Eliminations of oxiranes

Alkynyl)oxiranes also react with carbon nucleophiles to afford furan derivatives. Furanes of different types are obtained depending on the structure of the substrates. 7-Methyl-2-ethynyloxirane (95) reacts with acetoacetate to give the furan 97 by the elimination of formaldehyde from the cyclized product 96. The hydroxy ester of the alkylidenefuran 98 and the corresponding lactone 99 are obtained by the reaction of i-methyl-2-(2-propynyI)oxirane[40, 42]. 

The reaction of oxiranes with base can follow several paths, giving products of type (34-38 Scheme 27). (a) Formation of an oxiranyl anion (34) is rare (Section 5.05.3.5). (b) Nucleophilic ring opening to give (35) is common with unhindered bases (Section 5.05.3.4). (c) a-Elimination to give a carbene or carbenoid (36) is favored by alkyllithium bases and... 

The manufacture and uses of oxiranes are reviewed in (B-80MI50500, B-80MI50501). The industrially most important oxiranes are oxirane itself (ethylene oxide), which is made by catalyzed air-oxidation of ethylene (cf. Section 5.05.4.2.2(f)), and methyloxirane (propylene oxide), which is made by /3-elimination of hydrogen chloride from propene-derived 1-chloro-2-propanol (cf. Section 5.05.4.2.1) and by epoxidation of propene with 1-phenylethyl hydroperoxide cf. Section 5.05.4.2.2(f)) (79MI50501). 

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 ... 

The living nature of ethylene oxide polymerization was anticipated by Flory 3) who conceived its potential for preparation of polymers of uniform size. Unfortunately, this reaction was performed in those days in the presence of alcohols needed for solubilization of the initiators, and their presence led to proton-transfer that deprives this process of its living character. These shortcomings of oxirane polymerization were eliminated later when new soluble initiating systems were discovered. For example, a catalytic system developed by Inoue 4), allowed him to produce truly living poly-oxiranes of narrow molecular weight distribution and to prepare di- and tri-block polymers composed of uniform polyoxirane blocks (e.g. of polyethylene oxide and polypropylene oxide). 

Reaction of the carbanion of chloromethyl phenyl sulphoxide 409 with carbonyl compounds yields the corresponding 0-hydroxy adducts 410 in 68-79% yield. Each of these compounds appears to be a single isomer (equation 242). Treatment of adducts 410 with dilute potassium hydroxide in methanol at room temperature gives the epoxy sulphoxides 411 (equation 243). The ease of this intramolecular displacement of chloride ion contrasts with a great difficulty in displacing chloride ion from chloromethyl phenyl sulphoxide by external nucleophiles . When chloromethyl methyl sulphoxide 412 is reacted with unsymmetrical ketones in the presence of potassium tcrt-butoxide in tert-butanol oxiranes are directly formed as a mixture of diastereoisomers (equation 244). a-Sulphinyl epoxides 413 rearrange to a-sulphinyl aldehydes 414 or ketones, which can be transformed by elimination of sulphenic acid into a, 8-unsaturated aldehydes or ketones (equation 245). The lithium salts (410a) of a-chloro-/ -hydroxyalkyl... 

Tanaka and Kakiuchi (6) proposed catalyst activation via a hydrogen donor such as an alcohol as a refinement to the mechanism discussed by Fischer (7) for anhydride cured epoxies in the presence of a tertiary amine. The basic catalyst eliminates esterification reactions (8). Shechter and Wynstra ( ) further observed that at reaction conditions BDMA does not produce a homopolymerization of oxiranes. 

Although alcohols are oxidized by tetra-n-butylammonium persulphate when the reaction is conducted in dichloromethane, tetrahydropyranyl ethers have been produced (>90%) when attempts to oxidize the alcohol are conducted in tetrahydro-pyran (see Chapter 10) [ 19], Tetrahydrofuranyl ethers have been prepared by an analogous method [20,21 ]. Base-mediated elimination of halo acids from P-halo alcohols under phase-transfer catalysed conditions produce oxiranes in high yield (70-85%). The reaction has particular use in the synthesis of epihalohydrins from p,y-dihalo alcohols [22],... 

In an interesting catalysed conversion of trichloroethene by secondary amines into aminoacetamides, the initial steps are thought to involve the p-elimination of HC1 to produce dichloroethyne (Scheme 9.1), which reacts with the secondary amine under the wet conditions to produce the amide [35] the reaction does not work with N-alkylanilines. Such a mechanism is realistic, as it is well known [36] that trichloroethene is converted into the inflammable and explosive dichloroethyne by bases, and quaternary ammonium salts catalyse the formation of the alkyne when trichloroethene is reacted with oxiranes [37]. Chloroethynes have also been obtained by the catalysed reaction of terminal ethynes with carbon tetrachloride under basic conditions [38]. 

When both positions a to the C—Li bond of the lithiooxirane have no hydrogen to shift, a 1,2-alkyl shift (Scheme 58) can occur. This rearrangement has been observed recently in the case of oxiranes 125 derived from cyclopentenols. This 1,2-alkyl shift can occur on both sides of the lithiooxirane (Scheme 58, paths a and b). The resulting lithium enolate then undergoes a -elimination process of Li20, leading to diversely substituted cyclopentenones 126 and 127. 

Both the alio and manno epoxides (9) or (7), or their debenzyliden-ated analogs, undergo oxidation with methyl sulfoxide and boron trifluoride,104 but, unlike 5,6-anhydrohexofuranose derivatives (see p. 163), they do not yield a simple a-hydroxy carbonyl compound, as would be expected by analogy with simple oxiranes instead, the rearranged diulose (46) is obtained in low yield, possibly by elimination of methanol from the intermediate (47). 

Potassium selenocyanate with 2 (or the c-ido isomer) yieldsss l,2-0-isopropylidene-a-D-xy(o-hex-5-enofuranose (65) by elimination of the oxirane oxygen atom. A similar elimination had earlier been... 

Two parallel routes for the elimination of glycol formate are suggested, involving either reaction with H2 or with cocatalyst water. The detection of formic acid in the reaction products suggests another mechanism, with initial production of formic acid from H2 and C02, followed by reaction with the oxirane. This mechanism is not favored however since the yields of glycol formates varied substantially when various substituted oxiranes were reacted. This would not have been expected in a mechanism with formic acid as an intermediate. A third mechanism, not considered by the authors, could proceed through initial production of propylene carbonate, followed by reduction to the mono- or di-formate. 

A convenient one-step conversion of moderately activated nitroarenes to phenols was achieved in DMSO via nucleophilic nitrite displacement by the anion of an aldoxime.153 TTie resulting O-arylaldox-ime is rapidly cleaved to the phenol derivative under the reaction conditions. The reaction is also applicable to activated fluorides and even to 2-chloropyridine which, at 110 °C, is converted to 2-pyridone in 72% yield.153 A somewhat related process concerns the synthesis, in 82-92% yield, of 4-alkoxybenzoni-triles (45 R = Me, CH2-oxirane, CHrfh, CHMeCTfcMe from O-alkyl-4-nitrobenzaldoximes (44) via hydride-induced elimination of the alkoxide followed by alkoxy denitration (Scheme 17).154... 

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