Stereospecific alkynylation at the more hindered carbon of trisubstituted epoxides and concise syntheses of bis-THF acetogenins and analogues
Alkynyl aluminate reagents react cleanly at the tertiary position of trisubstituted epoxides in 75-80% isolated yield. The reaction is stereospecific with inversion at the substitution center. This work constitutes the first general method for the addition of a carbon nucleophile at the more congested site of a trisubstituted epoxide. The less efficient alkynyl alane reagents for the same transformation were also discussed. Bis-THF acetogenin bullatacin and analogues are each synthesized in 22 steps and 6.2% overall yield. The new synthesis takes a modular approach and features a cobalt mediated oxidative cyclization reaction, an efficient synthesis of epoxy butenolide, and a highly diastereoselective alkylation of a bis-THF aldehyde. The efficiency of the new synthetic sequence allows quick assembly of the series of compounds and the study of their structure and activity relationships (SARs) is currently underway.