Ring closing metathesis
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
---Homoallyl-substituted vinyl cyclopropanes 1a-c, which are readily prepared by reaction of allylindium reagents with α,β-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 from their ‘cis’ diastereomers by RCM to 5b and 5c - but only with moderate efficiency due to a competing homo-cross metathesis (‘dimerisation’) to give 7b and 7c, respectively. However, the four diastereomers of the (homoallyl)distyrylcyclopropane 14a obtained from indium-mediated reaction of dibenzylideneacetone and crotyl bromide undergo remarkable kinetic diastereoselection 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. Furthermore, the RCM product 15a is obtained rich in one diastereomer (ca. 90%). The kinetic diastereoselection and lack of dimerisation can be rationalised by considering developing transannular interactions and a pseudo-A1,3 strain model.
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