Epoxides, vinyl iodination

Vinyl azides have been prepared by treating epoxides (78) with azide ion to give azido alcohols (79) which are then dehydrated to the vinyl azide.30 This method also complements the iodine azide method since the epoxide route usually gives the isomeric vinyl azide. -Hydroxy azides (81) can also be prepared by the sodium borohydride reduction of a-azidoketones (80). 

In this synthesis (Scheme 6), the C2-symmetri-cal triacetonide of D-mannitol (32) is converted via the epoxide 33 and its nucleophilic addition product 34 to the propargylic alcohol derivative 35. From this intermediate, the Z-configured vinyl iodide 36 is stereoselectively obtained by hydroalumination/iodination. The Pd-catalyzed Heck cyclization then affords the isomerically pure product 37, which represents a potential building block for the synthesis of la,2y5,25-trihy-droxy-vitamin D, following the classical Wittig strategy of Lythgoe. 

It is apparent, then, that precursors for terminal vinyl azides are obtained following iodine azide addition to olefins only when rather unusual steric or electronic factors are operating. A number of other reaction sequences have been investigated in the search for a general approach to the terminal compounds. The opening of a terminal epoxide by azide ion leads to an azidohydrin which can be readily dehydrated (equation 4). Unfortunately, this reaction sequence... 

The manufacture of epoxy stabilizers begins with soya and linseed oils or oleate and other unsaturated fatty acid esters. When epoxidation is incomplete, residual unsaturation remains, leading to poor compatibility-stability. This results in exudation on products in the marketplace. Makers of epoxldized fatty acid esters strive to prevent such failure by ensuring more complete epoxidation and supplying products with the lowest possible iodine number. Vinyl formulators further protect themselves by diminishing the proportion of epoxidized stabilizer to an amount ca. 3% while maintaining plasticizer concentration at ca. 35%. 

Polyepoxide plasticizers comprise two basic types. The first type is the epoxidized soybean oils (with a maximum iodine number of 6 and an oxirane oxygen minimum of 6%), which have a good affinity to vinylic polymers [45]. Compared to plasticizers of the second type, the alkylepoxy stearates (butyl, octyl), they are less volatile and are less effective for low-temperature plasticization. Polyepoxide plasticizers have less affinity to PVC than primary plasticizers. They are efficient as heat and light stabilizers and for this reason they are used in synergetic mixtures to stabilize PVC. 

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