Cyclic substituted oxiranes

By contrast with the enantioselective deprotonation of meso oxiranes, there is only a limited number of reports on the kinetic resolution of unsymmetrically substituted oxiranes. This reaction involves the preferential recognition of one of the two enantiomers of a racemic mixture by a chiral reagent to provide both the starting material and the product in enantioenriched form . Several HCLA bases developed for enantioselective deprotonation have been tested as chiral reagent in stoichiometric or catalytic amount for the kinetic resolution of cyclic and linear oxiranes. 

Cationic polymerization of the unsubstituted oxirane (ethylene oxide) leads to the mixture of relatively low molecular weight (M < 103) linear polymer and up to >90% of cyclic oligomers, predominantly cyclic dimer (1,4-dioxane) 191,102]. The same behavior was observed for polymerization of substituted oxiranes, propylene oxide [103], epichlorohydrin [104], and other oxiranes having one or more substituents in the ring, although the distribution of cyclic fraction varied, depending on the structure of monomer. 

The polymerization of cyclic ethers like oxirane substituted oxiranes and bicyclic tetrahydrofurans proceeds with inversion of configuration on the carbon atom, stron y suggesting the 8 2 mechanism. 

Cyclooligomerization of substituted oxiranes has been found to be highly stereo-specific Cis-2,3-dimethyloxirane gave only one dioxolane-type dimer namely 2,cis-4,trans-5-trimethyl-2-ethyl-l,3-dioxolane isomer (plus other cyclic- and linear oligomers) ... 

In the nmr spectrum of oxirane, the protons 3 to the oxygen display a small chemical shift compared to that of the j3 protons of larger cyclic ethers this can only be explained by the shielding effect of the abnormal electron density, which assumes a low electron density around the oxygen, as suggested earlier on theoretical grounds. The proton chemical shifts for various substituted oxiranes are given in a number of reviews and handbooks. ... 

V.2.1.5 Pd-CATALYZED SUBSTITUTION REACTIONS OF ALKENYL EPOXIDES 1799 TABLE 2. Pd-Catalyzed Alkylation of Cyclic Vinyl Oxiranes... 

Crivello and Lam have demonstrated the use of diaryliodonium salts as photoinitiators for polymerization of electron-rich olefins, cyclic ethers, cyclic sulfides, lactones and spiro orthoesters, but a vast majority of their published work concerns polymerization of substituted oxiranes, illustrating the potential of such systems in photocuring of epoxy resins. Such polymerizations can be quite fast in the most favorable example, a 93% yield of polymer of Mn 10,700 was obtained from 3-vinylcyclohexene oxide after only 90 seconds of irradiation at room temperature with 4,4 -di-tert-butyldiphenyliodonium hexafluoroantimonate as initiator (3). The substituted salts are often preferred to the simple unsubstituted diphenyliodonium compounds for reasons of solubility (2). The use of diaryliodonium salts in combination with various dyes allows one to initiate cationic polymerization with visible light (5). 

For bulky substituted oxiranes, the kinetic enhancement in cyclics are also observed but the main products of cyclization are tetramers. ... 

In the case of polymers prepared from substituted heterocycles the tacticity of the chain is correlated with the configuration of true asymmetric carbons. As an example are given in Figure 1 isotactic, syndiotactic and heterotaetic polymers of a substituted oxirane. Up to now, the polymerization of cyclic monomers has never given polymers of syndiotactic structure (or even with a predominance of syndiotactic units). Therefore only isotactic polymers, e.g. polymers having all units of the same configuration, will be considered. 

In stereoselective antitheses of chiral open-chain molecules transformations into cyclic precursors should be tried. The erythro-configurated acetylenic alcohol given below, for example, is disconnected into an acetylene monoanion and a symmetrical oxirane (M. A. Adams, 1979). Since nucleophilic substitution occurs with inversion of configuration this oxirane must be trens-conilgurated its precursor is commercially available trans-2-butene. 

Aromatic and a-substituted aliphatic aldehydes react with oxiranes under neutral conditions to yield 1,3-dioxoIanes [35] whereas a-unsubstituted aldehydes undergo a simple aldol condensation under such conditions. In a somewhat similar manner, perfluorocarboxylic esters react with oxiranes to produce cyclic orthoesters (-50%) [36]. The corresponding reaction with non-fluorinated esters fails. 

Both, five- and six-membered cyclic carbamates have been synthesized by the reaction of 2-(l-haloalkyl)-oxiranes with C02 and primary aliphatic amines [81]. Notably, 5-substituted 2-oxazolidinones have been prepared in good yield (51-94%) by reacting 2-aminomethyloxiranes with C02 (0.1 MPa), in MeOH, at room temperature [82]. 

