Synthesis of trans-Cycloalkenes via Cyclopropanation and Rearrangement

Key words

rhodium - cyclopropanation - [3,3]-sigmatropic rearrangement - trans-cycloalkenes

Significance

There are few ways to access chiral medium-sized rings possessing a trans double bond (for selected examples, see: A. Deiters et al. Chem. Eur. J. 2002, 8, 1833; X.-N. Wang et al. J. Am. Chem. Soc. 2014, 136, 9802). Such motifs often exhibit planar chirality and may find application in the synthesis of complex polycyclic frameworks. The authors report an efficient procedure for the asymmetric synthesis of piperidine-fused trans-cycloalkenes 3 from triazoles 1 and meth­ylenecyclopropenes 2.

Comment

The reaction is initiated by the in situ formation of an α-imino rhodium carbenoid from triazole 1. Cyclopropanation of the exocyclic methylene group of 2 leads to the formation of spiropentane A, which can then undergo a thermal rearrangement under microwave irradiation to give trans-cycloalkene 3. The authors propose a concerted mechanism, which draws similarity to the retro-Claisen [3,3]-sigmatropic rearrangement.