Synthetically significant rings

It can be seen that the set of synthetically significant rings include the "primary rings" I, 2, 3, 4 and the "secondary rings" 5 and 6. 

At this point, the student should be able to find the "strategic bonds", looking first for all the primary rings, the set of synthetically significant rings, the maximum bridging ring, etc., as explained in Chapter 7. 

Draw the structures of twistane, tropinone, exo-brevicomin, patchouli alcohol, longifolene, sativene, luciduline and porantherine and search bifunctional relationships, rings, synthetically significant rings, bridgeheads, core bonds and strategic bonds of each one of them. 

Corey E.J. Petersson G.A. An Algorithin for Machine Perception of Synthetically Significant Rings in Complex Cyclic Organic Structures . J. Am. Chem. Soc. 1972, 94 (2), 460-465. 

In contrast to Nicolaou s synthetic plan, the retrosynthetic analysis of Holton s approach preserves the non-synthetically significant B ring and proceeds through disconnection of bonds which are involved in the D and C rings, to arrive finally to the bicyclo[5.3.1]undecane derivative 32, as the starting material. 

The only four- - six-membered ring interconversions of any real synthetic significance are those involving diketene. Base-catalyzed dimerization of diketene is a long-established and efficient method for the preparation of dehydroacetic acid (equation 161), while mild treatment with water in the presence of tertiary amine bases gives 2,6-dimethyl-4-pyrone (equation 162). 1,3-Dioxins are obtained from the acid-catalyzed condensation of diketene with ketones (equation 163). 

The aromatic pyrrole, indole, isoindole and carbazole rings can in general be obtained by oxidative aromatization of more highly reduced nonaromatic derivatives. In practice this method has been of most synthetic significance for indoles and carbazoles, where the corresponding 2,3-dihydroindoles (indolines) and 1,2,3,4-tetrahydrocarbazoles are accessible by a variety of synthetic pathways. 

Et3N(Bn)BH. It effects reductive ring opening of cyclic enol ethers. Perhaps this method is of little synthetic significance because it should be possible to achieve the reduction much more conveniently using simpler borohydride reagents in mildly acidic aqueous solutions. 

To direct a ring formation following the silylcupration is also synthetically significant. 

No synthetically significant (< eight-membered) secondary rings. 

Without doubt, it is possible to characterize the developments made in the area of pyridinium salt photochemistry as slow and far between. More recent investigations of this chemistry have begun to uncover its synthetic significance. However, a number of questions pertaining to the mechanism of the photocyclization process, the factors governing the rates of ring isomerization, and the regiocontrol of nucleophilic addition to the unsymmetricaUy substituted cation intermediates still need to be answered before the full potential of this unique excited-state process can be elaborated. 

Thiophene [110-02-1] and a number of its derivatives are significant in fine chemical industries as intermediates to many products for pharmaceutical, agrochemical, dyestuffs, and electronic appHcations. This article concentrates on the industrial, commercial, and economic aspects of the production and apphcations of thiophene and thiophene derivatives and details the main synthetic schemes to the parent ring system and simple alkyl and aryl derivatives. Functionalization of the ring and the synthesis of some functional derivatives that result, not from the parent ring system, but by direct ring cyclization reactions are also considered. Many good reviews on the chemistry of thiophene and thiophene derivatives are available (1 7). 

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