Reduction, of oxiranes

Fig. 6 Alkenes issued from the reduction of oxiranes in the presence of trialkylphosphites...

Reduction of oxirane oxime 106 with LAH in ether leads to pyrrolidine 107 as a 1 1 mixture of cis- and fraw5-isomers (equation 45) ". ... 

Table 7 Reduction of oxirane tosylates by Te under phase-transfer conditions < 1997T12131 >...

The past decade has also seen the publication of many articles on the catalytic reduction of oxiranes. An account of the mechanism and stereochemistry of the hydrogenolysis is presented in various reviews and papers discussed in the next section. A review that appeared recently on the reduction of oxiranes to alcohols covers all of these methods. 

Procedures involving the reduction of oxirane and ammonia in the vapor phase over an Al203-Si02 catalyst have been patented for the preparation of pyrazine, piperazine, and morpholine. The formation of quinoxalines is illustrated in Eq. 225. ... 

Opposite regioselectivity to that of regular metal hydride reductions has been observed in the NaBH4 reductions of oxiranes in the presence of triethylamine under photochemical conditions (hydride attacks the more highly substituted carbon) <92CC1133>. Diborane (B2H6) likewise tends to reduce oxiranes at the sterically more hindered oxirane carbon the mechanism and stereochemistry of the diborane reduction in connection with aliphatic oxiranes has been studied <82H(l8)28l>. 

As the hydrozirconation of alkene Z- and -29 is faster than the reduction of oxirane, the reaction of Cp2Zr(H)Cl with those vinyloxiranes 29 in CH2CI2 was reported to be an original alternative to optically active cyclopropyl carbinols 30 (Scheme 12.20) [24],... 

Reductive cleavage of oxiranes to alcohols by lithium aluminum hydride is an important reaction (64HC(19-1)199), but the most powerful hydride donor for this purpose is lithium triethylborohydride (73JA8486). 

Other syntheses of oxiranes which are occasionally useful and involve a species (64) are the dehydration of 1,2-diols (Scheme 69) (78TL5153, 7808(58)12) and the reductive coupling of aldehydes (Scheme 70) (B-73MI50500). 

Formation of oxiranes on the sterically more hindered side of the steroid ring system is usually carried out via /raw -halohydrins which afford oxiranes on treatment with base (c -Halohydrins yield ketones on exposure to base). Two general methods are available for the synthesis of tm s-halohydrins (1) the reduction of a-halo ketones and (2) the addition of a hypohalous acid to unsaturated steroids. 

Diaroylfuroxans react with an excess of diazomethane in diethyl ether at 20°C to form oxiranes 131 (Scheme 76). These compounds were reduced with LiAllTj (diethyl ether, reflux) to 1 -amino-2-arylpropan-2-ols in moderate yields (67JOC4050). Treatment of diacylfuroxans with LiAlH4 under similar conditions resulted in degradative reduction of the furoxan ring to arylethanolamines (67JOC1255). 

Oxidation 205-213 /J-Oximinosulphoxides, chiral 336 Oxiranes 169 reactions of 305 synthesis of 639 2-Oxo-l,2,3-oxathiazolidines 71 /1-Oxosulphones, synthesis of 169 Oxosulphonium ylides, reactions of 219 /3-Oxosulphoxides reduction of 347-349 synthesis of 337-340... 

The stereochemistry of the first step was ascertained by an X-ray analysis [8] of an isolated oxazaphospholidine 3 (R = Ph). The overall sequence from oxi-rane to aziridine takes place with an excellent retention of chiral integrity. As the stereochemistry of the oxirane esters is determined by the chiral inductor during the Sharpless epoxidation, both enantiomers of aziridine esters can be readily obtained by choosing the desired antipodal tartrate inductor during the epoxidation reaction. It is relevant to note that the required starting allylic alcohols are conveniently prepared by chain elongation of propargyl alcohol as a C3 synthon followed by an appropriate reduction of the triple bond, e. g., with lithium aluminum hydride [6b]. 

Carbonyl compounds, such as aldehydes [103, 179], (thio)ketones [31, 94, 180-183], carboxylic acids, and esters [183, 184] with 1 are reduced to alcohols after hydrolysis [5], except in stericaUy hindered cases (see Section 8.5) [185, 186]. Under the same experimental conditions the regioselective reduction of the oxirane ring with 1 gives also the corresponding alcohol [183, 187]. 

The rate of hydrozirconation of a terminal olefin is much faster than the reduction of the oxirane ring. This chemoselectivity was used in the preparation of various cycloalkylmethanols by hydrozirconation of alkenyloxiranes (Scheme 8-28) [217]. 

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