Diels-Alder condensation

Since the six carbons shown above have 10 additional bonds, the variety of substituents they carry or the structures they can be a part of is quite varied, making the Diels-Alder reaction a powerful synthetic tool in organic chemistry. A moment s reflection will convince us that a molecule like structure [XVI] is monofunctional from the point of view of the Diels-Alder condensation. If the Diels-Alder reaction is to be used for the preparation of polymers, the reactants must be bis-dienes and bis-dienophiles. If the diene, the dienophile, or both are part of a ring system to begin with, a polycyclic product results. One of the first high molecular weight polymers prepared by this synthetic route was the product resulting from the reaction of 2-vinyl butadiene [XIX] and benzoquinone [XX] ... 

It has long been assumed that the activity of corticoids which contain an additional ring anmilated to the 16,17 positions such a.s halcinonide (76), owed their activity to the hydrolyzed product. It is thus of note that full activity seems to be retained by a compound which incorpo-latcs a ring at that position held in place by carbon-carbon bonds. Diels-Alder condensation of halcinonide precursor 77, with benzocyclobutadiene, obtained by heating benzocyclobutane 78, Icad.s after hydrolysis of the acetate to naflocort (79) [17]. 

With the growing interest for the polynorbomene, photoresist polymer, and cyclic olefin copolymer, the synthesis norbornene or bicyclo[2,2,l]-2-heptene (NBN) has drawn significant attention because it is one of the most important precursor for these materials. Norbornene is produced by the reaction between ethylene and cyclopentadiene (CPD) via the Diels-Alder condensation process at elevated temperature and pressure [1,2]. 

Thus, Diels-Alder condensation of 9 with methyl vinyl ketone affords the bicyclic adduct 24. The new ring is formed by approach of the dienophile from the face containing the nitrogen bridge, since this is in fact the least hindered side of the molecule (9a). Reaction of the side Chain ketone with propylmagnesium bromide then leads to intermediate 25 demethylation of the... 

In the Diels-Alder condensation of the 2 neat endothermic dienes to give 5-ethylidene- and 5-methyl-6-methylene-bicyclo[2.2.1]hept-2-ene, there is a serious risk of explosive decomposition arising from local overheating of the reactor walls. This hazard is eliminated by the presence of various hydrocarbons and their mixtures as diluents. 

The asymmetric synthesis of (4R,93, 9aR)-4-phenyl-l-trimethylsilyloxy-9-vinylperhydropyrido[2,T ][l,4]oxazine with a high level of stereoselectivity in the cyclization of (3R,5R)-5-phenyl-3-phenylsulfanyl-4-(6-trimethylsilanyl-hex-4-enyl)-2-trimethylsilyloxy-morpholine was rationalized via AMI calculations <1998T10309>. AMI calculations suggested that the formation of l,6-dioxo-l,3,4,6,7,8-hexahydropyrido[2,Tf][l,4]oxazine-9-carboxylates 230 from acroyl chlorides 228 and 1,4-oxazinone 229 is favored over the hetero-Diels-Alder condensation (Equation 45) <1996JOC5736>. 

Diels-Alder condensation, in VDC polymer degradation, 25 712 Diels-Alder dimer, 23 381-382, 383 Diels-Alder maleic addition product,... 

As an example of this technique we consider 12C/13C isotope effects in the Diels-Alder condensation of isoprene and maleic anhydride (Equation 7.32). The terminal carbons of isoprene carbons are numbered 1 and 4, the methyl substituted carbon is number 2. The reaction proceeds via a concerted and slightly asynchronous mechanism. 

This fragmentation process suggests that a Diels-Alder condensation of the compounds corresponding to the two ions should afford xylopinine 19. 

Notice that the a-acetoxyacrylonitrile is in fact a "ketene equivalent" [30] in the Diels-Alder condensation with furan, since ketene itself undergoes [2 + 21-cycloadditions rather than [4 + 2] cycloadditions. 

The intermediates 2 and 4 can then be traced back to simple precursors via retro-Diels-Alder condensations, which are very well known stereospecific pericyclic reactions. 

