Spiro atom

The atom common to both rings is called the spiro-atom this may also be nitrogen, phosphorus, etc. 

In addition, it should be useful to mention that silicon as a spiro atom is able to conjugate its mutually orthogonal aromatic constituents, and thus, secures a release of an unpaired electron in a cation-radical. Scheme 3.61 shows such a phenomenon, revealed by Hirao et al. (2007). Obviously, vacant d orbitals of silicon provide the cation-radical with the possibility of delocalizing spin density. 

Scheme 61) [2b, 150], but can give rise to a variety of products. With unsym-metrically substituted alkenes of type 266, two regioisomeric products were obtained, but the isomer 268 bearing the ester group near the spiro atom was the minor component in all cases. Norbornadiene and norbornene react with 1 by the same mode to give formal [3 -F 2] cycloadducts 269 and 270, respectively, the latter as a 9 1 mixture of exo- and endo-isomers. 

The synthesis and stmctnral characterization of a series of spirocyclic organozincates containing two five- or six-membered metallacycles in which zinc is the central spiro atom, is shown in Scheme 1. Compound 8a was prepared via an elegant one-pot synthesis, starting from 1,5-dichloropentane, ZnCl2 and a lithium/sodinm alloy (1% sodinm) in diethyl ether as a solvent (eqnation 4 in Scheme 1). Snbseqnent treatment of a soln-tion of 8a with TMEDA afforded the corresponding TMEDA complex 8b of which the structnre in the solid state was unambiguously established by an X-ray crystal structnre determination. 

However, it is necessary to determine precisely the orientation of the dipole moments, and to ensure that the observed CD is not the resultant from a mixture of con formers. Moreover, even with a correct choice of the dipoles, there are often shortcomings. For instance, studies of l,l -spirobisindanes have shown substantial discrepancies between the theoretical and experimental results for several products of known configuration 27). It has been shown that the signs of the bands stand in direct relation to the configuration of the spiro-atoms, which means that the optical activity is really governed by the coupling mechanism including other types of interaction. 

Monospiro compounds consisting of only two alicyclic rings as components are named by placing spiro before the name of the normal acyclic hydrocarbon of the same total number of carbon atoms. The number of carbon atoms linked to the spiro atom in each ring is indicated in ascending order in brackets placed between the spiro prefix and the hydrocarbon name. 

The carbon atoms in monospiro hydrocarbons are numbered consecutively starting with a ring atom next to the spiro atom, first through the smaller ring (if such be present) and then through the spiro atom and around the second ring. 

If one or both components of the monospiro compound are fused polycyclic systems, spiro is placed before the names of the components arranged in alphabetical order and enclosed in brackets. Established numbering of the individual components is retained. The lowest possible number is given to the spiro atom, and the numbers of the second component are marked with primes. The position of the spiro atom is indicated by placing the appropriate numbers between the names of the two components. 

Monospiro compounds containing two similar polycyclic components are named by placing the prefix spirobi before the name of the component ring system. Established enumeration of the polycyclic system is maintained and the numbers of one component are distinguished by primes. The position of the spiro atom is indicated in the name of the spiro compound by placing the appropriate locants before the name. 

Polycyclic compounds containing more than one spiro atom and at least one fused polycyclic component are named in accordance with Part. 4 of this rule by replacing spiro with dispiro , trispiro , etc., and choosing the end components by alphabetical order. 

Alternate to A-41.6 and A-41.7)—Polycyclic compounds containing more than one spiro atom are named in accordance with Rule A-42.1 starting from the senior" end-component irrespective of whether the components are simple or fused rings. 

If at least one component of a mono- or poly-spiro compound is a fused polycyclic system, the spiro compound is named according to Rule A-41.4 or A-41.7, giving the spiro atom as low a number as possible consistent with the fixed numberings of the component systems. 

Heterocyclic spiro compounds are named according to Rule A-42, the following criteria being applied where necessary (a) spiro atoms have numbers as low as consistent with the numbering of the individual component systems (b) heterocyclic components have priority over homocyclic components of the same size (c) priority of heterocyclic components is decided according to Rule B-3. Parentheses are used where necessary for clarity in complex expressions. 

Spiro compounds are known to have joined rings in two perpendicular planes with a nodal common atom. This atom (the quaternary carbon) can prevent conjugative interactions between the two joined rings. In the case of two identical perpendicular 77-nctworks joined by a spiro atom, the orbitals of the halves may interact only if they possess the same symmetry. Such interactions lead to pairs of delocalized orbitals encompassing the... 

Acetals such as C are referred to as spiroketals because their acetal carbon is a spiro atom. (In a spiro compound two rings are connected by a single common atom that is called a spiro atom ). The intermediates in this acetalization are lactols B. They resemble those lac-tols whose rapid formation from y- or 5-hydroxyketones was shown in Figure 9.4. Note that because of the unfavorable reaction entropy, there is often no path hack from spiroketals to the open-chain form Usually, spiroketals cannot be hydrolyzed completely. 

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