Ring-Closing Reaction of Allenic/Propargylic Anions Generated by Base Treatment of Sulfonylallenes

Link: Org Lett ASAP

Shinji Kitagaki, Satoshi Teramoto, and Chisato Mukai*

Division of Pharmaceutical Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan

This is a report of a very neat and novel ring closing reaction. The reaction involves the allenic moiety substituted with an EWG and a strategically placed electrophile tethered to the allene.

The EWG could be sulfone, ketone and ester and electrophile could be halide, aldehyde and alpha,beta-unsat ester. The reaction conditions were quite mild and even TBAF could be used as a base, as well as a more potent base such as NaH. When EWG was sulfone, and the electrophile was either OTs group or I, the reactions proceeded to give the expected cyclic propargylic sulfone as shown in Table 1.

But when electrophile was an aldehyde, product of type 9 could be obtained in good yields, except in entry 14 of Table 2 where 10, instead of the expected 9, was also obtained in 26% presumably via addition of enolate of the aldehyde to the allene in the endo-mode.

In addition, when electrophile was alpha,beta-unsat ester and the base used was KOt-Bu, the expected product (12) was obtained in good yields.

Finally, when EWG was something else other than a sulfone, and the elctrophile was a terminal iodide, the reaction proceeded to give the ring-closing product of type 14 in modest to excellent yields (except 14c).