Selenium dioxide solvent effects

Selenium dioxide (SeOa) in tert-alcohols has been widely used for the dehydrogenation of 3-0x0 steroids, and particularly in the conversion of A -3-ketones into A dienones [240]. Other steroidal ketones suffer similar a,jS -dehydrogena-tion e.g. i2-ketones [241] and a 4,4-dimethyl-7-ketone [242]), but the same reagent in different solvents will also effect oxidation of certain steroidal ketones with adjacent methylene groups ( C0-CH2 ) into a -diketones (-CO CO-), a reaction which is common outside the steroid field. In ethanolic solution, for example, 5a-cholestan-3 one was oxidised by SeOa to give 5a-cholestane-2,3-dione [243], isolated in its enoHc forms. 

Cycloadditions Interestingly, sulfur dioxide participated as a dienophile in the [4+2] cycloaddition reaction with 1,3-dienes. In this manner, sulfur dioxide reacts similarly to the related selenium dioxide and the other sulfur dienophiles RN=S=0, RN=S=NR and R2C=S=0 (sulfines). However, the [4+2] cycloadducts derived from 1,3-dienes and sulfur dioxide are only obtained at low temperatures (—80 °C) in a kinetically controlled reaction and the cycloaddition reactions often require the presence of a Lewis acid (CF3COOH or BF3). Above —50 °C the Diels-Alder adducts undergo a cycloreversion and a cheletropic addition of the generated sulfur dioxide to the diene occurs with formation of the corresponding 2,5-dihydrothiophene-1,1-dioxides (sulfolenes). According to ab-initio computations, electrostatic solvent effects are predicted to be of importance in the control of the selec-tivities in this reaction . From linear dienes, the [4+1] cycloadducts are usually obtained. For example, from 1,3-butadiene and SO2 at -20 °C, the cyclic sulfone 25 is obtained in 95% yield. ... 

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