Michael addition stereospecific

The stereospecific base-cleavage of the trimethylsilyl group in 1,3-dithiane 1-oxides 499 enables to obtain the specifically deuteriated products 500 (equation 303), A nitro group in y-nitroalkyl sulphoxides 501 (obtained by the Michael addition of nitroalkanes to a, j8-unsaturated sulphoxides) is replaced by hydrogen by means of tributyltin hydride (equation 304). This reagent does not affect the sulphinyl function. The overall procedure provides an efficient method for the conjugate addition of alkyl groups to a, -unsaturated sulphoxides . ... 

Z, 6Z- 10 COOMe was produced by a short and stereospecific one-pot synthesis (Scheme 17) [25], via sequential addition of dipropyllithium cuprate 115 to two equivalents of acetylene, followed by Michael addition of the resulting conjugated dienyl cuprate 116 with methyl propiolate 117. The only sig-... 

Position 3 of the piperazine-2,5-dione nucleus has been alkylated by Michael addition of the enol to a suitable acceptor. This route has been successfully utilized for the total synthesis of bicyclomycin (83TL5627). The addition proceeded stereospecifically to give only one product (Scheme 30). 

The stereochemical aspects of the Michael addition of lithium dithioester enethiolates in the challenging acyclic series have been examined [362]. A stereospecific addition of a c/s-enethiolate to an ( )-enone furnishes the anti isomer (6). 

In the course of mechanistic studies it was established that aniline does not react with the cyclopropenones (153 and 154) even under reflux conditions. It was therefore assumed that the formation of (158) involves initial nucleophilic attack by the aminopyridine ring nitrogen on the electrophilic cyclopropenone ring. In this way 155 is formed, which is then transformed via the reactive intermediates (156, 157, and/or 161) to the prodticts. Kascheres et al. noted that the formation of 157 is formally a stereospecific trans addition of the 2-aminopyridines to the double bond of the cyclopropenone (153). Such sterospecificity has been observed in kinetically controlled Michael additions. 

Carbocyclization of m-alkcnyl-z-methoxybcnzy I lithiums to form five- or six-membered rings has been studied 101 the five-membered ring is formed with a cis-stereochemical relationship between the methoxy substituent and the adjacent methyl group. Intramolecular carbolithiation of vinyl sulfides at — 105°C in THF has been found to occur non-stereospecifically with regard to the newly formed C—Li centre.102. The stereochemistry of selective tandem Michael addition alkylation reactions of vinylphosphonates has been explored.103... 

Michael addition to a complexed styrene. A key step in a synthesis of 11-deoxy-anthracyclinone (3) involves the regioselective reaction of 1 with the carbanion of a protected acetaldehyde cyanohydrin to give 2 by stereospecific earo-addition. 

Ylides can cyclopropanate unsaturated systems which are susceptible to Michael additions, i.e. a,jS-unsaturated ketones, esters, amides, nitriles, sulfones, sulfonamides, and nitro compounds. Enhancement of electron withdrawal from the carbon-carbon double bond facilitates the reaction. The reaction is non-stereospecific. The intermediacy of zwitterions has generally been accepted, and hence the stereochemistry of the product may be predictable on the basis of the stepwise mechanism. Namely, the Michael addition of the ylide will occur predominantly from the less hindered side of the double bond in a given molecule and the subsequent cyclization will take place in the conformation which minimizes the non-bonded repulsions. 

Sulfides were also involved in a unique three-component, stereospecific Michael addition. The reaction described in Scheme 29 can be explained as follows regiospecific addition of the nucleophile to the exocyclic double bond of the a-methylenecyclopentenone, followed by trapping of the resulting enolate by methyl acrylate, and attack of the anion thus formed on the endocyclic double bond of the cy-clopentenone moiety to result, after protonation, in the bicyclo[2.2.1]heptan-6-one as a single stereoisomer (68% overall yield Scheme 29). 

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