Acid-labile epoxides

The use of the ionic liquid [bmim][BF4] further improved the Burgess epoxidation system [70]. Chan and coworkers found that replacement of sodium bicarbonate for tetramethylammonium bicarbonate and performing the reaction in [hmim][BF4] allowed for efficient epoxidation of a number of different olefins, including substrates affording acid-labile epoxides (such as dihydronaphthalene (99% yield) and 1-phenylcyclohexene (80% yield)). 

In order to prevent competing homoallylic asymmetric epoxidation (AE, which, it will be recalled, preferentially delivers the opposite enantiomer to that of the allylic alcohol AE), the primary alcohol in 12 was selectively blocked as a thexyldimethylsilyl ether. Conventional Sharpless AE7 with the oxidant derived from (—)-diethyl tartrate, titanium tetraisopropoxide, and f-butyl hydroperoxide next furnished the anticipated a, [3-epoxy alcohol 13 with excellent stereocontrol (for a more detailed discussion of the Sharpless AE see section 8.4). Selective O-desilylation was then effected with HF-triethylamine complex. The resulting diol was protected as a base-stable O-isopropylidene acetal using 2-methoxypropene and a catalytic quantity of p-toluenesulfonic acid in dimethylformamide (DMF). Note how this blocking protocol was fully compatible with the acid-labile epoxide. 

In searching for a relationship between antijuvenile hormone activities and epoxide chemical reactivity, we attempted to apply as a chemical probe the m-chloroperoxybenzoic-alkaline fluoride system, a reagent developed in this laboratory for preparation of acid labile epoxides (20). However, formation of hemiesters of 3,4-dihydroxy precocene, was the predominant reaction in the case of activated chro-mene structures. 

Rudler and coworkers reported that in the case of moderately acid-sensitive epoxides the use of biphasic reaction conditions (H2O/CH2CI2) proved to be sufficient in order to obtain the epoxides with good selectivity because under biphasic conditions the contact of epoxides with water is minimized. Because of the lability of pyridines under the reaction conditions employed, alternative and more stable cocatalysts such as pyrazole (12 mol%, biphasic conditions), bipyridine (6 mol%, biphasic conditions H20/CH2Cl2) and bipyridine-A,Af -dioxide (1.2mol%) were employed together with MTO (Scheme Pyrazole is stable against oxidation and with this additive... 

Regeneration of spent reagent is easy, and no loss in peroxide content is seen even over several recycles. Physical properties of the material are essentially unchanged. Labile epoxides, such as the industrially important a-pinene oxide can also be prepared in good yield under relatively mild conditions. This may be attributed to the heterogeneous nature of the system, which means that the solution phase is non-acidic. 

Most epoxides resist Jones oxidation with the exception of the very acid-labile ones,55 that is the ones able to generate a very stable carbocation on opening. 

The epoxidation of unsaturated materials is the most commonly used reaction of dioxiranes. The reaction, whether carried out in situ or using the solution method, gives high yields of epoxides, usually in rapid reactions. The isolated dioxirane method is particularly useful in cases where oxidation products are sensitive to the acidic conditions of the most commonly used epoxidation reagents or the basic conditions of the in situ method. Use of the isolated dioxirane permits reaction under neutral conditions so that labile epoxides are conveniently prepared. When the desired epoxide is sensitive to moisture, the dioxirane solution can be dried with molecular sieves for use in such epoxidations. The epoxidation of water-insoluble alkenes with the in situ method requires the use of a phase-transfer catalyst, but the reaction still proceeds in high yields. Dioxiranes, in situ generated... 

As shown in the Table, with 5 mol% of 1,1-dioxotetrahydrothiopyran-4-one as catalyst,10 epoxidation of various olefins (2-mmol scale) in a homogeneous acetonitrile-water solvent system with 1.5 equiv of Oxone at room temperature can be achieved in a short period of time with excellent yields of epoxides (80-97%) isolated by flash column chromatography.2 As the pH of the reaction is maintained at 7-7.5 by sodium bicarbonate, acid- or base-labile epoxides (entries 12-14) can be easily isolated without decomposition. More importantly, the in situ epoxidation of olefins can be performed on a large scale directly with 5 mol% of tetrahydrothiopyran-4-one, which is oxidized immediately by Oxone to 1,1-dioxotetrahydrothiopyran-4-one during the epoxidation reactions. For example, with 5 mol% of tetrahydrothlopyran-4-one, substrates 3,5 (20 mmol each) and 11 (100 mmol) were epoxidized with excellent isolated yields of epoxides (91-96%). 

Hydrogenation. The Pd-C(en) catalyst shows selectivity in hydrogenation such that an epoxide group is retained. The same system provides a means for the hydrogenation of alkenes that also contain an acid-labile TBS ether. ... 

The related imidophosphorane wherein Z = PhCH2N, and its acyclic analogue PhCH2N=P(NMe2)3, are very efficient catalysts for the protective acylation of alcohols in the presence of enol esters [96]. Acid-labile groups (such as acetal and epoxide) survive and groups such as TBS and disulfide [which undergo... 

When carrying out epoxidation reactions it should be remembered that the reaction is exothermic, that high concentrations of peroxy acids should be avoided, and that epoxides are acid-labile. 

Epoxy compounds. Epoxy compounds function both as plasticizers and stabilizers in flexible and semirigid PVC. As stabilizers, epoxies react with liberated HCl. In addition, they react with the polymer chain at labile-chlorine sites—either directly or catalytically by increasing the reactivity of the labile-chlorine site with metal salt stabilizers. Most epoxy stabilizers are derived from unsaturated fatty oils and fatty acid esters. Epoxidized soybean and linseed oils and epoxy tallate are commonly used products. Epoxy tallate also increases light stability. Epoxy compovmds can be formulated with metallic liquid stearates and, thus, can be sold to compounders as a one-package system if a constant ratio of stabilizer-to-epoxy is acceptable. However, since these epoxy compounds are also plasticizers, the balance of the formulation must be adjusted for this effect. 

Interaction between the coordinatively bound acid and a labile epoxide molecule in the second sphere of the iron ion, which leads to the formation of the reaction product. When a propylene oxide molecule forms an oxygen bridge, it does not take part in the reaction. 

Scheme 9.176. A representation of the reaction of the ethyl ester of cyclohexanone-2-car-boxylic acid with excess permethanoic acid (peracetic acid, CH3CO3H). The labile epoxide presumed to be produced from the corresponding enol is expected to rearrange to a ketone, which undergoes the Baeyer-Villiger reaction.

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