such reactants) with opening of the three-membered ring and consequent 3-(2-hydroxyethylation) and 3-(2-aminoethylation) of the heterocycle. Both ytterbium triflate and phenylboronic acid are good catalysts for reaction with epoxides under high pressure silica gel is also an effective catalyst, but reactions are slow at normal pressure and temperature. Reaction with aziridines can be catalysed by zinc triflate or boron trifluoride. °... 

The 1,3-dioxolans (234 R = CH2CI, Me, or Et) have been prepared under neutral conditions by the reactions of the corresponding epoxides with benz-aldehyde. The reactions are catalysed by halide ion and provide a mixture of cis- and rra s-2,4-disubstituted compounds. Using Lewis acid catalysts, the ds-isomer was preferentially formed whereas catalysis by lithium halides favoured the rrans-isomer. 

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