Homoallyl-substituted vinyl cyclopropanes 1a-c, which are readily prepared
by reaction of allylindium reagents with alpha,beta-unsaturated ketones and
aldehydes, undergo Ru-catalysed RCM reactions with Grubbs catalyst to give
[4.1.0]bicyclooct-2-ene (norcarene) type bicyclic products. Noncyclisable
'trans' homoallyl-substituted vinyl cyclopropanes 1b and 1c are separated f
rom their 'cis' diastereomers by RCM to 5b and 5c - but only with moderate
efficiency due to a competing home-cross metathesis ('dimerisation') to giv
e 7b and 7c, respectively. However, the four diastereomers of the (homoally
l)distyrylcyclopropane 14a obtained from indium-mediated reaction of dibenz
ylideneacetone and crotyl bromide undergo remarkable kinetic diastereoselec
tion in their RCM reactions to give a 4,7-dimethyl[4.1.0]bicyclohept-2-ene-
type product 15a. This process allows recovery of a single diastereomer (>
95%) of 14a without dimerisation being a significant side reaction. Further
more, the RCM product 15a is obtained rich in one diastereomer (ca. 90%). T
he kinetic diastereoselection and lack of dimerisation can be rationalised
by considering developing transannular interactions and a pseudo-A(1,3) str
ain model.