Decalin compounds, trans

Smissman and coworkers prepared a large number of trans-decalin compounds (82-90). The trans-decalin system is reasonably rigid and may be expected to provide nearly ideal dihedral angles however, the very large number of extra atoms makes these compounds highly lipophilic and creates a great deal of steric bulk. 

Complete hydrogenation of naphthalene in conventional processes produces mixtures of cis- and trans-decalin. The motivation for selective naphthalene hydrogenation comes from our accidental finding on zeolite-catalyzed isomerization of cis-decalin and from the need to tailor the formation of desired isomers from two-ring compounds. In our previous studies on naphthalene hydrogenation, certain catalysts show higher selectivity towards cis-decalin or trans-decalin. More recently, we found that zeolite-supported catalysts selectively promote the formation of cis-decalin or trans-decalin [Schmitz et al., 1996], as shown in Scheme 10. 

Although most attention has focussed on a cationic mechanism in the oxidative cyclization of squalene [20]. Breslow was concerned with the possibility that nature utilizes a free-radical pathway [21]. and studied the addition of benzoyloxy radical to trans, trani-famesyl acetate [22]. The benzoyloxy radicals generated by CuCl-catalyzed thermal decomposition and copper(II) benzoate was added to provide a termination mechanism. Excluding any intervention of a carbocationic process, Breslow obtained a tran -decalin compound (20 30% yield) bearing an exomethylene moiety. As pointed out by Breslow, despite a limited biochemical interest , this work evidenced a new synthetic reaction of considerable potential . An application shortly followed with the first example of a triple cyclization by Julia [23]. Triene isomers 40 were treated by benzoylperoxide in benzene and alforded after saponification alcohol 41 in 12% yield as a single diastereomer (relative stereochemistry confirmed by an X-ray analysis) with a similar tra 5-decalin system (A and B rings. Scheme 14). 

The relationship between c/s-decalin and trans-decalin is ste-reoisomeric. These two compounds are not interconvertable by ring flipping. 

It was also suggested that the relationship of the two forms (A) and (B) might be that of stereoisomerides of the cis-trans decalin type and a good deal of work was done to provide experimental evidence for this view or the alternative that they are structural isomerides. It was also found that of the methods used to synthesise the pyridocoline system all but one gave norlupinane (A) on reduetion, the exceptional formation of uorlupinane (B) being limited to reduction of the 1-keto compound by the Clemmensen method. 

This dry ozonation procedure is a general method for hydrox-ylation of tertiary carbon atoms in saturated compounds (Table 1). The substitution reaction occurs with predominant retention of configuration. Thus cis-decalin gives the cis-l-decalol, whereas cis- and frans-l,4-dimethylcyclohexane afford cis- and trans-1,4-dimethylcyclohexanol, respectively. The amount of epimeric alcohol formed in these ozonation reactions is usually less than 1%. The tertiary alcohols may be further oxidized to diols by repeating the ozonation however, the yields in these reactions are poorer. For instance, 1-adamantanol is oxidized to 1,3-adamantane-diol in 43% yield. Secondary alcohols are converted to the corresponding ketone. This method has been employed for the hydroxylation of tertiary positions in saturated acetates and bromides. 

Among the fused ring systems, the most important compound is decalin. This compound exists as the cis and trans forms and the existence of these two forms on the basis of non-planar structures was predicted by Mohr. He thought that the two decalins were formed by the fusion of two chair forms or two boat forms of cyclohexane and so he assigned, the following structures. 

The opening of the epoxide in the cij-decalin 24 by acetic acid leads exclusively to the hydroxyacetate 25 (through a kinetically controlled rrani-diaxial opening) rather than to the wanted diastereomer 26 (c/ the stereochemistry of the "southern" part of reserpine). To obtain the correct diastereomer the epoxy-lactone 22 is first formed (Scheme 8.6). Thus the conformation of the cij-decalin system, and therefore that of the substituents, is reversed. The kinetic tran -diaxial opening of the epoxide occurs in a regio- and stereoselective manner to afford compound 28 in which the substituents have the correct position and configuration (a-OH, P-OAc),... 

The second group of compounds are the oxa-decalins, which can exist in cis-39 and trans-39 anellation (Scheme 21). 2,4-Di-substituted 1,3-dioxa-decalins exist as a mixture of cis- isomers, the oxygen in O-in conformation... 

