Double retention, preferential

Radical decarboxylation of A-hydroxy-2-thiopyridone esters in the presence of an olefin results in the formation of a carbon-carbon bond. The monoester 5 is not prone to jS-elim-ination, and the faces of the dioxolane ring are encumbered by the methyl ester, so radical trapping occurs preferentially from the side opposite the methyl ester. This results in overall retention of configuration at the reacting carbon. Irradiation of ester 11 in the presence of methyl acrylate affords the tmns-eAkQm 12 after appropriate isolation. Once the first carboxylic acid has been modified, the second can be reacted similarly, whereby the stereochemistry of the first substitution controls the stereochemistry of the second. In this way the resulting product is obtained with overall double retention [11]. 

Copper(I) salts of enamines have been allylated with the 2-allyloxybenzimidazoles to give y,5-unsaturated ketones upon hydrolysis (79CL957). Primary allylic ethers react preferentially at their a-carbon with retention of double bond configuration whereas secondary allylic ethers react mainly at the y-carbon to afford alkenes of predominantly (E)-stereochemistry. 

The presence of an exocyclic double bond on aromatic [52] or heteroaromatic [53] compounds dictates the locoselectivity of the cycloaddition occurring preferentially at the alkene terminus, and sometimes with retention of stereochemistry [52b], Thus, the [4+2] photo-oxygenation of a vinylbenzene derivative, such as 60, leads to dihydrobenzo-1,2-dioxin in the primary step, which is followed by rapid oxygenation of the cyclohexadiene chromophore to give diendoperoxides as 61 and 62 (Sch. 34) [52c],... 

Towards electrophiles, the reactivity of vinylsilanes is similar to that of the corresponding alkene. However, incorporating a silicon substituent at the vinylic carbon of a ir-nucleophile markedly affects the cyclization outcome. Specifically, iminium ion-vinylsilane cyclizations occur cleanly to substitute, preferentially with retention of double-bond configuration, the iminium ion carbon for the silyl substituent. Both endocyclic and exocyclic modes of intramolecular electrophilic substitution have been demonstrated (Scheme 35). 

Lithium tetrahydridoaluminate/ cobaltous chloride/irradiation Phenols from cyclic phenolethers Retention of carhon-carhon double bonds Selective and preferential reduction... 

Preferential hydrogenation of carbon-carbon double bonds with retention of halogen... 

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