Oxirane reactions stereochemistry

The reaction of vinyloxiranes with malonate proceeds regio- and stereose-lectively. The reaction has been utilized for the introduction of a 15-hydroxy group in a steroid related to oogoniol (265)(156]. The oxirane 264 is the J-form and the attack of Pd(0) takes place from the o-side by inversion. Then the nucleophile comes from the /i-side. Thus overall reaction is sT -StM2 type, in the intramolecular reaction, the stereochemical information is transmitted to the newly formed stereogenic center. Thus the formation of the six-membered ring lactone 267 from 266 proceeded with overall retention of the stereochemistry, and was employed to control the stereochemistry of C-15 in the prostaglandin 268[157]. The method has also been employed to create the butenolide... 

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]. 

The analogous process involving allylic epoxides is more complex, as issues such as the stereochemistry of substituents on the ring and on the alkene play major roles in determining the course of the reaction [38]. Addition of the Schwartz reagent to the alkene only occurs when an unsubstituted vinyl moiety is present and, in the absence of a Lewis acid, intramolecular attack in an anti fashion leads to cyclopropane formation as the major pathway (Scheme 4.10). cis-Epoxides 13 afford cis-cyclopropyl carbinols, while trans-oxiranes 14 give mixtures of anti-trans and anti-cis isomers. The product of (S-elimi-... 

Yields are variable, but the reaction appears to be promoted by the presence of alkoxymethyl substituents on the oxirane ring. Reactions with p-propiolactones give lower yields (<10%). Cyclic carbonates can also be obtained from the zinc(II) promoted reaction of oxiranes with carbon dioxide [62] the stereochemistry of substituents on the oxirane are retained in the carbonate. 

These lithiooxiranes can be trapped by various electrophiles with retention of the configuration. The addition to aldehydes occurs with a low diastereoselectivity [but this can be enhanced by adding ClTi(OPr-/)3]. The reaction with enones occurs in a 1,2 fashion only. Intramolecular 1,4-silicon shift has also been reported. The reaction of the enantiomerically pure TMS-stabilized lithiooxirane 189 (Scheme 80) with an aldehyde has been used in a total synthesis of (-l-)-cerulenine. It must be noted that protodesi-lylation using TBAF (tetrabutylammonium fluoride) occurs with conservation of the oxirane stereochemistry. 

A5(10)-Estrenes, e.g. (9),20 are attacked predominantly on the /J-face with monoperphthalic acid (MPA). Although a number of factors may control this reaction, it should be noted that the corresponding 5a,10a-epoxide contains the unfavorable 9,10-syff arrangement in contrast to the 9,10-anti stereochemistry present in the /S-oxirane. 

The BF3-promoted rearrangement of several 4,5-epoxy-9-trimethylsilyldecalines having different relative stereochemistry and substitution at the oxirane ring is described. The presence of the silicon at C(9) favours two different main reaction pathways... 

The stereochemistry of the cyclopropyl as well as oxirane moiety in these P-lactams was determined based on the single crystal X-ray analysis of P-lactam 34 and the 4-nitrobenzoyl derivative (37) of 3-hydroxy-P-lactam 32.71 The extremely high diastereoselectivity observed in these cyclopropanation and epoxidation reactions can be explained by taking into account the highly organized transition state structures illustrated in Figure 1. The directing effect of the C-3... 

The reaction of oxiranes with carbonyl compounds in the presence of Lewis acids is an efficient way of preparing 1,3-dioxolanes (81S501). The reaction, shown in Scheme 33, proceeds with inversion of stereochemistry of the oxirane. A diol does not appear to be an intermediate even in the presence of water. Many Lewis acid catalysts are effective, but the use of anhydrous copper(II) sulfate in an excess of the carbonyl compound as solvent probably offers the mildest conditions. Since the copper(II) sulfate is insoluble, the reaction appears to be truly heterogeneous in nature and the mixture must be well stirred (78JOC438). 

The observed stereochemistry of addition suggests that oxiranes (epoxides) may be intermediates. Oxiranes may, indeed, be isolated from the reaction of certain alkenes with hydrogen peroxide in the presence of sodium tungstate. °... 

In contrast with other electrophilic additions, the peracid epoxidation is syn-stereospecific. With sterically strongly hindered alkenes the reaction takes place on the less sterically hindered side. In other cases, the stereochemistry of the reaction is affected by polar effects or the geometry of the transition state. Important conclusions regarding the mechanism of the reaction can be drawn from the steric pathways in the synthesis of the oxiranes. This has been dealt with comprehensively by Berti, who reviewed the topic up to 1971, with special emphasis on the peracid oxidation. A noteworthy account of the topic of peracid epoxidation is given in a review by Rebek. ... 

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