Ring size control

Figure 33 The cyclo(Adm-Cyst)3 adopts a figure-eight-like helical structure. The chiral amino acid, cystine, configuration determines the helix disposition (rigjit-handed or left-handed helix). Adamantane plays an important role as a ring size controlling agent. Taken from Ref. [163] with permission.

Tetrahydroxanthen-l-ones are obtained from the reaction of salicyl Af-tosylimines with alicyclic enones. Ring size controls the amounts of the Baylis-Hilhnan adduct which are also isolated <05SL2623>. 

Scheme 28 Chelate ring size-controlled oxidative addition - C-Br bond formation in bromination of Pt(II) diaryl complexes with Bt2...

Ring size controls die dihedral angle in cyclic allylamines. 47 is held with the orbitals overlapping and extensive ring enlargement caused by an allyl-type shift results (reaction 35) but the... 

Ring size controls the speed of cyclic ether formation... 

Concerted cycloadditions are observed with heterocyclics of all ring sizes. The heterocycles can react directly, or via a valence tautomer, and they can utilize all or just a part of unsaturated moieties in their rings. With three-membered rings, ylides are common reactive valence tautomers. Open chain 47T-systems are observed as intermediates with four-membered rings, and bicyclic valence tautomers are commonly reactive species in additions by large rings. Very often these reactive valence tautomers are formed under orbital symmetry control, both by thermal and by photochemical routes. 

There is no separate shift conversion system and no recycle of product gas for temperature control (see Figure 1). Rather, this system is designed to operate adiabatically at elevated temperatures with sufficient steam addition to cause the shift reaction to occur over a nickel catalyst while avoiding carbon formation. The refractory lined reactors contain fixed catalyst beds and are of conventional design. The reactors can be of the minimum diameter for a given plant capacity since the process gas passes through once only with no recycle. Less steam is used than is conventional for shift conversion alone, and the catalyst is of standard ring size (% X %= in). 

Entries 3 to 6 are examples of ester enolate alkylations. These reactions show stereoselectivity consistent with cyclic TSs in which the hydrogen is eclipsed with the enolate and the larger substituent is pseudoequatorial. Entries 4 and 5 involve SN2 substitutions of allylic halides. The formation of the six- and five-membered rings, respectively, is the result of ring size preferences with 5 > 7 and 6 > 8. In Entry 4, reaction occurs through a chairlike TS with the tertiary C(5) substituent controlling the conformation. The cyclic TS results in a trans relationship between the ester and vinylic substituents. 

The chromium-catalyzed coupling is extended to the intramolecular cycli-zation to the dials 43. The O-chelating silyl scavenger is used to control the stereoselectivity as shown in Scheme 25 [61,62]. The ring size effect on the stereoselectivity is also investigated. 

Macrocycles containing isoxazoline or isoxazole ring systems, potential receptors in host—guest chemistry, have been prepared by multiple (double, triple or quadruple) 1,3-dipolar cycloadditions of nitrile oxides, (prepared in situ from hydroxamoyl chlorides) to bifunctional calixarenes, ethylene glycols, or silanes containing unsaturated ester or alkene moieties (453). This one-pot synthetic method has been readily extended to the preparation of different types of macrocycles such as cyclophanes, bis-calix[4]arenes and sila-macrocycles. The ring size of macrocycles can be controlled by appropriate choices of the nitrile oxide precursors and the bifunctional dipolarophiles. Multiple cycloadditive macrocy-clization is a potentially useful method for the synthesis of macrocycles. 

The effect of /rfive-coordinate aluminum alkyls have been discussed.94 Studies have also focused on the equilibrium between four-coordinate and five-coordinate isomers in compounds of the type [R2Al /i-0(GH2) ER1x ]2 (n = Z, 3 ER1x = OR1, SR1, NRX2) (shown in Figure 7(h) for n = Z). Factors that control this equilibrium and hence the coordination around the aluminum include the steric bulk of the substitutent at the aluminum (R), and the Lewis base donor R1, the basicity of the neutral donor group ER1, and the chelate ring size (as determined by n).95... 

Hwang, W. W., Venkatasubrahmanyam, S., Ianculescu, a. G., Tong, A., Boone, C., and Madhani, H. D. A conserved RING finger protein required for histone H2B monoubiquitination and cell size control. Mol. Cell 2003, 11, 261-66. 

The extent to which cyclization occurs during polymerization depends on whether the polymerization proceeds under equilibrium control or kinetic control, the ring sizes of the possible cyclic products, and the specific reaction conditions. 

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