Epoxides electrocyclic ring-opening

Another carbenoid-typical reaction of a-lithiated epoxides is the 1,2-hydrogen shift, illustrated in Scheme 14. Two mechanistic pathways offer an explanation for the formation of the lithium enolate 94 First, the route via the a-ring opening of the epoxide followed by an 1,2-hydride shift in the carbene 93, and second, the electrocyclic ring opening of an oxiranyl anion 95 to an enolate anion 94. Both mechanisms are in accordance with different experimental... 

The formation of the oxepin is reasonably explained by an electrocyclic ring opening of rearranged epoxide 299 in a thermal reaction. As mentioned above, two routes to 299 are possible. If the rearrangement is concerted, a 1,5-sigmatropic reaction with inversion of the reaction center (oxygen) in 299 is photochemically allowed. It is possible to separate a nonconcerted process... 

The l,3-thiazole-5-thione (36) undergoes a regioselective 1,3-dipolar cycloaddition reaction with a carbonyl ylide. The ylide is thermally generated by the electrocyclic ring opening of the epoxide (37) to give the spirocyclic adducts (38a and 38b) <97HCA1190>. 

A mechanophore (blue in Fig. 2a) is a strategically designed chemical entity which responds to mechanical force in a predictable and useful manner (Fig. 2d-f). The polymer strand here acts as an actuator to transmit macroscopic force to the target. For a fully extended polymer chain, the maximum tension force is at the middle point of the chain contour. So the mechanophore should be incorporated into the middle of the chain with its active bond along the chain contotu (Fig. 2a) [15, 29, 32]. Examples of mechanochemical reactions include homolytic scission of weak bonds (diazo [33]), electrocyclic ring-opening (benzocyclobutenes [29], spiropyrans [32, 34 5], gem-dichlorocyclopropanes [46-49], ge/n-difluorocyclo-propanes [30, 50], and epoxide [51]), cycloreversion reactions (cyclobutane derivatives [52-56], Diels-Alder adducts [57, 58], 1,3-dipolar adducts [59, 60], and 1,2-dioxetanes [61]), dative bond scission [62-64], and flex-activated reactions [34, 65, 66], as recently reviewed by Bielawski [67]. 

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). ... 

Transformation of starting material to 6-17 involves a concerted elec-trocyclic transformation, followed by anion-promoted ring opening of the epoxide. The other transformations are one-step concerted processes. Explain the stereochemical preferences of each reaction. Why is the electrocyclic transformation of 6-17 to 6-19 favored when such reactions usually occur more readily in the opposite direction ... 

Many arene oxides are in dynamic equilibrium with their oxepin forms. The parent molecules, benzene oxide la and oxepin lb, are related as valence tautomers that interconvert by an allowed disrotatory electrocyclic reaction. Structural identification of la and lb was based initially upon spectroscopic evidence and chemical transformation to stable products of known structure. Thus the arene-oxide structure was inferred from its typical dienoid (4 + 27t cycloaddition) and epoxide (ring-opening, aromatization) reactions, while the oxepin structure was deduced by catalytic hydrogenation of the triene oxepin to form oxepane. 

Formation of an enolate from a lithiated epoxide could occur by two mechanisms a-ring opening (vide supra) followed by insertion of the carbene in the LiOC-H bond or electrocyclic P-ring opening (Scheme 18). There is some experimental support for both pathways [69,70]. 

Carbonyl ylides can also be prepared by electrocyclic epoxide openings. Under thermal and photochemical conditions, ring opening of an epoxide takes place through con- and disrotatory modes, respectively (Scheme 5.13). 

Electrophilic Cyclization of Trichloroacetimidates. Trichloroacetimidates derived from allylic " and homoallylic alcohols undergo electrocyclic ring closure when treated with a source of I" " (eq 5). Cyclization can also be triggered via the Lewis acid-mediated opening of epoxides. In at least one case the imidate proved more reactive than related reactions using carbamates. These are useful methods for the stereoselective introduction of nitrogen into cyclic and acyclic systems. 

Periplanone B is the most active sex pheromone found in the alimentary tract and excreta of the American cockroach Periplaneta americana. An elegant total synthesis of this germacrane sesquiterpene was achieved by SCHREIBER and SANTINI Cyclodecatrienone 1 is an obvious precursor. One of the oxirane rings arises from epoxidation of the enone CC double bond, the other from [2-1-1]-cycloaddition of a carbene to the carbonyl bond of the enone. Oxidation of the methylene group introduces the additional carbonyl double bond. The CC double bond of the enone results from an elimination of HX in the a-X-substituted cyclodecadienone 2, which, on its part, is feasible by substitution of cyclodecadienone 3. An electrocyclic opening of the cyclobutene ring in 4 provides the 1,3-diene substructure in 3. 

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