Convenient Synthesis of Ethenylcyclopropane and Some 2-Cyclopropylcyclopropane Derivatives¹

Abstract

Ethenylcyclopropane (5) was prepared from cyclopropyl methyl ketone (1) in four simple, easily scalable steps (bromination, reduction to the bromohydrin, acetylation of the latter, and reductive elimination with Zn/Cu) in an overall yield of 56%. 1-Bromo-2-cyclopropylcyclopropane (7) (approximately 1:1 mixture of cis-and trans-7) was prepared from 5 via the dibromide 6, and from 7 the boronate cis/trans-8 (ratio 1:1) was obtained in 96% yield. Rhodium(II)-catalyzed cyclopropanation of 5 with ethyl diazoacetate gave ethyl 2-cyclopropylcyclopropanecarboxylate (cis/trans-9, ratio 1:1.4) in 80% yield. The latter was also prepared in 79% overall yield by sequential cyclopropanation of buta-1,3-diene with ethyl diazoacetate [under Rh 2 (OAc) 4 catalysis] and diiodomethane/diethylzinc/trifluoroacetic acid. Curtius degradation of 2-cyclopropylcyclopropanecarboxylic acid (obtained by hydrolysis of the ester) gave 2-cyclopropylcyclopropanamine (1:1.2 mixture of cis-and trans-isomers) in 58% yield.

Enantioselective Synthesis of Biscyclopropanes Using Alkynes as Dicarbene Equivalents

“…Typically, two different strategies have been applied to assemble this unique and important motif: one is cyclopropanation of conjugated dienes and the other is cycloisomerization of 1,n‐enynes (Scheme 1B–C). [1g, 7] Traditionally, the former is mediated by carbenes and carbenoids and usually requires highly energetic diazoalkane precursors such as the cyclopropanation of diazo compounds, [7a–d] or using equivalent metal reagents, such as the diiodomethane‐mediated Simmons–Smith reaction [1g–h, 7g] . However, the one‐step synthesis of the biscyclopropanes from 1,3‐diene with the above strategies in an asymmetric manner still remains a challenge [7e, 8] .…”