Oxiranes isomerization

Upon slow warming of the matrix, the colour disappeared and a new species with A = 3.24 mT and gy = 2.0038 appeared, assigned to the formation of the chloro spin adduct [12] (32) after melting of the matrix at 240 K the characteristic solution epr spectrum of [12] was recorded. By y-radiolysis of the isomeric oxirane [13], which cannot sustain spin trapping, another way of direct matrix generation of PBN + was available and thus made possible further confirmation of these results (Zubarev and Brede, 1995). 

Maximum yield of epoxides can be expected with molecules, first 1,1-dimethy-lalkenes, which prefer addition to abstraction (allyl hydrogen removal).11,259,260 These oxidations are not steroselective mixtures of the isomeric oxiranes are formed in the reaction of isomeric n-octenes.261 This was interpreted as additional evidence of the formation of radical intermediate 28 allowing the rotation about the carbon—carbon bond. 

The isomeric oxiranes prepared by ring closure of l-bromo-l-phenyl-2-propan-ol (ref. 9) were separated on a Fischer concentric tube column (theoretical plate number 90) (b.p. cis (1) 355 K/13 mm Hg, trans (2) 361 K/13 mm Hg, ac-... 

The experimental results on the transformations of the two isomeric oxiranes in the gas phase (Table 2) indicate the formation of three isomeric carbonyl... 

Direct irradiation of 2,5-dihydrofuran (175) results in the formation of furan, tetrahydrofuran, and the isomeric oxiran (176).146 Certain 2,5-dihydrothiophen derivatives are similarly converted to the corresponding vinylthiirans,147 whereas 5,5-diaryl-2,5-dihydrofurans are reported to undergo di-7i-methane rearrangement.148 An unexpected transannular phototransformation has been observed on irradiation of the l,3-dioxolen-2-one (177)... 

Markowski, V. and Huisgen, R. (1976) Disrotatory photoconversion of cis,trans isomeric oxiranes to carbonyl ylides. Tetrahedron Letters, 17, 4643-4646. 

Similar treatment of the isomeric oxirane (3) fails however, treatment with anhydrous zinc bromide (2, 463-464) in ether at 0° for 3 hr. gives (4), (t)-5-bromo-3-mclhyl-2-pcntene-l-ol, in 73% yield (pure). 

Neben (E)- und Z)-2-Oxo-l,4-dime,thyl- t-acclyl-cydohexan (IV a und IVb, S. 675) wird das 7.11 r Ausgangsverbindung isomere Oxiran V (4-Oxo-2,2,6-trimethyl-l-oxa-spiro[2.5]octan) gefunden. 

Intramolecular trapping has been reported for the ester 568. Here, irradiation in a matrix at 10 K yields the photochemically reactive ylide 569. This can be transformed to the isomeric oxirane on irradiation . ... 

The isomerization of vinyl- or ethynyl-oxiranes provides a frequently exploited source of dihydrofurans or furans, but analogous conversions of vinylaziridines have not been applied so often. While most of the examples in Scheme 87 entail cleavage of the carbon-heteroatom bond of the original heterocycle, the last two cases exemplify a growing number of such rearrangements in which initial carbon-carbon bond cleavage occurs. 

The conversion of small rings to smaller ones, without loss, is not common. 3-Chloroazetidine isomerizes reversibly to 2-chloromethylaziridine (Section 5.09.2.2.5). Flash vacuum pyrolysis can convert isoxazoles to azirines (Section 5.04.4.3). More common is the isomerization of medium-sized, i.e. five- or six-membered rings, e.g. certain succinimides (Scheme 23) (81JOC27) to azetidinediones, or bicyclic 1,2-dioxetanes to bis-oxiranes (Section 5.05.4.3.2). 

These are discussed in (B-71MS4). Oxirane itself shows a strong molecular ion peak and a slightly stronger base peak at mje 29 (CHO ) due to isomerization to ethanal and loss of a methyl radical. Substituted oxiranes tend to show only weak molecular ion peaks, because of rearrangement and fragmentation. 

Oxiranes have been isomerized by palladium compounds to allylic alcohols and enones (79JA1623), and to 1,3-diketones (80JA2095). 

Molecular orbital calculations predict that oxirane forms the cyclic conjugate acid (39), which is 30 kJ moF stabler than the open carbocation (40) and must surmount a barrier of 105kJmoF to isomerize to (40) (78MI50500). The proton affinity of oxirane was calculated (78JA1398) to be 807 kJ mol (cf. the experimental values of 773 kJ moF for oxirane and 777-823 kJ moF for dimethyl ether (80MI50503)). The basicity of cyclic ethers is discussed in (B-67MI50504). 

The commonest of these for oxirane opening are amines and azide ion [amide ions promote isomerization to allylic alcohols (Section 5.05.3.2.2)]. Reaction with azide can be used in a sequence for converting oxiranes into aziridines (Scheme 49) and this has been employed in the synthesis of the heteroannulenes (57) and (58) (80CB3127, 79AG(E)962). 

Finally, metalated epoxides undergo isomerization processes characteristic of traditional carbenoids (Scheme 5.2, Path C). The structure of a metalated epoxide is intermediate in nature between the structures 2a and 2b (Scheme 5.2). The existence of this intermediacy is supported by computational studies, which have shown that the a-C-O bond of oxirane elongates by -12% on a-lithiation [2], Furthermore, experimentally, the a-lithiooxycarbene 4a (Scheme 5.3) returned cydo-pentene oxide 7 among its decomposition products indeed, computational studies of singlet 4a suggest it possesses a structure in the gas phase that is intennediate in nature between an a-lithiocarbene and the lithiated epoxide 4b [3],... 

