Ring-size selectivity

A triple RCM reaction cascade has been reported that allows for the construction of dihydrofurans with two adjacent dihydropyran or dihydrofuran rings from a common enantiomerically enriched acyclic precursor (Equation 55) <20010L1989>. Ring-size-selective metathesis reactions and the synthetic utility of enantiomerically pure l,5-hexadiene-3,4-diol, derived from D-mannitol, have been discussed in detail <2007ASC215>. 

An equally high ring-size selectivity is observed in the cyclization of 1,4-diketones such as 282 (Scheme 2.109). Here again an intramolecular crotonic condensation may yield two products, containing either a three- or a five-membered ring. The reaction, however, invariably follows the latter pathway and serves as a reliable method to synthesize cyclopentenone derivatives. A similar reaction for 1,6-diketones, for example 283, ultimately yields acylcyclo-pentenones. The competing pathway leading to the cycloheptenone derivative is virtually blocked. 

Scheme 8.15 Ring size-selective reduction of lactones using Sml2 in H20.

Some polar bonded-phase sorbents have also been explored for the HPLC separation of PAH. A silica-diamine packing was proposed as an alternative to the C.g-packing, using methylenechloride-hep-tane as the eluant (Chmielowiec and George (37)). The major advantages of this system would be a better ring size selectivity for alkylated PAH and the absence of water which eliminates splitting off for LC-MS analysis and hence increases sensitivity. 

Schmidt B, Nave S. Synthesis of Dihydrofurans and Dihydropyrans with Unsaturated Side Chains Based on Ring Size-Selective Ring-Closing Metathesis. Adv Syndi Catal. 2007 349(l-2) 215-230. 

Drawing-, text-, and structure-input tools are provided that enable easy generation of flow charts, textual annotations or labels, structures, or reaction schemes. It is also possible to select different representation styles for bond types, ring sizes, molecular orbitals, and reaction arrows. The structure diagrams can be verified according to free valences or atom labels. Properties such as molecular... 

Figures 9-32A and B [98] illustrate the correlation of wet pressure drop and system vapor rate at various liquid rates for No. 2 Nutter rings however, other available data indicate that other sizes of Nutter rings, Pall rings, and selected other packing shapes correlate in the same manner.

Finally, new tricyclic hexacoordinated phosphoranes with internal P-N coordination were synthesized by Swamy and coworkers by oxidative addition of cyclic phosphite precursors with quinones or with a combination of diols and (z-Pr)2NCl [57, 58]. Various ring sizes from five to eight membered were obtained showing the generality of the approach. A selection of compounds (47a-47e) is presented in Fig. 8. 

Electrophilic cyclizations are useful for closure of a variety of oxygen-, nitrogen-, and sulfur-containing rings. The product structure depends on the ring size and the exo-endo selectivity. The most common cases are formation of five- and six-membered... 

Benzyl carbamates have been used to form both five- and six-membered nitrogen-containing rings. The selectivity for N over O nucleophilicity in these cases is the result of the nitrogen being able to form a better ring size (5 or 6 versus 7 or 8) than the carbonyl oxygen. 

A wide range of ring sizes have been synthesized, from the very small [1.1.0] fused ring systems to the macro-bicyclic systems. The reactivity and synthesis of the fused ring systems is reviewed first followed by the nonfused compounds. As the synthetic methods used to produce the various examples in this class of compounds were well reviewed in CHEC-II(1996), the synthetic methods section, Section 12.12.8, will outline selected types of compound within this group that have received more attention over the review period. 

---