Bicyclic compounds, bridged

The relation of rates of reduction with NaBH4 to variations in structure in a wide variety of monocyclic and bridged bicyclic compounds has also been discussed for example, a methyl a to a ketone slows the rate of reduction. Brown ° stated that reactions should not be discussed in terms of axial and equatorial attack, since the rates simply reflect differences in the energies of the possible transition states and not enough is known about the transition state to analyze it. He accepted th concepts of SAC and PDC, but preferred to call them steric strain contrpl and product stability control. ... 

Ozoe Y, Matsumura F. 1986. Structural requirements for bridged bicyclic compounds acting on picrotoxinin receptor. J Agric Food Chem 34 126-134. 

According to Scheme 2.2Ilf, heating of nitrones (476a,b) in toluene gave bridged bicyclic compounds (477a,b), as the major reaction products (Scheme 2.235) (704). 

The first stereocontrolled synthesis of ( )-hirsutic acid (720) was achieved by Trost In this work, four of the seven asymmetric centers are fixed in the correct relative stereochemistry in bridged bicyclic compound 721 which in turn is formed by... 

Only a few of such reactions of these rings have been reported since CHEC-II(1996). However, Weaver and Tennant <1999TL8157, 2000TL9319> have demonstrated that imidazo[4,5-f]isoxazoles such as 19 undergo thermally driven reactions with acetylenic diesters (such as 22) to yield, rather than the expected bridged bicyclic compound 20 or the pyridine Woxide 21, the 2-pyrrol-2-ylimidazole 24 which the authors suggest proceeds via loss of a methyl oxalate unit. The proposed pathway is outlined in Scheme 1. 

In the case of a cyclic conjugated diene, the Diels-Alder reaction yields a bridged bicyclic compound. A bridged bicyclic compound contains two rings that share two nonadjacent carbons. For example, cyclopentadiene reacts with ethylene to produce norbornene. 

There are two possible configurations, endo and exo, for bridged bicyclic compounds resulting from the reaction of a cyclic diene and cyclic dienophile. A substituent on a bridge is endo if it is closer to the longer of the two other bridges, and it is exo if it is closer to the shorter bridge. Most of these reactions result in an endo product. However, if this reaction is reversible, and thermodynamically controlled, the exo product is formed. 

Elimination of CO and C02 from Bridged Bicyclic Compounds s c0-Carbonyl-1 /4/elimination... 

BREDT S RULE A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms. 

This time the spiro-bicyclic diketone (one C atom common to both rings) is preferred to the alternative bridged bicyclic compound because both rings are five-membered. 

A selection of important bridged bicyclic compounds is shown below, with the various ring sizes indicated in black. 

Endo position (Section 16.13D) A position of a substituent on a bridged bicyclic compound in which the substituent is closer to the longer bridge that joins the two carbons common to both rings. 

In bridged bicyclic compounds, a it bond between a bridgehead atom and its neighbor is forbidden due to ring strain unless one of the rings of the bicyclic compound has more than eight or nine atoms (Bredt s rule). Resonance structures in which such a tt bond exists are very poor descriptions of the compound. 

Bredt s rule In small bridged bicyclic compounds, double bonds are impossible at the bridgehead. Small means that the S number must be less than or equal to six. The S number is the sum of the number of atoms in the bridges of a bicyclic system. If the S number is equal to seven, then if the double bond is in a seven-membered ring it cannot be formed it can be formed, however, if it is in an eight-membered ring. 

Chemistry On the basis of the discussion by Matsumura and Ghlasuddin (6), several bridged bicyclic compounds were synthesized (Table III) Cycloaddition of allyl cations to isopropylidenecyclo-pentadiene yielded three epimers of 2,4-dimethyl-8-isopropylidenebicyclo[3.2.1]oct-6-en-3-one, i.e., the equatorial is-2,4-dimethyl [1], axial cis-2,4-dimethy1 [8], and trans-2,4-dimethyl [12] analogs, and two epimers of the 2,4-dibromo analog [7 and 11] Several epoxy and hydroxy analogs were prepared by epoxidation of double bonds and reduction of carbonyl groups, respectively ... 

Figure 5. Structure-activity relationships of bridged bicyclic compounds Reproduced from Ref. (57) (Copyright, 1986 Amer. Chem. Society).

The hydrogenation takes place at the least hindered face of the double bond, and usually opposite to the bridge of bridged bicyclic compounds . ... 

Cossy has shown that strained cyclopropanes and cyclobutanes situated alpha to a carbonyl group open via the ketyl radical anions formed during photolyses in the presence of amines [197, 210]. Moreover, strained ethers such as oxygen bridged bicyclic compounds have also been observed to undergo opening under these conditions, as shown by Eq. 174. 

In addition, several bridged bicyclic compounds have been examined as potential substrates (Fig. 16.5-32). In contrast to the regiodivergent behaviour of the [n.2.0] bicyclic compounds, in these cases, only one lactone product is usually obtained. This high selectivity compares favorably with the chemical Baeyer-Villiger oxidation of compounds of this type, which often afford regiomixtures[U9). In addition, the... 

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