Chemoselective vinyl epoxides

From cycloadduct 293, the aldehyde was homologated to 295. The cyclopentane ring of the core of englerin A was secured by means of an intramolecular Heck reaction from enol triilate 296 to furnish triene 297 (Scheme 18.62). The trisubstituted double bond in 297 was the most electron rich, and was chemoselectively and stereoselectively epoxidized from the less hindered a-face [52]. An Sn2 reductive opening of the vinyl epoxide using DIBAL furnished the... 

A one-sided attachment of a vinyl-functionalized salen monomer to an MCM-41 material was reported by Janssen (95). In the epoxidation of 1-phenylcyclohexene with PhIO in acetonitrile, the Mn-functionalized structure 7g gave an ee of 75%, which is the same as for the soluble Jacobsen complex and considerably higher than that obtained with 7f. Morever, the chemoselectivity, the olefin conversion, and the enantioselectivity remained unchanged over four consecutive cycles. 

The complementary nature of the purely basic and the palladium-assisted alkylation in combination with the chemoselective epoxidation of either of the two double bonds of 2-phenylsul-fonyl-1,3-dienes102 makes these substrates useful synthons for multicoupling reactions. By changing the order of nucleophilic addition and epoxidation, three isomers of the alkylated hydroxy vinyl sulfone are available one of them, that depicted, by palladium-promoted reaction. Without sodium hydride, or in the absence of a nucleophile, / -hydride elimination to the corresponding dienol occurs. 

A phase-transfer catalyst such as tetra-n-butyl ammonium bromide has been used to chemoselectively epoxidize 1,4-units rather than 1,2-units in PBD [127] or HTPB [128] in a biphasic system. The degree of conversion accessible with peracids was up to 90% before the epoxidation of 1,2-vinyl groups was possible. Side reactions such as epoxide opening or oxidatimi of terminal hydroxyl groups have not been observed [128]. Modified clays (e.g., Closite 30B, a montmorillonite modified with ammonium salts) were also found to be favorable for epoxidation of cis-l,4-PBD because no phase-transfer agents need to be separated and phase transfer was not the rate-determining step [129,130]. 

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