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Synthesis of 4H-pyrans

Recent Literature

An NHC-catalyzed [2 + 4] cyclization of alkynyl ester with α,β-unsaturated ketone provides highly substituted 4H-pyran derivatives in gooy yields. This strategy offers cheap and easily available starting materials, mild reaction conditions, and high atom economy.
F. Lu, Y. Chen, X. Song, C. Yu, T. Li, K. Zhang, C. Yao, J. Org. Chem., 2022, 87, 6902-6909.

Inexpensive and commercially available nano-powder magnetite or iron(III) oxide can be used as catalyst in a new, straightforward, and fast protocol for the construction of 4-substituted-4H-pyrans. The reaction implies a tandem process, involving an aldol condensation, a Michael-type addition, and a dehydrating annulation.
R. Cano, D. J. Ramón, M. Yus, Synlett, 2011, 2017-2019.