Cyclopentadienes Diels-Alder cyclization

On transforming the cyclopentenone 1431 into the trimethylsilyloxy diene 1432 the ensuing Diels-Alder cyclization gives rise to 69% tricychc compound 1433 [10] (Scheme 9.8). For Diels-Alder-reactions of thioaldehydes, selenoaldehydes, or unsaturated nitroso compounds with cyclopentadiene, see the reactions of 602 to 603 and of 605 to 606 in Scheme 5.48 and of 1092 to 1093 in Scheme 7.43. For Diels-Alder-reactions of silyloxyazadienes such as 510 with maleic anhydride to give 511, see Scheme 5.29. 

Chiral secondary amines such as nonracemic imidazolidin-4-ones have been found to be effective asymmetric organocatalysts in the Diels-Alder cyclization of cyclopentadiene and a,p-unsaturated aldehydes [60]. A tyrosine-derived imidazoli-din-4-one was immobilized on PEG to provide a soluble, polymer-supported catalyst 110. In the presence of 110, Diels-Alder cycloaddition of acrolein 112 to 1,3-cyclohexadiene 111 proceeded smoothly to afford the corresponding cycloadduct 113 with high endo selectivity and enantioselectivity up to 92% ee (Scheme 3.31) [61]. 

The intramolecular Diels-Alder cyclization of cyclohexadiene-l,2-dicarboxylic anhydride-derived diester-tethered triene (86) produces the cycloadducts in a ratio of 7 1 (Scheme 31). The intramolecular Diels-Alder reactions of amino acid-derived trienes (87) yield cycloaddition products (88) and (89) which are mainly cw-fused and derived from the exo-transition states (Scheme 32). A key reaction in the synthesis of the natural product momilactone A is the transannular Diels-Alder cyclization of the trans-trans-cis alkene (90) to the trans-syn-trans tricycle (91) (Scheme 33). The Diels-Alder cycloaddition of 1 l-oxapentacyclo[6,5,2,2 0 0 ]-heptadeca-4,14,16-triene-4,5-dicarboxylic anhydride (92) with cyclopentadiene proceeds with 5yn-facial selectivity to produce syn,endo and syn,exo cycloadducts (93) and (94)... 

A new synthesis of isoxazoles is by successive treatment of a ketoxime with butyllithi-um, the ester of a carboxylic acid and sulfuric acid, e.g. 1 -> 2 (94S989). Hitrovinyl oximes 3 (R1, R3 = alkyl or aryl) undergo oxidative cyclization to isoxazoles 4 by the action of DDQ or iodine/potassium iodide (94JHC861). Flash-vacuum pyrolysis of the 1,3-dipolar cycloadduct 5 of acrylonitrile oxide to norbornadiene results in a retro-Diels-Alder reaction to give cyclopentadiene and 3-vinylisoxazole 6 (94CC2661). 

Precomplexation of a,/ -unsaturated ketone 163 with ATPH in CH2CI2 at -78 °C, then cyclization with cyclopentadiene, resulted in the stereochemical reversal to furnish exo-164 as a major product, as shown in Sch. 128. Similarly, the Diels-Alder reaction with other dienophiles complexed with ATPH resulted in exo selectivity [168],... 

Diels-Alder adduct (2) of allyl alcohol with cyclopentadiene. The unsaturated alcohol was converted into 6-chloromethyl-3-oxatricyclo 3.2.I.O loctane (3). For cyclization of (3) to the tricyclic alcohol (4), lithium in tetruhydrofurane proved superior to sodium in toluene. The alcohol (4) has a compurutively strained cycio-butune ring, but was converted easiiy into the parent hydrocarbon by oxidation (CrOfl-Py) and Wolff-Kiihner reduction. 

Strained cage compounds can be made by intramolecular photocycloaddition of properly designed ene/enone substrates. In an interesting example from J. R. Scheffer, a ben-zoquinone derived Diels-Alder product is regioselectively transformed into a (halO cage compound. This is a more complicated version of the classical intramolecular photochemical cyclization reactions of benzoquinone derivatives (for example see B. Pandey s cage synthesis from a naphthoquinone-cyclopentadiene in Chapter 3). 

