1,6-ene-ynamides

Albeit no examples of a stable bicyclo[3.2.0]hept-5-ene obtained by a metal-catalyzed reaction from a 1,6-enyne have been observed, interestingly, following earlier work done with PtCl2 as catalyst,it was found that the gold-catalyzed reaction of 1,6-ene-ynamides gives cyclobutanones (equation 42), in a process that probably proceeds through unstable bicyclic enamines. 

One of the principal problems in the synthesis of cyclobutene compounds is the isolation process, because of the general instability of the final products. As a result, cyclobutanones are isolated via gold(I)-catalyzed cyclization of 1,6-ene-ynamides II-4 (Scheme 3.4) [9, 10]. The formation of cyclobutene intermediate II-5 only takes place if the ynamides are terminal or substituted by a trimethysilyl group (R = H, TMS). Skeletal rearrangement products were also obtained as minor products. Conversely, in the reaction of the same substrates catalyzed by PtCl2, the 1,3-dienes were isolated as the major products in good yields [11, 12]. 

Scheme 3.4 Synthesis of cyclobutanones via gold(I)-catalyzed cycloisomerization of 1,6-ene-ynamides II-4...

In addition, cyclobutanones are synthesized via platinum(II)-catalyzed cycli-zation of 1,7-ene-ynamides II-6 (Scheme 3.5). The cyclobutanones are fanned after hydrolysis of the corresponding bicycle[4.2.0] compounds II-7. 

Scheme 3.5 Synthesis of cyclobutanones via platinum(II)-catalyzed cycloisomerization of 1,7-ene-ynamides II-6...

The metathesis of ene-ynamides has been investigated by Mori et al. and Hsung et al. [80]. Second-generation ruthenium catalysts and elevated temperatures were required to obtain preparatively useful yields. Witulski et al. published a highly regioselective cyclotrimerization of 1,6-diynes such as 98 and terminal alkynes using the first-generation ruthenium metathesis catalyst 9... 

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