Observation on the synthesis of allenes by homologation of alk-1-ynes

Allene formation by gold catalyzed cross-coupling of masked carbenes and vinylidenes

“…Therefore, it seemed possible that 6 were the precursors of the observed allenes 7, as postulated for the Crabbé homologation (19,20,39). Palladium has also been shown to fragment preformed propargylamines to yield allenes.…”

“…For entries 11-15, the enamine is derived from pyrrolidine (Pyr). Only one distereomer was obtained for entries 16 -20. cording to multinuclear NMR spectroscopy, all of the allenes obtained by coupling the enamine, which bears R 2 ϭ Me and R 3 ϭ Ph, were obtained as a single diastereomer (entries [16][17][18][19][20]. The only serious limitation that was found is the absence of reaction when an internal alkyne was used, preventing the synthesis of tetrasubstituted allenes (see above).…”

Photocycloaddition of Arenes and Allenes

“…Thus we looked for an alternative route, taking advantage of the Crabbé homologation. 21 The salicylic aldehyde 7a was first alkylated in high yield with propargyl bromide (Scheme 3), and subsequently homologated into allene 1a. This latter reaction was carried out twice starting with 40 g of 8 yielding in total 36.6 g of photocycloaddition precursor 1a.…”

Enantioselective Copper‐Catalysed Propargylic Substitution: Synthetic Scope Study and Application in Formal Total Syntheses of (+)‐Anisomycin and (−)‐Cytoxazone

“…CrabbØ and co-workers found that terminal alkynes react with paraformaldehyde and diisopropylamine in the presence of copper(I) to form allenes. [22] The enantioselective copper-catalysed propargylic amination reaction provided the benzyl-substituted propargylic amines 7 f and 7 g (Scheme 8) in good yields and with high ee values. Even after prolonged reaction times no full conversion of the starting material was observed, which may be ascribed to catalyst deactivation.…”