Silylated nucleophiles, ring opening with

Lastly, Antilla has disclosed a novel asymmetric desymmetrization of a wide range of aliphatic, aromatic, and heterocyclic meso-aziridines with TMS-N3 promoted by 11 and related 12 (Scheme 5.31) [56]. Uniquely, this is one of only several reports of electrophilic activation of nonimine substrates by a chiral phosphoric acid. Mechanistic studies suggest that silylation of 11 or 12 by displacement of azide generates the active catalytic species A. Consequently, the aziridine is activated through coordination of it carbonyl with chiral silane A to produce intermediate B. Nucleophilic ring opening by azide furnishes the desymmetrized product and regenerates 11 or 12. 

An IV-heterocyclic carbene (similar to that in Scheme 15) has proved to be an effective catalyst for the nucleophilic ring opening of IV-tosylaziridines by silylated nucleophiles (MesSiX, X = N3, Cl, I).45 Yields range from 89 to 99% for reaction at the least substituted carbon, except when a phenyl group on one of the carbons of the aziridine ring induces predominant attack at the benzyl carbon. The stereochemistry is consistent with the SN2 mechanism and THF was found to be the best solvent for the reaction. 

The y-lactam 110 is prepared by the reaction of the lithium silyl-substituted ynolate 105 with the aziridine 108 activated by a p-toluenesulfonyl group. The initial product is the enolate 109, which can be acidified to yield the a-silyl-y-lactam 110. Intermediate 109 can be trapped by aldehydes to afford the a-alkylidene-y-lactams 111 via a Peterson reaction (equation 45) . These reactions may be considered to be formal [3 + 2] cycloadditions as well as tandem reactions involving nucleophilic ring opening and cyclization. 

Regiospecific nucleophilic ring opening of oxetanes to 3-silyl-oxyisocyanides with trimethylsilyl cyanide/Znl2 in dichloromethane has been reported, and this has led to the synthesis of amino-alcohols by hydrolysis of the isocyanide group. ... 

Treatment of an oxirane bearing no silyl group with a silyl nucleophile leads to the same result. Both trimethylsilylpotassium and dimethylphenylsilyllithium effect smooth conversion of oxiranes into alkenes with inversion of configuration, nucleophilic ring opening being followed by spontaneous syn-y6-elimination (Scheme 2.121) [341, 342]. 

Addition of alkyllithium to cyclobutanones and transmetallation with VO(OEt)Cl2 is considered to give a similar alkoxide intermediates, which are converted to either the y-chloroketones 239 or the olefinic ketone 240 depending on the substituent of cyclobutanones. Deprotonation of the cationic species, formed by further oxidation of the radical intermediate, leads to 240. The oxovanadium compound also induces tandem nucleophilic addition of silyl enol ethers and oxidative ring-opening transformation to produce 6-chloro-l,3-diketones and 2-tetrahydrofurylidene ketones. (Scheme 95)... 

In metal-free catalysis enantioselective ring-opening of epoxides according to Scheme 13.27 path B has been achieved both with chiral pyridine N-oxides and with chiral phosphoric amides. These compounds act as nucleophilic activators for tetrachlorosilane. In the work by Fu et al. the meso epoxides 71 were converted into the silylated chlorohydrins 72 in the presence of 5 mol% of the planar chiral pyridine N-oxides 73 (Scheme 13.36) [74]. As shown in Scheme 13.36, good yields... 

The ring opening reaction of A-tosylaziridines with certain silylated nucleophiles via a similar activation has been reported recently (Wu et al. 2006). 

The alkoxycarbenium ion 315, obtained by electrochemical oxidation of ct-silyl ethers under standard cation pool conditions, underwent ring opening by reaction with the nucleophile cyclohexenyltrimethylsilane to give 316 (Scheme 73) <20050L4717>. 

Compounds with an additional 2-vinyl group, easily available in two steps from a,J -unsaturated ketones, are of special interest. If the reactive vinyl ketone moiety is liberated, it can be trapped in situ by suitable nucleophiles, e.g. CH-acids, generating polyfunctional compounds or by a diene unit which undergoes an intramolecular Diels-Alder reaction (equation 93). Besides, radical additions to the vinylcyclopropane are also possible giving silyl enol ethers as ring-opened products . Future synthetic applications of theses processes are obvious. 

Narasaka found that optically enriched oxabicydic substrate 277 bearing a vinyl sulfide moiety reacts with a silyl enol ether or ketene silyl acetal in the presence of a Lewis acid to afford the protected cyclohexenols 278a and 278b, Eq. 175 [18]. The reaction was proposed to occur via a ring-opening and alkylation sequence which is equivalent to overall nucleophilic substitution with retention of configuration. Presumably, the nucleophile attacked the carbocationic intermediate from the exo face, because the methylene-OTIPS substituent was blocking the endo side. 

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