Structure bicyclic

A synthesis of ( )-cocaine proceeded through an initial cycloaddition of (526) to (527) to produce the bicyclic structure (528) (78JA3638). 

For bicyclic structures the von Baeyer name consists of the prefix bicyclo-, followed in square brackets by the numbers of carbon atoms separating the bridgeheads on the three possible routes from one bridgehead to the other, followed in turn by the name of the alkane (or other homogeneous hydride, or repeating unit hydride) containing the same number of atoms in the chain as the whole bicyclic skeleton (examples 55-57). Replacement nomenclature can be applied to hydrocarbon names (example 58). 

In addition to the intermolecular Paterno-Buchi reaction, the intramolecular variant has also been studied the latter allows for the construction of bicyclic structures in one step. For example the diketone 8 reacts quantitatively to the bicyclic ketone 9 ... 

The classic method for controlling stereochemistry is to perform reactions on cyclic substrates. A rather lengthy but nonetheless efficient example in the prostaglandin field uses bicyclic structures for this purpose. Bisacetic acid derivative S is available in five steps from Diels-Alder reaction of trans-piperylene and maleic anhydride followed by side-chain homologation. Bromolactonization locks the molecule as bicyclic intermediate Esterification, reductive dehalogen-... 

Another stereochemical feature of the Diels-Alder reaction is that the diene and dienophile partners orient so that the endo product, rather than the alternative exo product, is formed. The words endo and exo are used to indicate relative stereochemistry when referring to bicyclic structures like substituted norbornanes (Section 4.9). A substituent on one bridge is said to be exo if it is anti (trans) to the larger of the other two bridges and is said to be endo if it is syn (cis) to the larger of the other two bridges. 

Occasionally, addition products of 4//-l,2,4-triazole-3,5-diones or diazenedicarboxylic esters and oxepins have been obtained whose formation can be rationalized by an addition to the 2,4-diene system in the oxepin, e.g. formation of 10.190191 In these cases, the primary adduct usually cannot be isolated, because it undergoes a hetero-Cope rearrangement to a tricyclic or bicyclic structure in which the oxepin oxygen has become part of a carbonyl function.190 191,227... 

The addition of lithium dimethyl-, dibutyl-, diphenyl- or 1-butenylcuprate to 2d produced (JiS) 94% ee, ifiS) 95% ee, (jiR) 96% ec, and ifiR) 90% ee, respectively. In this case the difference between S and R results from the CIP selection rules and not to the steric course of the reactions. The conformation of the chiral auxiliary in 1 d is such that one IV-methyl group is axially, the other equatorially, arranged on the bicyclic structure. The reagents attack the double bond from the side opposite to the equatorial iV-methyl group. Other chiral auxiliaries such as a-c14 were less effective15. 

Loss of ketene from bicyclic structures (e.g., for camphor)... 

Geometric considerations in cyclopolymerization are optimal for 1,6-dienes (see 4.4.1.1). Instances of cyclopolymerization involving formation of larger rings have also been reported (see 4.4.1.4), as have examples where sequential intramolecular additions lead to bicyclic structures within the chain (see 4.4.1.2). Various 1,4- and 1,5-dienes are proposed to undergo cyclopolymerization by a mechanism involving two sequential intramolecular additions (see 4.4.1.3). 

Polymerization of 4-bromo-6,8-dioxabicyclo[3.2.1 ]octane 2 7 in dichloromethane solution at —78 °C with phosphorus pentafluoride as initiator gave a 60% yield of polymer having an inherent viscosity of 0.10 dl/g1. Although it is not described explicitly, the monomer used seems to be a mixture of the stereoisomers, 7 7a and 17b, in which the bromine atom is oriented trans and cis, respectively, to the five-membered ring of the bicyclic structure. Recently, the present authors found that pure 17b was very reluctant to polymerize under similar conditions. This is understandable in terms of a smaller enthalpy change from 17b to its polymer compared with that for 17a. In the monomeric states, 17b is less strained than 17a on account of the equatorial orientation of the bromine atom in the former, whereas in the polymeric states, the polymer from 17b is energetically less stable than that from 17a, because the former takes a conformation in which the bromine atom occupies the axial positioa Its flipped conformation would be even more unstable, because the stabilization by the anomeric effect is lost, in addition to the axial orientation of the methylene group. 

