Diequatorial trans product

The hydroboration of 3o -, 3jS-, 6o -, and 6/3-methoxyandrost-4-enes (75)-(78) has been shown to proceed predominantly trans to the MeO group, which parallels the behaviour of the corresponding alcohols. With 6-OMe derivatives (77) and (78), a small amount of Markovnikov hydration, giving 5-alcohols, has been observed. Epoxidation, osmylation, and bromination of 5/3-androst-3-enes (79) have been found to take place from the /3-face in the last reaction, diequatorial dibromide and bromohydrin accompany the axial addition products." ... 

A more striking example of the influence of conformation on the reaction outcome is seen in the nitrous acid deamination of 2-aminocyclohexanols which takes place by rearrangement of a group on the carbinol carbon that is anti to the developing carbocation. The deamination reaction is very fast and the products reflect the population of the chair conformers. The trans isomer exists mainly in the diequatorial conformer thus the only group anti to the amino... 

In 1999, Suzuki completed the synthesis of pradimicinone,63 the aromatic pen-tacyclic aglycone moiety common to both the pradimicin and benanomicin antibiotic classes. The key step in the approach involves a Sml2-mediated pinacol cyclisation of an axially chiral 2,2 -biaryldicarbaldehyde 54 (Scheme 7.24). This cyclisation proceeds to give the trans- 1,2-diol 55 in quantitative yield and with complete transfer of the axial chirality in the starting material to the central chirality of the product, which is obtained in enantiomerically pure form. The selectivity and chiral transfer were attributed to an i e,i e-cyclisation mode, giving the diequatorial product.63... 

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 axial and equatorial methyl and methylene absorptions have essentially equal peak areas. This spectrum is therefore consistent with the product s being the cis isomer, 11a. For the trans isomer one might expect a preference for the conformer with the diequatorial ethyl groups, lib. In this case the axial and equatorial absorptions should not have equal intensities. Such a preference is expected to be small, however, and the NMR spectrum must be regarded as consistent with the trans... 

Leland and Kotick(87) extended their work to include B/C cis and B/C trans series (82) with antagonist N-substituents (CPM and CBM) in place of methyl and with 8-alkyl substituents. The latter substituents were inserted by reaction of intermediates 87 and 88 with lithium methyl cuprate. Lithium ethyl cuprate gave little of the desired product, and it was found more convenient to proceed to 8-ethyl derivatives by hydrogenation of the more amenable 8-vinyl intermediate. Alkyl functions were assigned the diequatorial conformation by reference to studies on 8-alkylmorphinan-6-ones.<88)... 

Oxidation of cyclohexenes. 3-f-BulylcycIohexene (I) is oxidized by lhallium sulfate to the tran.s -diaxial diol (2, 73% yield), the trrtn.f-diequatorial diol (3, 10% yield), and an unidentified product (18 % yield). Oxidation of 4-/-butylcyclohexenc gives exclusively... 

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. ... 

Yamamoto and coworkers have demonstrated intra-molecular examples of the addition of allylic stannanes to imininm ions with good to excellent yields. The incorporation of chirality using (/ )-(+)-1-phenylethylamine provides for facial selectivity in the Lewis acid-promoted ring closure of 193. Trans-tetrahydropyranyl amine 195 is obtained as the major product using a variety of Lewis acids resulting from the synclinal arrangement 194, which leads to diequatorial substitution (Scheme 5.2.43). 

One major difference between cyclic and acyclic systems is the inability of cyclic systems to undergo rotation about carbon-carbon bonds, leading to relatively well-defined conformations. If trans-A-tert-h ity - -bromocyclohexane is used as an example, there is the usual equilibrating population of the two chair conformations (sec. 1.5.D). The diequatorial conformer is significantly more stable and accounts for the major portion of the equilibrium (sec. 1.5.B). According to Corey s calculations from Section 1.5.D, the equatorial form predominates (>99.99 <1, diequatorial/diaxial). To predict the major product, nucleophilic backside attack on both conformations must be considered, and the less sterically hindered pathway is usually preferred. In this case, the diequatorial conformation is more reactive. 

The importance of coplanarity of the eliminating fragments is emphasised by the large difference in the Arrhenius activation energies for elimination from cw-4-r-butylcyclohexanol (E (cis) = 21 kcal.mole" ) and rra/iJ-4-r-butylcyclo-hexanol (Epitrans) = 38 kcal.mole ) . In the trans isomer, elimination must occur from a diequatorial syn-clinal conformation, or a diaxial conformation with the bulky r-butyl substituent in an unfavourable axial conformation, or an initial epimerisation to the cis isomer, or via a boat conformation, all possibilities which involve much higher energy intermediates than the diaxial elimination from the cis isomer. The products of elimination from the four stereoisomers of 1-decalol are also consistent with anti stereospecificity ... 

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