Halogen derivatives Diels-Alder reactions

The diastereofacial selectivity of 2-azadienes with a stereogenic center attached to C-l in Diels-Alder reactions has been investigated (Scheme 63). The synthesis of monochloro derivatives 284 was readily achieved in excellent yields by halogenation of 139 with ALchlorosuccinimide. The... 

Amino substituted compounds, as well as being available via simple alkylation by halogenated dihydrothianes, may also be prepared by Diels-Alder reaction as described in Section 2.25.4.2.1, as may some alkoxy analogues. They are, however, frequently derived from dihydrothiopyranols, which are in turn derived from the ketones. A discussion of general chemical conversions within the area may be found in (79JOC3144) and references therein. 

As expected, the C-4—C-5 bond orders found for these derivatives of the 1,3-dithiolium system are even higher. Contrary to the theoretical expectations, however, Mayer and Gebhardt124 have recently found that halogens do not add to these compounds, and both isotrithione and isodithione are stable towards butadiene or furan under conditions in which vinylene carbonate readily enters into Diels-Alder reactions 176 under drastic conditions, isodithione gives an adduct with cyclopentadiene, but the structure of this adduct is not yet known. 

The aromatization of bicyclo[4.1.0]hept-3-enes gives access to a wide variety of cycloproparenes the substrates are readily available via Diels — Alder reaction of butadienes w ith cyclopropcnes. 1,2-Dihalo- and tetrahalocyclopropenes have been used for this purpose. In the aromatization step, it is important that the halo substituents arc located at Cl and C6 of the bicyclo[4.1. OJhept-3-ene. Halogens at C2 and C5 result in ring enlargement rather than aromatization on reaction with base. Oxidative bis-decarboxylation of bicyclo[4.1.0]hept-3-ene-l, 6-dicarboxylic acid was investigated as an alternative route to cycloproparenes however, the products were derived from carbenium ion capture after monodecarboxylation. 

Asymmetric Diels-Alder reaction, ene reaction, hydrogenation, halogenation by means of chiral olelln, such as camphor derivative 1 or 2. 

Metacyclophane (4a) also undergoes facile Diels-Alder reaction with weak dienophiles which did not react with la [66, 70]. Thus reaction of 4a with TCNE, DMAD, and MA furnished the [4 + 2] adducts 97a-c (Structures 18), respectively. Similarly, halogen substituted derivatives 4b-d (s. Scheme 9) gave the respective adducts 97d-j (Structures 18), though these were less reactive than the parent hydrocarbon 4a. This is ascribed to the electron-withdrawing effect of halogen substituents which lowers the HOMO level. 

Unfortunately, all of the above-described synthetic approaches towards HBC (1) suffer from serious drawbacks, such as harsh reaction conditions, a complicated experimental work-up, and low yields. Furthermore, under aluminum(III) chloride catalysis, dealkylation, migration of the alkyl substituents - or even chlorination of the aromatic system- occurs, which clearly limits the accessibility of functionalized HBC derivatives for further investigations and applications. In order to overcome these problems, the weaker Lewis acid iron(III) chloride in nitromethane was used instead of AICI3, and the reaction conditions were carefully optimized [55, 56]. In this way, access was obtained to a multitude of HBC derivatives 8 and 9 with diverse substitution patterns and symmetries bearing solubilizing alkyl chains and halogen substituents, starting from functionalized hexaphenylbenzenes. The sixfold symmetric hexaphenylbenzenes 10 were synthesized by the Co2(CO)g-catalyzed cyclotrimerization of substituted diphenyl-acetylenes 11 (Scheme 13.4a) [57], whereas the intramolecular Diels-Alder reaction... 

The best result was obtained from the o-tolyl substitution of phosphorus. The palladium catalysts derived from (62) afforded high enantioselectivities up to 95% ee in the Diels-Alder reaction of cyclopentadiene with N-acryloyl-oxazolidinone (15a). In addition, a similar result was obtained from the dibromo complex (PdBr2) and AgOTf as halogen scavenger provided 90% of enantioselectivity. 

Cuellar MA, Alegria LK, Prieto YA, Cortes MJ, Tapia RA, Preite MD (2002) Hetero-Diels-Alder reaction of halogenated quinones with a polygodial-derived azadiene. Tetrahedron Lett 43 (12) 2127-2131. doi 10.1016/S0040-4039(02)00246-0... 

Among the classical (summarized in [11]) reactions of cyanoacetylenes (salt and n-complex formation, nucleophilic addition of, inter alia, amines and alcohols, Diels-Alder additions with 2 (see Section 2.22.2), addition of halogens and hydrogen halides, etc.) their polymerization might deserve a second look since the products formed - polyacetylenes - have attracted much attention during the last two decades. Thus 1, several of its derivatives, and 2 have been polymerized with anionic initiators (triethylamine, sodiiun cyanide, butyllithium) to give black, low-molecular-weight polymers claimed to have structures like 33 [38-40], which is obtained from 2 by treatment with butyllithium. 

Reduction of the double bond Pt>lynicri/.ation Nilroacctyicncs Ptdynitro aliphatic compounds Nitration of hydrocarbons Substitution of halogen Electrolytic methods Addition reaction Michael addition Diels-Alder addithm Oxidative dimerizatum a, to-Dinitroalkanes gem-Dinitroalkancs Iriniiromclhane (nitroform) derivatives Properties of nitroform Manufacture of nitroform... 

In 1968, Tobey and Law reported on the intriguing reaction of furan, substituted furans, and cyclopentadiene with both tetrachloro- and tetrabromocyclo-propene (TBCP). The two unsaturated systems reacted smoothly to directly produce cycloheptanoid systems, the products of a formal [4-i- 3]-cycloaddition reaction. The authors proposed in their early manuscripts that the tetrahalocy-clopropenes underwent an initial thermal Diels-Alder cycloaddition to produce the cyclopropyl norbornene derivatives la and lb (Figure 2). However, these workers were unable to isolate or characterize the primary cycloadducts and instead observed a product which had spontaneously rearranged by way of a halogen atom migration to yield the bicyclo[3.2.1]octadiene nucleus 2a and 2b. 

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