From the kinetic viewpoint the polymerizability of 61 is considered to be higher than that of e-caprolactam, which is polymerized usually at temperatures above 135 °C63,64 Thermodynamically, the polymerization of 61 appears to be more favored than that of a-pyrrolidone, for which no polymerization is observed in THF63-65 The higher polymerizability of 61 may be attributed not only to its highly strained bicyclic structure but also to the activation of the anion 66 by the... 

Bicyclic structures of this type are more reactive than monocyclic or acyclic carbamates, indicating that a relatively rigid orientation of the carbonyl group is favorable to lithiation. Substituted formamidines can also be lithiated.60... 

A further test of the stereoelectronic theory of reactivity of phosphate esters has been attempted using measurements of the rates of displacement of 4-nitrophenate from the esters (23) and (24), their phosphorus epimers, and also (25), in aqueous methanol the introduction of the 4a-Me group into the system would, it was hoped, reduce the the flexibility of the bicyclic structures and so possibly eliminate the participation of twist-boat conformations. The presence of the 4a-Me group has no effect of the rate of displacement of the axial ArO group... 

The same nonpolar conformation can be achieved by conversion to bicyclic structures. 1,4-Cyclo-addition of ethylene to anthracene-9-carboxylic acid gives acid 68. Successive conversion to the N-methylamide, via the acid chloride, followed by reduction with lithium aluminum hydride produced... 

Captopril 678 and enalapril 679 are potent angiotensin converting enzyme (ACE) inhibitors used as antihypertensives. Molecular manipulation based on the enzyme model led to the discovery of some perspective bicyclic structures, for example, cilazapril 680 and compound 681, highly active antihypertensives in vivo. Compound 681 belongs to the most potent conformationally restricted ACE inhibitors and is often used as a model for molecular modeling studies <1996JA8231>. 

In contrast to 154 the isomeric bicyclo[4.4.1]undeca-3,5,8,10-tetraene-2,7-dione /bicyclic structure. Formation of the bis-Grignard reagent 163... 

There are only a few examples of bicyclic structures 194 described in the literature. On the other hand, similar tricyclic analogs containing one bridgehead nitrogen atom are not rare and some typical examples will be described here. 

Optimizations at various levels either gave pronounced bicyclic structures or a completely delocalized structure with no transannular bonding. 

This free-energy difference between the anomers, which can be estimated by the ratios observed in glycosidation and anomerization equilibria, is larger in derivatives of 4 than in those of D-glucofuranose. This result can be attributed to the lessened conformational flexibility of bicyclic structures. 

In a very thorough investigation of the reactions of hexaric acid lactones, and particularly dilactones, with sodium methoxide, F. Smith138,139 invariably obtained unsaturated products, at that time thought to be formed by isomerization and enolization. Hence, for alkyl D-glucofuranosidurono-6,3-lactones, also of bicyclic structure, similar reactions were believed24,25 to account for their unusual behavior in alkaline solutions. 

The stereochemistry of products 27 (Table 6, entries 2, 3, and 4) can be explained distinctly on the same basis that alkyl substituents R1, R2 are positioned in quasi equatorial positions in the transition structure. Exclusive formation of the all-cis isomer in the reaction of cyclohexyl acetate (entry 5) is further evidence to support the oxido-carbene interaction in a double-chaired bicyclic structure 28, as depicted in Scheme 13, Eq. 1. 

Evidently, the presence of a bicyclic structure hinders the ring inversion. 

Lautens also used this nickel-catalyzed hydroalumination methodology in the total synthesis of ionomycin 145. The starting compound was a [3.2.1]oxabicyclic alkene 143.144 Their rigid bicyclic structures can be used to introduce functional groups in a highly stereoselective manner. The synthesis of the key intermediate 144 involves the slow addition of DIBAL to the oxabicyclic alkene and the Ni(COD)2/(6T)-BINAP in toluene to afford 144 in 95% yield and 93-95% ee. (Scheme 18). 

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