Cyclobutanones, substituted cyclization

In the two separate, initial reports on the reactivity of Fischer carbenes with enynes, one study found cyclobutanone and furan products [59], while the other found products due to olefin metathesis [60]. These products have turned out to be the exceptions rather than the rule, as enynes have since been found to react with Fischer carbenes to produce bicyclic cyclopropanes quite generally. The proposed mechanistic pathway is included as part of Bq. (28), in which vinylcarbene 10, produced by insertion of the alkyne into the metal carbene, may then cyclize with the pendant olefin to metallacyclobutane 11, leading to product. The first reported version of this reaction suffered from extreme sensitivity to olefin substitution [Eq. (28) compare R=H, Me] often producing side-products due to metathesis (through 11 to yield dienes) and CO insertion (into 10 to yield cyclobutanones and furans) [61]. Since then, several important modifications have been developed which improve yield, provide greater tolerance for alkene substitution, and increase chemoselectivity for the bicyclic cyclopropane... 

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]. 

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