Cyclopentadiene dichloroketene

The base-catalyzed rearrangement of the cyclopentadiene-dichloroketene adduct gives tropolone. The reaction course passes through an SN2 displacement of the enol, fragmentative dehydrochlorination, and enolization [85],... 

Tropolone has been made from 1,2-cycloheptanedione by bromination and reduction, and by reaction with A -bromosuccinimide from cyolo-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. - ... 

A simple preparation of tropolone (253) was reported, in which the rearrangement of the 1 1 adduct (252) of dichloroketene and cyclopentadiene readily provide tropolone (253) by catalyzed with KOAc—HOAc 84). 

Attempts to add dichloroketene to 3//-pyrrolizine (1) were largely unsuccessful, giving Friedel-Crafts products (Section III,B,4) but in one case, in which the ketene was generated from trichloroacetyl chloride and activated zinc in ether, an adduct (294) (or its regioisomer) was obtained.123 The double bond in the 1 H-pyrrolizin-1 -one can act as a dienophile with cyclopentadiene ester 295 was isolated.26... 

Although the earliest syntheses of the Corey lactone, using a cyclopentadienyl-thallium species, were not very attractive, it was soon found that the lactone could be usefully generated by a Baeyer-Villiger reaction on a bicycloheptenone.8 In turn, this derives from an easy [2 + 2] cycloaddition reaction between dichloroketene and cyclopentadiene (Scheme 30.2).9 Additionally, an intermediate of the lactone could be resolved by way of the a-methylbenzylamine salt of its opened hydroxyacid, bringing the resolution step earlier in the synthesis than previously.10... 

It is possible that a [4+2] -cycloaddition between the dichloroketene carbonyl group and the cyclopentadiene occurs. This would be followed by a Claisen rearrangement to give the product observed. The lesson is that you should learn what we think we know, but not be afraid to question it. That is science. 

Ketene Cycloadditions. As we saw earlier [see (Section 6.3.2.8) pages 211 and 212], ketenes undergo cycloadditions to double bonds 6.118 (repeated below) to give cyclobutanones. In practice, the reaction is faster and cleaner when the ketene has electron-withdrawing groups on it, as in dichloroketene, and when the alkene is relatively electron-rich, as in cyclopentadiene. The product from this pair of reagents is the cyclobutanone 6.249. 

The energies and coefficients of the frontier orbitals of ketene are shown in Fig. 6.39. The regioselectivity in the reaction between cyclopentadiene and dichloroketene giving the cyclobutanone 6.249 is explained by the overlap from the large LUMO coefficient on the central atom of the ketene and the larger coefficient at C-l in the HOMO of the diene. 

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... 

A group at the Catholic University of Louvain independently developed the same process for generation of dichloroketene and characterized the product of its addition to cyclopentadiene as the bicyclic dichloroketone (3). They found that dichloroketene reacts also with cyclopentene to give (4). 

Lactone (16) is an important intermediate in the synthesis of prostaglandins. Reversing the Baeyer-Villiger rearrangement gives cyclobutanone (17) which is the adduct from ketene and cyclopentadiene. In practice, dichloroketene is much easier to make and handle than ketene itself so compound (8) is used as an intermediate, the chlorine atoms being removed with zinc. ... 

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. ... 

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... 

As the starting material for travoprost, Chirotech chose racemic bicyc-lo[3.2.0]hept-2-en-6-one, which is accessible in good yields by [2 + 2]-cycload-dition of cyclopentadiene with dichloroketene, followed by a zinc reduction. [230]... 

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. ... 

Grieco synthesized (326) regiospecifically from cyclopentadiene and dichloroketen and dechlorinated it to (327), which was converted into lactone (328) with aqueous HjOj and thence into (329) with di-isobutylaluminium hydride. Sequential treatment of (329) with the ylide from PhjP —Pr Br" in DMSO, which serves to introduce the cis-double bond, and oxidation gave ketone (330). After isomerization of (330) to (331), reaction with methyl-lithium gave (332) which on chromium trioxide oxidation gave cis-jasmone (Scheme 13) in 40 % overall yield. 

---