Transoid stereochemistry

Standard depictions for steroids, such as those in Scheme 1, overlook the three-dimensional nature of the molecule. The structures in Scheme 2 represent the stereochemical arrangement implicit in the more customary formulas. The junction between rings B and C and also that between rings C and D involve transoid stereochemistry. It is of note in passing that the latter in fact comprises a disfavored trans hydrindane fusion. The ring junction between rings A and B is also transoid but can in selected cases assume a cisoid configuration (2-2). Note further that the stereochemical... 

Subsequent work established that the rearrangement occurred via a transoid stereochemistry, a geometric arrangement that permitted an allowed lirs - - 27ns -h 2os reaction (Scheme 9.45). ... 

An example of the use of NMR spectroscopy to ascertain with reasonable certainty the stereochemistry of a series of enamines has been provided by Paquette (25). Based on a study of the NMR spectra of the endo- and exo-5-norbornene-2-carboxaldehydes (168), the enamine mixtures were estimated to contain 80 to 90 % of the transoid form (170). 

The use of UV-VIS spectra to analyse dienes and polyenes was historically the first method of choice. The spectra of isolated non-conjugated polyenes is actually the superposition of the spectrum of each one of the double bonds. For each double bond the spectrum depends on the various substituents and also on its location in the molecule. It also depends on the stereochemistry, since conjugated double bonds have either E or Z configuration around each jr-bond but also a cisoid and transoid conformer3 around the single bond marked as s-cis and s-trans4. 

The hydroxyl in (42-6) is then acylated with p-toluenesulfonyl chloride exposure of this to a base leads to elimination to form the 9,11 olehn (43-1). It should be noted that the hydroxyl group in the hrst-obtained fermentation product is equatorial and would eliminate only with great difficulty as it lacks a transoid proton at the adjacent position. Reaction of (43-1) with A -bromosuccinimide in an aqueous base leads to the addition of the elements of hypobromous acid. The stereochemistry of the reaction... 

The known preference for transoid elimination of the elements of water from alcohols such as (6-3) controls the stereochemistry of the product. The arrangement in the starting material of the groups about the incipient olefin actually determines the steric identity of the product. The two rotamers of alcohol (6-3) that have the trans hydrogen and hydroxyl shown as their Newman projections (6-3a) and (6-3b) are equally probable since they differ only in the placement of the remote basic ether. The dehydration in fact gives a mixmre of the trans isomer (7-2) and the cis isomer (7-3) presumably from rotamers (6-3a) and (6-3b), respectively. Reaction... 

Although the relative stereochemistry of the B,C ring fusion of compound II was not discussed in the original paper, it is always instructive to treat this point in cycloaddition reactions, and some valid predictions may be advanced on theoretical grounds only. Assuming that a transoid C=C bond is produced during the destruction of the cyclobutane fragment, the only model of inter-... 

Polish chemists have continued to study the stereochemistry of sparteine and its derivatives in relation to the all-chair conformation (15) ( cisoidal ) and the chair,chair, boat, chair conformation (16) ( transoidal ). Infrared studies indicated that the hydrate of 10-oxosparteine hydrochloride is in the transoidal conformation, cf. (16), whereas the corresponding acid salts of 15-... 

Figure 12.16. Dependence of allylic coupling constants on stereochemistry. Solid lines Transoid (Jbx) dashed lines Cisoid Ja - 0 is the... 

The reaction of benzylamine with 2,3-acetonitrile solvent was examined in the presence of one molar equivalent of siivernitrate no acid catalyst was added. The reaction took a different course Instead of forming SI, 54, or 55, the dioxane ring remained unopened and a 9,l0-diaza-l,4,5,8-tetraoxaperhydroanthracene denvative (61a) was obtained (42% yield). This reaction was also observed with pyridylmethylamine, in the absence of either silver nitrate or add catalyst, to yield 61b (67% yield). The structures 61a,b are in agreement with their spectral data. An x-ray crystal structure determination established the structure of 61a. Both 61 and the related 49 exhibit cis-transoid-cis ring stereochemistry. 

The chiral oxazolidinesulfonium salt with both cis and tram configurations 194 was demonstrated to be a good or excellent chiral auxiliary for the diastereoselective synthesis of hd 5-epoxyamides 196, which afforded epoxy alcohols 197. Interestingly, the diastereoselective outcome depended mainly on the stereochemistry of the C-4 position of the oxazolidine moiety. This diastereoselection can be rationalized by considering the transoid conformer 195, as the attack of the aldehyde from its opposite si face is hindered by the phenyl group. The reaction carried out with 4-(S)-194 afforded ent-197 (13EJ05561). 

Steric reasons direct the bromine into the less hindered trans situation in the haloketone 136. Due to the necessary coaxial alignment of the orbitals in the transition state of displacement of bromine in 757, the carboxyl group is in a transoid situation. Thus, the stereochemistry in the haloketone 136 is preserved in the final product 91. The same argument explains the cis-product later on in Reaction scheme 89. 

Recently, a few heterogeneous Rh catalysts have been reported. Kopaczynska et demonstrated that rhodium nanoparticles stabilized by polyvinylpyrrolidone exhibit catalytic activity in the polymerization of PA. The stereochemistry of the polymer produced with this catalyst is purely cis-transoidal. The progress in polymerization can be monitored by atomic force miaoscopy (AFM) and transmission electron microscopy (TEM). This report includes the first detection of a spectacular helical poly(PA) using AFM imaging. Son and co-workers reported that the nanopartides composed of the (benzoquinone)Rh(cod) complex and aluminum compounds catalyze the polymerization of PA. The catalyst nanopartides can he recovered hy centrifugation, and the recovered nanopartides show almost the same artivity. 

There is also the added possibility of head-to-head rather than head-to-tail arrangements. For the free-radical polymerization of both isoprene and chloroprene it is found that the trans-1,4 structure predominates. The tendency for most simple diene monomers to form the trans structures may be because it is thought that the monomers exist mainly in transoid conformations and that this stereochemistry is retained during polymerization. 

---