Synthetic Study towards Providencin: Stereocontrolled Synthesis of the Furan-Substituted Cyclobutanol Segment.

This study explores the synthesis of unique furanocembranoid-type marine diterpenoid, providencin. Providencin features a highly oxidized structure with two furan rings, two oxirane rings, and a bicyclo[12.2.0]hexadecane framework. Its potential as a lead compound for drug development has drawn attention to its total synthesis, particularly focusing on the challenging right-half segment involving a highly substituted cyclobutane ring. We developed a novel synthetic strategy for the fragment using a [2 + 2] cycloaddition reaction of lithium ynolates with α,β-unsaturated lactone, successfully constructing a bicyclic cyclobutene structure. Stereoselective hydrogenation of cyclobutenes was achieved by using Crabtree's catalyst under high pressure H₂ atmosphere. After further transformation, the synthesis of the furan-substituted cyclobutanol fragment having a formyl side chain was accomplished.