Retrodien reaction

An interesting mechanistic issue was raised by Firestone on the aqueous Diels-Alder reaction between 2-methylfuran and maleic acid in water, which is found to be 99.9% stereospecific.80 By adding heavy atom (defined as any below the first complete row of the periodic table) salts to the aqueous media, it was found that addition of heavy but not light atom salts reduced the degree of stereospecificity significantly in the retrodiene reaction. The results suggest that a large portion of the Diels-Alder reaction occurs via diradical intermediates (Scheme 12.2). 

A versatile new and general method was developed for the synthesis of six-membered 1,3-heterocycles. In comparison with the general reaction conditions of retrodiene reactions, a very mild retro Diels-Alder (RDA) decomposition was found to occur when the norbornene-d/eAro- or -diendo-fused dihydrooxazinone 474 was heated at melting temperature or refluxed in different solvents (e.g., chlorobenzene). Cyclopentadiene splitt off and the 2-aryl-6//-l,3-oxazin-6-ones 475, earlier unknown representatives of the simple 1,3-oxazines, were obtained (84S345, 84T2385). 

Free enols usually result from photochemical processes (e.g. Norrish type II reactions) or fast thermal decompositions [e.g. retrodiene reaction (36) (Ripoll, 1979)]. By the latter reaction, different methyl-substituted enols [60] (R1, R ... 

Cyclic carbonyl ylides, formed from diazo amides or diazo anhydrides through intramolecular carbene addition to the carbonyl group, react with the triple bond of a dipolarophile to produce bicyclic adducts. The latter undergo a retrodiene reaction, splitting off an alkyl isocyanate or carbon dioxide to give furan derivatives. 

Kwart, H., King, K. The reverse Diels-Alder or retrodiene reaction. Chem. Rev. 1968, 68, 415-447. 

Retrodiene reaction [679] of the Diels-Alder adduct 313 of 2-benzyl-4-methyloxazole and propynol, by splitting off acetonitril, also affords the desired alcohol (Reaction scheme 221). 

In the next Reaction scheme 266 it needs a retrodien reaction of an inactive chrysanthemum-propargylate 402 and a benzyloxazole 403 for generation of resme-thrin 404 [883]. 

In the case of TROP of 1-methyl-1-norbornenylmethyl-MSCB performed at 220°C, the process was accompanied by a partial retrodiene reaction of the nor-bornene pendant fragments resulting in elimination of cyclopentadiene and formation of allyl side groups [33],... 

Diels-Alder adduct with cyc/o-CgCU Retrodiene reaction >160... 

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 theimolysis of 2-substituted 2,5-dihydrothiophene 1,1-dioxides leads to ( ) conjugated dienes via cycloieversion followed by the concerted cheletropic extrusion of sulfur dioxide. The thermolysis of the a, -alkylated sulfone (96) gives the intennediate, which loses SO2 to give 1-dodecadien-l-yl acetate (98), a component of the sex pheromone of the red bollwoim moth. Tliis procediue has been extended to the thermolysis of a,3-dialkylated sulfones in order to obtain ( , )-1,4-disubstituted-1,3-dienes (equation 46). Similar processes have been used for the syntheses of alkaloids. The synthesis of an Elaeocarpus alkaloid, elaeokwine A (100), makes use of the retrodiene extrusion of sulfur dioxide to give the 1,3-diene intermediate (99) that is subsequently consumed by an intramolecular imino DA reaction (equation 47). o-Xylylene (102) has been generated by rDA expulsion of SO2 from benzo-fiised 3,6-dihy(ho-l,2-oxathiin 2-oxide (101). ... 

Silanones, the silicon analogs of ketones, are produced via rDA reactions. One attempt to prepare a suitable DA precursor for retrograde decomposition to a silanone met with unexpected results. The desired DA reaction between 2,2-dimethyl-l-oxa-2-silacyclohexa-3,S-diene (251) and perfluoro-2-but-yne was complete in one day at room temperature. The observed product was o-bis(trifluoromethyl)ben-zene, as the initial adduct apparently underwent retrodiene decomposition to yield the intermediate dimethylsilanone (252) (equation 111). The occurrence of this retro ene process at room temperature was not consistent with the analogous extrusions of silenes and disilenes that require elevated temperatures. However, the reaction sequence was substantiated by comparison with its carbon analog in which tetramethylpyran and dimethyl acetylenedicarboxylate react at room temperature to afford only acetone and the corresponding phthalate. Stable adducts that extrude silanones are also known. Reactions of 2-sili yrans and nudeic anhydride provide stable adducts, such as (253), that decompose upon thermolysis... 

The PE spectroscopic evidence for silaethene as the retrodiene dissociation product51 rests on the coincidence between precalculated and observed ionization energies, on the band intensity increase upon addition of argon to the gas stream, which apparently reduces secondary reactions such as oligomerization, and on the vibrational fine structure of the first PES band, which resembles that of the iso(valence)electronic ethene and precludes with high probability the presence of carbene or silylene isomers. Possibly, the silaethene radical cation H2Si=CH 2e possesses a twisted ground-state equilibrium structure like H2C=CH 2 with the two lowest doublet states coupled via the torsional vibration v6 (cf. Reference 10). 

Retroaldol reaction, ring opening by - 26, 227 Retro-benzilic acid-type rearrangement imidazoles by - 28, 959 Retrodiene scission... 

Weis CD (1962) A diene-retrodiene-diene reaction. The synthesis of 2,3,5,6-tetrakis-(trifluoromethyl)-7-oxahicyclo[2.2.1]-2,5-heptadiene. J Oig Chem 27 3693-3695... 

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