Abstract
To investigate the preparation of allenes by means of 1,6-addition reactions to 2-alken-4-ynones, 3-ethynyl-2-cycloalkenones (5) were synthesized and reacted with a variety of organocopper nucleophiles. Such nucleophiles undergo regiospecific 1,6-additions to generate intermediate allenyl enolates (10), which are regiospecifically and stereoselectively protonated to yield a preponderance of (Z)-3-alkenyl-2-cycloalkenones (6-8). The reaction is sensitive to the nature of the organocopper reagent used; higher order cyanocuprates derived from organolithium reagents are the reagents of choice. The intermediate allenyl enolates (10) can be trapped as their enol triflates (13), which in turn can be converted into enallenes (14). Non-organocopper nucleophiles also can add to the terminal carbon of the alkynyl moiety of the enynones studied, but the mechanism may be distinct from that of 1,6-addition: (phenylthio)lithium adds to 3-ethynyl-2-methyl-2-cyclopentenone (5a) by a syn-carbometalation-like mechanism and upon protic quenching yields a preponderance of (E)-2-methyl-3-(2-(phenylthio)ethenyl)-2-cyclopentenone (6h).