Cyclobut-2-enones from Alkynes via Dichlorocyclobut-2-enones

Article abstract of DOI:10.1002/hlca.19870700208

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 equlibrium mixture of the two; this isomerization does not occur with LiCl.The Cl-atoms of both 2a,b and 3a,b and 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.