抄録
The reaction of vitamin D with sulfur dioxide (SO_2) and its application to structural modification of vitamin D are described. Vitamin D reacted with SO_2 to give the 6,19-cycloadducts in quantitative yield as a mixture of two C (6) - epimers (5 and 5'). The SO_2-adducts (5 and 5') were reverted back to the starting vitamin D in high yields via the (5E)-isomer (6) by thermolysis (90-120℃) followed by dye-sensitized photochemical isomerization. Thus SO_2 was proved to be an useful protecting group of the reactive conjugated triene part of vitamin D. By the SO_2 adduct formation, the 6- and 19-positions of vitamin D are activated and introduction of an electrophile to these positions under basic conditions is made possible. The vitamin D-SO_2 adducts were conveniently used in the following structural modifications of vitamin D : 1) synthesis of vitamin D compounds labeled at the 6- and 19-positions from ready-made vitamin D, 2) syntheses of 6- or 19-substituted vitamin D derivatives, 3) syntheses of C_<22>-vitamin D synthons from vitamin D_2 as useful starting materials for numerous side chain modified vitamin D compounds, 4) protection of the vitamin D triene function from acid-catalyzed isomerization.
