Alkyne transannular

In the reaction of disubstituted alkynes, 1,3-migration of the acetate takes place to give allenyl esters that can be versatile substrates, especially for [3,3]-Cope rearrangements.333 1,5-Enynes have proved to be versatile substrates for the preparation of perfumery agents such as sabinol334 and sabina ketone.335 Transannular systems undergo similar reactions.336... 

The mesoionic compound 41 was further used in a reaction with dimethyl acetylene dicarboxylate (DMAD) to produce a nine-membered cycloadduct 44 which is formed by a reaction cascade of double addition of the alkyne and transannular ring opening of the intermediate 43 (Scheme 7) <20030BC978>. 

A mechanistic scheme involves intramolecular single-electron transfer (SET) from the enamino moiety toward carbon 5 of the ring, and subsequent transannular interaction in zwitter-ionic biradical 259 with formation of stabilized zwitter-ion 260. Opening of the oxetane ring in 260 leads to cyclobutene 256, while secondary photolysis of the latter gives olefin 257 and alkyne 258 (Scheme 100). 

Electrochemically generated NO3 has been reported to add to medium-ring alkynes and alkynones to furnish bicyclic ketones and epoxy ketones, respectively.50 The postulated mechanism involves the addition of N03 to the alkyne followed by transannular cyclization/elimination of NO3. 

Finally, cerium(IV) ammonium nitrate can serve as the radical source by itself, generating NO3 radicals If by photolysis. The addition of such radicals to cyclo-alkynes 20 initiates an interesting tandem reaction [12]. Transannular hydrogen atom abstraction by the vinyl radical 21 affords the intermediate 22, which undergoes a 5-exo cyclization to the radical 23. In the last step, the ketone 24 is formed by elimination of NO2 in moderate yield thus, the overall sequence can be described as a self-terminating radical reaction (Scheme 7). 

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). 

One of the most important transannular reactions of cyclic alkynes is the Bergman cycliza-lion [67] of enediynes (Scheme 8-15). Nicolaou et al. have shown that the crucial distance between the termini of the enediyne system, allowing spontaneous Bergman cyclization at room temperature, is in the range of 3.1-3.2 A [9cj. Questions about the energy and structure of the biradicaloid intermediate and the mechanism of the biradical formation have stimulated considerable theoretical and experimental work [68]. These investigations and the importance of the Bergman cyclization reaction for the action of a new class of antitumor antibiotics will not be discussed here, as the enediynes are dealt with in considerable detail in Chapter 7. 

The reactions of thiepanes and related compounds fall into two categories (1) reactions on the sulfur atom and (2) those occurring at another part of the molecule. The first category includes transannular reactions in which the sulfur atom participates, and the second category includes reactions of thiacycloheptyne with the alkyne part being able to form adducts and reactions of the substituents on the sulfur atom (see Section 9.03.7). These reactions are of synthetic and mechanistic interest. 

Intermolecular [5+2+1] annulation of VCP 57, alkyne 58, and CO afforded cyclooctadienone 59, which underwent transannular closure to provide bicyclo[3.3.0]octenone 60 (Scheme 2.47) [67]. 

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. 

These studies show that if the two 1,3-butadiyne units are in close proximity on one side, only three of the four alkyne units are involved in the transannular ring closure. 

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