Michael reaction sigmatropic reactions

Allylic nitro compounds undergo [2.3]sigmatropic rearrangement to afford rearranged alcohols, as shown in Eq. 7.4346 and Eq. 7.44 47 Because the allylic nitro compounds used in these reactions are readily prepared either by the Henry reaction or the Michael addition, these reactions may be useful in organic synthesis. 

Increasing importance has to be attributed to modem tandem (or cascade ) techniques—reaction sequences that can be performed as a one-pot procedure because the first reaction step creates the arrangement of functional groups needed for the second to occur. Schemes 5-7 present some in situ preparations for iminium species, which can then react further with appropriate nucleophiles that are already present (preferably in the same molecule). Most elegantly, in situ generation of iminium ions for tandem processes was performed by a 3,3-sigmatropic (aza-Cope-type) rearrangement (Scheme 5), but also by initial Michael-type addition reactions to vinyl-substituted Atio... 

During 1989-93 lithium perchlorate iethyl ether (LiC104 EtiO, LP-DE) was studied as a reaction medium in organic synthesis when it was observed that cycloadditions, sigmatropic rearrangements, Michael additions and aldol condensations carried out in LP-DE occurred quickly and selectively under mild reaction conditions [33]. In addition, LP-DE allowed the reaction and subsequent work-up to be carried out under essentially neutral conditions. 

Alternative, also stereoselective, routes to allenic steroids take advantage of cationic cyclization reactions [108] or [2,3]-sigmatropic rearrangements [109]. For example, the allenic Michael acceptor 112 was prepared with 57% chemical yield by reaction of mestranol (111) with diethyl chlorophosphite and was found to inhibit the sterol biosynthesis of the pathogen responsible for Pneumocystis carinii pneumonia (PCP), the most abundant AIDS-related disease (Scheme 18.36) [110]. 

The Michael adduct of 6-(l-methylhydrazino)-3-methyl-2-(methylsulfanyl)pyrimidin-4(3//)-one with dimethyl acetylenedicarboxylate, a compound containing an enehydrazine moiety, is cyclized in Tetralin at 200 °C to give methyl 6-amino-3,8-dimethyl-2-(methylsulfanyl)-4,7-dioxo-3,4,7,8-tetrahydropyrido[2,3-c/]pyrimidine-5-carboxylate in moderate yield. The reaction is presumed to proceed through an initial [3,3]-sigmatropic rearrangement with cleavage of the N —N bond followed by cyclization and elimination of methanol.193... 

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