Oxirane reactions ring opening regioselectivity

The effect of the BF3 activation on the regioselectivity of the ring opening of vinylic and acetylenic oxiranes is dramatic, as in these conditions the reaction occurs exclusively on the allylic " or the propargylic " position, and still with inversion (Scheme 40). This excellent regioselectivity allows the direct stereospecific preparation of homoallylic and homopropargylic alcohols in excellent yields. In the case of acetylenic oxiranes, a remarkable difference in the reactivity of cis and trans oxiranes has been evidenced, the former being more reactive. 

The two mono-formates are formed in comparable amounts, suggesting little regioselectivity in the oxirane ring opening reaction. Propylene carbonate, 1,2-propanediol, and formic acid were also found as reaction prod-... 

The ring opening of several oxiranes by various thiosilanes (promoted by tetrabutylammonium fluoride as catalyst) occurs regioselectively under mild conditions.32 With the amphiphilic reagents isothiocyanatotrimethylsilane and O-trimethylsilyl thioacetate the sulfur centre exclusively attacks the oxiranes. A plausible reaction mechanism was suggested to explain the role of F as promoter. 

Epoxides undergo a repertoire of fairly well-characterized reactions, perhaps the most typical being the nucleophilic ring opening process. Since the epoxide in question is usually found in a compound with multiple functionality, many new methodologies seek to optimize chemo- and regioselectivity. Fortunately, the oxirane lone pair provides a convenient handle to effect activation of the heterocycle toward C-0 bond cleavage thus, efficient catalysts are of particular interest. 

Furthermore, in respect to the regioselectivity of the ring opening reaction of oxiranes, electronic as well as steric factors can play a role. These general considerations stimulate the use of zeolites and non zeolitic molecular sieves as heterogeneous catalysts for such rearrangement reactions in the liquid or in the gas phase in a slurry reactor and in a continuous fixed bed reactor, respectively. 

Phenyl-substituted oxiranes may be reduced in DMF akin to benzyl ethers by direct and indirect methods the regioselectivity is slightly higher in the indirect reduction [76]. Benzoyloxiranes can be reduced in MeCN to aldoles addition of acetic acid prevents a retroaldol reaction catalyzed by the EGB [77]. The ring opening is analogous to the reduction of 2-alkoxyacetophenone to acetophenone. 

The rate and regioselectivity (C-3 attack) of the oxirane ring opening of 2,3-epoxy alcohols by nucleophiles are markedly increased by the addition of Ti(OPr )4. Thus, the reaction of 2,3-epoxyhexanol with excess diethylamine in the presence of Ti(OPi )4, 1,2-diol (132) and 1,3-diol (133) were formed in 90% yield in a ratio of 20 1, while, in the absence of the alkoxide, (132) and (133) were obtained in 4% yields in a ratio of 3.7 1 (Scheme 58)." ... 

Regioselective C-2 amination of 2,3-epoxy alcohols can be realized by intramolecular oxirane ring opening of the carbamates prepared by the reaction of the epoxy alcohols with isocyanates." -" In some cases, however, acyl transfer is a serious side reaction (Scheme 59)." ... 

Table 2 Regioselectivity (%) of ring-opening reactions of trans-oxiranes.

A 5-exo reaction can be observed when ,/Tepoxycyclohexenone is treated with tin(lV) chloride. After a regioselective oxirane ring opening, cyclization follows via axial attack of the alkyne group. The terminal vinyl cation is trapped by chloride and the bicyclo[4 3.0]nonan-2-one system 1 is formed stereoselectively in 94% yield5. 

MgBr2 effects a regioselective oxirane ring-opening by bromide and recyclization by SNi-displacement of Br via amino nitrogen according to ref [13]. The reaction proceeds with a net retention of configuration. 

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