Hexamer cyclization reaction

When the cyclization reaction is carried out under basic conditions with KOH in refluxing xylene, the hexamer is isolated in nearly quantitative yield (Scheme 7.2).13... 

As already discussed in Section 2.2, crystalline dimethylsilanediol 53 can be prepared by hydrolysis from hexamethylcyclotrisilazane 51, from dimethoxydimethyl-silane [40], and from octamethylcyclotetrasilazane (OMCTS) 52. The most simple preparation of 53 is, however, controlled hydrolysis of dimethyldichlorosilane 48 in the presence of (NH4)2C03 or triethylamine [41]. Likewise, hydrolysis of hexam-ethylcyclotrisiloxane 54 and of octamethylcyclotetrasiloxane 55 eventually gives rise to dimethylsilanediol 53. In all these reactions the intermediacy of the very reactive dimethylsilanone 110 has been assumed, which can be generated by pyrolytic [42, 43] and chemical methods [44—46] and which cyclizes or polymerizes much more rapidly, e.g. in contact with traces of alkali from ordinary laboratory or even Pyrex glassware [40, 47] to 54, 55, and 56 than trimethylsilanol 4 polymerizes to hexamethyldisiloxane 7. Compound 111 is readily converted into dimethylsilanone 110 and MesSil 17 [46] (Scheme 3.6). 

In the synthesis of the core structure of the diynene antitumor antibiotics esperamicins and calicheamicins by Danishefsky et al. [108], an acetylene-aldehyde coupling reaction was used to achieve the cyclization. As shown in Scheme 60, reaction of a toluene solution of acetylene aldehyde 182 with potassium hexame-thyldisilazide at —78 °C for 20 min afforded a 52% yield of a 10 1 ratio of 183 and 184. 

Although the mechanism of the base-induced formation of calixarenes has been studied in some detail, the reaction pathways remain uncertain. The most intuitively reasonable proposal is that the immediate precursor of any particular calixarene, regardless of size, is the linear oligomer carrying the requisite number of aryl residues. Another proposal, however, postulates that calix[8]arenes, for example, arise from intermolecularly hydrogen-bonded dimers (hemicalixarenes) formed from a pair of crescent-shaped, intramolecularly hydrogen-bonded linear tetramers. Calix[4]arenes, formed under considerably more strenuous conditions, have been postulated to be the result not of direct cyclization of the linear tetramer but of reversion of the calix[8]arene. The cyclic octamer is viewed as the product of kinetic control, and the cyclic tetramer is viewed as the product of thermodynamic control. The particular efficacy of KOH and RbOH for the formation of calix[6]arenes suggests that the hexamer is the product of template control. 

Nierengarten et al. revealed the reaction mechanism of pillar[5]arenes using 1,4-dialkotybenzene derivatives (Scheme 2.8d). To synthesize pillar[6]arenes, a co-cyclization of 2,5-bis(bromomethyl)-l,4-diethotybenzene with 1,4-diethotybenzene was performed. However, the product obtained was not cyclic hexamers (pillar[6]arenes), but cyclic pentamers (pillar[5]arenes). This... 

---