Convergent synthesis of the IJKLM-ring part of ciguatoxin CTX3C

Convergent synthesis of the IJKLM-ring part ( 2 ) of ciguatoxin CTX3C has been achieved from the I-ring and the L-ring parts ( 4 and 5 ) in total eight steps in 27% overall yield. The carbanion derived from 4 , stabilized by a dimethyldithioacetal S-oxide group, was readily reacted with aldehyde 5 to give an adduct, which was facilely transformed into the corresponding α,e-dihydroxy ketone 3 . The JK-ring formation from 3 under reductive conditions followed by oxidative M-ring cyclization efficiently led to the pentacyclic ether 2 . Improved synthesis of 6 , a synthetic intermediate for 4 , was also established.