Selenite esters

Compound 87 isolated in a trapping experiment681 undergoes a [2,3] sigmatropic rearrangement to yield a selenite ester (88). Hydrolysis of the latter gives the product allylic alcohol. 

Selenium-mediated allylic oxidations producing allylic alcohols have been discussed above however, in some cases oxidation proceeds further to give the a, -unsaturated carbonyl compounds directly, or mixtures of alcoholic and ketonic products. That the regioselectivity observed in these allylic oxidation reactions closely resembles that found in classical selenium dioxide oxidations is in accord with initial formation of the intermediate allylic alcohol before in situ oxidation to the carbonyl compound. This process was studied by Rapoport and was explained mechanistically as an elimination of the intermediate allylic selenite ester via a cyclic transition state, analogous to Ssi (rather than 5n20 solvolysis (Scheme 21). Of the two possible transition states (78) and (79), the cyclic alternative (78) was preferred tecause oxidation exclusively yields trans aldehydes. 

Thionyl chloride, as well as other inorganic acid chlorides, reacts with polyols to form mixed esters (see under Sulfate esters. Chapter III). In the presence of pyridine, partial chlorohydrin formation may occur (95), Selenium oxychloride forms a selenite ester upon reaction with mannitol (96), Phosphorus pentachloride yields unsaturated chlorohydrins of mannitol and galactitol which have the composition CeHeCU (97), Extremely interesting are the so-called complexes of alditols with various inorganic polybasic acids, their salts, or anhydrides in aqueous solutions. Complexes with boric, molybdic, tungstic, and other acids, as well as the oxides of antimony and arsenic, have been reported. It is believed that these complexes are true esters with one or more moles of alditol, a chelate type of structure being involved at some point. For the hexitols a compound with boric acid such as the following is postulated (98),... 

However, aliphatic alcohols can be alkylated by diazomethane if boron trifluoride etherate,788 hydrogen tetrafluoroborate,789 or aluminum chloride788 is added to the reaction mixture. Further, aliphatic alcohols can be methylated without further ado by diazomethane if hexane or heptane replaces ether as the solvent.790 Also the hydroxyl groups of hydroxy acids such as tartaric acid and their esters are smoothly methylated by diazomethane in ether 791, 792 and aliphatic alcohols can be methylated by diazomethane in the presence of sulfur dioxide or sublimed selenium dioxide, this reaction proceeding by way of the unstable alkyl methyl sulfite or selenite, respectively.793... 

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