Enynol acetate

A interesting variation on this theme employing the isomeric enynol acetates (Scheme 24) has been developed by Rautenstrauch. The cyclizations are induced by a Pd" catalyst in warm acetonitrile. The proposed mechanism is intriguing. Reaction is initiated by an anchimerically assisted palladation to (35) followed by opening the dioxolenium ion to a pentadienylic cation (36). The closure of (36) is analogous to the silicon-directed Nazarov cyclization in the ejection of the Pd" electrofuge from (37). Both secondary and tertiary acetates can be employed as well as both acyclic and monocyclic systems. 

Rautenstrauch reported another mechanistically intriguing example. Treatment of enynol acetate 111 with a palladium(II) catalyst in warm acetonitrile resulted in the formation of cyclopentenone 115. The proposed mechanism involves generation of divinyl cationic species 113, followed by electrocyclization, and elimination of the palladium(ll) electrofuge in a manner comparable to the silicon-directed Nazarov cyclization (see Section 3.4.5.1). 

Intramolecular alkoxycarbonylation of alkynols is parallel to what has been described for alkenols except that functionalization of the triplebond produces a double bond. No lactone formation is observed in the Pd(II)-catalyzed oxidative cyclization-carbonylation of alkynes. Instead [(methoxycarbonyl)methylene]tetrahydrofurans are selectively formed [134, 135]. Moreover, starting from an enynol, furan-2-acetic ester is obtained resulting from a final aromatization step [136]. 

Z)-Enynols can also be employed as substrates when ruthenium or iridium catalysts are used <2006ASC1671>. Furan-2-acetic esters are obtained by a Pd(n)-catalyzed oxidative cyclization-alkoxycarbonylation of (Z)-enynols <1999JOC7693>. In analogy, 2-furan-2-ylacetamides are obtained in an aminocarbonylation with secondary amines (Equation 22) <2006S4247>. 

Startg. enynol stirred at room temp, overnight with dicobalt octacarbonyl in a hydrocarbon solvent complex (Y 88%) allowed to react 15 min. at 0 with a fluoroboric-acetic acid mixture diol (Y 91%) treated with Fe(N03)3 9H20... 

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