Cyclopentadiene carboxylic ester

The formation of Thiele s ester 6.217 is a remarkable example of several of the kinds of selectivity that we have been seeing in the last few sections, all of which can be explained by frontier orbital theory. The particular pair of cyclopenta-dienes which do actually react together 6.215 and 6.216 are not the only ones present. As a result of the rapid 1,5-sigmatropic hydrogen shifts [see (Section 6.3.1.3) page 197], all three isomeric cyclopentadiene carboxylic esters are present, and any combination of these is in principle possible. As each pair can combine in several different ways there are, in fact, 72 possible Diels-Alder adducts. 

The second variant is the reaction of a pyran-2-one (methyl coumalate, 313) as dienophile with a cyclopentadiene (314). The balance between the dienic and dienophilic capabilities of pyran-2-one is tipped in favour of the latter by the electron-withdrawing properties of the carboxylic ester group and this fine balance enhances the potential of these compounds as synthons (72CC388). 

Fascinating rearrangements of aziridines have been applied to the synthesis of diterpene alkaloids by Wiesner and co-workers (Scheme 40). For example, the ester 217, prepared from cyclopentadiene carboxylate (215) and the benzyne precursor 216 by a Diels-Alder reaction, was converted to the aziridine 218 by treatment with benzenesulfonyl azide in 83% yield. When the aziridine 218 was heated with water for 24 h, the hydroxy ester 219 was obtained in 97% yield subsequent oxidation with the Jones reagent afforded the ketone 220. This rearrangement is analogous to that of the benzenesulfonylaziridine of norbomene. ... 

In the reaction of sodium methoxide with 3-trichloromethylpyridine the nicotinecarbaldehyde acetal (379 Scheme 69) is formed, whereas by action of methoxide on 2,6-dichloro-3-trichloromethylpyridine the orthoester (380) is accessible.TTie l,l-dichloro-2-acylcyclopropanes on treatment with sodium methoxide undergo ring enlargement to yield the cyclic orthoester (381 equation 178). A mixture of isomeric cyclopentadiene carboxylic orthoacid esters results when the bis(methylthio)fulvene (382 equation 179) is reacted with excess alkoxide. i... 

Dienes do not react with carbonyl compounds unless the latter are activated by electron-withdrawing substituents such as carboxyl groups. Cyclohexa-1,3-diene, for example, adds diethyl mesoxalate (1) at 120 °C to form 2 (equation 2)2. Other cycloadditions of this ester with various dienes, which were carried out in a sealed tube at 130-135 °C, are shown in equations 3 and 43. It is noteworthy that no product was isolated from the action of diethyl mesoxalate on cyclopentadiene it was suggested3 that the cycloadduct reverted to its components at the high temperature required for the reaction. 

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). 

The concept of using an ester auxiliary which also contains a handle suitable for chelation was first disclosed in 1984/1985. Thus TiCU-promoted addition of cyclopentadiene to the acrylate of ethyl (S)-lactate (379) proceeded readily at -63 C to give (with a 39 1 endolexo preference) a 93 7 mixture of norbomenes (381a) and (382a), from which the major product (381a) was isolated by MPLC (Scheme 93, Table 23, entry 1). Mild saponification of adduct (381a) with LiOH in aqueous THF and purification via iodolactonization/elimination provided pure (l/ ,2/ )-5-norbomene-2-carboxylic acid. 

The process is quite general for simple dienes and aldehydes. For example, the reaction of acrolein with cyclopentadiene, cyclohexadiene, or 2,3-dimethyl-l,3-butadiene gives cycloadducts with 8(F-84 % ee and exolendo = 12/88-< 1/99. The a-substituent on the dienophile increases the enantioselectivity (acrolein compared with methacro-lein). When there is /3-substitution in the dienophile, as in crotonaldehyde, the cycloadduct is almost racemic. On the other hand, for a substrate with substituents at both a and ji positions, high ee is observed, as for 2-methylcrotonaldehyde and cyclopentadiene (90 % ee, exolendo = 97/3). The active boron catalyst is beheved to have the structure shown in Eq. (8), with a five-membered ring and a free carboxyl group. The latter seems not to be crucial for the enantioselectivity because eomparable results are obtained when the carboxylic group is transformed into an ester. 

The transformation of an active CH compound into the corresponding diazo derivative with -toluenesulfonyl azide has been designated a diazo transfer reaction and possesses a variety of preparative uses. The method has been useful for the syntheses of diazo derivatives of cyclopentadiene, 1,3-dicar-bonyl compounds, 1,3-disulfonyl compounds,1,3-keto-sulfonyl compounds, ketones, " carboxylic acid esters, and /3-keto imines. Further reaction of these diazo intermediates can lead to azo compounds,"- " 1,2,3-triazoles, and pyrazolinones. ... 

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