Bicyclization of Enynes

Scheme 37 Zr-promoted bicyclization of enynes, dienes, and diynes. Some natural products synthesized by these reactions.

This catalytic tandem carbene addition-bicyclization of enynes has been very recently applied to the synthesis of fluorinated bicyclo[ .0.l]-amino esters.176... 

Asymmetric Pauson-Khand bicyclization.1 Efficient asymmetric Pauson-Khand bicyclizations of enynes are possible using (lS,2R)-( + )-phenylcyclohexanol (13, 244 14, 128-129) as the chiral auxiliary. Thus the (E)-enol ether (1) derived... 

Synthetic Applications of the Zirconium-promoted Bicyclization of Enynes... 

It is clear from the results presented above that the Zr-promoted bicyclization of enynes is applicable to the synthesis of a wide variety of monocyclic, bicyclic and polycyclic compounds, including terpenoids. At present, however, the number of such applications is still very small. 

Another promising application is to use the Zr-promoted bicyclization of enynes for controlling the stereochemistry of aliphatic carbon centers, as exemplified by the synthesis of a key intermediate (26) for invictolide (27) shown in Scheme 3. 

Although the number of examples of the Ti-promoted bicyclization of enynes is very limited, the reaction appears to proceed satisfactorily in some favorable cases, as shown in equations (23) and (24). The presumed titanacyclic intermediates do not appear to have been characterized. 

The same type of reaction can also be effectively applied to the stereoselective bicyclization of enyne iodides65. 

Negishi, E.-i., Ma, S., Sugihara, T. and Noda, Y. (1997) Synthesis of hydrazulenes via Zr-promoted bicyclization of enynes and transition metal-catalyzed or radical cyclization of alkenyl iodides. Efficient synthesis of ( )-7-e/n -)3-bulnesene. J. Org. Chem., 62,1922-3. 

Negishi E, Holmes SJ, Tour JM et al (1989) Novel bicyclization of enynes and di5mes promoted by zirconocene derivatives and conversion of zirconabicycles into bicyclic enones via carbonylation. J Am Chem Soc 111 3336-3346... 

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