Dichloroketene generation

Cycloadditions with reactive ketenes therefore can he observed only when they are prepared in situ and in the presence of the alkene to which they shall he added. Dichloroketene generated in situ is the best reagent for intermolecular [2+2]-cycloadditions. Dichloroketene is poorer in electrons than the parent ketene and therefore more reactive toward the relatively electron-rich standard alkenes. The reason is that the dominating frontier orbital interaction between these reactants involves the LUMO of the ketene, not its HOMO (see Section 15.2.4). 

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)... 

Oja-Dichloro-y-lactones Epoxides such as styrene oxide undergo C-O bond insertion with dichloroketene generated in situ. 

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. 

Cycloaddition of 2,3-dihydro-1,4-dioxin (10) to 1,1-dichloroketene, generated from zinc and trichloroacetyl chloride by sonication, leads to 7,7-dichloro-2,5-dioxabicyclo[4.2.0]octan-8-one (124), which upon acid hydrolysis affords semisquaric acid (125) <90S583>. 

Dichloroketene generated from trichloroacetyl chloride and activated zinc in ether reacted with 3H-pyrrolizine (1) to give the adduct ((44) or its regioisomer) <71ACS2447>. The double bond in methyl-lH-pyrrolizin-l-one-2-carboxylate reacts with cyclopentadiene forming (45) <82CB706>. 

Treatment of allyl sulfide 111 with dichloroketene, generated in situ by reductive elimination of chlorine from trichloroacetyl chloride, results in an intramolecular ketene Claisen rearrangement giving 779 with high, 2-syn selectivity (94% de). Reductive dechlorination and subsequent lactonization affords chiral butyrolactone 780, with an optical purity that exceeds 95%. 

Note that adducts had been obtained previously from trisubstituted alkenesand dichloroketene generated with activated zinc. ... 

The enol precursor 151 was prepared in three steps by addition of Steri-col 150 onto trichloroethylene, which was followed by the formation of a sensitive ynol through addition of n-BuLi and excess allyl iodide (Scheme 43). The ynol was then partially hydrogenated with Pd on barium sulfate, furnishing the required enol ether 152 contaminated with 8% of the corresponding dihydro enol ether. [2+ 2]-Cycloaddition between 152 and dichloroketene, generated in situ from trichloroacetyl chloride, led to the cyclobutanone 153 as the major diastereomer with a 93 7 diastereocontrol. 

Dichloroketene, generated by the ultrasound-promoted dechlorination reaction of thrichloro- REACTIONS OF... 

The first report concerning the additive Pummerer reaction of a,P-unsaturated sulfoxides with dichloroketene appeared in 1981 [220], and work in this area has recently been reviewed [221]. Simple a,P-unsaturated sulfoxides such as (282) react with dichloroketene (generated from the addition of trichloroacetyl chloride to zinc) to produce diastereoisomerically pure c -y-butyrolactones such as (283) (Scheme 5.93) [220]. 

Dichloroketene, generated by the ultrasound-promoted dechlorination reaction of thrichloroacetyl chloride with zinc, adds to the carbon-carbon double bonds of poly(methyl-l-phenyl-1-silane-c s-pent-3-ene) [271]. This can be illustrated as follows ... 

Dichloroketene generated in situ by zinc dechlorination of trichloroace-tyl chloride reacts by a highly stereoselective [2 4- 2] cycloaddition with the alkene 135 giving the cyclobutanone 136 (Eqns (4.79)), which is used in the synthesis of a variety of other polycyclic products. ... 

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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