Diels-Alder adduct, pyrolysis

DIDEUTERIODIAZOME THANE, 53, 38 Diels-Alder adduct, pyrolysis, 50, 37... 

The Diels-Alder adduct of sulpholene and cyclopentadiene is a useful starting material for substituted diene synthesis121. The diene moiety is unmasked by retro-Diels-Alder reaction and sulphur dioxide extrusion under flash vacuum pyrolysis conditions (equations 74 and 75)122,123. 

The endo- and e.vo-Diels - Alder adducts 37a and 38a, which are the products from the reaction of cyclopentadiene and (7 )-angelica lactone 39a, have been alkylated under the usual conditions (LDA, THF, — 78 °C) with iodomethane, ethyl, butyl and benzyl bromide26. The alkylations are all completely diastereoselective as is expected in these tricyclic examples and yields are in the range of 80-95%. Pyrolysis of 37b and 38b at around 250-290°C for 2-6 days led to the butenolides 39b. 

The flash-pyrolysis technique has been especially useful for the synthesis of unstable benzo[c]furans the 1-methyl, 1,3-diinethyl, and 1-benzyl derivatives have been prepared in this way in quantitative yield. The UV and H-NMR spectra are in accord with their structures. These compounds are extremely reactive, resinifying rapidly on standing at room temperature. With 7V-phenylmaleimide, the Diels-Alder adducts 88 (endo, mp 201-201.6°C, 18%), 89 (exo, mp 153-153.5 C, 14%) and 90 [endo, mp 215 217 C. 60% (includes both endo and exo)] are obtained 87 (R = R = H, =... 

By analogy, flash vacuum pyrolysis of a p-chlorobenzoate ester (378, Ar=4-C1C6H4) (90TL1487) produced oxazole-4,5-xylylene (379), which could be trapped with S02 to give adduct (381). The unstable intermediate also gave a Diels-Alder adduct (380) with methyl acrylate (for the similar reactions see Section 3.3.3.3.3). 

The Diels-Alder adduct 13 of silapyranes with maleic anhydride was also used as an alternative precursor for 10 (Scheme 3)19. In the pyrolysis of 12, a 2-siloxetane intermediate 14 was proposed as a transient precursor leading to dimethylsilanone 10 (or ll)19-21. 

Pyrolytic fragmentations of other suitable precursors also lead to dimethylsilanone (97). For example, 6-oxa-3-silabicyclo[3.1.0]hexane 96 is reported to be split into 97 and 1,3-butadiene upon pyrolysis104. Other possible routes to 97 consist in the pyrolysis of (allyloxy)dimethylsilane (99)105 or Diels-Alder adduct 98104. Matrix-isolated 97 gives its trimer hexamethylcyclotrisiloxane when the matrix is warmed up to 35-40 K. The SiO stretching vibration of 97 was found at 1210 cm-1. This frequency fits the calculated force constant and bond order and has to be considered as evidence for significant double bonding in 97104. Octamethylcyclotetrasiloxane, allyl(allyloxy)dimethylsilane and 2,2,6-trimethyl-2-silapyrane failed as precursors for 97 and only the SiO molecule (cf Section V.B) and CH3 radicals were found on the matrix holder105. 

The flash vacuum pyrolysis (FVP) of 1-allyl-l-methyl-l-silacyclopent-3-ene gives rise to elimination of propene and formation of silole 3 identified as its dimer 5, and by its Diels-Alder adducts with maleic anhydride and hexafluorobutyne. However, the silole 3 has been detected as a monomer by the MS-FVP technique6. 

Thus cyclopropabenzene (61a) can be obtained in 45% yield from flash-vacuum pyrolysis of the Diels-Alder adduct 60a formed from l,6-methano[10]annulene (59a) and dimethyl acetylenedicarboxylate. In like manner the benzo analogue 59b provides cyclopropa[a]naphthalene (83%) and the dibenzo derivative 59c gives cyclopropa[/]phenanthrene. Whereas 61a and its linear naphtho[h] analogue are stable entities, 61b suffers explosive decomposition on melting and 61c decomposes at temperatures in excess of — 70° C. 

The cage compound (301) is formed efficiently on irradiation of the adduct (302).Irradiation of the ene dione (303) affords the cage compound (304) by a (6 + 2)-cycloaddition. The Diels-Alder adducts (305) are photochemically reactive. Irradiation of (305) affords either the cage compounds (306) by a (2+2)-cycloaddition reaction, a process favoured by the adducts (305 a,b), or the oxetanes (307). The structures (306, 307) were verified by 2L ray diffraction studies The quinone adduct (308) also undergoes photochemical cyclization to afford the cage diketone (309). This material was subjected to flash vacuum pyrolysis to yield the diketone (310) important in the synthesis of perhydroindacenes. ... 

Cyclopentadiene is obtained from the light oil from coal tar distillation but exists as the stable dimer, dicyclopentadiene, which is the Diels-Alder adduct from two molecules of the diene. Thus, generation of cyclopentadiene by pyrolysis of the dimer represents a reverse Diels-Alder reaction. See Figs. 1 and 2 for nmr and infrared spectra of dicyclopentadiene. In the Diels-Alder addition of cyclopentadiene and maleic anhydride the two molecules approach each other in the orientation shown in the drawing above, as this orientation provides maximal overlap of ir-bonds of the two reactants and favors formation of an initial ir-complex and then the final e do-product. Dicyclopentadiene also has the endo-configuration. 

A new approach to the synthesis of 3-acetyl-5-methoxynaphthoquinone 221 involves the pyrolysis of the polycycle 218 derived from the Diels-Alder adduct 188 (Section IV.B.l). The regiospecific Fries rearrangement of diacetoxynaphthalene 219 leads to the naphthol 220 whose oxidation gives the desired product 221 as a key intermediate for the synthesis of naturally occurring antibiotic pyranoquinones (equation 100) . 

The usefulness of the approach for the preparation of highly reactive cycloproparenes is further illustrated by its application to benzocyclopropene-2,5-quinone In this case, flash-vacuum pyrolysis of the precursor (500 °C) afforded only polymeric material pyrolysis in a static system at 255 C in the presence of 3 equivalents of anthracene for 10 minutes gave the quinone as the Diels —Alder adduct with anthracene (12%), while unreacted starting material (41 %) and dimethyl phthalate (16%) were also recovered. 

Methylenecyclopropene has also been generated by the pyrolysis of 2-methylenecyclopropyl phenyl sulfoxide and 2-methylenecyclopropyl methyl phenyl sulfinylide in particular, the elimination of methyl phenyl sulfide from the latter compound occurred at room temperature to form a Diels-Alder adduct in the presence of cyclopentadiene. 

Baldwin and Lopez have formed both aromatic and aliphatic thioaldehydes by pyrolysis of thiosulfinates like 7, and they were able to trap these species as Diels-Alder adducts [Eq. (7)].22... 

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