Electrophilic and radical transannular cyclizations of 5-cyclodecenone to give either hydronaphthalene or hydroazulene products

Abstract

The transannular cyclization of the E and Z double-bond isomers of 5-cyclodecenone were investigated in order to determine the regio- and stereochemical preferences of the unsubstituted ring system. Electrophilic cyclization of the E isomer under either protic or Lewis acid conditions led to hydronaphthalenols with a preference for the trans ring fusion, while the Z led to only cis-fused hydronaphthalenols. Cyclization of the ketyl radical generated from the ketone led exclusively to a cis-fused hydroazulenol, regardless of double-bond geometry, although the E isomer was considerably more reactive than the Z isomer. The stereochemistry of the ring fusion in the products from (E)-5-cyclodecenone can be rationalized by cyclization through its lowest energy conformations in which the carbonyl oxygen is anti to the alkene hydrogen at C6, leading to the trans-fused hydronaphthalenol, and syn to the alkene hydrogen at C5, leading to the cis-fused hydroazulenol. For (Z)-5-cyclodecenone, molecular mechanics calculations found two low energy conformations, only one of which brings the alkene and the carbonyl groups close enough for their reaction with each other. In this conformation, the alkene hydrogens at C5 and C6 are syn to the oxygen of the ketone, leading to a cis ring fusion regardless of whether 1,5- or 1,6-cyclization is observed. The difference in regiochemistry in radical versus electrophilic cyclizations is explicable on the basis of the differences in mechanism for the two reaction pathways. The radical cyclizations are kinetic in nature with the ketyl radical adding to the proximate C5 alkene carbon in a very exothermic step, akin to the cyclization of 1-hexenyl radicals. The stereochemistry of the acid-induced cyclizations can be explained through the intermedicacy of either nonclassical or contact ion pairs, the regiochemistry reflecting the greater stability of the hydronaphtalene ring system over the hydroazulene. A system of nomenclature for unambiguously labeling each of the low energy conformations of (E)-5-cyclodecenones is also proposed