Trans diaxial product

Most addition reactions of electrophilic reagents to double-bonds which are known to proceed through an epoxide-like intermediate (i,e. the three-membered ring intermediate 14h and, therefore, must follow the same principle, yielding the trans-diaxial product lj> (7). 

The oxyanion must open the epoxide to give a trans diaxial product 48a by attacking the less hindered end of the epoxide. This is inevitably from the endo face but attack at the other end of the epoxide would have to be from right inside the fold. Being a cis decalin, the product can equilibrate to the equatorial conformer 48b and the arrangement for cyclisation is perfect 51. 

Now the epoxide is opened with HBr to give the only possible trans diaxial product (Chapter 18). The role of the bridge in fixing the conformation of the ring is more important in this stereospecific reaction because the bromide ion is forced to attack from the top face. The alcohol is protected as a silyl ether. 

Electrophilic Addition.—rrans-Diequatorial diols are not readily prepared from olefins in the steroid series. The ring-opening of epoxides generally gives trans-diaxial products, whereas acyloxonium ions (65), formed during the Prevost... 

The opening of cyclohexene epoxides is controlled by the need to get trans diaxial products. This is the reverse of the reaction discussed in Problem 6 and is described in full in the text on pp. 468-70. 

The anion CIO cannot be an electrophile one protonation makes it neutral HOCl but it becomes electrophilic enough to attack an alkene only after a second protonation. Attack of the chlorine end of ClOHj evidendy occurs from underneath the alkene, presumably because the two substituents (Me and AcO) are on the top surface. Attack by water must then be from the opposite (top) face but why does it occur at the less substituted carbon This is a conformational question we must get the trans diaxial product for the correct alignment of orbitals (Chapter 18). 

The question asks for a conformational drawing of the product and indeed that is necessary. The. 3 lactone bridge must be diaxial as that is the only way to reach across and the carboxylate group T.ust attack the iodonium ion from an axial position too. You can regard this as the formation of -. e trans diaxial product as in the opening of a cyclohexene oxide with a nucleophile (pp. 468-70). 

Cyclic sulfates can be prepared from cyclic but not acyclic carbohydrate units by reaction with sulfuryl chloride in pyridine [98] or phenyl chlorosulfate and sodium hydride [99]. The reactivity of cyclic sulfates is quite similar to that of epoxides. Thus, they can be used to form fluoro or azido derivatives of polysaccharides by highly regiospecific approaches, usually, but not always, leading to trans-diaxial products from which the sulfate can be easily removed by mild, acid-catalyzed hydrolysis [100,101]. 

The chair antiperiplanar mode leads initially to the trans diaxial product (2), which typically will equilibrate via chair-chair interconversion with the trans diequatorial conformer (3). The twist boat antiperiplanar initial product (4) will commonly undergo rotation to form the more stable chair conformer (5). Thus both the upper and lower pathways of equation (1) can result in trans diequatorial products. Structures (3) and (5) are enantiomeric these pathways can be distinguished through the use of an appropriate stereochemical marker (substituent). 

Although kinetic control typically favors the trans diaxial product, the trans diequatorial product is commonly thermodynamically preferred. If the epoxide can be regenerated due to reversibility under particular reaction conditions, a small amount of competing twist boat opening can in principle allow eventual product equilibration. An analogy is found in the classical steroid diaxial to diequatorial dibromide interconversion that is believed to occur via reversible formation of a bromonium ion intermediate that slowly drains to the thermodynamically favored product through the kinetically disfavored antiperiplanar twist boat intermediate. ... 

The opening of cyclohexene epoxides is controlled by the need to get the trans diaxial products. To get the right answer we need merely to draw the only possible trans diaxial (i.e with CN and O" diaxial) product from each of these conformationally fixed trans decalins. Cyanide must, of course, open the epoxide with inversion so the OH group in the products is on the same side as the oxygen atom in the original epoxides. 

This is an epoxide of the type we discussed on p. 836 it has no choice but to open to the trans diaxial product. 

Now for the second nitrogen substituent. Bromination of the less electron deficient end of the diene with N-bromoacetamide in the presence of SnBr4 leads to an intermediate brom-onium ion which is opened by the acetamide by-product at the more reactive end adjacent to the alkene, giving a trans diaxial product. 

Addition of chlorine or bromine to cyclohexene and its derivatives gives a trans diaxial product because only axial positions on adjacent atoms of a cyclohexane ring are anti and coplanar. The initial trans diaxial conformation of the product is in equilibrium with the trans diequatorial conformation, and in simple derivatives of cyclohexane, the trans diequatorial conformation is more stable and predominates. Because the original bromonium ion can form on either face of the double bond with equal probability, both trans enantiomers are formed as a racemic mixture. 

Let us consider the bromination of (i )-4-f rf-butylcyclohexene. Recall that in derivatives of cyclohexane in which interconversion between one chair conformation and the other is not possible or is severely restricted, the trans diaxial product is isolated. If a cyclohexane ring contains a bully alkyl group, such as fert-butyl (Section 2.6B), then the molecule exists overwhelmingly in a conformation in which the fert-butyl group is equatorial. Reaction of bromine with enantiomerically pure (R)-4-fert-butylcyclohexene occurs at both faces of the six-membered ring. Because bromine atoms must add in an axial manner, each bromonium ion intermediate reacts with bromide ion to give the same product. In the favored chair conformation of this product, fert-butyl is equatorial, the bromine atoms remain axial, and only a single diastereomer is formed. 

In the rigid trans-decalin system where conformational inversion is prevented, only the trans-diaxial product is formed. The initial attack would be expected to occur from the axial direction to maximize overlap with the tt electrons. The subsequent attack on bromotrichloromethane must be stereoselective and probably occurs from the axial direction to avoid a gauche interaction with the trichloromethyl group. 

---