Spiro carbon

The novel highly substituted spiro[4.4]nonatrienes 98 and 99 are produced by a [3+2+2+2] cocyclization with participation of three alkyne molecules and the (2 -dimethylamino-2 -trimethylsilyl)ethenylcarbene complex 96 (Scheme 20). This transformation is the first one ever observed involving threefold insertion of an alkyne and was first reported in 1999 by de Meijere et al. [81]. The structure of the product was eventually determined by X-ray crystal structure analysis of the quaternary ammonium iodide prepared from the regioisomer 98 (Ar=Ph) with methyl iodide. Interestingly, these formal [3+2+2+2] cycloaddition products are formed only from terminal arylacetylenes. In a control experiment with the complex 96 13C-labeled at the carbene carbon, the 13C label was found only at the spiro carbon atom of the products 98 and 99 [42]. 

The absorption bands for both quinoid and dipolar structures have been calculated by the PPP method.2,12 The calculations for a more simplified model of the colored form of some spirobenzopyrans using the normal parameters are shown in Table 2.12 In this case, the spiro carbon in the indoline moiety is ignored in the 7i-electron system, and the quinoid structure is assumed. 

Fluoran compounds have an optically active spiro-carbon atom. Consequently, some fluoran compounds, especially those having an alkylamino group of four or more carbon atoms at 3 -position, have been found to exhibit crystal modifications as determined by X-ray diffraction. Each crystal modification reveals different physical properties such as melting point, solubility, and affinity with acidic compounds, resulting in different characteristics regarding use for carbonless copying papers, thermosensitive recording papers, and the like. 

The analysis presented on the next page, which requires consultation of reference 4 with respect to finer details of Sequence Rule 4, shows that the spiro-carbon is a stereogenic center type 1 of the rare type X(FFGG) (see Section 1.1.3.5.). 

The examples 6a and 6b are added as a test of spatial vision one is achiral (point group. S ), one is chiral (point group D2). This problem is best solved (6a ), 6b S4) by projecting the models on a plane through the spiro-carbon and perpendicularly arranged to the obvious two-fold axis. 

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

In the assignment of the resonances of the carbon atoms of rings A and B compound 3 served as a convenient model. However, the spiro carbon atom present in the alkaloids caused significant changes in the resonances of the carbon atoms of ring B. Thus for example in 101, C-6 and the A-methyl group were shielded by —4.9 and —7.0 ppm, respectively, as a result of y steric interactions with C-8 and C-13, and C-14 was deshielded by +14.3 ppm. Other spiro compounds are known to exhibit similar behavior 66) and qualitatively similar effects were found in other alkaloids of this series 63). 

An X-ray structure of the photochromic spirobenzothiopyran 46 shows an elongated bond from sulfur to the spiro carbon atom while the bond from S to the aromatic ring is shortened. Irradiation at 365 nm cleaves the former bond and generates the zwitterionic blue-green merocyanine with an s-trans, s-trans conformation in the solid state (Equation 3). In solution, NMR studies indicate an s-trans, s-cis conformation <2002JOC533>. 

Grosu et al. also synthesized a variety of new 5,5-disubstituted (96MI71) and 2,2,5,5-tetrasubstituted 1,3-dioxanes (98M723) the compounds exhibit anancomeric structures with the bulkier substituents in an equatorial conformation. The same authors also studied the stereochemistry of trispiro- (98M59), dispiro- (96T12783), and spiro- 1,3-dioxanes (97T6215), when the substituents are close to the spiro carbon atoms, they induce anancomericity of the heterocycle, and when they are further away, the 1,3-dioxanes proved to be flexible. 

If you try to draw these spirocyclic acetals you will soon find there is a trick to getting them to look right the spiro carbon has to be one of the four that aren t at the "point of either ring otherwise one ring ends up looking flat. 

Butylcalix[n]arenes can be converted into monospirodienones 43 by mild oxidation (tetrabutylammonium tribromide) under alkaline conditions.89 The structure of two examples 43a,b has been established by single-crystal X-ray analysis.90 Their chirality is mainly due to the asymmetrically substituted spiro carbon atom, however, compound 43a has also some similarity with calix[4]arenes having three different phenolic units in the order AABC (see below). Under neutral conditions the oxidation of r-butylcalix[4]arene leads to the formation of 44 which has been also confirmed by single-crystal X-ray analysis.91... 

In a third category of cyclic 3, y-enones of synthetic potential, the chromophore moieties are linked by a spiro carbon typical examples are a 3, y-unsaturated 8-diketone and 3, y 8,e-unsaturated spirocyclic ketones. " A representative of the latter category is (53), which upon triplet sensitization with Mich-ler s ketone affords the isomers (54) and (55) in a ratio of about 2 1 and a total yield of 82% (Scheme... 

Spiro ring systems are named as spiro[x.y]aIkanes where the parent alkane corresponds to the total number of carbons in both rings, and x and y refer to the number of carbons that join the shared carbon (the spiro carbon), written in order of increasing size. When substituents are present, the rings are numbered beginning with a carbon adjacent to the spiro carbon in the smaller ring. 

All three syntheses of XX create the spiro carbon by the use of variations of the Pictet-Spengler reaction. This long-known and valuable method of constructing tetrahydroisoquinolines (6) consists in the acid-... 

All of the aromatic alkaloids (except for erythratine) and -erythroi-dine possess two asymmetric carbon atoms—the spiro carbon (C-5) and the carbon substituted by methoxyl (C-3). a-Erythroidine has an additional center of asymmetry at C-12. Chemical, spectroscopic, and crystallographic methods have been used to assign relative and absolute configurations at the asymmetric centers. 

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