Synthesis and circular dichroism of steroids with 2,3-dihydro-1-benzofuran and 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 4H-benzopyran derivative 14 with known absolute conformation were synthesized by a stereocontrolled sequence. The same helicity rule was found to be valid for both chromophores; the P/M helicity of the heteroring leads to a negative/positive CD within the alpha-band. On the basis of this rule the absolute configuration of norneolignans 24-26 isolated from Krameria cystisoides was also revised.