Retro-Diels-Alder reaction ketones

To avoid the retro-Diels-Alder reaction, 56 was dihydroxylated prior to the introduction of the bromine atom (57). Removal of the acetonide group followed by cleavage of the diol afforded a bis-hemiacetal. Selective reduction of the less-hindered hemiacetal group gave 58. The remaining hemiacetal was protected, and the ketone was converted to an enol triflate, thus concluding the synthesis of the electrophilic coupling component 51. 

Interestingly, we were intrigued by the ESI mass spectrum of the compound, as the observed base peak consisted of [M-S02+Na]+. This led us to explore a thermal retro-Diels-Alder reaction that could afford the desired enone 69. It is noteworthy that the chemistry of cyclic enol-sulfites would appear to be an under-explored area with a few references reporting their isolation being found [57]. At last, we were also able to prepare epoxy ketone 70 from 69 in three steps, albeit epoxidation did not take place unless the TES group was removed. Spartan models reaffirmed our initial conformational assessment of enone 69 and epoxy ketone 70, which contain sp3-hybridized C8a and s/r-hybridized C8b (p s e u d o-. v/r - h y b r i d i zed C8b for 70) at the AB-ring junction (Fig. 8.12) and displayed the desired twisted-boat conformation in A-ring. 

The cycloadduct of an a,a -dioxo-thioketone with antracene,84 treated at low temperature with the Nysted reagent and TiCl4, transforms a ketone group to the corresponding methylenic group. This product is thermically decomposed via retro Diels-Alder reaction into the unstable a, p-unsaturated-a -oxo derivative (Scheme 10). 

Titanacyclobutenes, prepared readily from Tebbe reagent and alkynes, react with aldehydes and ketones to form insertion products which undergo facile retro-Diels-Alder reaction to afford substituted 1,3-dienes (equation 107)185. 

Another method to prepare allenyl ketones uses flash vacuum pyrolysis of the heterocycles 121 (Scheme 7.19) [163], This elimination of carbon monoxide is at least formally a cheletropic reaction. Highly reactive allenes such as esters and nitriles of type 124 or unsubstituted butadienal can be generated if retro-Diels-Alder reaction of 123 or similar precursors, respectively, is performed by flash vacuum pyrolysis [164]. 

The analogous transformation of 125, also realized by flash vacuum pyrolysis, gave rise to allenic oximes 126 [165], which are not directly accessible by the classical route starting from allenyl ketones and hydroxylamine (see Section 7.3.2) [122], Because compounds 125 are prepared from allenyl ketones and furan by [4 + 2]-cycloaddition followed by treatment with hydroxylamine, the retro-Diels-Alder reaction 125 —> 126 is in principle the removal of a protecting group (see also Scheme 7.46). 

Particularly good yields of the cydoadduct 329 are obtained if R1 = R2 = H is valid for the allenyl ketone 328 [165]. The Diels-Alder products 329 can undergo many chemical transformations, for example to the oximes 330, which yield the modified allenes 331 after a subsequent flash vacuum pyrolysis. The oximes 331 generated by retro-Diels-Alder reaction are not available from ketones 328 and hydroxylamine hydrochloride directly [122] (see also Scheme 7.19). 

They noted that in dimethoxyethane or in iso-octane (path a), the major product was dicarbonylcyclopentadienylcobalt (2) which must arise as a result of a retro Diels-Alder reaction of the norbornadiene (which would lead to the formation of acetylene and cyclopentadiene). When the solvent was changed to an aromatic hydrocarbon such as benzene or toluene (path b), the major cobalt-containing product was shown to be a complex derived from Co4(CO)i2, with three CO ligands on an apical cobalt being replaced by a molecule of the aromatic solvent (3). The group noted that they were also obtaining hydrocarbon and ketonic products derived from norbornadiene, acetylene and carbon monoxide .1,2... 

Formation of a double bond between carbon and a heteroatom or between two heteroatoms can be accomplished by a hetero retro-Diels-Alder reaction. Aldehydes, ketones and carboxylic esters can be obtained by this retrodiene process. A series of ct,3-unsaturated aldehydes (152) was prepared by Funk using the facile bis-hetero retro-Diels-Alder reaction of 4-alky 1-4//-1,3-dioxins (151) as shown in equation (66). This retrodiene process involves exceptionally mild conditions, refluxing in toluene, in order to unmask the a,3-unsaturated aldehyde. The diversity of this process is illustrated by the partial list of reactions shown. The same mild conditions can be used to prepare a,3-unsaturated ketones by reflux-... 

The concise enantioselective total synthesis of (+)-monomorine I, a 3,5-dialkyl-substituted indolizidine alkaloid, was completed by S. Blechert et al. using a sequential cross-metathesis double reductive cyclization strategy. The enedione substrate was prepared in two steps. The Stetter reaction between the masked equivalent of acrolein and butyl vinyl ketone was followed by a retro Diels-Alder reaction under flash vacuum pyrolysis (FVP) conditions. 

Dicyclopentadiene is a feedstock for both the fragrance and polymer industries. It forms spontaneously from cyclopentadiene by a Diels-Alder reaction, and a retro-Diels-Alder reaction can be used to regenerate cyclopentadiene from it. A number of minor fragrance ingredients are produced by Diels-Alder reaction of the monomer with a variety of activated olefins in which the activating group X, is usually an aldehyde, ketone, ester or nitrile. However, the main fragrance uses stem from the dimer. 

Cycloreversion reactions, especially the retro-Diels-Alder reaction, also yield diagnostic product ions. Fragmentation reactions of each class of compounds (e.g., alkanes, ketones, alcohols, esters) are also discussed (Section 6.6). Upon El, each class of molecules fi agments in a typical fashion that can identify the precursor structure. 

In 1971 at the University of Strathclyde in Glasgow, Professor Peter Pauson reported on the retro-Diels-Alder reaction of norbomadiene 3 induced by dicobalthexacarbonyl complexes of acetylene or phenylaeetylene 1 to provide dicarbonylcyclopentadienylcobalt complexes 4 in high yield. Almost as an after thought, he mentions In addition to the above products, the reaction of norbomadiene with complexes 1 yields hydrocarbon and ketonic products derived from norbomadiene, acetylene and carbon monoxide. ... 

A plausible mechanism for the present benzannulation is shown in Scheme 15.8. As shown in Scheme 15.4, the intermediate 16" would be formed from benzopylilyum intermediate 14 and alkyne 2. Protonolysis of the C—Cu bond of 16" by a Brpnsted acid (HA) such as CF2HCO2H, followed by the attack of A to the carbon of RCO would produce 22. It would undergo the retro Diels-Alder reaction to give 19 together with the compound eliminated, 23. When the reaction was conducted in the absence of a proton source, the protonolysis of the C—Cu bond does not proceed efiflciently, and the naphthyl ketones are produced directly from 16", as in the case of the AUX3-catalyzed reaction. 

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