Stereochemistry of epoxidation

To control the stereochemistry of epoxidation at the 10,11-double bond in intermediates in prostaglandin synthesis, a bulky protective group was used for the C15-OH group. Epoxidation of the tribenzylsilyl ether yielded 88% a-oxide epoxidation of the tri-/ -xylylsilyl ether was less selective. ... 

The stereochemistry of epoxidation of 5j5-steroids is changed from to predominantly a by the presence of an a-oriented hydroxyl group. The magnitude of this hydroxyl effect is less in polar solvents, as shown by the... 

Bartnicki EW, CE Castro (1969) Biodehalogenation. The pathway for transhalogenation and the stereochemistry of epoxide formation from halohydrins. Biochemistry 8 4677-4680. 

I he stereochemistry of epoxide 4 reveals that it has arisen through Solution... 

The stereochemistry of epoxidation by using chiral ketones (3 and 4) as catalysts can be explained by spiro transition state, in which jt-electrons of the olefin attack the lone-paired electrons concurrently attack the it -orbital of double bond to give the epox ide (Figure 6B.4) [12,13]. The observed effect of the size and location of the olefinic substituents on enantioselectivity (Figure 6B.3) is compatible with the proposed transition-state model [10a],... 

Bycroft et al. [83] have prepared a series of novel 6-spiro-epoxypenicillins 43,44 (Scheme 12) by the reaction of diazopenicillanate 41 with oxalyl chloride followed by reactions with various nucleophiles. These compounds notably exhibit (i-lactam inhibitory and antibacterial properties [84] depending on substituents and stereochemistry of epoxides. These inhibitors possess side-chains, which are highly con-formationally restricted but structurally similar to those of some active penicillins. 

The relative rates and stereochemistry of epoxidation reactions of 5-substituted-adamantan-2-ones with two sulfur ylids (methylenedimethylsulfurane and its oxy-sulfurane analogue) have been studied in DMSO and in benzene.318... 

Sharpless epoxidation of alkenylsilanols.1 Allylic silanols also undergo highly enantioselective Sharpless epoxidation. This reaction furnishes simple epoxides such as styrene oxide in high optical purity. Thus reaction of fram-(3-lithiostyrene il) with ClSi(CH,)2H gives 2, which can be oxidized to the alkenylsilanol 3. ShaTp-less epoxidation of 3 gives the epoxide 4, which is converted to styrene epoxide 5 by cleavage with fluoride ion. The stereochemistry of epoxidation of 3 is similar to that of the corresponding allylic alcohol. 

The previous assignment of stereochemistry to isolongifolene epoxide (149)66 has been confirmed by epoxidation studies with analogous substrates.67 It has also been concluded that the factors controlling the stereochemistry of epoxidation are the bicyclohepty moiety and the C-2/3 methyl group. 

Bearing in mind the stereochemistry of epoxide 11, propose a mechanism for its acid-catalysed ring opening under the conditions of step o. 

Hydrogen peroxide or t-butyl hydroperoxide may be used in the presence of a catalyst such as sodium tungstate(VI) or vanadyl acetylaceto-nate [ MeC0CH=C(0 )Me 2V0] for the epoxidation of allylic alcohols. The stereochemistry of the hydroxyl group has a profound effect on the stereochemistry of epoxidation. A system which has been applied to allylic alcohols, to make optically active epoxides, utilizes titanium(rV) isopropoxide, t-butyl hydroperoxide and either of the enantiomeric forms of diethyl tartrate. This system forms chiral epoxides of predictable stereochemistry. When the reactivity of epoxides is combined with the... 

