Triple bond transannular

Transannular participation of a triple bond has been examined by Hanack and Heumann (86). Acetolysis or solvolysis in aqueous acetone of cyclodecyn-5-yl-l-tosylate 104a gave, besides some unidentified unsaturated hydrocarbons, a mixture of the rearranged ketones 105 and 106. None of the unrearranged... 

A large number of cyclic acetylenes have been synthesized to investigate the proximity interactions between two triple bonds or between a triple bond and other unsaturated systems. Such transannular proximity interactions have been observed in some cyclic acetylenes of medium ring size. However, no appreciable evidence of the interaction was detected in the electronic spectra of diacctylenes 1 2 - and 3... 

The interaction between a triple bond and an aromatic ring was first studied with paracyclophynes (10) > but only ambiguous evidence was observed in their spectra . The first positive evidence for such a transannular interaction was shown in the spectra of naphthalenophapolyynes (11) . Here all the absorption bands appear at the same positions, but their intensities in the cyclic monomer 11a are markedly decreased as compared with the intensities per unit chromophore of the cyclic dimer 11b and trimer 11c. This hypochromism of 11a was explained in terms of a dispersion... 

Many ring systems have been prepared by cycloaddition of acetylenic compounds, following concerted or multi-step intra- or intermolecular reaction mechanisms. In-particular, the placing in close proximity of triple bonds or a triple bond and another unsaturated system, such that intramolecular cycloaddition might lead to four-seven-membered rings, would seem of interest. This section deals with transannular carbon-carbon bond formation of triple bonds in acyclic and cyclic systems. 

On treatment with electrophilic reagents such as bromine and hydrogen bromide, 38 gave diphenylbenzofulvenes (45) through transannular bond formation between proximate triple bonds (44) , whereas addition of bromine to 41 yielded the... 

Base-catalysed prototropic rearrangements of cyclic polyacetylenes have often been used to synthesize annulene and dehydroannulene systems. In such reactions, transannular carbon-carbon bond formation between proximate triple bonds occasionally takes place to give polycyclic compounds. Treatment with a base of cyclic polyacetylenes 57 and 59 gave benzenoid aromatic compounds 58 and 61, with the desired dehydro[16] and [I8]annulenes, respectively. ... 

Similarly, biphenylene (63) was obtained in ca. 7% yield, from treatment of the thermally unstable tetrayne 8a with potassium /-butoxide, probably by transannular bond formation between the triple bonds closely placed in the intermediate 62 . ... 

It is well known that the transannular carbon-carbon bond formation is often induced in catalytic hydrogenation of severely strained cyclic acetylenes where triple bonds are closely placed to each other. Hydrogenation of the highly strained tetrayne (69) with 10% Pd-C gave the polycyclic compounds, 70 and 71, in addition to the expected hexadecahydro compound (50% yield). On the other hand, Birch reduction of 69 gave another perhydro product (72) in good yield. From a similarly constructed tetrayne (73) a polycyclic compound (74) was obtained by catalytic hydrogenation over platinum . 

Addition of carbenium ions to C —C double bonds and C—C triple bonds, as well as to allylic systems, is of great interest in organic chemistry. Extensive investigations due to the importance of terpenes in biosynthesis (e.g., generation of steroids, transannular reactions of humulene. etc.) led to the development of a great number of reactions employing this method of C —C bond formation. 

Among the very few reported possibilities offered by transannular cycle contraction of cyclododecenyl radicals to construct 9,5-, 8,6- or 7,7-bicyclic systems, the radical tandem approach to the taxanes planned by Pattenden, from the substituted A-ring precursor 134, introduces a new conceptual strategy, described as E in Scheme 29. First, a 12-e Jo-dig macrocyclization involving alkyl radical 135 occurs easily on the triple bond of the ynone moiety. The produced vinyl radical (major conformer is presented) 136 cyclizes in a 6- n-endo)-exol - n-exo)-endo transannular manner to assemble the unusual tricyclo[9.3.1.0 ]pentadecane framework 137 as a 6 1 ratio of diastereomers (Scheme 38) [55]. 

The elaboration of a polyunsaturated 5 9-fused ring system based on transannular addition of an alkenyllithium species to a triple bond following the Br—Li exchange of a bromoalkene is clean and elegant. 

A second functional group can undergo a transannular reaction with the triple bond. It can also alter the reactivity of the acetylenic unit by transannular orbital interactions. 

If the cyclic alkyne is strained, Le., if the triple bond is bent, electron density is pushed to the outside of the ring. Thus, if the in-plane orbitals of the triple bond are involved, the exocyclic and the transannular attack of the triple bond are electronically different reactions (Fig. 8-2). 

Another transannular C-C bond formation reaction which starts with the attack of an electrophile at the triple bond is the formation of bityclic systems from tyclic diynes shown in Scheme 8-13, where H2SO4 induces the transannular addition of H2O to a cyclic diyne... 

The diyne cyclizations discussed so far involve the formation of an aromatic system together with the biradical. Surprisingly, though, a new kind of diyne- to -biradical cyclization has been discovered only recently, where the transannular interaction of two triple bonds in a diazacy-... 

Recently, the high reactivity of alkynes has been exploited to form polycarbo-cyclic and heterocyclic frameworks by multiple bond formations via tandem reactions [106-109]. Since DBAs contain multiple triple bonds, they are expected to serve as precursors of highly unsaturated polycyclic compounds if transannular bond formations take place with reasonable selectivity. Constraint due to the cyclic structure may well direct the reactivity and selectivity in bond formation to an unusual manner, leading to unconventional polycyclic aromatic compounds. 

Our interest in the topic of this chapter was stimulated by earlier spectroscopic investigations [12] on cyclic diynes with two parallel triple bonds in close proximity and transannular reactions between the triple bonds [13, 14]. We were further inspired by the results of Wegner [15] who found out that parallel aligned 1,3-butadiyne units (Scheme 7.1) yielded, upon irradiation or heating, a 1,4-addition in a topochemical reaction [15, 16]. 

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