Photoreactivite Acétalique en Serie Hétérocyclique et Osidique

The acetalic photoreactivity of cyclic acetals was studied by some intramolecular acetalic hydrogen abstraction using photochemical ways (NORRISH. II type and BARTON - like reactions). The regioselectivities and stereoselectivities of these reactions were deduced from photochemical transformations of different tetrahydropyrannic cyclic ketoacetals, oxoalkglglycosides and hydroxyalkylglycosides. The abstraction of γ-acetalic hydrogen is not stereoselective by excited carbonyl groups but regiospecific to give lactones in good yields. With hydroxyalkyl hypoiodites, this γ-H abstraction is stereoselective to yield anomeric orthoesters with predominant retention of configuration. By δ-acetalic hydrogen abstraction, the photocyclization of oxoalkylglycosides leads to hydroxy spiro ketals with retention of configuration. The influence of differents substrates, of carbonyl substituents and temperature effects were described. In osidic series (gluco, manno, 2-deoxy), β-anomers are more photoreactive than α-ones. All these results can be interpreted using stereoelectronic effects at the acetalic center for the photocyclization reactions.