Synthetic studies toward the total synthesis of nopsan-4-ol through radical rearrangement

Abstract

Core ring skeleton of Quadronoids was synthesized through tandem radical reaction sequence, which involved the rearrangement. And this reaction was used as the key step in the total synthesis of Nopsan-4-ol, Suberosenone and formal total synthesis of Quadrone. The strong patchouli-like and woody smelling essential oil of the rhizomes of Echinops giganteus var.lelyi C.D Adams (compositae) contains only sesquiterpens, which are mainly triquinanes. But Nopsan-4-ol is component of this oil, it has different skeletal, tricyclo $[4.3.2.0^{1,5}]$ undecane. Endiyne 6 and 7, prepared from b-keto ester via 6 and 7 steps, undergo tributyltin mediated radical reaction to form tricyclo $[4.3.2.0^{1,5}]$ undecane 15 and 8. If the resulting undecane 15 and 8 were satisfied following two conditions, cationic rearrangement from 5-ring to 6- ring occurred. First, one is the presence of tin moiety doing the deprotection of TMS-ethers to diol. The other is the acidic condition. The first step of rearrangement is protonation of the tertiary hydroxy group, and generation of cation and rearrangement of cation. The driving force for the rearrangement is the existence of the tin group.