Oxirane moiety

One convention (Dickson. 1968) for oxygen heterocycles sets the coulomb integral at z 2f) and the resonance integral at Eor the oxirane moiety,... 

The bis(diene) (46) adds dienophiles preferentially on the side syn to the oxirane moiety (Scheme 35) (80X149). This may be due to formation of a charge-transfer complex by donation of electron density from oxygen into an antibonding orbital on the dienophile. 

The stereochemistry of the cyclopropyl as well as oxirane moiety in these P-lactams was determined based on the single crystal X-ray analysis of P-lactam 34 and the 4-nitrobenzoyl derivative (37) of 3-hydroxy-P-lactam 32.71 The extremely high diastereoselectivity observed in these cyclopropanation and epoxidation reactions can be explained by taking into account the highly organized transition state structures illustrated in Figure 1. The directing effect of the C-3... 

Functionalized polymers of the type B and C (Scheme 1) can be formed via polyaddition processes of bifunctional reactants, without splitting off of low molecular weight compounds. Most syntheses of stabilizers have been based on reactions of bis-isocyanates with H-acid nucleophiles. Some reactions of oxiranes may be listed here too (syntheses involving oxiranes are listed in Sect. 3.1.1.2 if polymerization aspects are more evident syntheses of stabilizers formed via reactivity of oxirane moieties attached to an oligomeric or polymeric chain are classified as reactions on polymers. Sect. 3.2.2.1... 

Many authors elucidated functionalization of polymers containing reactive oxirane moieties. Epoxidized NR, BR, IR and/or the respective model hydrocarbons, poly (butadiene-co-isoprene, various epoxy resins, poly (2,3-epoxypro-pyl methacrylate) and its copolymers or grafted systems were mostly exploited. Stabilizers based on epoxidized unsaturated rubbers are of the top interest. The mechanism of the functionalization process was studied in details by means of 3,4-epoxy-4-methylheptane and 1,2-epoxy-3-ethyl-2-methylpentane as model compounds [289]. The ring opening of the asymmetric oxirane is regiospecific. Aliphatic primary amines attack the least substituted carbon atom and can be involved in crosslink formation. Aromatic primary and secondary amines are less reactive than aliphatic ones because of their lower basicity the attack on the least substituted carbon atom is however preferred too. 

The other mode of activation of the oxirane moieties is through cationic polymerization, which will generate a network for polyfunctional epoxidized glycerides and thermoplastic materials with fatty acids bearing a single epoxy group [43],... 

The absolute stereochemistry of the C-12 and C-13 oxirane moiety of laureoxolane (157), a colorless unstable bromoether obtained from extracts of Laurencia nipponica, was determined on the basis of a chiral synthesis of 156, a degradative derivative of 157. The C-5 to C-8 unit with two asymmetric centers at C-6 and C-7 of 157 corresponds to (25, 35)-l-benzyloxy-3,4-epoxy-2-butanol (142). Elongation of 142 using butyllithium and copper cyanide followed by the creation of a new epoxide provides 152. Lithium acetylide ethylenediamine complex addition to 152 and subsequent ketalization affords the acetylenic acetonide 153, which is coupled with (2i ,35)-l,2-epoxy-3-benzoyloxypentane (154) to furnish 155. Subsequent five-step transformation of 155 provides 156 [60] (Scheme 37). 

TGBG phases with large temperature ranges were also obtained in a remarkable series of materials first prepared by Takatoh [54] and later added to by Lamb [55]. The materials synthesized were derivatives of terphenyl. Using Sharpless chiral epoxidation of hexen-2-ol they made liquid crystals with terminal oxirane moieties which introduced two sequential chiral centers into the terminal aliphatic chain, see general structure 11. 

Scheme 4.6 Two-step acrylation of an oxirane moiety incorporated into a triglyceride chain [25, 26]...

Scheme 4.11 Mechanism of conversion of an oxirane moiety into a primary OH group through carbonatation [37]...

A series of ten withanolides, isolated from Physalis minima, were evaluated for their cytotoxic properties against HCT-116 and NCI-H460 cell lines. It was found that withaphysalin C (31) was active against both the cell lines [72]. The stmcture-activity relationship (SAR) studies of the C3dotoxic withanolides from Physalis minima and Physalis angulata indicate that the a,)S-unsaturated ketone moiety in ring A and 5jS-6/i oxirane moiety in ring B are important contributors in the cytotoxicity of withanolides. The absence of these two structural features may result in a loss or reduction of the cytotoxicity of withanohde [4]. 

A diastereospecific synthesis of alkenyl enolates (13) and (15) was reported as shown in Scheme 15.1 [5]. Condensation of alkenylzirconocene (8) with lithiated sulfonyloxirane (10) affords zirconate (11), which causes migration of the alkenyl group from the zirconocene into the oxirane carbon accompanied with elimination of the sulfonyl group. Thus formed intermediate (12) regiospecifically afforded the corresponding alkenyl enolates (13) or (15) depending on the substituents on the oxirane moieties. Further reactions of these alkenyl enolates (13) and (15) with aldehydes were demonstrated. 

Epoxides are versatile organic intermediates due to the reactive oxirane moiety, which can be opened by various nucleophiles or undergo elimination, reduction, or rearrangements to a multitude of more elaborate intermediates with the retention or inversion of chirality [1, 2], Enantiopure epoxides, in particular, play an important role in organic synthesis and the pharmaceutical industry [3,4], In recent decades, there have been enormous advances in the development of catalysts, including chemical catalysts and biocatalysts, for the production of chiral epoxides [5-10],... 

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