Can. S. Chem. 54, 3536 (1976).4-(l-Hydroxyeth~l)-3,3,5,5.tetramethylcyclohexanone has been prepared to serve as a model for 3,3,5,5-:etramethyicyc1ohexanone on which the reduction stereochemical product ratio may be experimentally determined. Reduction with sodium borohydride in z-propanol gives two epiiners in a 95.5:A.j ratio. The major isomer is shown to be the less stable by equilibration over Raney nickel. Proton and '3C nmr spectra confirm that this major epimer is ch-4-(I-hydroxjethyl)-3,3,5,5-tetramethylcycIohexanol and show that this compound exists predominantly in the conformation with an axial hydroxyethyl group and an equatorial hydroxyl group. It is concluded that reduction of this model compound proceeds at least 95.5'; by equatorial attack. Chem. 54, 3536 (1976).O n a prepare (l'hydroxy-1 Cthyl>4 titramethyl-3,3,5,5 cyclohexanone pour servir de modkle pour la t6tra111ethyl-3.3,5,5 cyclohexanone; avec ce composi le rapport stereochimique des produits de reduction peut $Ire determine experimentalement. La reduction avec le borohydrure de sodium dans ie propanol.2 donne deux Cpimkres dans le rapport 95.5:4.5. Faisant appel h l'6quilibration slar du nickel de Raney, on montre que l'isomere prkpondkrant est le moins stable. Les spectres rmn du proton et du '3C confirnlent que l'epimkre form6 d'une f a~o n prepondirante est (l'hydroxy-1 CthylF4 tetramethyl-3,3,5,5 cyclohexanol cis et on montre que ce compos6 existe d'une f a~o n prefkrentielle dans la conformation avec un groupe hydroxyethyle en position axiale el un groupe hydroxyle en position Cquatoriale. O n conclut que la reduction de ce compost! modkle se produit avec a u rnoins 9.5"; d'attaque Cquatoriale.[Traduit par le journal]Reduction of cyclohexanon~es with sodium borohydride is a reaction that displays wellknown and interesting stereoselectivity. In 1953 Barton was able to make the qualitative generalization that unhindered cyclohexanones yield primarily the equatorial aicohol, but that hindered cyclohexanones show the opposite stereoselectivity, yielding primarily the axial alcohol (1). This generalization has since been amply confirmed, but in spite of the synthetic and mechanistic activity in this area, the origin of the stereoselectivity still remains in doubt. Among simple cyclohexanones, the ketone a t the hindered extreme of the spectrum has been 3, 3,5,5-tetramethylcyclohexanone, and it has frequently been an irritating point that, because of the symmetry of this molecule, its stereoselectivity remains unknown. Estimates, in fact, have varied from 0.57, t o 30% equatorial alcohol (for a discussion of these estimates, see ref.2). In this paper we report the use of a ciosely related model compound to obtain an experimental value for the stereoselectivity of reduction of 3,3,5,5-tetramethylcyclohexanone by sodium borohydride in 2-propanol.The criterion in the selection of a model compound was clearly the introduction of a substituent to break the symmetry of 3,3,5,5-tetramethylcyclohexanone, but one which would have the minimum per...
