Comparison of amine-induced cyclization of 6-chloro-1-hexynylphosphonate and isobutyl 7-chlorohept-2-ynoate

Hemant Kumar Srivastava, Abed Al Aziz Quntar*, Abdulatif Azab, Morris Srebnik, Avital Shurki

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

8 Scopus citations


The reaction of 6-chloro-1-hexynylphosphonate with primary and secondary amines afforded exclusively 2-aminocyclohexenylphosphonates in 62-85% isolated yields. In contrast, reaction of various amines with isobutyl 7-chlorohept-2-ynoate in acetonitrile at 70 °C gave (E)-sec-butyl 2-(1-alkylpiperidin-2-ylidene)acetates in 65-78% isolated yields. Calculations offer an explanation for the difference in the behavior of the two compounds classes. It is shown that C-C cyclization in the alkyne-phosphonate group occurs via an initial formation of a zwitterionic intermediate, which is stabilized by both an inductive effect of the phosphonate group and a newly formed hydrogen bond. The alkyne-carboxylate group, on the other hand, proceeds via enamine formation as a result of the smaller inductive effect of the carboxylate combined with involvement of an allene-like resonance form. This resonance form both delocalizes the negative charge in the zwitterionic intermediate making it to be less available for attack, and affects the geometry thus preventing formation of the stabilizing hydrogen bond. Hence, the zwitterionic intermediate of the alkyne-carboxylates is less stable leading to formation of an enamine, which is followed by N-C cyclization to give the azaheterocycles.

Original languageAmerican English
Pages (from-to)4389-4395
Number of pages7
Issue number22
StatePublished - 30 May 2009

Bibliographical note

Funding Information:
The research in AS Lab was partially supported by The Israel Science Foundation (grants No. 1317/05 and 1320/05), Applied Funds of the Hebrew University and the David R. Bloom Foundation for Pharmacy.


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