Alicyclic ring systems, carbon

In semicydic allenic hydrocarbons, one of the terminal allene carbon atoms is part of an alicyclic ring system, as illustrated by the general structure 37 in Scheme 5.3. Numerous hydrocarbons of this type are known, some of them carrying more than one allene group, such as in the case of the conjugated bisallenes 127 and 129 (see Scheme 5.17), and many of them are described in the review literature [2] and will not be repeated here. However, since Chapter 6 on cycloallenes does not treat these derivatives, some new developments in this area will be briefly presented, limited to the two cases in which cydopropane rings form the end groups of the allene moiety, i.e. 246 and 249. 

Oxidative Reactions at Carbon 1.09.2.4.1 Alicyclic ring systems... 

To increase the yields of the ring closure reactions, a new method was developed that was successfully applied for the synthesis of alicyclic fused systems of both the parent oxazolidine-2-thione and tetrahydro-1,3-oxazine-2-thione (85S1149). As an example, the synthesis of 2-thioxoperhydro-l,3-benzoxazine 103 is described. The dithiocarbamate 101, prepared from the amino alcohol 100, carbon disulfide and triethylamine, was treated with ethyl chloroformate in the presence of triethylamine, to give the thioxo derivative 103 via the transition state 102 (85S1149). In this way, the fused-skeleton thioxooxazines (91, X = S, 92) can be prepared with considerably higher yields (50-70%) than by the earlier methods (85S1149). 

Aromatic compounds are unsaturated cyclic molecules that possess additional stability as a result of the arrangement of 7i-electrons associated with the unsaturation of the ring system. This book will concentrate on the chemistry of benzene (4) and its derivatives and related polynuclear hydrocarbons. Aromatic compounds are also known as arenes they can becarbocydic, indicating that the ring skeleton contains only carbon atoms, or heterocyclic, with at least one atom other than carbon in the ring. These heteroatoms are typically N, O or S. Heterocyclic compounds, which can be aromatic or alicyclic, are covered in another book in this series. 

Five-membered alicyclic ring substrates eliminate more readily than the six-membered analogs (92, below). This may be due to the greater thermodynamic stability of cyclopentene (heat of hydrogenation 1.7 kcal/mole less than cyclohexene ) or the reduction in steric interactions on elimination in the cyclopentene series due to a decrease in the eclipsing interactions between substituents on adjacent carbons . Alternatively, a lower population of the favourable conformation for elimination from the six-membered systems, due to the preference of large axial groups to occupy the equatorial position, accounts for the observed differences in reactivity. 

An important characteristic of medium-sized alicyclic rings is the ease with which transannular reactions arising from proximity of atoms occur (e.g. see 103). In the preferred conformation of the four-carbon bridge in the longifolane (155) and longibornane (156) systems, the proximity of the C-3, C-10 and C-2, C-9 positions, respectively has been responsible for interesting transannular reactions, both of homolytic and heterolytic types. 

The second expansion is very much less stereoselective and produces a mixture of alkenes in the ratio (E,E)/(E,Z)-5 l. Here should be noted the use of a stabilized ylide in the second stage of Scheme 3. Generation requires only an amidine base (DBU) but the rearrangement proceeds at room temperature, some 70 °C above the most favoured unstabilized system. The ylidic carbon may also be part of the thiopyran ring, in which case, as equation (11) shows, the products are alicyclic (79JOC3230). 

Alicyclic compounds - Aliphatic compounds having a carbocy-clic ring structure which may be saturated or unsaturated, but may not be a benzenoid or other aromatic system. [5] Aliphatic compounds - Acyclic or cyclic, saturated or unsaturated carbon compounds, excluding aromatic compounds. [5] Alkali metals - The elements lithium, sodium, potassium, rubidium, cesium, and francium. 

In general terms, lignite and anthracite coals appear to contain much lower proportions of the volatile organic componnds (10) than the bituminous coals, although alkaline extraction of lignite (and bituminous coal) will yield organic acids of various types that can be characterized. On the other hand, the hexane-soluble portion of the pyridine extracts that were obtained at 50°C (120°F) from coal (carbon content 83.6% w/w) have been identified as alkylated chrysenes as well as alkylated picenes in addition to a mixture of C2g, C29, and C30 paraffins. There were also indicates of the presence of an alicyclic or methyl-substituted five-ring (cat-condensed) system as well as l,2,5,6-dibenzanthracene(s) (Table 10.1). 

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