Correspondence cyclization, butadiene

Further variations of the general scenario described in Scheme 4 consist in trapping adduct radical 48 before oxidation occurs7. This can be achieved if intramolecular radical additions are possible, as is the case in radical 62. Oxidation of 62 to the corresponding allyl cation is slower than 6-ew-cyclization to the chlorobenzene ring to form radical 63, which subsequently is oxidized to yield tetrahydronaphthalene 64 as the main product (equation 27). This sequence does not work well for other dienes such as 2,3-dimethyl-1,3-butadiene, for which oxidation of the intermediate allyl radical is too rapid to allow radical cyclization onto the aromatic TT-system. 

An approach to the synthesis of angularly substituted polycyclics through the Diels-Alder cycloaddition of dihydrothiophenes has been devised (69JA7780). The easily prepared 2,5-dihydro-4-methoxycarbonyl-2-thiopheneacetic acid methyl ester (316) was heated at 180 °C with excess butadiene to yield (317). Desulfurization and double bond reduction of the cycloadduct with W-5 Raney nickel gave (318) which was characterized by conversion to the corresponding diacid and comparison with an authentic sample. Dieckmann cyclization of (318) is known to lead to the 5-methyl-1-hydrindanone (319 Scheme 68). The use of other dienes in the [4 + 2] cycloaddition process will, of course, produce more highly functionalized hydrindanones. 

Fig. 7-30. Examples of proposed leucochromophoric and chromophoric structures. Aryl-coumarones (1) and stilbene quinones (2) are thought to be formed from stilbenes after oxidation. Butadiene quinones (3) could arise from oxidation of hydroxyarylbutadienes being formed from phenolic pinoresinol structures during kraft or neutral sulfite pulping. Cyclization may yield intermediates which are further oxidized to cyclic diones (4). A resonance-stabilized structure (5) results from the corresponding condensation product formed during pulping. o-Quinoid structures (7) are oxidation products of catechols (6) formed during alkaline or neutral pulping processes.

Two potassium atoms transfer an electron each to butadiene forming a dianion transmetallation with o-xylene then gives the potassium-bonded carbanion, which inserts butadiene. A second transmetallation with o-xylene liberates the potassium-stabilized benzylcarbanion, which is the actual catalytic species and generates o-pentenyltoluene. This can then be cyclized to 1,5-dimethyltetralin, which, after dehydrogenation to the corresponding naphthalene and isomerization to the 2,6-isomer, affords 2,6-naphthalenedicarboxylic acid by oxidation. 

A similar reasoning can be applied to hexatriene to be converted into cyclohexadiene. The only significant change is that the number of tt electrons becomes six corresponding to six conjugated C atoms, and so there is one more node in the frontier orbitals. Accordingly, the symmetries of the HOMO and LUMO at the terminal C atoms are different from those for butadiene, and cyclization occurs by the disrotatory mode or by the conrotatory mode, respectively, for a thermally or a photochemically controlled reaction. The general rule is that the thermal cyclization reactions of a A 7r-electron system will be conrotatory for A =4 and disrotatory for A =4g-f-2 (g = 0,1,2,...). For photochemical cyclizations these relationships are reversed. 

In 1987, Hill and co-workers (75) reported a clever synthesis of the pentacyclic cephalotaxine analog 246 starting from the nitrostyrene derivative 98 (Scheme 42). The Diels-Alder adduct 244, obtained by the reaction of butadiene sulfone with 98, was treated with methyl acrylate to give a single stereoisomer of the nitro ester, which was reduced with zinc in etha-nolic HCl to yield the lactam 245 and further reduced by Red-AI to the corresponding pyrrolidine. Pictet-Spengler cyclization with formaldehyde gave the pentacyclic amine 246. Alternatively, the reduced pyrrolidine obtained from 245 could be formylated, cyclized to the iminium salt by a Bischler-Napieralski protocol, and finally reduced with sodium borohy-dride to 246. Nearly identical sequences have also been reported by Bryce... 

Enantiomerically pure samples of / -vinyI- -butyrolactone (98) were prepared starting from the diastereomerically pure bromoacetal 96 (obtained by separation of the 1 1 mixture of products from the corresponding vinyl ether, 1,2-butadiene-4-ol, and NBS) via purification of the major cyclization product 97. The bromoacetal obtained from 1-ethoxypropene was used in an approach to the dihydroagarofuran framework [66]. Bridged pyranosides were synthesized from cyclic iodoacetals [67]. Bicyclic acetals may be prepared with relative ease epialboatrin (100) was synthesized via a successful hypophosphite-mediated radical cyclization of the cyclic bromohydrin 99 [68] (Scheme 35). In one of the early examples reported by Ueno, bromoacetals obtained from butoxyallene, allylic alcohols, and NBS underwent efficient radical cyclization reactions providing easy access to a-methylene-y-butyrolactones after Jones oxidation [69]. 

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