The isomerization of acetylenic oxiranes cis- and trows-91 to allenic ketone 94 has recently been described (Scheme 5.18). It is proposed that the rearrangement proceeds via a dilithium ynenolate [33]. 

A comparison of the configuration of the substrates and reaction products shows that the oxiranyl anions arc configurationally stable under the reaction conditions. Only one example is known in which isomerization was observed. When the ci.v-tm-butyl-substituted epoxysilane27 was metalated and quenched with 2-cyclohexenone, addition product 27 was obtained under inversion of the anionic center. Presumably the strain created in forcing the ter/-butyl and the trimethylsilyl group cis on the oxirane ring facilitates the isomerization process13. 

The strict geometrical requirements for elimination can be put to further use, as illustrated by elegant procedures for the geometrical isomerization of alkenes. Trimethylsilyl potassium (10) and phenyldimethylsilyl lithium (11) both effect smooth conversion of oxiranes into alkenes, nucleophilic ring opening being followed by bond rotation and spontaneous syn fi-elimination ... 

In the case of the cyclopentane oxirane 424, fluorination [KHFj, Me0(CH2)20H] proceeded to give preferably one fluoroalcohol, 426 (61%) over the isomeric one, 425 (7%), possibly by the influence of the benzyloxy-methyl group. Similarly (KHF2, ethylene glycol, 160°), another oxirane, 427, was converted into 428 (30%). 2-C-(Fluoromethyl)-/ T6>-inositol (429)... 

Alkenylbenzotriazoles 865 are readily prepared by isomerization of the corresponding allyl derivatives catalyzed by Bu OK. Lithiated compounds 865 are treated with electrophiles to provide a-substituted derivatives 866. Epoxidation of the double bond with ///-chloroperbenzoic acid converts intermediates 866 into oxiranes 867 that can be hydrolyzed to furnish a-hydroxyketones 868 in good yields (Scheme 140) <1996SC2657>. 

To link the two half moieties of the molecule, a Julia-Kocienski olefmation was carried out between the C19 building block 59 (again prepared by syn-SN2 -substitu-tion of a propargylic oxirane with DIBAH) and the C20 building block 60, formed via oxidation of 58 with Mn02 (Scheme 18.19). Although this reaction initially led to the formation of the Z-isomer as the major product, the latter was readily isomerized at room temperature to the desired all-trans-polyene peridinin (6). 

Another isomerization reaction of arene oxides is equilibrium with oxe-pins [5], Here, the fused six-membered carbocycle and three-membered oxirane merge to form a seven-membered heterocycle, as shown in Fig. 10.2. An extensive computational and experimental study involving 75 epoxides of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons has revealed much information on the structural factors that influence the reaction rate and position of equilibrium [11], Thus, some compounds were stable as oxepins (e.g., naphthalene 2,3-oxide), while others exhibited a balanced equilibrium... 

Turning to enzymatic hydration, we see from the data in Table 10.1 that phenanthrene 9,10-oxide Fig. 10.10, 10.29) is an excellent substrate for epoxide hydrolase. Comparison of enzymatic hydration of the three isomeric phenanthrene oxides shows that the Vmax with the 9,10-oxide is greater than with the 1,2- or the 3,4-oxide the affinity was higher as well, as assessed by the tenfold lower Km value [90]. Furthermore, phenanthrene 9,10-oxide has a plane of symmetry and is, thus, an achiral molecule, but hydration gives rise to a chiral metabolite with high product enantioselectivity. Indeed, nucleophilic attack by epoxide hydrolase occurs at C(9) with inversion of configuration i.e., from below the oxirane ring as shown in Fig. 10.10) to yield the C-H9.S, 10.S )-9,10-dihydro-9,10-diol (10.30) [91],... 

The -deprotonation reaction was initially considered as the normal mode of oxirane isomerization in basic media with a-deprotonation appearing as an alternative pathway when the principal process was slowed. Recent studies based on kinetic studies, calculations and labeling experiments, along with advances in the determination of the organolithium... 

Nevertheless, whereas the base-promoted isomerization of simple linear oxiranes and cyclohexene oxide occurs via a -deprotonation mechanism, recent denterinm-labeUng experiments demonstrate that the LDA-mediated rearrangement of cyclopentene oxide in nonpolar solvents furnishes the corresponding cyclopentenol through an a-deprotonation route (Scheme 7) . 

Based on this feature, aggregation states of transition-state structures for base-promoted isomerization of oxiranes have been established by kinetic studies of LDA-mediated isomerizations of selected oxiranes in nonpolar media in the presence of variable concentrations of coordinating solvents (ligands). Results reported provide the idealized rate law V = [ligand]" [base] [oxtrane] for a-deprotonation and v = fc[ligand]°[base] / [oxirane]... 

The treatment of the deuteriated cis oxirane 32 by EDA in HMPA yields exclusively the nondeuteriated alcohol 33. Indeed, complexation of the lithium cation by HMPA prevents the formation of the six-center transition state. The isomerization thus follows a more common E2 process, i.e. anti -elimination. 

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