Polycyclic systems, structure

The chemistry of mononuclear isothiazoles has been developed since 1956 without the aid of thiohydroxylamine, the preparation of this very unstable substance having only recently been reported. Bicyclic and polycyclic systems involving the isothiazole (1,2-thiazole) structure have long been known and were fully reviewed in 1952, but little new work has been reported since then and the present review... 

Besides some new cyclosilanes with side chains and cage-like compounds, we were able to isolate the decalin analogue cyclosilane Si Me in a yield of about 20%. We characterized the compound by the usual spectroscopic methods, investigated the structure, and found that this compound is able to form a radical anion [12]. In former investigations we were not able to form such radical anions with linearly connected polycyclic systems [13]. 

As it is a bridged polycyclic system, twistane has "strategic bonds" (bonds 11 and 12 in the conventional numbering adopted in structure l), the disconnection of which leads to rather simple intermediate precursors. 

Fused polycyclic systems are very numerous and diverse. They are named using the structures and names of their smaller components and the concept of ortho-fusion, which is purely a formal operation encountered in nomenclature. This concept is essential to the naming of larger systems, and is the formation of one bond by the condensation of two bonds belonging to two different cyclic systems, one of them being a mancude ring. 

Extensions of the strain theory have been made, with varying success, to other hydrocarbon ring structures, saturated and unsaturated, to ring compounds in which the hydrogen atoms have been variously substituted, and to rings containing atoms other than carbon, as well as to bieyclic and polycyclic systems,... 

Since the term cryptand only refers to crown ether derived polycyclic receptors, polycyclic systems deriving from other structural motifs are commonly termed molecular cages or molecular capsules, although other names such as nanospheres, nanoflasks, temple-type receptors, etc., can also be found. The word cage well illustrates the overall architecture of many of these receptors, consisting of a floor and a roof connected by at least three bars.1 For self-assembled receptors, on the other hand, the term molecular capsule is often more appropriate. 

A more challenging aspect of the study of the trithiapentalene structure has been the synthesis of polycyclic systems containing more than three sulfur atoms in line. For these compounds, partial bonds of different strengths are involved and the use of formulae with quadricovalent sulfur does not help very much. In such cases, formulae are drawn with dicovalent sulfur atoms, with single bonds between those sulfur atoms whose distance is relatively short, with the optional addition of dotted lines to indicate the weakest bonds, as shown in formulae (5), (6) and (7). In this chapter we shall omit these dotted lines, the partial bonding between sulfur atoms being implied by the fact that the four or five sulfur atoms are aligned in the formula. 

Arene activation by metal complexation is not always necessary for the cyclisation of samarium ketyls on to aromatic systems. Studies carried out by Reissig demonstrated the utility of these reactions for the synthesis of a wide variety of polycyclic systems, although yields and stereoselectivities depend highly on the substrate structure.68 One well-behaved system is represented by the cyclic y-naphthyl ketones 50-54, which cyclise with excellent diaster-eocontrol, thereby incorporating four- to eight-membered rings into the tetracyclic structures 55-59 (Scheme 5.38).68 Tetracyclic product 56 (n = 1) is of particular interest because of its steroid-like structure. 

---