Epoxide intramolecular

When the water ligand of 5b is replaced by propenol as axial donor or in case of equatorial coordination of propenol at the metal center, intermediates are formed which could undergo intramolecular epoxidation through TSs S-6 and S-7b, respectively (Figure 11). Here, the prefix S indicates a transition... 

The asymmetric lithiation/substitution of Af-Boc-Af-(3-chloropropyl)-2-alkenylamines 395 by w-BuLi/(—)-sparteine (11) provides (5 )-Af-Boc-2-(alken-l-yl)pyrrolidines 397 via the allyllithium-sparteine complexes 396 (equation 106) . Similarly, the piperidine corresponding to 397 was obtained from the Af-(4-chlorobutyl)amine. Intramolecular epoxide openings gave rise to enantioenriched pyrrolidinols. Beak and coworkers conclude from further experiments that an asymmetric deprotonation takes place, but it is followed by a rapid epimerization a kinetic resolution in favour of the observed stereoisomer concludes the cyclization step. 

The key bond closure in Scheme 13.21 is somewhat similar to that used in Scheme 13.20 but is performed on a bicyclo[4.4.0]decane ring system. The ring juncture must be cis to permit the intramolecular epoxide ring opening. The required cis ring fusion is established during the catalytic hydrogenation in step A. 

Silylation followed by selenium dioxide oxidation converted 13 into 14. Epoxidation of the derived TES ether proceeded by addition of oxygen to the more open face of the alkene, leading to 15. Ozonolysis followed by diastereoselective one-carbon homologation provided 17. This set the stage for intramolecular epoxide opening by the carboxylate, to give 2, in which all of the stereogenic centers of tetrodotoxin have been established. 

Intramolecular epoxidation is also possible with this system.2 Thus treatment of the ortho ester (1) of oleic acid with H202 in CH2C12 leads to the epoxide 2 in 40% yield, probably via the intermediate a. 

In the Halcon epoxidation process, the reaction of the zirconium(IV) methyl trialkoxide (201) with 02 yields the epoxy alkoxide (203), via intramolecular epoxidation of the coordinated allyl alcohol by the incipient methyl peroxide complex (202).630... 

Narjes, F. Bolte, O. Icheln, D. Konig, W. A. Schaumann, E. Synthesis of vinylcydopro-panes by intramolecular epoxide ring opening. Application for an enantioselective synthesis of dictyopterene A. /. Org. Chem. 1993, 58, 626-632. 

Oxidatively generated oxocarbenium ions have been used for intramolecular epoxide activation. Cascade reactions to form oligotetrahydrofuran products that demonstrated a strong preference for the exo-cyclization pathway were achieved in good yields when disubstituted epoxides were used as substrates. High stereoselectivity was observed in these reactions, with complementary diastereomers being formed from diastereomeric (g) epoxides.257... 

Cyclization of hydroxy epoxides to tetrahydrofurans or tetrahydropy-rans.1 CSA is the most efficient and convenient catalyst for this intramolecular epoxide opening. The cyclization of trans-epoxides is particularly useful because it can lead to either 5- or 6-membered cyclic ethers as shown in equation (I) and (II). 

Other synthetically useful intramolecular epoxide ring openings have been reported. For example, the strained methylene epoxide 79, derived from DMD epoxidation of the corresponding allene 78, undergoes spontaneous isomerization to the lactone 81 via attack of the... 

The regioselective intramolecular epoxidation of the peroxy ester (163), which can be prepared from famesol, has been effected by treating it with Cu(OCOCT3)2 (equation S ). This reaction provides a convenient route for the preparation of the 6,7-epoxide (164), which cannot be synthesized from farnesol by conventional methods or even by template-directed epoxidation using Mo(C(5)6/rBHP. 

Several examples of intramolecular epoxide formation have been reported. - In general, a keto sulfide is reacted with a trialkyloxonium tetrafluoroborate to yield a sulfonium salt, which cyclizes upon treatment with base. A particularly clever variation involves the in situ generation of an allylic ylide (Scheme 2). ... 

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