Epoxide ring opening tetrachloride

More traditional carbon nucleophiles can also be used for an alkylative ring-opening strategy, as exemplified by the titanium tetrachloride promoted reaction of trimethylsilyl enol ethers (82) with ethylene oxide, a protocol which provides aldol products (84) in moderate to good yields <00TL763>. While typical lithium enolates of esters and ketones do not react directly with epoxides, aluminum ester enolates (e.g., 86) can be used quite effectively. This methodology is the subject of a recent review <00T1149>. 

Denmark, S. E. Barsanti, P. A. Wong, K.-T. Stavenger, R. A. (1998) Enantioselective ring opening of epoxides with silicon tetrachloride in the presence of a chiral lewis base., J. Org. Chem., 63 2428-2429. 

Buono, Gerard (2001). On the Beneficial Effect of ortho-Methoxy Groups in the Asymmetric Ring Opening of meso Epoxides with Silicon Tetrachloride Catalyzed by Chiralortho-Methoxyphenyldiazaphosphonamide Lewis Bases. Angewandte Chemie, International Edition in English. 40 4536. 

Catalytic Asymmetric Ring Opening of meso-Epoxides with Silicon Tetrachloride... 

Denmark et al. have extended this study to the enantioselective ring opening of meso-epoxides with silicon tetrachloride catalyzed by chiral phosphoramide 71 (Table 11). In this case, enantiomerically enriched chlorohydrins have been synthesized in enantiomeric excesses varying from 7 to 87 depending on the structure of the considered substrate [68]. 

Nucleophilic Attack at Halogen. Further studies have been reported of the reactions of diols with the triphenylphosphine-carbon tetrachloride reagent. It has now been applied to 1,2-diols (in the presence of potassium carbonate) to form epoxides and to the trans-6 o (84), the nature of the product depending on the relative amounts of phosphine and diol present. The major product of reactions involving equimolar quantities of phosphine and diol is (85). The cyclodehydration product (86) is formed in only poor yield. In the presence of carboxylic acids, the triphenylphosphine-carbon tetrachloride system causes ring-opening of epoxides with the formation of c -enol esters, the reaction presumably proceeding via nucleophilic attack by the oxirane at an acyloxyphos-phonium intermediate. ... 

The reductive ring opening of 330a with sodium cyanoborohydride/titanium tetrachloride in acetonitrile occurs with no ester reduction whatsoever to provide 421 in 83% yield. Subsequent conversion to the tosylate followed by reduction with lithium borohydride/lithium triethylborohydride affords in 61% yield the crystalline diol 422. Lithium aluminum hydride or sodium borohydride reduction of the tosylate of 421 fails to produce clean reductions to 422. Epoxide ring closure of 422 is achieved with two equivalents of sodium hydroxide in methanol to fiimish in 93% yield (2 S, 3i )-2-benzyloxy-3,4-epoxybutan-l-ol (423) [140] (Scheme 94). 

Among the more interesting developments in ring-opening reactions of epoxides (oxiranes, oxacyclopropanes) has been the observation that the process can be made enantioselective by the use of suitable Lewis acid-base pairs. Thus, with silicon tetrachloride in the presence of HMPA [(Me2N)3P=0], chlorohydrins can be generated. Then, if one enantiomer of a chiral phosphoramidate (such as the bis-... 

In addition, this class of chiral phosphoramides could catalyze the enantioselective ring opening of epoxides with silicon tetrachloride (SiCfl) [4]. For instance, the reaction of ds-stilbene oxide with SiCl, proceeded smoothly in the presence of 1 to afford the desired chlorohydrin in an enantiomericaUy enriched form (Scheme 7.3). The reactive ion pair shown in the scheme was proposed to be generated through the activation of SiCl, by the coordination of one phosphoramide 1, serving to activate the epoxide it was confirmed that bis-phosphoramide 2 was far less selective in this reaction. 

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