Skeletons Considered

In this last case, the main skeleton considered could be the cationic trimethine cyanine and the third ring with its independent chain could be named as a substituent at the a or j3 position. When this substituent is of an ionic nature, it could be named after the corresponding ring, for... 

Some theoretical works report the geometric features of penams skeleton, considering the strain of the bicyclic system and the bridgehead nitrogen pyramidality (see Section 2.03.3.1). For instance, the two possible conformations (axial and equatorial with respect to the C-3 carboxyl group) of the penicillins thiazolidine ring have been localized as two minima by ab initio calculations employing several theoretical methods (Hartree-Fock (HF), density functional... 

The problem indicates this acid formed from the alcohol, C, is identical to A. note that in the synthesis of the carboxylic acid from C, a carbon atom must be added. (Preparation of carboxylic acids through a Grignard reagent will add one carbon atom to the skeleton.) Consider... 

When there is a change in the carbon skeleton, consider all of the C-C bond-forming reactions and all of the C-C bondbreaking reactions that you have learned so far. 

This chapter includes both the earliest and the most recent methods for the total synthesis of steroids, In accordance with the methods of building up the steroid skeleton considered in it, the presentation of the material is divided into five sections. 

The total syntheses of steroids by the methods of building up the skeleton considered in this chapter were developed as early as the beginning of the thirties the workers concerned had to open up new paths which frequently led to dead ends. Consequently, in this chapter a fairly large number of ingenious but not completely successful, or even completely unsuccessful, attempts at total synthesis are described. However, they have considerable value as illustrations of new approaches to the building up of the rings and the formation of the centers of asymmetry (see, for example. Schemes 6, 7, 10, 13-15, 18, 21, 24-28, 30, 31, 49, 51, 52,55). 

Syntheses of Azasteroids. Syntheses of 6-, 8-, and 9-aza-steroids have been carried out by the method of building up the steroid skeleton considered in this section (Schemes 92 and 93). 

LEER suffer from an artificial separation of a molecule into skeleton, reaction site, and substituent. The physicochemical effects mentioned in section 3.4.1 and the methods presented in section 7.1 for their calculation consider a molecule as a... 

A basic assumption in such additivity schemes is that the interactions between the atoms of a molecule are of a rather short-range nature. This fact can be expressed in a more precise manner The law of additivity can be expressed in a chemical equation [1]. Let us consider the atoms (or groups) X and Y attached to a common skeleton, S, and also the redistribution of these atoms on that skeleton as ejqjressed by Eq. (1). 

The silyl enol ethers 209 and 212 are considered to be sources of carbanions. and their transmetallation with Pd(OAc)2 forms the Pd enolate 210. or o.w-tt-allylpalladium, which undergoes the intramolecular alkene insertion and. 1-elimination to give 3-methylcyclopentenone (211) and a bicyclic system 213[199], Five- and six-membered rings can be prepared by this reaction[200]. Use of benzoquinone makes the reaction catalytic. The reaction has been used for syntheses of skeletons of natural products, such as the phyllocladine intermediate 214[201], capnellene[202], the stemodin intermediate 215[203] and hir-sutene [204]. 

Consider the alkyl group having the same carbon skeleton as (CH3)4C All the hydrogens are equivalent replacing any one of them by a potential point of attachment is the same as replacing any of the others... 

Introduction of Nitrogen into a Terpenoid Skeleton. The acetate-derived fragments (35) mevalonic acid (30), which yields isopentenyl pyrophosphate (31) and its isomer, 3,3-dimethyl ally pyrophosphate (32) a dimeric C -fragment, geranyl pyrophosphate (33), which gives rise to the iridoid loganin (34) and the trimer famesyl pyrophosphate (35), which is also considered the precursor to C q steroids, have already been mentioned (see Table 3... 

Most heterocyclic anions may be considered to be derived by loss of a proton from a parent compound, which is therefore the conjugate acid. Such anions have at least one unshared pair of electrons at the anionic site. They are named by appending the suffix -ide , with elision of a terminal e (lUPAC recommendation RC-83.1.1), as in (190)-(193). The site may be specified by a locant placed immediately before the suffix, and so chosen as to be as low as possible consistent with the numbering of the skeleton of the parent compound. The locant may be omitted in order to designate an equilibrating mixture of positionally isomeric anions, which is what one usually obtains in practice. The anion of piperidine is often informally referred to as piperidide . 

Valence bond theory (Chapter 7) explains the fact that the three N—O bonds are identical by invoking the idea of resonance, with three contributing structures. MO theory, on the other hand, considers that the skeleton of the nitrate ion is established by the three sigma bonds while the electron pair in the pi orbital is delocalized, shared by all of the atoms in the molecule. According to MO theory, a similar interpretation applies with all of the resonance hybrids described in Chapter 7, including SO S03, and C032-. 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

This statement can be proved very easily by considering four different types of general POP properties and their variability as a function of the side substituents on the polymer skeleton, i.e. glass transition temperature (Tg), morphology, solvent solubility and limiting oxygen index (LOI). The values of these parameters are reported in Tables 5-8. 

Additional lack of chain flexibility is introduced in the polyphosphazene skeleton when polyspirophosphazenes are considered. These materials are obtained by reacting 2,2 -di-hydroxy biphenyl or l,r-binaphthyl derivatives with polydichlorophosphazene [305], leading to the formation of polymers having the structure shown in Formula below. 

Of course, not all the phosphazene polymers that have been synthesized are equally important. Many of them, in fact, have a mere academic or speculative interest, and will not be described in this article. A few other classes of POPs, however, do occupy an important place in phosphazene history, and have been seriously considered for industrial development and commercialization. These polymers are basically those in which the properties of the inorganic -P=N- skeleton overlap to the highest extent those of the phosphorus side substituents. In the successive sections of this article we will describe in some detail the most important classes of polyphosphazenes that fulfil this condition. 

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