Cyclobut‐2‐enones from Alkynes via Dichlorocyclobut‐2‐enones

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Abstract
The [2 + 2]‐cycloaddition of dichloroketene (prepared in situ from CCl3COCl and Zn(Cu)) with three alkynes 1a–c to form 2,3‐dimethyl‐(2a), 2,3‐diethyl‐(2b) and 3‐butyl‐4,4‐dichlorocyclobut‐2‐enone (2c) proceeds rapidly in the absence of POCl3. The primary products 2a–c rearrange in situ to the 2,4‐dichlorocyclobut‐2‐enones 3a–c under the influence of ZnCL2 produced during the reaction. ZnCl2 converts both 2a and 3a into a 4:6 equilibrium mixture of the two; this isomerization does not occur with LiCl. The Cl‐atoms of both 2a, b and 3a, band of 2c may reductively be removed with Zn(Cu) in AcOH/pyridine to afford the alkylcyclobutenones 4a‐c. Without pyridine, this reduction gives ca. 1:1 mixtures of the double‐bond isomers 4 and 5 in low yields. The cyclobutenones 2c and 4c may be deuterated by CD3COOD in the presence of pyridine. D‐Atom is introduced into 2c at C(4) and at C(γ), and into 4c at C(2) and C(4). A mechanism for this deuteration is considered, which does not involve a cyclobutadienolate 7, but rather a cyclobutenolate of type 8. The reductions of 2 and 3 to 4 might also pass through the same type of intermediate 8.