Cyclopentadiene, 1,4-addition oxygen

The enantiomerically pure (—)-2-norbornenone s, a key intermediate for (+)-bre-feldin A19 is prepared via a cyclopentadiene addition to the acrylate Id [a 97 3 mixture of (S)-endo- and (R)-exo-adducts was obtained19] or to Is. Replacement of the ether oxygen in the neopentyl side chain by a methylene group (e.g.. It, lu) decreases the diastereoselectivity20,... 

Both the carbon-carbon and carbon-oxygen double bonds of fluoroketenes can take part in [2+2] cycloadditions, but with cyclopentadiene, only cyclo butanones are produced via concerted [2 +2 ] additions [J34] (equation 58) Cycloadditions involving the carbon-oxygen double bonds to form oxetanes are discussed on page 855 Difluoroketene is veiy short lived and difficult to intercept but has been trapped successfully by very electron rich addends to give 2 2 di fluorocyclobutanones m moderate yields [/55] (equation 59)... 

Violent explosions which occurred at —100 to —180°C in ammonia synthesis gas units were traced to the formation of explosive addition products of dienes and oxides of nitrogen, produced from interaction of nitrogen oxide and oxygen. Laboratory experiments showed that the addition products from 1,3-butadiene or cyclopentadiene formed rapidly at about — 150°C, and ignited or exploded on warming to —35 to — 15°C. The unconjugated propadiene, and alkenes or acetylene reacted slowly and the products did not ignite until +30 to +50°C [1], This type of derivative ( pseudo-nitrosite ) was formerly used (Wallach) to characterise terpene hydrocarbons. Further comments were made later [2],... 

Complexes 17-19 can be written in one valence structure as a, /3-unsaturated carbonyl compounds in which the carbonyl oxygen atom is coordinated to a BF2(OR) Lewis acid. The C=C double bonds of such organic systems are activated toward certain reactions, like Diels-Alder additions, and complexes 17-19 show similar chemistry. Complexes 17 and 18 undergo Diels-Alder additions with isoprene, 2,3-dimethyl-1,3-butadiene, tram-2-methyl-l,3-pentadiene, and cyclopentadiene to give Diels-Alder products 20-23 as shown in Scheme 1 for complex 17 (32). Compounds 20-23 are prepared in crude product yields of 75-98% and are isolated as analytically pure solids in yields of 16-66%. The X-ray structure of the isoprene product 20 has been determined and the ORTEP diagram (shown in Fig. 3) reveals the regiochemistry of the Diels-Alder addition. The C-14=C-15 double bond distance is 1.327(4) A, and the... 

However, a number of addition reactions may compete with these abstraction reactions. A significant fraction of the cyclopentadiene may react to form oxygenated compounds, ... 

The synthesis69 started with c s-cyclopentene-3,5-diol7°) 26 which had been obtained in a stereospecific reaction by addition of singlet oxygen to cyclopentadiene and reduction of the intermediate cycloperoxide. 

Diels-Alder addition of cyclopentadiene (1.3 mol equiv., CH2CI2) to the purified dienophile (416) proceeded smoothly at 5 °C in a virtually quantitative endo fashion syn to the H-bonded sulfoxide oxygen atom (assuming that formula 416 depicts the reactive conformation). Stereoisomerically pure adduct... 

In most of the cyclic compounds that we have studied so far—benzene, naphthalene, cyclohexanol, cyclopentadiene—the rings are made up only of carbon atoms such compounds are called homocyclic or alicyclic compounds. But there are also rings containing, in addition to carbon, other kinds of atoms, most commonly nitrogen, oxygen, or sulfur. For example ... 

Carbanion formation. Sprinzak" found that Triton B in pyridine solution converts compounds of the cyclopentadiene series into carbanions highly sensitive to air oxidation. He prepared a solution of the base in pyridine by adding the solvent to the commercial 40% solution in methanol and evaporated the methanol at reduced pressure. Addition of this solution to a solution of fluorene in pyridine produced an orange-yellow solution which absorbed oxygen rapidly after stirring for 40 min. 

Polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and pentacenes, as well as cyclopentadienes, cyclohexa-l,3-dienes, cyclo-hepta-l,3-dienes, and furans, have been found to be suitable diene systems to which the singlet oxygen adds as a dienophile in a 1,4-cycloaddition reaction. Thus, endoperoxides (transannular peroxides) and, in the case of furans, ozonides of the corresponding cyclobutadienes are the primarily produced, more or less stable addition products (2, 21, 22). 

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