Oxidative C-H Bond Activation:Study on DDQ-Induced Intramolecular Cyclization Reactions and Application to the Formal Synthesis of Neopeltolide

摘要

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) has been found to be an excellent reagent for oxidatively activating benzylic C-H bonds, and the in-situ generated oxocarbenium ions can be captured by nucleophiles appended in the molecules. A variety of aromatic methyl ethers with different substituents are shown to be reactive with DDQ, and common protecting groups are well tolerated. The enol acetate group is the most reliable and productive nucleophile compared with other candidates and provides the 2,6-disubstituted tetrahydropyrone product as exclusively the cis isomer. This method has been successfully applied to the macrocyclic systems. The valuable macrocyclic oxocarbenium ions are readily formed when treating the substrates with DDQ. The reactions are generally slower than those of acyclic substrates but the yields are almost unaffected. The ring size has no influence on the diastereoselectivity of cyclization.A convergent formal synthesis of natural product neopeltolide was accomplished from three building blocks by utilizing the DDQ mediated cyclization reaction as the key transformation. A novel intramolecular hydrosilylation-Tamao oxidation reaction is employed to synthesize the precursor for the key step. This approach to the natural product has minimized the involvement of protecting groups and conveniently provided an opportunity to access the natural product analogues through double bond functionalizations.