Diaxial bromo derivative

Bromine addition (Bfj or NBS) to A -S-keto-steroids afforded exclusively diaxial bromo-derivatives (6/5, 7a), which were converted with base into the 4-bromo-A " -3-keto-analogues. °... 

A -Benzylepimine 160 reacted with a mixture of ammonium and tetrabutyl-ammonium bromide or iodide to give the respective diequatorial bromo (193) or iodo (194) derivatives as the sole products. This formation is the result of an equilibrium between the starting epimine and the diaxial halo derivative, which is formed initially by aziridine-ring cleavage. This equilibrium could be set up readily because of the instability of the diaxial halo derivatives and their tendency to undergo back-cyclization of the aziridine ring. To prove the existence of the equilibrium, the diaxial bromo derivative 191 was prepared by the action... 

All cleavage reactions of the epimines having the D-manno configuration afforded solely the diaxial isomers of the products, except for the reaction of N-benzyl derivative 199 with a mixture of Bu4NBr and NH4Br in toluene, wherein which 2-bromo derivative 202 of the opposite configuration on carbon C-2 was... 

All reactions mentioned in Table XII produced only diaxial isomers of the cleavage products, except for the reaction of V-benzylepimine 163 with a Bu4NBr + NH4Br mixture, which gave the diequatorial bromo derivative 308. Again, its formation is evidently the result of an equilibrium existing between diaxial isomer and the epimine, as already reported for the 2,3-D-allo epimine... 

The experimentally observed preference for the diaxial conformation both in the chloro and bromo derivatives (AG° = +0.3 kcal mol"i) shows the influence of a conformational effect that stabilizes trans-ax, or destabilizes trans-eq, in Scheme 3.11, and which probably originates from the favourable antiparallel orientation of the bond dipoles in the diaxial arrangement [30]. 

The addition of A,V-dibromobenzencsulfonamidc to hexose-derived glycals afforded, after reduction of the N —Br bond of the initial adducts, a mixture of diastereomeric 2-bromo-l-phenylsulfonylamino derivatives913 b. In the case of tri-O-benzyl-D-glucal the diastereoselectiv-ity in favor of the diaxial adduct 7 was improved by the alternative procedure using iodonium di-.cyw-collidine perchlorate and benzenesulfonamide (see Section 7.2.6.1.4.). 

The relative stabilities of axial and equatorial substituents are revealed in an interesting set of data for equilibration of the bromo-substituents in 5a,6j5-dibromocholestane 3) and its 3 -substituted derivative (5) [40]. In the dibromide (3) not only do the bromine atoms occupy axial positions and so suffer considerable steric compression, but the structure is further destabilised by the syw-diaxial interaction between the axial bromine at C(6> and the angular methyl group at C(io). A solution of this dibromide exhibits mutarotation and... 

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