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DOI: 10.1055/sos-SD-023-00504

Danheiser, R. L.; Dudley, G. B.; Austin, W. F.Science of Synthesis, (200623558.

The four-electron electrocyclic ring opening of 4-alkynylcyclobutenones represents the most widely employed method for the generation of ketenes of this product subclass. The vast majority of the cases described to date involve 4-alkynyl-4-hydroxycyclo­butenones or closely related alcohol derivatives, although a few examples have been reported of ring openings involving 4-cyano-[‌183‌,‌185‌] and 4-chloro-substituted[‌190‌,‌191‌] alkynylcyclobutenones. As discussed in Section, donor substituents are predicted to rotate outward in the thermal version of this conrotatory electrocyclic process.[‌40‌] On the other hand, this torquoselectivity is reversed in the photochemical variant of the electrocyclic ring opening, as illustrated in Scheme 79.[‌192‌,‌193‌] Thus, irradiation of cyclobutenones of the type 256 induces electrocyclic cleavage to form enynylketenes 257, which then cyclize to afford lactones of the type 258 in modest yields. The 4-alkynyl-4-hydroxycyclobutenones 256 are prepared by the addition of the appropriate lithium acetylide to dimethyl squarate.

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