2-Methylenecyclobutanones (2-MCBones), used to be difficult to access, but can now be easily achieved by a green and stereospecific Ca(OH) 2 -catalyzed direct and simple aldol condensation of cyclobutanone and aldehydes under mild conditions. The obtained (E)-2-MCBones should be a class of potentially useful building blocks in synthesis as they could readily undergo an interesting (PhSe) 2 -catalyzed Baeyer-Villiger (BV) oxidation with H 2 O 2 at room temperature to give the versatile 4-methylenebutanolides. Mechanistic studies revealed that the BV reaction most possibly proceeded via the initial formation of benzeneseleninoperoxoic anhydride [PhSe(O)O] 2 O, which then converted to benzeneseleninoperoxoic acid PhSe(O)OOH as the active oxidant, followed by its selective addition to the CvO bond of 2-MCBones and then a selective C-C bond cleavage and rearrangement to give 4-methylenebutanolides.Scheme 1 Three structurally-related building blocks. † Electronic supplementary information (ESI) available: Detailed experimental procedures, condition screening tables, characterization data, 1 H, 13 C NMR and NOESY spectra of the products, and 77 Se NMR spectroscopic studies on the involved organoselenium species. See

Cheap, abundant but seldom-employed Ca(OH)2 was found to be an excellent low-loading (5–10 mol%) catalyst for Claisen-Schmidt condensation of aldehydes with methyl ketones under mild conditions. It was interesting that dilute aqueous ethanol (20 v/v%) was unexpectedly discovered to be the optimal solvent. The reaction was scalable at least to 100 mmol and calcium could be precipitated by CO2 and removed by filtration. Evaporation of solvent directly afforded the product in the excellent 96% yield with high purity, as confirmed by its 1H NMR spectrum.

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Neutral Conditions. -(YU*, L.; LUAN, J.; XU, L.; DING, Y.; XU, Q.; Tetrahedron Lett. 56 (2015) 44, 6116-6119, http://dx.

The products are obtained as (E)-isomers exclusively. -(YU*, L.; WU, Y.; CAO, H.; ZHANG, X.; SHI, X.; LUAN, J.; CHEN, T.; PAN, Y.; XU, Q.; Green Chem. 16 (2014) 1, 287-293, http://dx.