Saturated Ring Systems

Saturated ring systems lose side chains at the alpha carbon. Upon fragmentation, two ring atoms are usually lost. 

Alternative ( soft ) ionization techniques are not usually required for aromatic isothiazoles because of the stability of the molecular ions under electron impact. This is not the case for the fully saturated ring systems, which fragment readily. The sultam (25) has no significant molecular ion under electron impact conditions, but using field desorption techniques the M + lY ion. is the base peak (73X3861) and enables the molecular weight to be confirmed. 

It must be noted that the saturated ring system, the 1 -chloro-A -phosphirane was synthesized [84] a cyclic phospheniiun cation was postulated which is stable towards ring opening [85]. A corresponding cation could not be isolated, due to a methanid shift from a trimethylsilyl substituent. 

Unsaturated five membered 2-chalcogenons with two heteroatoms at 1,3-positions (6) will show versatile reactivities with acceptors, since E in 6 must be very electron rich with the formation of a stable cyclic 671 electron system (6A).27,28 A saturated ring system 7 is expected to show similar trend due to the stabilization by the allylic X-C-Y framework with 471 electrons (7A). Arduengo reported the TBP formation of 1,3-dimethyl-4-imidazoline-2-thione (8), a typical example of 6 (X = Y = NMe and E = S), with bromine (8 Br2), together with the reactions.36... 

Shown below are the aromatic ring systems and names of compounds in this book used for Method 1. There are two types of these. The ori/zo-fused and the ortho- and peri-fused rings. For each of the ori/zo-fused ring systems the corresponding saturated ring system and the name obtained by Method 2 is shown. 

Two publications appeared during the past period reporting oxidation of partially saturated ring systems to fully conjugated analogues. These transformations are shown in Scheme 16. 

Most partially saturated ring systems, 2,3-dihydro-l,4-dioxin 10 (sometimes named as 1,4-dioxene), 2,3-dihydro-1,4-dithiin 11, 2,3-dihydro-l,4-oxathiin 12, 2,3-dihydro-l,4-benzodioxin or 1,4-benzodioxane 13, 2,3-dihydro-l,4-benzodithiin 14, and 2,3-dihydro-l,4-benzoxathiin 15 are well investigated. Ring numbering for compounds 10-12 is followed as shown, independently of the presence of substituents. 

Both germanium and tin form saturated ring systems of the type (R2MNR ) that are synthesised in a similar manner to cyclosilazanes however, they are limited to four- and six-membered rings (n = 2 or 3, respectively). Transamination reaetions with the elimination of a volatile amine are also used to prepare eyelostannazanes [eqn (10.22)]. ... 

The next main section deals with thermodynamic aspects. It starts by consideration of the intramolecular forces between heterocyclic molecules which influence melting and boiling points, solubility and chromatographic characteristics. This is followed by a section on stability and stabilization, including thermochemistry and conformation of the saturated ring systems, and then a discussion of aromaticity. 

Rule 1. Use the name norbomane for the fundamental saturated ring system, which is bicyclo[]2.2.1]] heptane. Use the name bornane for the saturated larger parent compound, which is 1,7,7-trimethylbicyclo[]2.2.1]] heptane. Form a series of names based on these two new terpene names. 

Draw, number and letter the saturated ring system found in many steroids. Describe the most common conformations of the rings found in saturated steroid ring systems. 

The second major type of sigma-bonded boron-nitrogen-carbon heterocycles contains boron and nitrogen as annular atoms of saturated ring systems. Though several types of such azaboracycloalkanes are known 7>... 

X-ray studies also have been carried out on saturated and partially saturated ring systems. For example, structures of three hexahydrofuro[3,4-c]furan-type lignans ((63), (64), (65)) have been determined <92AX(C)2240>. Compound (63) was obtained as colorless needles by acetylation of... 

Most of the work described so far deals with the synthesis of members of the many possible ring systems. Both saturated and partially saturated ring systems have been prepared, but most information comes from the fully conjugated ring systems. 

Azine approach. Saturated ring systems with a bridgehead nitrogen atom are best prepared from 3,4,5,6-tetrahydroazine 1-oxides. The latter can be classified as nitrones and as such will undergo 1,3-dipolar cycloaddition reactions. 

The naming of these compounds is a bit odd. Pyran refers to the six-membered oxygen-containing heterocyclic ring system with two double bonds. It is not aromatic though compounds like pyrones are. The compound with only one double bond is therefore dihydropyran, and the saturated ring system istetrahydropyran. 

One of the most important classes of Michael initiated ring closure processes in the construction of carbo- and heterocycles are stepwise cycloaddition reactions where a metal induces dipolar behavior in otherwise unreactive organic compounds to be reacted with activated olefins. In this area, Pd-assisted cycloaddition reactions which involve zwitterionic zr-allylPd complexes of type I (linear type), II, or III (Pd-Trimethylenemethane (TMM) type and analogs) as reactive dipole partners are popular methods that provide highly functionalized, saturated ring systems often with high stereocontrol and atom economy (Scheme 1). Discovered in the early 1980s, they have been extensively covered in the review literature [8-16]. 

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