Decalin stereoisomers

Feed Catalyst Temp. (°C) Partially converted product, composition (%) Decalin stereoisomers, distribution (%) ... 

Among the most important of the bicyclic hydrocarbons are the two stereoisomeric bicyclo[4.4.0]decanes, called cis- and rrani-decalin. The hydrogen atoms at the ring junctions are on the same side in c/5-decalin and on opposite sides in tmns-AecdXm. Both rings adopt the chair confonnation in each stereoisomer. 

Decalin unit 121, an intermediate in the total synthesis of compactin, has been prepared by intramolecular cycloaddition reaction [117] of trienone-carboxylic acid 122 carried out under either thermal conditions or microwave irradiation. The desired cxo-adduct 123 was the major stereoisomer (Equation 2.34). Similar results were observed in the cycloadditions of the corresponding esters. 

In another similar approach (40, 72) Wrobel and Gol biewski obtained the methyl ether (150) in the same way. The Ullmann condensation with methyl 4-hydroxycinnamate afforded a mixture of stereoisomers 160 and 161. Catalytic reduction of the trans-fused quinolizidone (160) gave a mixture of axial and equatorial epimers (162 and 163) in 4 1 ratio. Hydrolysis of the axial hydroxyester (162) followed by lactonization with thionyl chloride in chloroform yielded racemic decaline. 

Decalin has two saturated six-membered rings fused together that is, the rings share a common bond. The hydrogens at the ring junctions may be attached on the same side or on opposite sides, so there are two stereoisomers civ-decal in and /remv-decalin. Neither of these is chiral. 

In substituted irans-decalins, one of a pair of epimeric substitution products is necessarily axial and the other equatorial. In cfs-decalins, the possibility of inversion of the ring conformation permits an equatorial arrangement for each of a pair of epimers. If two or more substituents are present in m-decalin, there will be at least one axial group in some of the stereoisomers, and it then is necessary, when selecting the most stable conformation, to avoid conformations that show the effect seen in XVII, where an axial group is inside the L-shape and very close to three axial hydrogen atoms. 

This stereoelectronic effect may explain other cases of different mass spectra of stereoisomers. The more pronounced loss of axial versus equatorial methyl groups in heteroatom-containing decalin systems, which has been attributed... 

More common is use of intramolecular Heck reactions to fashion ckv-decalins. Several examples are found in our current program aimed at the total synthesis of complex cardenolides [14,15]. In a model study for a proposed synthesis of (-t-)-ouabagenin (48), trienyl triflate 45 was converted to pentacycle 47 in good yield (Scheme 6-8) [16]. The stereochemical outcome of this cyclization was rationalized by the preference for an eclipsed orientation of the two reacting bonds, which is readily achieved in a cyclization geometry (46), and led to the cis stereoisomer. This example also highlights the stability of... 

Note that trans- and cw-decalin are stereoisomers and not conformational isomers cw-Decalin is less stable than rran -decalin by 2.7 kcal/mol. 

Stereoisomers of the trans-decahydro-quinoline (228) (273) and the trans-decalin (229) (88) displayed extremely low orders of muscarinic effect, with the 2,3-ira/is-diaxial isomer of (229) being the most potent of the four stereoisomers of this structure (0.06%the potency of acetylcholine). 

Simple rotations of groups about carbon—carbon bonds do not interconvert cis- and tw r-decalins. They are stereoisomers and they have different physical properties. 

Solid-phase techniques are also used for synthesis of compounds with trans-decalin motif where the Robinson annulation is used for synthesis of some natural product inspired structures such as that of 75. The protocol uses an immobilized solid phase bound Nazarov reagent that reacts with the enamine of the starting ketone under basic conditions. After cyclization the product is released from the solid phase by using TBAF at room temperature overnight. For the products 75a-c only one stereoisomer was formed (de > 98%) with modest 22-38% yields. 

Let us look more closely at trans-decalin, by far the more common stereoisomer of decalin. An important feature of this bicycloalkane is that each ring is locked into one chair conformation neither ring can invert to its alternative chair. This means, for example, that if an —OH group is equatorial in a decalinol (a decalin alcohol), it remains equatorial it cannot become axial because the cyclohexane ring is locked into this one conformation. Likewise, if an —OH group is axial, it remains axial. 

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