Block et al.194 examined the effects of trimethylsilyl substitution on the first vertical ionization potentials by photoelectron and Penning ionization electron spectroscopy studies of a range of cyclic and noncyclic sulfides and ethers. It was shown that substitution of oxirane 218 with a trimethylsilyl substituent as in 219 lowered the ionization potential by 0.90 eV (20.8 kcal/mol), while similar substitution of dimethyl ether 220 in 221 lowered the ionization potential by 0.64 eV (14.8 kcal/mol). By comparison, the effects of silyl substitution on sulfur lone-pair ionization potentials was found to be smaller thus the ionization potential of dimethyl sulfide 222 is lowered by 0.37 eV upon trimethylsilyl substitution in 223, and the trimethylsilyl-substituted thiirane 225 is lowered by 0.59 eV relative to thiirane 224. The raising of the energy of the sulfur lone-pair electrons in the thiirane 225 is also apparent from its UV spectrum, where there is a bathochromic shift in the absorption maximum compared to the parent 224. 

The 7-octen-l-ols substituted with a rigid cyclic moiety (cyclopropane or phenyl) react with bis(collidine)iodonium and -bromonium hexafluorophosphates to afford oxocanes in modest to good yields (Scheme 12). If the cyclic component is an oxirane or a dioxolane ring, the yields are lower. The cyclizations are carried out in dichloromethane (DCM) at room temperature <2003EJ0463>. 

For the sake of comparison, the regio- and stereo-selectivity of some nucleophilic openings of vinyl-oxiranes with organometallic reagents derived from copper, lithium, sodium and other metals indicate the control avail le under some conditions. Complete control of diastereoselectivity in the opening of cyclic vinyloxiranes is available, for example by utilizing palladium(0)-catalyzed conditions in the reaction of die sodium salt of dimethyl malonate with cyclic vinyloxiranes.Increased substitution on the vinyl portion of the vinyloxirane leads to isomerization with opening, as in the case of the disub-stituted vinyloxirane (150 equation 49). ... 

Although the C(17)-oxiran (327) failed to react with most nitrogenous bases (NH3, NH2, phthalimide ion, urea, etc.), guanidine effected substitution at C(20), which was followed by expulsion of ammonia to give the oxazoline (328). This compound could not be hydrolysed, but was deaminated by nitrous acid to give the 17/S,20-cyclic carbonate (329). 

Chromic acid oxidation of olefins can rarely be used for the preparation of oxiranes because they occur as intermediates that rapidly undergo further transformation. From an investigation of the mechanism of oxidation of triaryl-substituted olefins, it was concluded that a carbonium ion or cyclic chromate ester is a possible intermediate. Selective epoxidation of compounds containing conjugated double bonds is attainable by means of chromic-acid oxidation (Eq. 48) 535 Exclusively cis product was obtained from a highly substituted octalin with Na2Cr04, KMn04, or ozone (Eq. 49). ... 

Three-membered cyclic compounds can generally be prepared from difunctional compounds by 1,3-elimination. In the formation of oxiranes from 2-substituted alkanols, the alkoxide produced in the basic medium participates in an internal nucleophilic attack, which promotes the departure of the substituent on the adjacent carbon atom leading to ring-closure. A stereochemical condition for the reaction is that the reacting groups should be in an antiperiplanar conformation relative to each other (Eq. 51). 

With regard to the reagents used in this type of cyclopropanation, 1,2-dibromoethane has been used most frequently. l-Bromo-2-chloroethane, however, may give better yields in some cases. 2-Chloroethylsulfonium iodide is used in the reaction of cyclic ketones to give the spiro derivatives . The use of vinylogous derivatives of 1,2-dihalides allows a synthesis of vinylcyclopropanes (equation 89) . (Chloromethyl)oxirane provides substituted cyclopropylmethanols (equation Oxiranes may also be used in place of... 

Mixed substituted orthoesters, e.g. (383 equation 180), can be obtained by addition of alcohols to ketene 0,0-acetals. With the aid of phosphoranes containing a ketene O,(7-acetal structure, various orthoesters, e.g. (384) and (385) (Scheme 70) were prepared. " Cyclic orthoesters (386)-(390) (Scheme 71) are formed in cycloaddition reactions of ketene 0,0-acetals with aldehydes, ketones, 7.848 gj.yj cyanides, oxiranes, a-keto esters, o-diketones or ketones (with irradiation), a,3-unsaturated aldehydes and ketones (under pressure or catalyzed by ZnCh), diazoaceto-phenone, 7 diazoacetone and azodicarboxylate. ... 

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