Cyclohexane silyl enol ether

Organic synthesis 74 [OS 74] Reaction between 4-bromobenzaldehyde and the silyl enol ether of cyclohexane [15]... 

In detail, the first step is a Cope rearrangement - a [3,3]-sigmatropic rearrangement involving nothing but carbon atoms 167. This step is unfavourable because it transforms a stable cyclohexane into an unstable It, C-dccadicnc. The product 168 is a minor component in the equilibrium. What drives the reaction forward is a favourable Claisen-Ireland rearrangement on the silyl enol ether 168. This step is favourable because it creates a carbonyl group (the ester in 165) at the expense of an alkene. 

Recently, the thermal cyclization of silyloxy-substituted trienes was investigated108. Silylation of dienone 60 a under a variety of conditions affords predominantly the desired ( )-silyl ether 61 a [d.r. (EjZ) 7-11 1], 1.6-ECRC of a mixture [d.r. (EjZ) 11 1] via flash vacuum pyrolysis (0.5 M in cyclohexane, 0.8 mL/min, 4-m quartz tube, 365 "C, N,) affords 65% of the cyclized silyl enol ether 62a. 15% of the predominantly Z-acyclic silyl ether 61 a [d.r. (EjZ) 1 9], and about 20 % of rearranged products. Hydrolysis of the mixture in aqueous acid gives 63a in 72 % isolated yield. Pyrolysis of a mixture of silyl enol ethers 61 b [d.r. ( /Z) 69 31] affords recovered 61b, which is predominantly Z, and 68% of 62b. Hydrolysis yields 63b in 55% yield. 

The reaction of alkenes with trialkylsilanes (639) and carbon monoxide in the presence of Co, Ru and Rh catalysts leads to silyl enol ethers which are extended by one carbon atom. Since this reaction is also applicable to cycloalkenes, one can synthesize diethyl-methylsiloxymethylene cyclohexane (640) in 89% yield (equation 325)354. 

The silyl-tethered compound 69 undergoes a tandem radical cyclization followed by desilylation and oxidation of the sulfide and elimination to give bicyclic 70 further elaborations (by way of 71) convert this intermediate to cyclohexane derivative 72, transformed in five steps to 73, an intermediate in Woodward s reserpine synthesis (Scheme 13). PCC oxidation of the enol ether function of 72 yielded 74. 

The same authors have shown that independently prepared cyclohexanone enolate on treatment with borane yields trans-cyclohexane-l,2-diol in 45% yield. 2-Methylcyclohexanone, which can give two enolates under appropriate conditions, may thus be selectively converted into either a mixture of trans-diols (112) and (113) in a ratio of 42 58 or the trans-diol (114). Enol silyl ethers may also be employed as substrates for this reaction. 

---