Spiro structure

There have been a number of computational studies of the epoxidation reaction. These studies have generally found that the hydrogen-bonded peroxy acid is approximately perpendicular to the axis of the double bond, giving a spiro structure.75 Figure 12.8 shows TS structures and Ea values based on B3LYP/6-31G computations. The Ea trend is as expected for an electrophilic process OCH3 < CH3 CH = CH2 < H < CN. Similar trends were found in MP4/6-31G and QCISD/6-31G computations. 

FIGURE 6.28 Influence of the reaction of the first o-QM (to a pyrano or spiro structure) on the reactivity of the second o-QM from 33 according to computations only spiro/pyrano, but no pyrano/pyrano or spiro/spiro pairs are formed in the oligomeric products.56... 

Chapter 4 is concerned with a technically important group of leuco compounds which like the spiropyrans are not formed by reduction of the parent dye, but by formation of a spiro structure from the dye in such a way that the newly created sp3 center destroys the conjugation, and hence, the color of the chromophore. These are the phthalides (spirolactones) and the position of equilibrium is determined by pH rather than a redox process. Such materials are used mainly as color formers in pressure-sensitive... 

The synthesis of 2-thioxo-1,3-0,/V-heterocycles connected to glycopyran and glycofuran scaffolds involves the formation of fused, anchored or spiro structures. The synthesis of such compounds is an attractive challenge from a chemical point of view but also from the growing interest concerning their biological properties. 

An important finding is that all peroxo compounds with d° configuration of the TM center exhibit essentially the same epoxidation mechanism [51, 61, 67-72] which is also valid for organic peroxo compounds such as dioxiranes and peracids [73-79], The calculations revealed that direct nucleophilic attack of the olefin at an electrophilic peroxo oxygen center (via a TS of spiro structure) is preferred because of significantly lower activation barriers compared to the multi-step insertion mechanism [51, 61-67]. A recent computational study of epoxidation by Mo peroxo complexes showed that the metallacycle intermediate of the insertion mechanism leads to an aldehyde instead of an epoxide product [62],... 

Density functional calculations reveal that epoxidation of olefins by peroxo complexes with TM d° electronic configuration preferentially proceeds as direct attack of the nucleophilic olefin on an electrophilic peroxo oxygen center via a TS of spiro structure (Sharpless mechanism). For the insertion mechanism much higher activation barriers have been calculated. Moreover, decomposition of the five-membered metallacycle intermediate occurring in the insertion mechanism leads rather to an aldehyde than to an epoxide [63]. 

Polymer chains based on spiro structures have been studied as another route to heat-resistant polymers [Kurita et al., 1979]. A spiro structure is a double-strand structure (Sec. l-2c) in which the uninterrupted sequence of rings have one atom in common between adjacent rings. (Adjacent rings in ladder polymers have two or more atoms in common.) An example of a spiro polymer is the polyspiroketal synthesized from 1,4-cyclohexanedione and pentaerythri-tol (Eq. 2-249). The low solubility and intractability of spiro polymers makes it difficult to synthesize or utilize high-molecular-weight polymers. 

More complex bulky amines, especially dendrimers, have been developed to enhance the properties of the HTL materials. These include starburst products such as ST638 and componnds with spiro structures (Spiro-TAD) from Covion. These are shown in Figure 3.34. 

One mechanistic matter that has caused quite a bit of general consternation about a decade ago concerns the experimental evidence for the involvement of diradical intermediates (proposed as sources for the observed radical products) in dioxirane epoxidations, which were thought to be formed through induced peroxide-bond homolysis by the alkene. Nonetheless, rigorous experimental and high-level theoretical work disposed such radical chemistry in the epoxidation of alkenic substrates. The latter computations unequivocally confirm the established concerted mechanism, in which both CO single bonds in the incipient epoxide are concurrently formed by way of an asynchronous, spiro-structured transition state for the oxygen transfer. 

An interesting spiro compound (59) that contains the thietane ring was obtained in minor amounts by dimerization of dimethylketene and subsequent treatment with P2S5. Pyrolysis of the spiro structure produced 60 and the thietanethione 61, which can also be prepared by base-catalyzed rearrangement of 62, a process that can be carried out as well with 63 to give the 2-thietanone 60. The solvent and the basicity of the catalyst are important parameters in this rearrangement. 

