Cis-decaline system

The Birch reductions of C C double bonds with alkali metals in liquid ammonia or amines obey other rules than do the catalytic hydrogenations (D. Caine, 1976). In these reactions regio- and stereoselectivities are mainly determined by the stabilities of the intermediate carbanions. If one reduces, for example, the a, -unsaturated decalone below with lithium, a dianion is formed, whereof three different conformations (A), (B), and (C) are conceivable. Conformation (A) is the most stable, because repulsion disfavors the cis-decalin system (B) and in (C) the conjugation of the dianion is interrupted. Thus, protonation yields the trans-decalone system (G. Stork, 1964B). 

Starting from quinic acid, a highly substituted, cis-afi unsaturated nitrile oxide has been synthesized and used in a 1,3-dipolar cycloaddition, to afford a precursor of the cis -decalin system of branimycin (468). 

The first precise evaluation of the anomeric effect was realized by Descotes and co-workers in 1968 (22). These authors have studied the acid catalyzed isomerization of the cis and trans bicyclic acetals 6 and 6 and found that, at equilibrium, the mixture contains 57% ci s and 43% trans at 80°C. The cis isomer is therefore more stable than the trans by 0.17 kcal/mol. The cis isomer 5 has one (stabilizing) anomeric effect whereas the trans isomer 6 has none. Steric interactions in cis acetal 5 were estimated tobel.65 kcal/mol (one gauche form of ri-butane, 0.85 kcal/mol and an OR group axial to cyclohexane, 0.8 kcal/mol). By subtracting an entropy factor (0.42 kcal/ mol at 80°C) caused by the fact that the cis acetal S exists as a mixture of two conformations (cis decalin system), they arrived at a value of 1.4 kcal/mol for the anomeric effect. 

Onocerane triterpenes2 This reagent was used in the first synthesis of the unsym-metrical a,X-onoceradienedione (8) for cyclization of two famesol derivatives, (2) and (6), to A/B-trans, 9/10-cis decalin systems. 

Neirgenicin (136) is an antibiotic isolated from Nocardia argentinemis. In the biosynthetic studies of nargenidn (136) a pentaketide precursor (133) was incorporated into 136 without modification of the stereochemistry and E-olefine via cis decaline system (135). The incorporation of the tetraene precursor (134) indicates that the decaline system (135) would be generated by the enzymatic Diels-... 

Alkenylenones have not been as widely utilized as the corresponding 2- and 3-alkenylenones. The photocycloaddition of various allenes attached to cyclohexenones and cvclopentenones at the 4-position has been reported and two examples are shown. Irradiation of the cyclopentenone 1 resulted in the formation of the straight adduct 2 and product 3 in a 1 1 ratio140. Compound 3 is formed by reaction across the 1,2-double bond of the allene function. Photocycloaddition of the cyclohexenone derivative 4141, as a 3 2 mixture of anti- and, vv -diastereomers at — 70 C, resulted in quantitative cycloaddition of the anti-diastereomer to yield the cis decalin system 5. The syn-diastereomer gave three minor photoproducts which were not characterized. 

A possible mechanism for the cyclization is indicated in A- C. It will be noted that the stereochemistry of the product (CCCXCI) thus formed is that of the natural series of alkaloids at Ci4 This results from the fact that the amino group of CCCXC must be equatorial since its adjacency to a ketone allows it to epimerize to the more stable configuration after its formation. The alkylation reaction can obviously form the new carbon-carbon bond only cis to the nitrogen atom, and the result is a cis decalin system in CCCXCI, even though the precursor (CCCXC) undoubtedly has a trans decalin system. 

Diels-Alder reactions to construct the cis-decalin system. Metathesis of triene 214 in the presence of 5 mol % of 4 in toluene gave the corresponding cw-decalin system 215 in quantitative yields. The authors reported on the ring-closing metathesis of several additional trienes differing only in the stereochemistry of the groups with similar success. 

Various bicyclic and polycyclic compounds are produced by intramolecular reactions] 127]. In the syntheses of the decalin systems 157 [38] and 158 [128], cis ring Junctions are selectively generated. In the formation of 158, allyhc silyl ether remains intact. A bridged bicyclo[3.3. l]nonane ring 159 was constructed... 

The same regioselective and stereospecific reactions are observed in decalin systems. The 3/3-formate 605 is converted into the a-oriented (j-allylpalladium complex 606, and the hydride transfer generates the fra .s-decalin 607, while the cis junction in 610 is generated from the 3tt-formate 608 by attack of the hydride from the /3-side (609). An active catalyst for the reaction is prepared by mixing Pd(OAc)2 and BU3P in a 1 I ratio with this catalyst the reaction proceeds at room temperature. The reaction proceeded in boiling dioxane when a catalyst prepared from Pd(OAc)2 and BujP in a 1 4 ratio was used[390]. 

Tetralin-cis-Decalin (Syz). Co-oxidations of this system were undertaken because the internally consistent results of Russell and Williamson (32) (rt = 1.89 zb 0.01 rd = 0.01 zb 0.002 rtrd = 0.019) indicated an extraordinary difference in selectivity of the two alkylperoxy radicals. Oxidations of cw-Decalin (7% trans by GLPC, from K and K Laboratories, passed over aluminum before use) and Tetralin were carried out in sealed tubes and analyzed as described for styrene-Tetralin. 

The cis bicyclic chloroamine 313 reacted at a faster rate than the corresponding decalin system 313, (NCH3 replaced by CHj) and gave exclusively the cyclohexene iminium salt 314 while the chloroamine 315 reacted to give a mixture of other products. 

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 frans-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. 

Just as in the decalin system, each ring junction could be cis or trans, but it turns out that all steroids have all trans-junctions except where rings A and B join which is sometimes cis. Examples are cholestanol (all trans) and coprostanol (A and B fused cis). 

Cook has developed in recent years a tandem PKR towards the synthesis of pentalenes, that is, linear [5.5.5.5] systems. Their first studies used suitable diendiynes 141-143 that gave the corresponding tetracyclic bis-cyclopentenones. These were described as precursors of dicyclopenta[a, d] (144) and [a, e] pentalenes (145). The same methodology allowed the synthesis of [5.6.6.5] systems (146) where the central decaline system was cis fused (Scheme 43) [150-152]. This group succeeded in tuning up efficient photochemical catalytic conditions for these reactions [153]. 

Discussion of the conformation and relative stabilities of bicyclic diacetals with cis ring junctions requires consideration of the possibilities of H-inside and 0-inside conformations of the flexible ring system (XVIII and XIX) as well as of axial and equatorial positions of residues. One possibility that may be excluded is an inside axial position for a substituent of the type XVII shown for cis-decalin, as this would be extremely unstable compared to other conformations. 

In the deprotonation step, the axial proton will be lost, though this doesn t affect the conformation of the molecule. In the elimination step the OH group must be axial so that the C-C bond is parallel to the p orbitals in the n system. The molecule is a cis-decalin so it has a choice o conformations. Don t be disheartened if you got the wrong conformation first time - we took several tries before getting it right. 

Hydroxylation of alkanes preferentially occurs at the more nucleophilic tertiary C—H positions, but some of these systems using 2-mercaptoethanoF or metallic iron powder d hydroxylate adamantane with preference for the secondary position. However, none of these systems hydroxylates cis-decalin with retention of configuration at the hydroxylated atom. ... 

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