Unsaturated epoxidation

Substituted epoxides are attacked by organocopper reagents at the least hindered carbon atom and form alcohols (C.R. Johnson, 1973A). With a, 9-unsaturated epoxides tram-allylic alcohols are produced selectively by 1,4-addltion (W. Carruthers, 1973 G.H. Posner, 1972). 

The methodology used in the preparation of RU 486 (84) and other ll -steroids is shown. Conjugate addition of a cuprate reagent to the a,P-unsaturated epoxide (85) provides the liP-substituted steroid (86) stereospecificaHy (131). Subsequent steps lead to the synthesis of RU 486 (84). 

Crystallinity is low the pendent allyl group contributes to the amorphous state of these polymers. Propylene oxide homopolymer itself has not been developed commercially because it cannot be cross-baked by current methods (18). The copolymerization of PO with unsaturated epoxide monomers gives vulcanizable products (19,20). In ECH—PO—AGE, poly(ptopylene oxide- o-epichlorohydrin- o-abyl glycidyl ether) [25213-15-4] (5), and PO—AGE, poly(propylene oxide-i o-abyl glycidyl ether) [25104-27-2] (6), the molar composition of PO ranges from approximately 65 to 90%. 

In mosl allylation reactions, only a catalytic amount of CuCN-2LiCl is required [41]. Use of die chiral ferrocenylamine 104 as a catalyst makes enables asymmetric allylation of diorganozinc reagents to be effected witli allylic chlorides iScbeme 2.3G) [78]. Related electropb des such as propargylic bromides [79] and unsaturated epoxides [80] also undergo Su2 -substitution reactions iScbeme 2.37). 

Pineschi and Feringa reported that chiral copper phosphoramidite catalysts mediate a regiodivergent kinetic resolution (RKR) of cyclic unsaturated epoxides with dialkylzinc reagents, in which epoxide enantiomers are selectively transformed into different regioisomers (allylic and homoallylic alcohols) [90]. The method was also applied to both s-cis and s-trans cyclic allylic epoxides (Schemes 7.45 and 7.46,... 

Epoxytin compounds are obtained by either hydrostannylation of unsaturated epoxides or condensation of organotin alcohols with epichlorohydrin 70). Extensive applications of these compounds in plastic industry have been suggested. 

A rather new concept in the context of domino radical cydizations has been developed by Gansauer and coworkers utilizing titanocene-complexes for the radical opening of unsaturated epoxides. The titanocene-catalyzed reactions [61] of 3-145 primarily led to radical 3-146, which underwent a subsequent intermolecular addition to a present a,(3-unsaturated carbonyl compound to form bicyclic carbocy-cles of type 3-148 via the intermediate 3-147 after aqueous work-up (Scheme 3.38) [62]. From a kinetic point of view, the reaction is remarkable since the intermolecular addition of simple radicals to a,(3-unsaturated carbonyl compounds is not an easy task, as highlighted above. 

As a first step towards this goal, we managed to develop a titanocene-catalyzed 5 -exo cyclization of radicals derived from suitably unsaturated epoxides. The mechanism of the cyclization is depicted in Scheme 12.18 [31],... 

Scheme 2.37. Substitution reactions of propargylic bromides and unsaturated epoxides with organozinc reagents.

A detailed mechanistic study of acid-catalysed monocyclization of 5,6-unsaturated epoxides, such as (66), has now provided compelling evidence for a pathway in which the oxirane C—O cleavage and the C—C bond formation are concerted. These experimental results are now further supported by theoretical evidence for a concerted mechanism of the oxirane cleavage and A-ring formation in epoxysqualene cyclisation, obtained at the RHF/6-31G and B3LYP/6-31 + G levels. The chemical pathway thus parallels the mechanism of the enzymatic cyclization that plays a role in the biosynthesis of isoprenoids. 

Moderate to good yields of a,j8-unsaturated epoxides are obtained, allowing aromatic and aliphatic aldehydes to react with dialkyltelluronium allylide (the dusobutyl derivatives are the reagents of choice). ... 

Unsaturated epoxides are reduced preferentially at the double bonds by catalytic hydrogenation. The rate of hydrogenolysis of the epoxides is much lower than that of the addition of hydrogen across the carbon-carbon double bond. In a, -unsaturated epoxides borane attacks the conjugated double bond at -carbon in a cis direction with respect to the epoxide ring and gives allylic alcohols [660], Similar complex reduction of epoxides occurs in a-keto epoxides (p. 126). 

SCHEME 76. Opening of a, -unsaturated epoxides with copper-zinc reagents... 

Isomerization of u,fi-unsaturated epoxides. This base is superior to LDA or lithium diethylamide for isomerization of a,/ -unsaturatcd epoxides to allylic alcohols with a trans double bond.1--Examples ... 

Allylation of nucleophiles. jr-Allyl palladium complexes (a) of a,/S-unsaturated epoxides react with active methylene compounds to give 1,4-adducts [allylic alcohols, equation (I)].10... 

Cyclization of unsaturated epoxides.1 Reaction of the epoxy alcohol (2) derived from linalool with cobaloxime(I), (1), forms the P-hydroxycobaloxime 3. 

Coupling of Arl and unsaturated epoxides. Palladium-catalyzed coupling of CftHsI with unsaturated epoxides in which the two groups are separated by one or more carbon atoms can provide arylated allylic alcohols as the major product. Highest yields obtain with Pd(OAc)2 as catalyst, 1-3 equiv. of an alkali metal formate as... 

Millar, J.G., Giblin, M Barton, D. and Underhill, E. W. (1990a). 3Z,6Z,9Z-Trienes and unsaturated epoxides as sex attractants for geometrid moths. J. Chem. Ecol., 16, 2307-2316. 

Cyclization of epoxy alkenes.2 The reaction of 8,e-unsaturated epoxides with 2 equiv. of Cp2TiCl in THF results in cyclization to cyclopentanemethanols. The reaction is compatible with keto and nitrile groups but not aldehyde groups. 

Allylic bromides 383 and 384 were the relais substances for the synthesis of coriamyrtin (9) (Scheme 43). The allylic bromides were eliminated with potassium ferf-butoxide in toluene affording diene 387. 1,4-Addition with W-bromosuccini-mide in polar solvent at room temperature yielded the unsaturated a-bromo-5-hydroxy unit of 388, which formed the unsaturated epoxide 389 by base treatment. Epoxidation with peracid occurred as expected from the convex face. The cyclic ether moiety of bisepoxide 390 was oxidized to the lactone 368 with ruthenium... 

Asteromurin A (22), a compound isolated from a scale insect was the next target of Yamada , picrotoxane synthesis project. An intermediate of the synthesis of tutin (11), the unsaturated epoxide 395, was the relais substance. 

Epoxyannelation. The enolate of cyclohexanone reacts with this reagent (1) to form the a,(3-unsaturated epoxide 2, with displacement of dimethyl sulfide, and a mixture of two sulfur-containing products (3). The reaction to give an epoxide is general for enolates of unhindered, cyclic ketones, but is not observed with enolates of acyclic ketones. However, it is possible with enolates of aldehydes. An example is the conversion of phenylacetaldehyde into 4 and 5, in somewhat low yield. 

Reduction of a,fi-unsaturated epoxides. Allylic alcohols can be prepared by-reduction of a,/3-unsaturated epoxides. Although yields are only moderate (40-h.S"/o), the method is stereoselective in that the (Z)-isomer is formed. The reaction is believed to involve conjugate reduction. ... 

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