Dichloroketene Cycloaddition with cyclopentadiene

DicWoroketene is particularly reactive, and reductive dechlorination of the product with zinc and acetic acid allows access to the cyclobutanone from formal addition of ketene itself. Thus, cycloaddition of dichloroketene with cyclopentadiene, followed by dechlorination and Baeyer-Villiger oxidation gave the lactone 173, a usehil precursor to various oxygenated cyclopentane products (3.117). Intramolecular cycloaddition reactions of ketenes can allow the formation of bicyclic and polycyclic products using otherwise unstable ketene intermediates. ... 

The first models for asymmetric induction in [2 + 2] cycloadditions are ketimines derived from precursors such as (25), which react with both cyclic and acyclic alkenes to give adducts with high enantiomeric excesses. Full details have been reported on the regio- and stereo-specificity of the addition of dichloroketene to 1-substituted cyclohexenes. Methyl (phenylthiomethyl)-ketene (26) provides a further example of a ketene which, after cycloaddition to cyclopentadiene, undergoes facile opening of the cyclobutane that is thus formed to provide vicinally substituted cyclopentene derivatives. ... 

Tropolone has been made from 1,2-cycloheptanedione by bromination and reduction, and by reaction with N-bromosuccinimide from cyclo-heptanone by bromination, hydrolysis, and reduction from diethyl pimelate by acyloin condensation and bromination from cyclo-heptatriene by permanganate oxidation from 3,5-dihydroxybenzoic acid by a multistep synthesis from 2,3-dimethoxybenzoic acid by a multistep synthesis from tropone by chlorination and hydrolysis, by amination with hydrazine and hydrolysis, or by photooxidation followed by reduction with thiourea from cyclopentadiene and tetra-fluoroethylene and from cyclopentadiene and dichloroketene. - The present procedure, based on the last method, is relatively simple and uses inexpensive starting materials. Step A exemplifies the 2 + 2 cycloaddition of dichloroketene to an olefin, " and the specific oycloadduot obtained has proved to be a useful intermediate in other syntheses. " Step B has been the subject of several mechanistic studies, " and its yield has been greatly improved by the isolation technique described above. This synthesis has also been extended to the preparation of various tropolone derivatives. - " ... 

S.2 Cycloadditions Involving Ketenes We next present two examples of cycloadditions involving ketenes that invoke a bifurcating PES. The cycloaddition of ketene with cy clopentadiene had long been thought to simply give the formal [2-1-2] product, such as the reaction of cyclopentadiene 67 with dichloroketene 68... 

Ketene itself is usually made by high-temperature pyrolysis of acetone but some ketenes are easily made in solution. The very acidic proton on dichloroacetyl chloride can be removed even with a tertiary amine and loss of chloride ion then gives dichloroketene in an ElcB elimination reaction. If the elimination is carried out in the presence of cyclopentadiene a very efficient regio- and stereospecific [2 + 2] cycloaddition occurs. 

An attractive jasmone synthesis comes from Grieco. [89] A bicyclic ketone is obtained from the [2 -i- 2]-cycloaddition of dichloroketene to cyclopentadiene, followed by reduction with zinc. After Baeyer-Villiger oxidation, reduction to... 

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