Heptachlor is produced commercially by the free-radical chlorination of chlordene in benzene containing from 0.5% to 5.0% of fuller s earth. The reaction is run for up to 8 hours. The chlordene starting material is prepared by the Diels-Alder condensation of hexachlorocyclopentadiene with cyclopentadiene (Sittig 1980). Technical-grade heptachlor usually consists of 72% heptachlor and 28% impurities such as trans-chlordane, cis-chlordane, and nonachlor (HSDB 1990a). 

Now, dienes with shielded donble bonds can be involved in diene synthesis. The presence of donor gronps at the double bond normally prevents its involvement in conventional Diels-Alder condensations. These reactions do take place with the cation-radicals. Cyclic adducts are formed in high yields (80-90%) and under mild conditions. Polymerization, which usnally decreases the yield is inhibited completely within the framework of the cation-radical variant (Bellville et al. 1981). The stereoselectivity of the addition, which is usually typical for diene condensation, does not change in the cation-radical version and even increases. The positional selectivity also increases. The regiose-lectivity is enhanced, as well. Bauld et al. have discovered and explained these effects (Bellville and Banld 1982, Bellville et al. 1981, 1983, Banld and Pabon 1983, Pabon and Banld 1984). 

Diels-Alder condensation of benzo[6]thiophene 5,5-dioxide with butadiene or hexachlorocyclopentadiene yields the adducts 66 and 67, respectively, examples of the l,4,4a,9b-tetrahydrodibenzothiophene... 

The molecular ion is relatively abundant (30-50%), as is the (M — ion. The latter may be represented as either (a) or (b) (Scheme 32). The clue which points out unequivocally to an isopavine nucleus is the presence of the (M — 43) + ion in intensities varying from 30 to about 60%. It is formed by the retro-Diels-Alder condensation of the molecular ion, resulting in loss of a CH2=NCH3 unit. This particular ion is practically absent in the spectrum of a pavine (77). An additional peak of moderate intensity is associated with the (M — 86) ion which is formed by a subsequent loss of a methyl radical and carbon... 

New possibilities in hetero-Diels-Alder condensation have been opened by the introduction of highly active l-methoxy-3-trimethylsilyloxy-, 4-benzoyIoxy-l-methoxy-3-trimethylsilyloxy-, and 2-acetoxy-l-alkoxy-3-trimethylsilyloxy-l,3-butadienes ( Danishefsky dienes, 5). These compounds readily react under atmospheric pressure, in the presence of Lewis acids, with normal aldehydes (e.g., acetaldehyde, benzaldehyde, furfural) to furnish 2,3-disubstituted or 2,3,5-trisubstituted derivatives of 2,3-dihydro-4H-pyran-4-one 7 capable of readily functionalizing to sugars (Scheme 5) [26]. This approach... 

Two new approaches to the synthesis of deoxynupharidine (14) and its C-l and C-7 epimers were reported. Arata et al. (67) made use of the Mannich reaction of a suitable derivative of isopelletierine and 3-furylaldehyde ( )-7-epideoxy-nupharidine (15) and ( )-l-epideoxynupharidine (8) were proved to be the main products of the reaction. The synthetic route is shown in Scheme 11. Intramolecular Diels-Alder condensation of 1-Azadienes was shown (68) to be a stereoselective route to the total synthesis of (-)-deoxynupharidine (14). The key steps are shown in Scheme 12 from synthon A in four steps alkaloid 14 was obtained. 

An earlier off-odor problem that surfaced was the presence of 4-phenyl-cyclohexene in styrene-butadiene coated paper (Koszinowski et al. 1980). This compound was created by a Diels-Alder condensation reaction involving a molecule of styrene and butadiene and is differentiated by its odor from the isomeric 3 and 1-phenyl-cyclohex-ene compounds which cannot be formed by such a condensation reaction. The recognition threshold of this compound in the headspace over an aqueous solution lies around a concentration of 10 pg/kg (10 ppb). The typical odor of this compound at concentrations of 4-phenyl-cyclohexene in paper over 4 mg/kg (4 ppm) is easily identified. AGC determination in this concentration range is also possible without difficulty and its identification with MS using the relative molecular mass of 158 and one of the retro Diels-Alder decomposition product fragments at m/e = 104 (styrene) and mJe = 54 (butadiene) is definitely possible. 

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