Bridged polycycles

Intramolecular reactions of electron donor and acceptor sites in cyclic starting materials produce spirocyclic, fused, or bridged polycyclic compounds. 

Intramolecular reactions between donor and acceptor centres in fused ring systems provide a general route to bridged polycyclic systems. The cts-decalone mesylate given below contains two d -centres adjacent to the carbonyl function and one a -centre. Treatment of this compound with base leads to reversible enolate formation, and the C-3 carbanion substitutes the mesylate on C-7 (J. Gauthier, 1967 A. Belanger, 1968). 

Scheme 3.68. Cyclization/ring expansion for the construction of bridged polycyclic systems.

Chupakhin and co-workers <1999IZV1573> reported the formation of a bridged polycycle starting from tetra-zolo[ 1.5-//]pyrimidine 1 (Scheme 12) this compound was treated with resorcinol in the presence of boron trifluoride etherate at room temperature to yield a 1,3-addition product on the pyrimidine ring 63 with participation of one... 

Intramolecular Heck reactions.6 Heck intramolecular coupling of alkenyl or aryl iodides substituted by 3-cycloalkenyl group is an attractive route to fused, spiro, and bridged polycyclic products. Coupling is achieved with a Pd-phosphine catalyst such as Pd[P(QH5),]4 in combination with a base, N(C2H5)3 or NaOAc. The coupling tends to produce a mixture of two isomeric alkenes, in which the newly formed bond is allylic or homoallylic to the ring juncture. 

These transformations were applied to develop a new promising method for synthesis of various bridged polycyclic systems66, viz. ketones 160 and 161. Tropone reacts with butenyl magnesium bromide (—78 °C, 75%) to form a mixture of 2-(3-butenyl)dihydrotropones 158 and 159, the pyrolysis of which (200-210 °C, neat or in heptane solution) leads to 60% total yield of the isomeric homoprotoadamantenones 160 and 161 and the tricyclic ketone 162 in a ratio of 58 18 24, respectively (equation 49)66. 

Finally, bridged polycyclic systems containing heteroatoms should be menctioned and they are treated in the next chapter, as an appendix to Corey s rules for the selection of "strategic bonds" (see 7.2.2). 

Curran s retrosynthetic analysis of fused and bridged polycyclic systems through homolytic disconnections... 

We have already referred to the retrosynthetic analysis of dissonant open chain molecules (see Heading 5.5). In this chapter we will deal with Curran s ideas in connection with fused and bridged polycyclic systems present in many natural products. Emphasis on cyclisations leading to 5-membered rings is maintained because ... 

Twistane, as an example of a non-natural product, which is a chiral, bridged polycyclic compound, completely deprived of functionality. 

As it is a bridged polycyclic system, twistane has "strategic bonds" (bonds 11 and 12 in the conventional numbering adopted in structure l), the disconnection of which leads to rather simple intermediate precursors. 

Reaction 7 —> 9 is general for synthesis of bridgehead azides of bridged polycyclic hydrocarbons. Another route which was studied as a general synthesis is shown in Fig. 2.14. 

Head-to-tail intramolecular dimerization of nonactivated vinyl groups can also occur in compounds in which the tether is only two atoms long under conditions of flash-vacuum pyrolysis (FVP). Under such conditions strained bridged polycyclic derivatives can be prepared.78 The formation of 18 is the result of intramolecular head-to-tail cycloaddition of the Cope rearrangement intermediate derived from 1,2-divinylacenaphthene.78... 

Electron transfer may also be important in a series of photosolvolysis/rearrangement of certain bridged polycyclic compounds. The efficiency of photochemically induced solvolyses (186), for example eq. 60,... 

A reaction between a-diazo ketones and a,(1-unsaturated aldehydes under rhodium(ll)-catalysis provides a route to epoxy-bridged tetrahydropyran-4-ones <2002JOC8019>. This methodology allows entry to functionalized spiro-dioxa-bridged polycyclic frameworks <2002TL3931, 2003T8117>. 

Compounds in which ring heteroatoms are members of another fused ring and bridged polycyclic compounds are, with a few exceptions, not covered in this chapter. 

These may be classified into four chemical groups Chlorinated hydrocarbons which include dico-phane (DDT) and its analogues (e.g. dicofol and methoxychlor), hexachlorocyclohexMie isomers (e.g. lindane), and bridged polycyclic chlorinated compounds (e.g. chlordane) ... 

Chlorinated Hydrocarbons These insecticides, e.g. dicophane and its analogues, isomers of hexachlorocyclohexane, and bridged polycyclic chlorinated compounds, may be chromatographed satisfactorily without prior deri-vatisation. A typical separation is shown in Fig. 1, using the dual detection system. Although greater... 

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