1,1-Cyclopropane diesters with donor substituents undergo formal [3+3] cycloaddition with various silyl nitronates under Yb(OTf)₃ catalysis in the presence of a proton scavenger to give target N-silyloxy tetrahydro-1,2-oxazines in 56–98% yield. The reasons determining the reaction diastereoselectivity are discussed. The resulting tetrahydro-1,2-oxazines are transformed into pyrroline-N-oxides via a novel TfOH-promoted silanol elimination reaction.

This review on atypical angucyclinones possessing an aromatic cleavage of the C-ring covers literature between 1995 and early 2020. The unusual framework of the middle C-ring, “broken” as a result of biotransformations and oxidations in vivo and bearing an sp³-C connection, is of interest for biosynthetic investigations. The reported 39 natural compounds (55 including stereoisomers) have been analyzed and arranged into three structural groups. The biosynthetic origin of all these compounds has been thoroughly reviewed and revised, based on the found connections with oxidized angucyclinone structures. The data on biological activities has been summarized. Careful consideration of the origin of the structure allowed us to outline a hypothesis on the biological function as well as prospective applications of such atypical angucyclinones.