Ring under high dilution conditions

Condensation of dicesium 2-thioxo-l,3-dithiole-4,5-diselenolate with fo/s-alkylating polythioethers under high dilution conditions afforded the TTF-containing macrocycles possessing soft donor sites and 12-, 15-, and 18-membered rings <%JCS(P1)1995>. 

The use of azodicarboxylates as a route to dioxaphosphoranes continues to attract attention. In the most recent contribution, triphenylphosphine and di-iso-propyl azodicarboxylate (43) are shown to react with prcpane-1,3-diol and butane-1,4-diol in THF at 0°C under high dilution conditions to give the expected six-and seven-membered-ring phosphoranes (44 ab)36. In more concentrated solution however, cyclic oligomers are formed. Substituted and ccnformationally restricted 1,3- and 1,4-diols form the expected cyclic phosphoranes without recourse to high dilution techniques. 

The 10- to 20-membered ring diols have also been [V2Cl3(THF)5]2[Zn2Cl6] under high dilution conditions (Equation (50))... 

Ring expansion of the six-membered ketosulfide (371) produces both 5-thiocanone and 4-thiocanone [372 v 1700 cm-1 (neat)], but in low yields (70JOC584). The photochemistry of these ketones has been studied and leads to a variety of products in low yields (70JOC584). 3-Thiocanone (373 b.p. 66-68 °C/0.35 mmHg, v 1708 cm-1) is formed in 44% yield by the dihalide method, but in only 3% yield by the Dieckmann reaction under high dilution conditions (70JPR1058). 

The kinetics of 5-exo and 6-endo acyl radical cyclizations have been investigated under a variety of reaction conditions.28 The presence of the 6-endo product was found to arise either by a direct cyclization (2.0 x 104 s 1) or by a ring expansion (4.2 x 103 s ) from the 5-exo radical product (1.6 x 10s s 1). Consequently, cyclization in the presence of high concentrations of fast H-donors (e.g. B113S11H) furnished 5-exo products whereas reactions under high dilution conditions or with poor H-donors gave rise to 6-endo products. 

Dienes are cyclized by intramolecular metathesis. In particular, cyclic alkenes 43 and ethylene are formed by the ring-closing metathesis of the a,co-diene 46. This is the reverse reaction of ethenolysis. Alkene metathesis is reversible, and usually an equilibrium mixture of alkenes is formed. However, the metathesis of a,co-dienes 46 generates ethylene as one product, which can be removed easily from reaction mixtures to afford cyclic compounds 43 nearly quantitatively. This is a most useful reaction, because from not only five to eight membered rings, but also macrocycles can be prepared by RCM under high-dilution conditions. However, it should be noted that RCM is an intramolecular reaction and competitive with acyclic diene metathesis polymerization (ADMET), which is intermolecular to form the polymer 47. In addition, the polymer 47 may be formed by ROMP of the cyclic compounds 43. 

Figure 2. Under high-dilution conditions, die reaction of 1 with isophthaloyl chloride 2 yields tetralactam macrocycle 3, the catenane 4, and octalactam ring 5.

A major drawback of alkene metathesis is lack of control over the stereochemistry of the newly formed double bond. For unstrained systems, E/Z ratios are virtually unpredictable. Alkyne metathesis, on the other hand, can always be combined with subsequent Lindlar hydrogenation, thereby giving access to stereochemically pure 2-olefins. In 1998, Ftirstner and Seidel were the first to report a ring-closing alkyne metathesis [7]. Under high-dilution conditions (0.02 m) and reduced pressure (20 mbar, removal of 2-butyne, solvent 1,2,4-trichlorobenzene (b.p. 214 °C)) the Schrock catalyst was applied to assemble macrocydic... 

---