Diynes cobalt-mediated cyclization

This synthetic methodology was used to prepare (+)-estrone in 5 steps and 12.5% yield from 2-methylcyclopentenone where the benzocyclobutene was prepared by a cobalt-mediated cyclization of 1,5-diynes and acetylenes. The reaction type was also used in an alkaloid synthesis. 

In previous works this group had observed a competition between the PKR and a [2 + 2 + 2] cyclization in the second reaction step of three triple bonds. Thus, when reacting linear triynes 174 under catalytic, high CO pressure, cobalt mediated PKR conditions, they obtained mixtures of products 175 coming from two [2 + 2 + 1] cycloadditions, and 176 from a [2 + 2 + 1]/ [2 + 2 + 2] tandem reaction. When the triple bonds were ether linked, the latter was the favored reaction, while with substrates lacking oxygen atoms, the iterative PKRs was the major pathway (Scheme 51) [166]. When the reaction was performed intramolecularly between a diyne and an alkyne, the only reaction products were the result of a [2 + 2 + 1 ]/[2 + 2 + 2] tandem cycloaddition [167,168]. 

A one-step construction of the stemodane framework that resulted in a formal synthesis of the diterpene stemodin (185) was realized by the cobalt-mediated intramolecular cyclization of ene-diyne 182 (Scheme 7.40) [55], Three stereocenters in 183, two of them quaternary, were produced with complete specificity during this [2 - - 2 + 2] cycloaddition. 

Indole N-substituted diyne tetracobalt complexes 1467 undergo a Lewis acid-mediated dimerization-cyclization reaction through the indole 3-position to afford indolophanetetrayne cobalt complexes 1468 (Scheme 280) <20030L1003>. 

Cobalt(I)-Mediated [2+2+2] Cyclization of Allene-Diynes A Diastereoselective Approach to 11-aryl Steroid Core. 11-Aryl-Substituted Steroid Systems by Co-catalyzed [2+2+2] Cyclization of Allene-Diynes... 

---