Five-membered cycles

On the other hand, when the oxidative carbonylation of a ,a -disubstituted propynylamines was carried out in the presence of an excess of CO2, the intermediate carbamate species could undergo cyclization with incorporation of CO2 into the five-membered cycle, either by direct nucleophilic attack of the carbamate oxygen to the triple bond coordinated to Pd(II) (Scheme 33, path a) or through the intermediate formation of a palladium carbamate complex followed by triple bond insertion (Scheme 33, path b). Carbon monoxide insertion into the Pd - C bond of the resulting stereoisomeric vinylpalladium intermediates then led to the final oxazolidi-none derivatives. 

Ring Enlargement. The ring enlargement of five-membered cycles to six-membered ones is possible over pure metallic sites. This reaction is, however, generally slower than any of the cyclization processes. We believe that it proceeds by some kind of radical type bond shift mechanism (see Section III,A) rather than via ionic intermediates owing to some kind of acidic properties of platinum assumed by Lester (108). 

The formation of an intramolecular dihydrogen bond should be accompanied geometrically by the creation of a ring of size four to six, with the five-membered cycle preferred. 

When SO2CI2 was used as a reagent the corresponding five-membered cycle (108) breaks down and so provides a useful route to aziridines (Scheme 16) <92TL3013> see also <9lJOC3l77>. 

Single crystal X-ray diffraction data for disubstituted 1,3,2-dioxathiolane V-oxides 12 (R = c-C6Hn) <1995AXC129>, 12 (R = Ph), 13, and 14 <1996AXC739> revealed half-chair (envelope) conformations of the five-membered cycles with the S=0 group in a pseudoaxial position and other substituents in pseudoequatorial positions. 

On the basis of similar five-member cycles were obtained the r 5(7i)-complex compounds of type 556 [7, vol.3 8]. In this respect, we note that the r "(7i)-metal-binding shown by 557-560 is formed in complexes of five- and six-member boron-containing carbocyclic ligands (n = 5 and 6, respectively) [8,942,954b]. A variety of similar structures are presented in Ref. 184f. 

The complex 669 is interesting due to the fact that, according to x-ray diffraction data, the six-member metal-cycle (but not five-member cycle 672) exists in its structure [243] ... 

At the same time, five-membered cycles are formed in the low-molecular copper(II) — glycilglycine system [44], Thus a knowledge of the local chelating fragment conformation allows us to predict PCMU with a specified size of the metallocycles. 

Bifunctional nucleophilic reagents, such as ketones, enamines, / -diketones and their derivatives, amidines, thioureas, thioamides, dithiocar-bamates, and others, are very often employed in cyclizations with azine derivatives, resulting in the formation of annelated five-membered cycles (84UK1648 85KGS1011). The structure of all these 1,3-dinucleophiles feature a double bond conjugated with an anionic center or with a heteroatom bearing the lone pair of electrons (Scheme 5). 

Vinylcyclopropanes are also suitable starting materials for the construction of five-membered cycles. The vinylcyclopropane rearrangement first discovered in the late 1950s191 192) immediately became the subject of intense mechanistic investigations 173 193,194) ancj was use(j for tjje CyCiopentene anelation 165.171 195 197> Gr cycloheptane anelation 198 200). This reaction was successfully applied to vinylcyclopropanes bearing a silyl moiety by Paquette 137,138> and was found particularly useful for the synthesis of vinylsilanes as part of a five-membered cycle. These have been in some cases subjected to electrophilic substitution 137,138 201 202) (Schemes 58 b, 65, 82-84). The ring expansion takes place when the thermolysis is conducted between 570° and 670 °C and only occurs with those compounds which do not possess 137)sterie inhibition around the carbon-carbon bond (Scheme 76, compare a and e). Incorporation of the double bond (Scheme 76, compare a and e). Incorporation of the double... 

PAH are constituted by two or more aromatic rings joined together or separated by a five-membered cycle. The studied PAHs and their structures are shown in Fig. 40. In addition to these 16 PAHs from the USEPA list, benzo[b]fluorene and benzo[e]pyrene are also studied. 

The difference is not so marked for benzo[b]fluoranthene and benzo[k]fluoranthene, which have a five-membered cycle in their structure. Nevertheless, a slight bathochromic effect can be noticed for the less-condensed isomer (Fig. 47). 

Some PAH molecules include in their structure a five-membered cycle. When an aromatic ring is added to the linear chain containing the five-membered cycle, a bathochromic shift and hyperchromic effect are observed. 

In this section, for we are the first time discussing both single and multiple SN bonds (Table 10) In a five-membered cycle of thiadiazole, H2C2N2S, the -S-N= bonds are 0.06 A shorter than in S(NMb2)2 and about 0.1 A shorter than the Schomaker-Steven-son estimate. These molecules are usually regarded as aromatic systems, which explains... 

Vinylcyclopropanes are also suitable starting materials for the construction of five-membered cycles. The vinylcyclopropane rearrangement first discovered in the late 1950s immediately became the subject of intense mechanistic investigations 193,194) fpj. jjjg cyclopentene anelation les. 171,195,197) cycloheptane... 

For example, pyrrolidine enamines are more nucleophilic than piperidine and morpholine-based enamines. This difference in reactivity of cyclic amines originates from variations in pyramidalization at nitrogen. The nitrogen atom in a five-membered cycle is much more flat than the nitrogen atom in a six-membered cycle. 

This is not a comprehensive listing, and other substructures leading to five-membered cycles will be presented at the end of this section. 

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