The addition of allyl magnesium and allyl indium reagents to a key TBS protected norbornenyl building block, synthesised in 6-steps from commercially available 1,1-dimethoxy-2,3,4,5-tetrachlorocyclopentadiene, has been achieved providing the syn addition products with high diastereoselectivity. The subsequent exposure of the addition products to metathesis conditions, in the presence of ethene, then provided cis fused[3.0.3]-carbocycles with very high regioselectivity, via a Ring Rearrangement Metathesis (RRM) transformation.