Ring diaxial

Since ring closure is only possible when the starting material is diaxially substituted, this has to mean that ring diaxial can flip to opening is similarly only possi-diequatoriai proc(UCf diaxial. This... 

The greater stability of an equatorial methyl group compared with an axial one IS another example of a steric effect (Section 3 2) An axial substituent is said to be crowded because of 1,3 diaxial repulsions between itself and the other two axial sub stituents located on the same side of the ring... 

Iodine isocyanate was used to synthesize the first steroidal aziridine, 2, 3 -iminocholestane (95). from 5a-cholest-2-ene (91). This reaction sequence which is believed to proceed through a three-membered ring iodonium ion (92) illustrates the limitation of pseudohalogen additions for the synthesis of -aziridines. The iodonium complex forms from the least hindered side (usually alpha) and is opened tmK5-diaxially to give a -oriented nitrogen function. The 3a-iodo-2 -isocyanate (93) is converted by treatment with... 

Iodine azide, on the other hand, forms pure adducts with A -, A - and A -steroids by a mechanism analogous to that proposed for iodine isocyanate additions. Reduction of such adducts can lead to aziridines. However, most reducing agents effect elimination of the elements of iodine azide from the /mwj -diaxial adducts of the A - and A -olefins rather than reduction of the azide function to the iodo amine. Thus, this sequence appears to be of little value for the synthesis of A-, B- or C-ring aziridines. It is worthy to note that based on experience with nonsteroidal systems the application of electrophilic reducing agents such as diborane or lithium aluminum hydride-aluminum chloride may yet prove effective for the desired reduction. Lithium aluminum hydride accomplishes aziridine formation from the A -adducts, Le., 16 -azido-17a-iodoandrostanes (97) in a one-step reaction. The scope of this addition has been considerably enhanced by the recent... 

Diaxial repulsion (Section 3.10) Repulsive forces between axial substituents on the same side of a cyclohexane ring. 

CHa-ring interactions might also be expected to control the conformational preferences of dimethylcylohexanes. It might even be anticipated that the energy differences between a diequatorial conformer and a diaxial conformer will be twice the equatorial-axial energy difference in methylcyclohexane. 

Chair cyclohexanes are conformationally mobile and can undergo a ring-flip, which interconverts axial and equatorial positions. Substituents on the ring are more stable in the equatorial position because axial substituents cause 1,3-diaxial interactions. The amount of 1,3-diaxial steric strain caused by an axial substituent depends on its hulk. 

The stereoselectivity of these reactions has been interpreted in terms of chair-like six-membered ring transition states in which the substituents a to tin adopt an axial position, possibly because of steric and anomeric effects. The cc-substituted (Z)-isomers are less reactive because the axial preference of the a-substituent would lead to severe 1,3-diaxial interactions17. 

Treatment of benzyl 2-acetamido-3,4-anhydro-2-deoxy-6-C>-trityl-a-D-allopyranoside (412) with tetrabutylammonium fluoride [BU4NF in refluxing (refl.) MeCN, 24 h] gave, under diaxial ring-opening, benzyl 2-acetamido-2,4-dideoxy-4-fluoro-6-f9-trityl-a -D-gulopyranoside (413), in... 

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