Cyclic tetraacetylenes

Dialkyl 1,3-diacetyIenes usually absorb in the ultraviolet region, Amax 226, 239 and 253 nm. Of a series of cyclic tetraacetylenes (8), two higher homologues, 8c and 8d, show absorption spectra similar to that of acyclic compounds, whereas the trimethylene homologue (8b) demonstrates strikingly different features in its absorption spectrum (0 a red shift of 9nm and (//) appearance of a new band at 246 nm. This is attributed to the marked transannular electronic interaction in 8b, in... 

The jp-carbons of 4a are deshielded by 14-5 p.p.m. from the chemical shift (8T3 p.p.m.) of those of cyclotridecyne. This remarkable effect has been ascribed to partial olefinic character of the triple bonds caused by large molecular deformation . In fact the c/.y-oIefinic configuration has recently been confirmed by X-ray structure analysis of 25 As to the chemical shifts of acetylenic jp-carbons in cyclic tetraacetylenes, both of the inner and outer sp-carbons of 8b are deshielded by ca. 3 p.p.m. relative to the corresponding carbons of acyclic diacetylenes (Figure 5). 

Figure 5. Carbon-13 chemical shifts of cyclic tetraacetylenes (8), 8 (TMS) . ...

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