Dichloroacetyl chloride, dichloroketen from

Fig. 15.35. Prototypical in situ generation of dichloroketene through NEt3-mediated /(-elimination of hydrogen chloride from dichloroacetyl chloride. Regioselective [2+21-cycloaddition II of this dichloroketene with an alkene whose C=C double bond contains a conjugated substituent at C-a, but not at C-/J.

Fig. 12.31. Orientation-selective [2+2]-cycloaddition with in situ generated dichloroketene I the dichloroketene is generated by way of an NEt3-mediated fi-elimination of HC1 from dichloroacetyl chloride.

The resulting enolate is not stable as it can eliminate chloride ion, a good leaving group, to form a ketene. This works particularly well in making dichloroketene from dichloroacetyl chloride as the proton to be removed is very acidic. 

The same reaction has been also carried out with heterocyclic aminomethyleneketone derivatives of, e.g., 4-pyranones273, chromanones and derivatives and 4-piperidones276 (equation 202). A useful dienophile is dichloroketene, produced from dichloroacetyl chloride and triethylamine. It enables the enaminone oxygen to be incorporated into the functional group of a lactone instead of a sultone (equation 202). 

Cyeloaddition to silyl enol ethers. As expected, dichloroketene, generated from trichloroacetyl chloride and activated zinc, reacts with silyl enol ethers to form 3-silyloxy-2,2-dichlorocyclobutanones, usually in good yield. Cycloaddition does not obtain with dichloroketene generated from dichloroacetyl chloride and triethylamine. In some cases only acyclic products are formed, but these may arise by ring opening of intermediate cyclobutanones. ... 

A practical extension of the reaction uses ketenes generated in situ in the presence of a carbonyl compound. Thus the highly reactive dichloroketene generated in situ from the reaction of dichloroacetyl chloride with triethylamine has been trapped by aldehydes (equation 6) or activated ketones (equation 7)... 

Synthesis of fi-lactones. Dichloroketene (generated in situ from dichloroacetyl chloride and triethylamine) reacts with aliphatic and aromatic aldehydes in ether at low temperatures (<20°) to give a,a-dichloro-/3-lactones.4 Esters of a-keto-carboxylic acids also undergo the reaction. 

Preparation. Dichloroketene has been generated in situ by dehydrohalogena-tion of dichloroacetyl chloride with trimethylamine (1, 222) and from trichloro-acetyl bromide by dehalogenation with copper-activated zinc (1, 1287). 

The (5)-phenylethylamine-derived norbomene 14 also serves as the starting point for the synthesis of the natural alkaloid, (-)-y-iV-normethylsky-tanthine (79) [26]. The key transformation of the 2-azabicyclo[2.2.1]heptane skeleton into the 3-azabicyclo[4.3.0]nonane backbone, embodied in the natural alkaloid, involves the ketene amino Claisen rearrangement as reported by Roberts et al. [27] (see Scheme 2.3 and the prior equation in section 2.3.1). Treatment of 14 with dichloroketene, generated in situ from dichloroacetyl chloride and Hunig s base, at 2°C for 16h, gives lactam 80 in... 

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