The nitration of chiral alkanes producing racemic nitro compounds, and that of cis-and frans-decalin, both leading to the formation of trans-9-nitrodecalin,170 support the involvement of alkyl radicals. 

The nearly equal amount of cis and trans products formed from 1,5-dimethylcyclo-hexene is explained by the almost equal degree of hindrance of the homoallylic methyl group with the catalyst surface in the alternate adsorption modes.63 64 Another interesting example is the platinum-catalyzed hydrogenation of isomeric octalins.65-67 If syn addition to the double bond is assumed, in principle, both cis- and mms-decalin are expected to result from l(9)-octalin, but only the cis isomer from 9(10)-octalin. In contrast with these expectations, the isomers are produced in nearly the same ratio from both compounds. Transformation in the presence of deuterium revealed that most of the products contained three deuterium atoms. This was interpreted to prove that the very slow rate of hydrogenation of 9(10)-octalin [Eq. (11.9)] permits its isomerization to the 1(9) isomer. As a result, most of the products are formed through l(9)-octalin [Eq. (11.10)] ... 

Sterically induced CH bond polarizations are also reflected by the shieldings of carbon atoms in crs-decalin relative to its trans isomer [230]. In the 9-methyl derivatives, steric repulsion between the methyl- and the syn-axial 2,4-hydrogens shields the attached carbon atoms relative to the parent frans-decalin. In the cis isomer, shielding of the bridgehead carbons relative to those in the trans compound clearly reflects steric interaction of hydrogen atoms in the 2,4- und 6,8-positions. 

At this point, it probably will be helpful to construct models of cis- and trans-decalins to appreciate the following (a) The two compounds cannot interconvert unless C-C or C-H bonds first are broken, (b) traw -Decalin is a relatively rigid system and, unlike cyclohexane, the two rings cannot flip from one chair form to another. Accordingly, the orientation of the substituent is fixed in the chair-chair conformation of trans-decalm, (c) The chair-chair forms of cw-decalin are relatively flexible, and inversion of both rings at once occurs fairly easily (the barrier to inversion is about 14 kcal mole-1). A substituent therefore can interconvert between axial and equatorial conformations (Figure 12-24). 

Four model compounds, n-undecane, tetralin, cis/trans decalin and mesitylene, and a natural gas condensate from the North Sea were also cracked. Analyses and the reference code key of the coal-based feedstocks and the gas condensate are given in Table 1. Paraffin, naphthene, and aromatic-type analyses were calculated from gas chromatographic analyses of the partially hydrogenated anthracene oil and gas condensate whereas, mass spectrometric analysis was performed on the two coal extract hydrogenates and their further hydrogenated products. 

Six alkaloids belong to this group decaline (49), demethyldecaline (50), vertaline (51), demethylvertaline (52), lagerine (53), and methyllagerine (54). In the first two alkaloids the quinolizidine ring is trans-fused and in the latter four it is cis-fused. These compounds are listed in Table III (7, 2, 77, 25, 36, 41, 42). Decaline and vertaline were isolated by Ferris from Decodon verticillatus (2). 

This sequence serves to exemplify the formation and aspects of reactivity of toluene-p-sulphonate esters in monosaccharide systems, and further to illustrate the selective protection afforded to hydroxyl groups by the formation of cyclic acetals by reaction with carbonyl compounds. Thus reaction of methyl a-D-glucopyranoside (26) with benzaldehyde in the presence of zinc chloride gives the 4,6-acetal (27) (Expt 5.118), wherein two fused six-membered rings of the trans-decalin type are present. As a cognate preparation the reaction of benzaldehyde with methyl a-D-galactopyranoside results in a similar conversion to a 4,6-acetal, but in this case the product is the conformationally flexible system of the cis-decalin type, the most likely conformation being that shown below. 

Notice that wc have confidently assigned the configuration of these compounds without ever being able to see the yellow proton at the ring junction. Since nitrogen can invert rapidly, we know that this decalin-like structure will adopt the more stable trans arrangement at the ring junction. 

Compounds of this type with a trans ring junction present no problems. The bicyclic system is comparable to axial positions at every point of substitution. The substituents at acetal carbon atoms will be equatorial. 

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