Two plus two equals a bicycle: A highly efficient acid-catalyzed intramolecular [2+2] cycloaddition of ene-allenones affords strained bicyclo[n.2.0] frameworks, which contain vicinal all-carbon quaternary and tertiary centers (see scheme; Tf: trifluoromethanesulfonyl), under mild conditions with excellent yields and chemo-, regio-, and diastereoselectivities.
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Zhao et al. (2009) studied this question.