Oxiranes regioselectivity

Protection of 194 as a p-methoxybenzylether and subsequent epoxydation led to the trans-epoxide 195, which was transformed into the unsaturated aldehyde 196 by a three-reaction sequence, including regioselective oxirane opening with a 1,3-dithiane anion, hydrolysis of the dithioacetal formed, and dehydration. Chlorite promoted aldehyde oxidation, methyl ester formation, and removal of the hydroxyl protections delivered methyl (+)-shikimate 197 in a remarkable 12% yield from 193. 

Alcohols can be obtained from many other classes of compounds such as alkyl halides, amines, al-kenes, epoxides and carbonyl compounds. The addition of nucleophiles to carbonyl compounds is a versatile and convenient methc for the the preparation of alcohols. Regioselective oxirane ring opening of epoxides by nucleophiles is another important route for the synthesis of alcohols. However, stereospe-cific oxirane ring formation is prerequisite to the use of epoxides in organic synthesis. The chemistry of epoxides has been extensively studied in this decade and the development of the diastereoselective oxidations of alkenic alcohols makes epoxy alcohols with definite configurations readily available. Recently developed asymmetric epoxidation of prochiral allylic alcohols allows the enantioselective synthesis of 2,3-epoxy alcohols. 

Diethyl 2,3-epoxy-4-oxoalkylphosphonates can be converted on reaction with thioureas and thioamides into diethyl 2-hydroxy-2-(5-thiazolyl)ethylphosphonates or into 2-(5-thiaz-olyl)vinylphosphonates (Scheme 4.49)."" Cleavage of both systems into phosphorus-free thiazoles can be readily achieved with CsF in DMF. Reaction of diethyl 2,3-epoxy-4-oxoalkylphosphonates with 2-mercaptobenzimidazole yields oc-substituted enones arising from regioselective oxirane opening and subsequent dehydration (Scheme 4.49). °° These derivatives exist, mainly or exclusively, as cyclic tautomers. The cyclization step depends on the reaction conditions. °°... 

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. 

Raifeld YE, Nikitenko AA, Arshava BM. Compounds of ( -PrO)3TiX as novel reagents for regioselective oxirane ring opening. Tetrahedron Asymm. 1991 2(11) 1083-1084. 

Oxiranes with a single perfluoroalkyl chain are regioselectively opened by nucleophilic reagents at the more accessible carbon [d4] (equation 30). 

The oxirane ring in 175 is a valuable function because it provides a means for the introduction of the -disposed C-39 methoxy group of rapamycin. Indeed, addition of CSA (0.2 equivalents) to a solution of epoxy benzyl ether 175 in methanol brings about a completely regioselective and stereospecific solvolysis of the oxirane ring, furnishing the desired hydroxy methyl ether 200 in 90 % yield. After protection of the newly formed C-40 hydroxyl in the form of a tert-butyldimethylsilyl (TBS) ether, hydrogenolysis of the benzyl ether provides alcohol 201 in 89 % overall yield. 

Regioselective anbd Chemoselective Synthesis of Halohydrins by Cleavage of Oxiranes with Metal Halides," Bonini. C. Righi, G. Synthesis, 1994, 225... 

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]. 

In the case of Pt and Pd, the surface oxidation can also take place, but the resulting metal ions can be easily reduced in the presence of hydrogen. According to this fact, the observed difference in the regioselectivity of the rupture of the oxirane ring on Ni and Cu or Pt and Pd can be explained on the basis of the different stability of the metal ions in a hydrogen atmosphere.289... 

Moreover, propargyl oxiranes 202 were found to react with samarium diiodide and ketones to form a,a -dihydroxyallenes 203 with moderate to high anti-diastereo-selectivities (Scheme 2.62). Aurrecoechea and co-workers [99] reported this reductive coupling to proceed smoothly in the absence of a palladium catalyst, i.e. a direct electron transfer from the samarium(II) to the substrate has to take place in order to generate an allenyl/propargyl samarium intermediate of type 184/185, which is then regioselectively trapped by the electrophile. 

D. Wistuba, V. Schurig, Enantio- and Regioselectivity in the Epoxide-Hydrolase-Catalyzed Ring Opening of Simple Aliphatic Oxiranes. Part I , Chirality 1992, 4, 178 - 184. 

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