Electrophiles cross-cyclization with

Cross-cyclization with electrophiles 31, 479 Crotonaldehyde as reactant 9, 851s32... 

An all carbon conjugated ladder polymer (graphite ribbon) was synthesized by a novel electrophile-induced cyclization reaction to provide fused benzenoid aromatic hydrocarbon in quantitative yield [161]. Suzuki cross-coupling of dieneyne 105 with 1,4-didode-cylbenzene-2,5-diboronic acid (106) gave rigid-rod polymers 107, which was further treated with TFA to produce 108 graphite ribbon as a yellow/orange solid. 

As described above, our synthetic strategy involves the convergent construction of the central cyclopentanone ring with a carbonylative cross-coupling reaction and a photo-Nazarov cyclization reaction (Chart 2.2). The electrophilic coupling component 51 was synthesized by an intramolecular Diels-Alder reaction [34] and the nucleophilic coupling component 52 by a vinyiogous Mukaiyama aldol reaction [35]. 

The [2,3- ]> [2,3- ]. and [i,Z-b] isomers of 2,3-disubstituted furopyridines can be obtained from alkynes and o-iodoacetoxypyridines by a palladium-catalyzed cross-coupling reaction followed by an electrophilic iodocyclization reaction or a palladium-catalyzed cyclization in the presence of CO, as illustrated in Scheme 32 for [2,3-3] and [3,2-3] derivatives. When the pyridine ring contains strongly electron-withdrawing substituents such as carboethoxy, the reaction proceeded smoothly with benzyloxyalkynylpyridines <20020L2409>. 

Dihydropyrroles have recently become readily available by ring-closing metathesis. For this purpose, N-acylated or N-sulfonylated bis(allyl)amines are treated with catalytic amounts of a ruthenium carbene complex, whereupon cyclization to the dihydropyrrole occurs (Entries 6 and 7, Table 15.3 [30,31]). Catalysis by carbene complexes is most efficient in aprotic, non-nucleophilic solvents, and can also be conducted on hydrophobic supports such as cross-linked polystyrene. Free amines or other soft nucleophiles might, however, compete with the alkene for electrophilic attack by the catalyst, and should therefore be avoided. 

Electrophilic cyclization of a wide variety of (Z)-enynols affords substituted (Z)-5-(l-iodoylidene)-2,5-dihydrofur-ans with high regio- and stereoselectivity under mild reaction conditions. To access the cyclization precursors 27, a zirconium-mediated cross-coupling reaction of three different components involving an alkyne, a ketone, an alkynyl bromide in a one-pot procedure has been developed (Scheme 34) <2005JOC6999>. 

As shown below, cross-coupling of add chlorides with THP-protected propargyl alcohol derivatives gave rise to the corresponding alkynone, which underwent acid-assisted electrophilic addition of hydrogen halide with concomitant deprotection and cyclization, affording 3-halofurans. If RB(OH)2 was added into the reaction system before work-up, the reaction can provide Suzuki-coupled products in moderate yields. 

If the initial product is treated with NH4CI, one obtains the corresponding proton-substituted products. If I2 is employed, the corresponding vinylic iodides are obtained. This reaction is suggested to proceed by initial cross-coupling to form an enyne, followed by cyclization by palladium, and finally electrophilic cleavage of the carbon-tin bond (Eq. 39). 

In conclusion of these cyclization methodologies, we should note that the classical cyclization reactions are generally more atom economical than RCM, with its obligatory loss of at least two carbon atoms in the form of ethylene, while cross-couplings are even less efficient with loss of both nucleophilic and electrophilic activating groups. 

A novel series of 2,5-disubstituted boryl thiazoles 10 is prepared by means of Hantzsch cyclizations of thioamides with monohalogenated boryl aldehydes 9a/b (14JA17669).These aldehydes are derived from the unsubstituted a-MIDA boryl aldehyde 8 by means of electrophilic halogenation (NBS or NCS, pyrrolidine). Application of these novel boryl thiazoles 10 is demonstrated in cross-coupling reactions (e.g., 11-12). The optimal coupling conditions utilize the Lipshutz surfactant TPGS-750-M in water. The 2,5-disubstituted boryl thiazole intermediate 10 is a versatile intermediate for medicinal chemistry and material science. 

Treatment of mono-alkylacetylenes with anhydrous hydrogen chloride yielded mainly trans-l,3-dialkyl-l,3-dichlorocyclobutenes. This cyclization involves initial formation of a vinyl cation by protonation of the acetylene, followed by reaction with a second molecule of the acetylene to give a cyclobutenyl cation. When mixtures of acetylenes were used, both crossed- and self-addition products were formed. The reaction of propene, propyne, and anhydrous hydrogen chloride gave, inter alio, tra/w-l,3-dichloro-l,3-dimethyl-2-isopropylcyclobutane. In this case, electrophilic addition of the iso-propyl cation to the propyne to give the alkylated vinyl cation must initiate the cyclization process. 

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