Hydro-halo-addition

Addition of Hydrogen Halides Hydro-halo-addition... 

The introduction of 1,2-bromochlorocyclopropene (27) by Billups as a synthon for cycloproparenes constituted a major achievement and made a large number of previously unaccessiblecycloproparenes, lacking substituents at Cl, available. The cyclopropene 27 is labile and decomposes at room temperature, but nevertheless undergoes addition to a variety of dienes around -20 °C. In some cases reaction times of several days or even weeks are necessary in order to obtain complete conversion. The stereoselectivity for the addition of 27 to open-chain dienes is usually exo, as in the case of the tetrahalogenocyclopropenes. Bis-(hydro,halo-geno)elimination is carried out with r-BuOK in THF, as in the case of the adducts of tetrahalogenocyclopropenes. 

For the preparation of halogen acids in which the substitution is in the beta or gammapositiOYi the reaction of the unsaturated acids of the ethylene series is usually employed. As ethylene by the addition of hydro-bromic acid yields brom ethane, so in like manner, unsaturated acids of the ethylene series take up halogen acids and pass to the mono-halo-genated saturated acid,... 

The Darzens glycidic ester condensation involves the condensation of an aldehyde or ketone with an a-halo ester, in the presence of a base, to afford an a,3 epoxy ester (a glycidic ester ). The first synthesis of a glycidic ester was reported by Erlenmeyer in 1892 and is illustrative of the general reaction, as shown in equation (1). The reaction was subsequently developed and generalized by Darzens. Glycidic esters, in addition to undergoing transformations normally associated with epoxides, afford upon hydro-... 

---