Cyclic acetylenes medium-ring

The reaction of 1,2-dibromocycloalkenes (13) with magnesium in the presence of l,3-diphenylbenzo[c]furan (16) affords a cycloalkyne adduct (17). Also, the mercuric oxide oxidation of bishydrazone, which has been used extensively in the preparation of open-chain and medium or large cyclic acetylenes " , is adapted for small ring bishydrazones (18) . Treatment of 18 with mercuric oxide in the presence of 16 or of phenyl azide results in the adducts, 17 and 19, respectively. It seems... 

Medium-ring acetylenes (8- to 11-membered) with one triple bond are isolable compounds. Eight- and ten-membered cyclic acetylenes containing more than one unsaturated bond have been prepared and isolated. Isolable substituted cycloheptyne derivatives will also be discussed in this section. 

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... 

Meier has provided an excellent review of medium- and large-ring cyclic acetylenes. Cycloheptyne and cyclo-octyne may be obtained by pyrolysis of (144 n = 7 or 8), which can be readily obtained by selenium dioxide oxidation of the corresponding semicarbazones the photochemistry of cyclo-octyne in the presence of molybdenum hexacarbonyl is discussed. Compound (145) reacts with sodium methoxide to give (146) the acetylene (147) is proposed as an intermediate. 

The two-carbon ring expansion which involves the [2 + 2] cycloaddition of enamines of cyclic ketones with electron-deficient acetylenes followed by thermal rearrangement of the resulting fused cyclobutenes (see Section II.E) has been successfully used in the synthesis of medium-size heterocycles. Examples include the preparation of compounds 407246, 408247 and 409248. 

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