196. Long-range effects in alicyclic systems. Part I. The rates of rearrangement of some steroidal dibromides

Abstract

The rates of mutarotation of some steroidal 5a : 6P-dibromides have beeii investigated. 501 : 6p : 225 : 23f-Tetrabromostigmastane reacts a t only 75% of the rate observed with 5c( : 6P-dibromo-cholestane and -stigmastane. The rate of rearrangement of 5a : 6P-dibromodeoxytigogenin is similar. These anomalies represent long-range effects in systems which are conformationally unambiguous.

Stereoelectronic Effects on Type I 1,2‐Dyotropic Rearrangements in Vicinal Dibromides

“…In the particular case of dyotropic reactions in vicinal dibromides, there are several important papers on the nature of the experimentally observed mutarotations in 1,2‐dibromocyclohexanes4 and 5,6‐dibromocholestanes5, 6 (Scheme ). Thus, in a seminal work Grob and Winstein5 discussed the possible mechanisms of these reactions and concluded that “…︁mutarotation does not involve dissociation of the steroid dibromide to bromonium ion and bromide ion, followed by return of external bromide ion.…”

Verification of stereospecific dyotropic racemisation of enantiopure d and l-1,2-dibromo-1,2-diphenylethane in non-polar media

“…Remarkably, they concluded that the two bromines migrate simultaneously and intramolecularly via an intermediate or TS 3 (typically represented by A) with essentially equivalent Br's having very little charge separation. Reetz 4 named, classified, and defined such "dyotropic" rearrangements 5 as uncatalysed 6 pericyclic processes in which two sigma bonds interchange under orbital symmetry control: dyotropic rearrangements involving the stereospecific exchange of two migrating groups in 1,2-anti conformations necessarily lead to inversion of configuration at both positions.…”

“…Reetz 4 named, classified, and defined such "dyotropic" rearrangements 5 as uncatalysed 6 pericyclic processes in which two sigma bonds interchange under orbital symmetry control: dyotropic rearrangements involving the stereospecific exchange of two migrating groups in 1,2-anti conformations necessarily lead to inversion of configuration at both positions. [3][4][5] The observation of rearrangements of substituted 1,2-dibromocyclohexanes and other alkyl dibromides 7 as well as several computational studies [8][9][10] support the generality of such vicinal dihalide reactions. However, there has been no direct experimental verification of a concerted dyotropic rearrangement of an acyclic 1,2-dibromide.…”

“…We 3 ) spectra in the aromatic region, and by their 100-102 and 240-242 ˚C melting points, respectively. The meso compound is much less soluble than its dlisomer.…”

Reactivity Studies on Natural Products: Iv. Dissociation Constants of the Cyanohydrins of Some Steroid Ketones

“…A double bond, which is exocyclic to these two rings, has less effect and a double bond, one more carbon atom remote and in another ring, has no measurable effect. Bartoil and co-workers (17,18) have recently shown the operation of long-range effects in the mutarotation of steroid 5,6-dibronlides and in the condensation of 3-lceto ti-iterpenes with benzaldehyde and has introducecl the term "conformational transmission" for the transmission of distortioils through a molecule by flexing of valency angles and alteration of atomic co-ordinates. Since no electronic interactions are involved and the effect is purely a steric one, it will manifest itself in an entropy term, and Barton and Head observed no difference in the energy of activation for the dibromides studied, but found small differences in the Irequency factor.…”