A -(Aroylamino)norbornene carboxylates (317) are cyclized by treatment with thionyl chloride and triethylamine. The tricyclic 1,3-oxazinones (318) thus formed undergo retro-Diels-Alder reactions when heated at 150°C to give 2-aryl-l,3-oxazin-6-ones (319) and cyclopentadiene (Scheme 91) <84S345>. 

Mechanistic and kinetic studies have been made of Diels-Alder reactions in SCCO2. For the reaction of maleic anhydride and isoprene in the presence of aluminum chloride in SCCO2, a two-step mechanism in which a <7-bond is formed followed by cyclization was suggested. For the reaction of ethyl acrylate and cyclopentadiene in SCCO2, good correlation was found between the density and the temperature-normalized rate constant. ... 

A full report has been published on the preparation of fused valerolactones from cyclopentadiene anions by condensations with epoxides, carboxylation, and finally cyclization (Scheme 26). Diels-Alder reactions of 3-hydroxy-2-pyrones usually lead to benzene derivatives by decarboxylation of the intermediate lactone. It has now been shown that these lactones can be isolated in good yields by suppression of the CO2 loss by carrying out the cycloadditions under a high pressure of carbon dioxide. 

Many different cyclization reactions have been reported in solid inclusion compounds. An example of a thermal reaction of this type is the Diels-Alder reaction between maleic anhydride and cyclopentadiene within the thiourea tunnel structure, which is reported to occur with high conversion and high stereoselectivity. In this reaction, the two guest components were cocondensed with the vapor of thiourea at 77 K. On subsequent heating, the Diels-Alder reaction occurs with 90% conversion, and with more than 95% of the product comprising the endo isomer of the Diels-Alder adduct. 

The carbonylative cyclization of 2-iodophenols with norbornadiene or norbornene was also carried out. In 1989, Catellani s group studied the annulation of 2-iodophenol with norbornadiene to coumarin (Scheme 3.11a). ° In this method, the elimination of cyclopentadiene via retro-Diels-Alder reaction was involved in the formation of the terminal product. Fiaud and co-workers reported the palladium-catalyzed carbonylative cyclization of 2-iodophenol with norbornene in 1997 (Scheme 3.11b). They reported that the selectivity for the production of the two regioisomers can be controlled. Catellani and co-workers studied the intramolecular cyclization of ort/zo-iodophenyl 3-butenoate to 4-methylcoumarin with a palladium catalyst. 85% of 4-methylcoumarin was produced in the presence of benzonitrile and carbon monoxide, which is compulsory (Scheme 3.12a). Silva, Costa and their co-workers reported a tandem process for the synthesis of coumarins from 2-iodophenols and enoates with the assistance of a palladium catalyst (Scheme 3.12b). The general scope of this Heck-lactonization involves E- and Z-enoates as substrates. It was shown that this reaction is sensitive to steric hindrance around the double bound in... 

Diels-Alder reaction of cyclopentadiene and butadiene affords a mixture of exo-5-vinyl-2-norbornene (la) and e do-5-vinyl-2-norbornene (lb) [1], Preparative GC separation [1] of these isomers encounters difficulties in obtaining the individual isomer in pure form and in large quantities. An alternative approach of separation via thermal isomerization [2], in which lb gets transferred to 4,7,3a,7a-tetrahydro-lff-indene, whereas la remains unchanged, is also not successful. This is because it is difficult to prevent la being contaminated by unreacted lb. As no other method is available for their separation, Inoue has reported [3] that hydroboration of 1 with 9-BBN, followed by oxidation with alkaline hydrogen peroxide results in the formation of alcohols 2a and 2b. The iodoether cyclization only of the endo isomer takes place. The sequence of approach is delineated in Scheme 29.1. 

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