Dowd-Beckwith ring expansion

What is Dowd-Beckwith ring expansion?

In 1987, Dowd and Beckwith reported the Dowd-Beckwith ring expansion simultaneously. The Dowd-Beckwith ring expansion, also referred to as the Dowd-Beckwith type ring expansion, Dowd-Beckwith rearrangement, Beckwith-Dowd type rearrangement, Beckwith-Dowd expansion, Beckwith-Dowd ring expansion, or Beckwith-Dowd ring enlargement, is a general method to expand one-carbon rings in cycloketones.

Dowd-Beckwith ring expansion - general reaction scheme - Dowd-Beckwith type ring expansion - Dowd-Beckwith rearrangement - Beckwith-Dowd type rearrangement - Beckwith-Dowd expansion - Beckwith-Dowd ring expansion - Beckwith-Dowd ring enlargement
Dowd-Beckwith ring expansion

R = alkyl
X = Br, I, SePh

This is accomplished by introducing a halomethyl group to cyclic β-keto esters, followed by radical dehalogenation, such as the addition of a mixture of tributyltin hydride and 2,2′-azobis(isobutyronitrile)  AIBN in refluxing benzene solution. After radical dehalogenation, the carbon radical joins to the neighboring carbonyl group, leading to an unstable cyclopropyloxy radical that forces the cyclopropyl ring to break and form a larger cycloketone.

The ester group in the cyclic β-keto ester can promote alkylation, activate radical addition to the carbonyl group, and aid in the opening of the cyclopropyloxy radical. The acyl radical cyclization is irreversible on any kinetically significant time scale, but radical cyclization to the carbonyl group may fail if a highly congested transition state is involved. This reaction has been extensively studied and expanded to prepare middle-size cycloketones via four-carbon ring expansion by Dowd.

The utilization of this reaction for the synthesis of cyclic ketones is extensive.

References

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