Synthesis and circular dichroism of steroids with 2,3-dihydro-1-benzofuranand 4H-benzopyran chromophores; revision of the absolute configuration of some norneolignans from Krameria cystisoides

Starting from cholesterol the 2,3-dihydrobenzo[b]furans 12a, 12b, and the 4 H-benzopyran derivative 14 with known absolute conformation were synthesize d by a stereocontrolled sequence. The same helicity rule was found to be va lid for both chromophores; the P/M helicity of the heteroring leads to a ne gative/positive CD within the alpha-band. On the basis of this rule the abs olute configuration of norneolignans 24-26 isolated from Krameria cystisoid es was also revised.