An expeditious route to eight- and nine-membered carbocycles based on a RCM-ring fragmentation strategy

The presence of a temporary one-atom internal tether in 1,9-deca- and 1,10-undecadienes allows their efficient ring-closing metathesis (RCM). Cleavage of the bridging tether of the resulting bicycles provides eight- or nine-membered carbocycles, medium-sized rings that are difficult to assemble using other currently available procedures.

Unmasking the 6,7,5-tricarbocyclic frame of

Opening of the bridging ether of 8-oxabicyclo[3.2.1]oct-6-ene systems 1,7-fused to a five-membered carbocycle that bears a double bond conjugated to that of the oxabicycle can be induced at low temperaure by the addition of an organolithium reagent. If this double bond is unsubstituted, the opening occurs via an anti-1,6-addition pathway, and if substituted it takes place by means of a more classical syn SN2' addition.

Sulfinyl-directed diastereoselective

Introduction of a homochiral p-tolylsulfinyl group at an appropriate position in the alkene accelerates the thermal [5C + 2C] intramolecular cycloaddition to beta-silyloxy-gamma-pyrones and, most importantly, leads to excellent levels of diastereodifferentiation. The resulting adducts can be easily desulfinylated to give optically active 8-oxabicyclic[3.2.1]octane intermediates which by virtue of their rich functionalization might be susceptible to elaboration into enantiomerically pure natural products containing seven-membered carbocycles and tetrahydrofurans.