3‐Alkyloxycyclobutanones as Useful Synthons for Heterocycle and Carbocycle Synthesis

Abstract

Substituted 3-alkyloxycyclobutanones are highly valuable intermediates in organic synthesis, which demonstrate interesting and rich reactivity in different types of reactions due to the strain energy. Under Lewis acid catalysis, 3-alkyloxycyclobutanones can form zwitterionic intermedi-ates and participate in a series of cycloaddition reactions as three-or four-carbon synthons. This Focus Review will mainly discuss formal [4+2], [3+2], and [3+3] annulations as well as other types of transformations with 3-alkyloxycyclobutanones.Scheme 1. Synthesis of 3-alkoxycyclobutanones. DIPEA = diisopropylethylamine; TBME = tert-butyl methyl ether.Scheme 2. Synthesis of 2-monoalkyl-or 2-monoaryl-3-ethoxycyclobutanones.Scheme 3. Synthesis of disubstituted 3-alkoxycyclobutanones.[a] G. Scheme 4. Matsuo's unexpected discovery. LA = Lewis acid. Scheme 9. A practical example for the utility of the alkyne-cobalt complex. Scheme 10. Asymmetric cycloaddition of chiral cyclobutanones. Scheme 11. CÀC insertion of silanes into cyclobutanones. Tf = trifluoromethanesulfonyl. Scheme 12. Possible mechanism for 1,5-hydride transfer. TBDPS = tert-butyldiphenylsilyl. Scheme 13. CÀC bond insertion of silylenol ethers into cyclobutanones. Scheme 14. CÀN insertion into 3-ethoxycyclobutanones. Scheme 15. Application of CÀN insertion into 3-ethoxycyclobutanones in natural product synthesis. DME = 1,2-dimethoxyethane. Scheme 16. Intramolecular cycloaddition of 3-ethoxycyclobutanones. Scheme 17. [3+2] cycloaddition reaction to give pyrazoles. Bz = benzoyl; Cbz = carboyxbenzyl. Scheme 22. Synthesis of unsymmetrical and symmetrical 1,10-phenanthrolines. Scheme 23. Synthesis of b-enamino ketones. Fmoc = 9-fluorenylmethoxycarbonyl.