In this chapter, the structures and chemistries of 1,3-dioxins, 1,3-oxathiins, and 1,3-dithiins are described, including both their fully saturated forms (1, 7, and 13) as well as their benzo analogs (6, 11, 12, and 17). The formally fully unsaturated monocyclic structures (4, 9, 10, and 16) contain only one endocyclic double bond with further unsaturation being accomodated by exocyclic double bonds (2, 3, 5, 8, 14, and 15), for example, by the introduction of a carbonyl group. Well known and intensively studied are the Meldrum s acid derivatives 18 and 19. In addition, 1,3-dioxane, 1,3-oxathiane, and 1,3-dithiane moieties can be part of spiro structures as well as hi- and tricyclic analogs. And finally, both the structures and chemistries of the corresponding sulfoxides and sulfones are also reported. 

It was established that the oxygen transfer takes place via an Sn2-like transition state (a), rather than a pathway which resembles a 1,3-dipolar addition (b). Comment on the relative merits of the two transition states using principles of orbital interaction theory. A spiro structure similar to (a) for the transition state has been located by ab initio calculations (Bach, R. D. Owensby, A. L. Gonzalez, C Schlegel, H. B. McDouall, J. J. W., J. Am. Chem. Soc., 1991, 113, 2338). 

Mechanistic studies showed that epoxidation catalyzed by MTO/H2O2 occurs as direct oxygen transfer via transition states of spiro structure likely involving both mono- and bisperoxo complexes.1233 1245-1247... 

In some cases, a spiro structure (138) is obtained instead of, or in... 

The composition of 2-C-spirocyclopropyl-2-deoxy-L-arabinose was found64 (by l3C-n.m.r. spectroscopy) to be pyranoses, furanoses and acyclic forms in the ratios of 10.6 2.9 1.0. This is, actually, not a sugar having a fused ring but one with a spiro structure. The proportion of the acyclic form is surprising apparently the spiro arrangement introduces strain into both the pyranose and the furanose forms. 

Although the conformationally rigid, N-spiro structure created by two chiral binaphthyl subunits represents a characteristic feature of 1 and related catalyst 9, Maruoka and coworkers have generally used their (S,S)- and (R,R)-isomers. Surprisingly, however, when the diastereomeric (R,S)-lc was used for asymmetric benzyla-tion of 2, the reaction was found to proceed very slowly, such that even after 60 h the... 

Despite the impressive catalytic properties of 29 (and its analogs), the conforma-tionally rigid N-spiro structure can also be a drawback, in particular with regard to conformational adaptation and the difficulty of modification. Addressing this issue,... 

The reaction stereoselectively affords the 2 -R isomer as the major product (d.e. 33%). The absolute configuration of the 2 carbon was determined by NOESY analysis, in particular consistent n.0.e.s between H-l and H-l b and between H-l a and H-2 protons were observed. Treatment of 16 with zinc and acetic acid, afforded 17, that was transformed in the C-fiructoside 18 (Scheme 6). Compound 18 can be considered as an a- or a P-C-fructoside, in which both the hydroxymetyl arms can be further manipulated. An interesting example of this is the formation of the new spiro structure 20, as reported in Scheme 6. 

A concerted, spiro-structured, oxenoid-type transition state has been proposed for C-H oxidation by dioxiranes (Scheme 5). This mechanism is based mainly on the stereoselective retention of configuration at the oxidized C-H bond [20-22], but also kinetic studies [29], kinetic isotopic effects [24], and high-level computational work support the spiro-configured transition structure [30-32], The originally proposed oxygen-rebound mechanism [24, 33] was recently revived in the form of so-called molecule-induced homolysis [34, 35] however, such a radical-type process has been experimentally [36] and theoretically [30] rigorously discounted. 

All chiral molecules do not have asymmetric centres. For example, some substituted allenes and spiro structures have no symmetric centre but are still chiral (Following fig.). The substituents at either end of the allene are in different planes, and the rings in the spiro structure are at right angles to each other. The mirror images of the allene and the spiro structures are non-superimposable and are enantiomers. 

The Descostes group [14] has used this strategy of cyclization for the synthesis of the analogues of the ionophore antibiotic with anomeric spiro-structure 46 by irradiating the 4-hydroxyl-2-butanone glucoside 45 in benzene (Scheme 8.13). 

Lubimin, a phytoalexin found in several Solanaceae, has been assigned the spiro-structure (57) on n.m.r. and biosynthetic evidence,83 whilst the new sesquiterpenoid skeleton of taylorione (58) from the liverwort Mylia taylorii is thought84 to be biosynthesized via an aromadendrene-type precursor which may also generate the co-occurring myliol (59). 

The reaction of 108 with the sodium salt of tetraphenylcyclo-pentadiene, on the other hand, yields only two products, namely 146a and another compound which has been provisionally assumed to possess the spiro structure (149a). The formation of 149a can be explained by the attack of a carbanion on the disulfide bridge of 108, followed by the formation of the sodium salt of 148a, and finally a... 

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