Epoxidation of allylic alcohols with peracids or hydroperoxide such as f-BuOaH in the presence of a transition metal catalyst is a useful procedure for the synthesis of epoxides, particularly stereoselective synthesis [587-590]. As the transition metal catalyst, molybdenum and vanadium complexes are well studied and, accordingly, are the most popular [587-590], (Achiral) titanium compounds are also known to effect this transformation, and result in stereoselectivity different from that of the aforementioned Mo- and V-derived catalysts. The stereochemistry of epoxidation by these methods has been compared for representative examples, including simple [591] and more complex trcMs-disubstituted, rrans-trisubstituted, and cis-trisubstituted allyl alcohols (Eqs (253) [592], (254) [592-594], and (255) [593]). In particular the epoxidation of trisubstituted allyl alcohols shown in Eqs (254) and (255) highlights the complementary use of the titanium-based method and other methods. More results from titanium-catalyzed diastereoselective epoxidation are summarized in Table 25. 

An investigation has been made of the stereochemistry of epoxidation of the sesquiterpene isolongifolene. The a-epoxide structure 29 has been proved by x-ray crystallography. 

Dialkylsulfoxonium ylides like (33) are stabilised by the oxygen atom. They are therefore less reactive than the corresponding dialkylsulfonium ylides, e.g. (21). The difference is reflected in several important respects. In studies of the stereochemistry of epoxide formation using the rigid 4-t-butylcyclohexanone molecule (34) (Scheme 15) as substrate, it was found that the more reactive sulfonium ylides like dimethylsulfonium methylide (21) reacted very quickly by axial attack to form mainly the kinetically controlled epoxide (35).2a... 

Mg-Al oxides with a Mg/Al ratio of 5, calcined at 400 °C, are the most active catalysts for the reaction of CO2 and styrene oxide, and DMF is the best solvent. Using this Mg-Al oxide, various kinds of epoxides could be quantitatively converted into the corresponding cyclic carbonates. This addition reaction proceeds with retention of the stereochemistry of epoxides the reaction of CO2 with (R)- and (S j-benzyl glycidyl ether gave and ( Sj-4-(benzyloxymethyl)-l,3-dioxolane-2-one with >99% e.e., respectively. 

The relative stereochemistry of epoxides can be inverted by eqnilibration with cyanate anion ... 

The stereochemistry of epoxide formation from ethylenes-tE was investigated also by taking the IR spectra of dideuterioethylene oxides prepared from gem-C2H2D2, from... 

The stereochemistry of epoxidation with peroxycarboxylic acids has been well studied. Attack and addition of oxygen occurs preferentially from the less hindered side of the molecule. Norbornene, for example, gives a 96 4 exo endo ratio. Other examples where this general trend is followed are summarized in a review by... 

Figure 3.78 Stereochemistry of epoxide formation in the conversion of hyoscyamine into scopolamine via 6 3-hydroxyhyoscyamine.

For the general case, a complicated scheme of alternate pathways has been pro-posed to convert the two half-chair conformations of a 3-substituted cyclohexene into two pairs of conformationally isomeric bromonium ions [(262X ( 63) and (264X (265)] and thence to four pairs of adducts (Scheme 4). This scheme is considered below, with reference to the stereochemistry of epoxide formation and hypobromous-add addition to olefins. A small allylic equatorial substituent does not greatly influence the site of electrophilic attack, and when the substituent is bulky the preference is for formation of (262). An axial substituent exerte a mild preference for (263) over (264). The nucleophilic step can also be influenced by shielding of the substituent An electron-with-... 

The stereochemistry of epoxide ring-opening by benzoic acid has been studied using /m j-4-methoxystilbene oxide (297). The monobenzoate formed was converted into the keto-benzoate (298), whose structure had previously... 

The stereochemistry of epoxidation of homoallylic alcohols has been established by reduction of the products with AIH4. The structure of resulting diols (334) and (335) demonstrates that the original peroxy-acid attack had occurred from the same side of the double bond as the OH group further peroxy-acid oxidation of (334) occurred from the opposite side owing to... 

Although (848) is an anti-Bredt system, stabilization of the positive charge is possible by electron donation from the strained anti double bond. The stereochemistry of epoxidation, hydroboration and osmylation of the double bond in (850), isolongifolene, and related molecules is controlled largely by the bicycloheptyl moiety and results in predominantly endo-attack. The regiospecific fragmentation of the C-1—C-12 bond in patchoulol (851) by lead tetra-acetate has been used to provide... 

---