Octalin

The chiral copper reagent 24 is an effective catalyst not only for intermolecular, hut also for intramolecular Diels-Alder reactions, as shown in the following schemes (Scheme 1.41, 1,42, 1.43). Synthetically useful octalin and decalin skeletons were synthesized in high enantio- and diastereoselectivity. The synthetic utility of this intramolecular Diels-Alder reaction has been demonstrated hy a short total synthesis of isopulo upone [23, 33d]. 

Application of this catalytic process was extended to asymmetric intramolecular Diels-Alder reactions. Synthetically useful intermediates with octalin and decalin skeletons were obtained in high optical purity by use of a catalytic amount of the chiral titanium reagent [45] (Scheme 1.57, Table 1.25). The core part of the mevi-nic acids was enantioselectively synthesized by use of this asymmetric intramolecular reaction [46] (Scheme 1.58). 

The reduction of an aromatic system under controlled conditions is an important source of cycloalkanes. The procedure given here employs a solution of lithium in a mixture of low-boiling amines to accomplish that end and affords a mixture of octalins as product. The mixture may be separated by selective hydroboration (Chapter 4, Section III). 

As previously described, a mixture of and J -octalins can be prepared by the reduction of naphthalene or Tetralin. Another route to this mixture is the dehydration of a mixture of 2-decalol isomers. This latter route has certain advantages in that one can avoid the handling of lithium metal and low-boiling amines. Moreover, 2-decalol is available commercially or can be prepared by the hydrogenation of 2-naphthol (5). In either case a comparable mixture of octalins is obtained, which can be purified by selective hydroboration to give the pure J -octalin (Chapter 4, Section III). 

In judging hindrance, it is useful to view the molecule in its three-dimensional, folded configuration. For instance, 17 can be reduced without undue difficulty, whereas 18 requires extreme conditions (Raney Ni, 2(WC, 200 atm) (7), a difference not expected from planar representations of the molecule. Saturation of A -octalin (17) may largely go through a prior isomerization to A -octa in, despite an unfavorable equilibrium 121). 

Der ungesattigte Ester I wird in Acetonitril/Tetrabutylammoniumacetat an Quecksilber zu Octalin II cyclisiert4 ... 

A concerted four-center cis addition leads to (52) and a trans adduct a trans addition, possibly via protonium species, leads to (53) and a cis adduct a stepwise cationic addition leads to (54) and a mixture of cis and trans adducts. Recent studies by Marshall and Wurth strongly indicate that intermediate (54) is correct. Irradiation of octalin (55) in aqueous /-butyl alcohol (DaO)-xylene results in formation of the equatorially deuterated alcohols (56) and (57) and the equatorially deuterated exocyclic olefin (58) ... 

With octalin (55), rotation to the trans chair form (59) followed by protonation from the outside face of the double bond at the less substituted carbon atom leads to the tertiary cation (60) with net equatorial introduction of the proton. Rotation to a trans boat form would lead to net axial protonation<84) ... 

To add another complication recently, (R)-(—)-10-methyl-A1(-9)-octalin (7) has been prepared and its hydrogenation studied over Pt, Pd, and Rh catalysts.91 Like the apopinenes and the (R)-(—)-4-methylcyclohexene, this R enantiomer may undergo double bond migration to its 5 enantiomer, which... 

Reduction of tetralin to octalin with lithium and ethylenediamine proceeds slowly, but if heated to 85°C it becomes violent, with rapid evolution of hydrogen. 

Because the cis-decalin molecule extends its two methine carbon-hydrogen bonds on the same side in contrast to frans-decalin, the carbon-hydrogen bond dissociation of adsorbed decalin would be advantageous to the cis-isomer on the catalyst surface (Figure 13.17). A possible reaction path by octalin to naphthalene in dehydrogeno-aromatization of decalin will be favored to the cis-isomer, since its alkyl intermediate provides the second hydrogen atom from the methine group to the surface active site easily. 

Tetrasubstituted Alkenes. Tetrasubstituted alkenes lacking electron-withdrawing substituents undergo facile ionic hydrogenation to alkanes in very good yields. Simple examples include 2,3-dimethyl-2-butene,208,214 1,2-dimethyl-cyclopentene, 1,2-dimethylcyclohexene,229 and A9(10)-octalin.126,204,212... 

The reduction of A9(10)-octalin to cis- and /ran.v-decalins occurs with cis to trans stereoselectivities that vary with the nature of the organosilicon hydride employed. The ratios are 0.28-0.59 with n-butylsilane, 0.67 with diethylsilane,204 0.34212 or 0.72204 with triethylsilane, 0.67 with diphenylsilane, 0.77 with diphen-ylmethylsilane,212 1.3 8204—1.80127,212 with triphenylsilane, 0.54 with triisopen-tylsilane, 1.17 with tricyclopentylsilane, 2.57 with tri-.vcc-bulylsilane, 3.35 with di-ferf-butylsilane, 4.88 with di-ferf-butylmethylsilane, and 13.3 with tri-rm-butylsilane.204 Opinions differ about the mechanistic significance of these changes in isomer ratios.204,212... 

The dienaldehyde 117 cyclizes to a mixture of the octalins 120 and 121 on treatment with concentrated orthophosphoric acid. It was suggested that the reaction is initiated by formation of the cation 118, which undergoes ring-closure to the bicyclic cation 119. Proton loss in two alternative ways leads to the products (equation 62)72. 

Treatment of trans,trans-2,6-ociddiene (122) with deuteriated formic acid HCO2D in the presence of deuteriosulphuric acid gave the cyclized formate ester 123. A concerted mechanism (equation 63) was proposed for this reaction73. The stereospecific ring-closure of the 1,4-cyclohexadiene derivative 124 in acetic anhydride/perchloric acid affords the octalin 125, which was isolated as the diacetate 126 (equation 64)74. 

Another example of the formation of an octalin is the conversion of the cyclohexenone 127 into the enol acetate 128 by the action of acetic anhydride and perchloric acid in the presence of acetic acid (equation 65)75. The acid-induced ring-closure of the cyclopentane derivative 129 gives a 85% yield of a mixture of the octahydroazulenes 130 and 131... 

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