In bicyclic systems

The thiones are readily desulfurized with Raney nickel to give the corresponding unsubstituted compounds in bicyclic systems in the 2-, 4- and 7-positions, and in tricyclic systems such as (95). The 2-methylthio derivatives may be similarly desulfurized. Thione groups in the 4-position, but not the 2-position, in pyrido-[2,3- f ]- and -[3,2- f]-pyrimidines may be replaced directly with ammonia or amines. 

In bicyclic systems, bridgehead hydrogens are most resistant to replacement by fluorine Cobalt trifluonde converts l//-nonafluorobicyclo[3.2.0]hept-6-ene to... 

The exo reactivity of 2-norbomanone 25 in nucleophilic addition (such as reduction with hydride) is a classical example of the facial selectivity of carbonyl groups in bicyclic systems [80]. 

Reductive cleavage of the N-N bond in bicyclic systems containing a ring junction hydrazine subunit presents a facile route to medium-sized heterocycles containing two or more nitrogen atoms and the subject was duly covered in CHEC-II(1996) <1996CHEC-II(8)747>. No substantial progress has been reported since its publication. 

The reaction is an important one, especially in bicyclic system where it leads to ring expansion... 

The double )5-scission pathway becomes dominant in bicyclic systems (Equations (7)-(9) and Scheme 13). Thus, cyclopentene ozonide (69) gives cyclopropane (Equation (7)) <68TL329l>. Photolysis of the ozonide derived from 1,4-benzodioxins (70) provides a method for the preparation of labile o-benzoquinones (71) (Scheme 13) <87JOC56l6>. Photolysis can also provide a route to unstable compounds and transient species such as the aziridine-2,3-dione (72) (Equation (8)), identified at 77 K using infrared spectroscopy <80JA6902>. Relatively unstable azacarbapenems (73) have been prepared by photolysis of tricyclic compounds containing a cyclobutene ozonide (Equation (9)). On silica, the 1,2,4-trithiolane (74) underwent photo-equilibration (Equation (10)) with the 1,3-dithetane (75) and sulfur. 

For a review of rearrangements in bicyclic systems, see Hogevcen van Kruchlen Top. Curr. Chem. 1979, 80, 89-124. For reviews concerning caranes and pinancs see, respectively, Arbuzov Isaeva Russ. Chem. Rev. 1976, 45, 673-683 Banthorpe Whittaker Q. Rev. Chem. Soc. 1966, 20. 373-387. 

Similar reactions have been reported in bicyclic systems where the intramolecular reaction of an alcohol with a simple alkene leads to relief of ground state strain5. 

The use of diimide to selectively reduce the C=C bond in bicyclic systems has received great attention as an efficient route to prostaglandin endoperoxides [86b]. It can be employed also at low temperatures and photooxygenation at —78 °C followed by diimide reduction at the same temperature has been used to trap unstable singlet oxygen adducts [73b,86d],... 

The intramolecular hydrogen abstraction has been largely developed for the case of the nitro group [22,37], and is well known for some derivatives of o-nitrotoluene, for o-nitrobenzyl alcohols, ethers and esters [38-40] as well as on the acetals of o-nitrobenzaldehyde [41], With these compounds, the nn (singlet or triplet) state abstracts a hydrogen and the biradical relaxes to the aci-nitro form (the first intermediate is actually observed in bicyclic systems where formation of the nitronic acid is sterically hindered) [41], which under basic conditions dissociates to the corresponding anion [42],... 

The impossibility of planar bridgehead carbons means that double bonds can never be formed to bridgehead carbons in bicyclic systems. This principle is known as Bredt s rule , but, as with all rules, it is much more... 

It was shown that an enol intermediate was initially formed in the decarboxylation of l -ketoacids and presumably in the decarboxylation of malonic acids. It was found that the rate of decarboxylation of a,a-dimethylacetoacetic acid equalled the rate of disappearance of added bromine or iodine. Yet the reaction was zero order in the halogen . Qualitative rate studies in bicyclic systems support the need for orbital overlap in the transition state between the developing p-orbital on the carbon atom bearing the carboxyl group and the p-orbital on the i -carbonyl carbon atom . It was also demonstrated that the keto, not the enol form, of p ketoacids is responsible for decarboxylation of the free acids from the observa-tion that the rate of decarboxylation of a,a-dimethylacetoacetic acid k cid = 12.1 xlO sec ) is greater than that of acetoacetic acid (fcacw = 2.68x10 sec ) in water at 18 °C. Enolization is not possible for the former acid, but is permissible for the latter. Presumably this conclusion can be extended to malonic acids. 

As was pointed out earlier, rearrangements in bicyclic systems of the norbomyl type are extraordinarily facile4, involving cationic intermediates1. If a-bridged species are not involved, the barriers for such interconversions must be very low. 

The less highly substituted bond of a siloxycyclopropane is quantitatively opened by mercury(II) acetate to afford -mercurio ketones. In the same pot these are transformed to a-methylene ketones in virtually quantitative yield on treatment with one equivalent of palladium(II) chloride in the presence of lithium chloride and lithium carbonate (2 equiv each). Catalytic amounts of palladium(II) chloride (0.1 equiv) are sufficient in the second step, if two equivalents of copper(II) chloride is added as an oxidant. Mechanistically, the second step involves trans-metalation to a j -palladio ketone followed by /i-hydride elimination. In bicyclic systems it is sometimes necessary to add triethylamine to avoid HPdCl induced double-bond shifts in the reaction product. Examples are the rearrangements of 18, 20 and 22. ... 

Twist, Bend, and Length of the Olefinic Double Bond in Bicyclic Systems... 

The conformational limitations that are imposed upon phosphorus in bicyclic systems are illustrated by the surprising conversion of (153) into (154), in which attack occurs at the methyl rather than benzylic carbon, owing to the inability of the benzylic group to become apical in the trigonal-bipyramidal intermediate. 

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