Fused structures

The numbering of the complete fused structure does not depend on the original numbering systems of its components. Assignment of numbering requires (i) correct orientation of the structure and (ii) application of a rather intricate set of priority rules. For details, the lUPAC Rules should be consulted, but in outline the procedure is as follows. 

Ring fusion seems to occur in the quinoxaline derivative (28), which has been stated to exist in red and blue-black forms. Other derivatives of type 28 are reported. Attempts to prepare 5,6-furo-xanobenzofuroxan by pyrolysis of the azide (29) met with no success. An early example in the literature of such a linear fused structure was shortly afterward revised to the angularly fused type (17). 

The 1 R,6R,7R,8S-as-fused structure and conformation of 102 were elucidated on the basis of their NMR spectroscopic data. The observed formation of only one sulfonium salt in this cyclization reaction was remarkable in that either sulfur atom might have been expected to participate in tosylate displacement. The H NMR spectrum of salt 102 shows a large three-bond scalar coupling of 10.6 Hz between H-6 (5 = 4.736) and H-7 (5 = 4.606) this indicates that they have an almost antiperiplanar relationship. The equatorial orientation of H-6 and the 3C6 conformation of its six-membered cycle are consistent with the strong NOEs observed between H-7 and both H-2axja and and... 

Conformational analysis of the novel bicyclo[3.3.0]octasilane and bicyclo[4.2.0]octasilane systems was carried out by X-ray crystallography (Figs. 19-21). The bicyclo[3.3.0]octasilane framework of trans-17 has a trans-fused structure of cyclopentasilane rings. The trans-fused structure is... 

A Russian team carried out the synthesis of 3-benzoyl-6-methylthiazolo[2,3-z][l,2,4]triazin-5(4/f)-one 62 and found that it was difficult to differentiate between this /-fused structure (as shown here) and the possible isomeric -fused one <1996IZV2971>. The problem was solved by X-ray elucidation. Similarly, in the case of the cyclic sulfone 64 <2002TL4099>, X-ray analysis decided the exact structure of this product. 

Zemplen s work was revisited by Bromund et al43 by exploring the reaction in diverse aldose series (D-galacto, D-xylo, L-arabino). Similar bicyclic fused structures of OZT on pyran backbones were proposed (Scheme 20). 

With a view to increasing the selectivity in OZT formation (i.e. reducing the number of products shown in Scheme 23), selective hydroxyl protections were performed. Protection of the C-l alcohol would induce limitation to fused structures (Scheme 25),10 whereas 3-O-protection would only allow formation of s/hro-derivatives (Scheme 26).47... 

Oxathiazolidin-3-ones lose CO2 and S at room temperature to give open chain products (see Section 4.13.6.1.1). Fused structures can undergo recyclization <84CHEC-I(6)897>. 

The structures of the compounds were verified by H NMR spectroscopy. Two doublets of benzene ring protons (7 = 9 Hz) characteristic of mutual ott, ti-positions indicated angularly ring-fused structures. The shifts of NMR peaks and the features of the IR spectra (containing deformed out-of-plane vibrations at 800 cm ) were characteristic of 1,2,3,4-tetrasubstitution and so also confirmed the angular ring fusion. 

The majority of methods for the synthesis of thieno[2,3-c]pyridines and related fused structures can be tentatively divided into two groups based on the construction of the pyridine or thiophene rings, respectively. 

Intramolecular reactions occur by the same principle if they can, but the contortion required to get the bridged cyclobutane 28 from 29 is too much and the molecule prefers to add the wrong way round and give the fused structure 30 instead.10... 

Numerous applications of IR spectral data have been reported for derivatives of (5,5)-fused ring systems, especially in connection with the problem of tautomeric equilibria. For example, the IR spectra of 5,6,7,8-tetrahydro-l//,3H-pyrrolo[l,2-c][l,3]oxazole-1,3-diones (24 X=0) and corresponding thiazole-l,3-diones (24 X = S) show no carbonyl absorption, but broad bands at 3300 cm-1 due to the associated OH group, indicating that the fused structures (24) exist in the enolic forms rather than their diketonic tautomers (81BJC1844). 

Furopyridines have a fused structure containing both a 7t-excessive furan ring and a n-deficient pyridine ring. Interesting results were obtained during the Birch reduction of the four different furopyridines, and a mechanistic interpretation for the results was presented. An example is shown in the following scheme <02JHC335>. 

M. Shiozaki, Y. Kobayashi, M. Arai, and H. Haruyama, Synthesis of 6-cpt-trehazolin from D-ribonolactone Evidence for the non-existence of a 5,6-ring fused structural isomer of 6-epi-trehazolin, Tetrahedron Lett., 35 (1994) 887-890. 

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