Medium-sized cyclic

To extend the scope of asymmetric transannular C-H insertions, more highly functionalized medium-sized cyclic epoxides have been investigated. A triad of cydooctene oxides 34, 36, and 38, possessing protected diol units, gave the expected alcohols 35, 37, and 39 (Scheme 5.10) [17, 18] an asymmetric synthesis of (-)-xialenon A has been achieved starting from alcohol 39 [19]. In comparison,... 

The acetal [1,2]-Wittig rearrangement protocol is also applicable to the synthesis of medium-sized cyclic ethers. For example, a reaction of the 9-membered cyclic acetal 37 with lithium piperidide provides the 8-membered ring ether 38 in good yield along with high diastereoselectivity (equation 20) . 

In the case of medium-size cyclic oxiranes, in the presence of diamine ligands, the decomposition of the oxiranyllithium intermediate can be prevented at low temperature and the lithium species can be trapped by various electrophiles (Scheme 52). The use of (—)-sparteine 112 has led to the corresponding functionalized oxiranes in good ee s. 

TABLE 9.2. AEst for Smaller and Medium-Size Cyclic Carbenes ... 

The partial steps of the conjugate addition in aminocatalytic reactions are in dynamic equilibrium, and thus products are formed under thermodynamic control. This fact is translated also in the geometry of the enamine intermediates, leading to the product, which can be either E or Z (Fig. 2.9). The geometry of the enamine depends on the catalyst structure and also on the substrate. Whilst proline-catalyzed reactions form preferentially, with a-alkyl substituted ketones, the. E-isomer, enamines derived from pipecolic acid afford an approximate 1 1 mixture of the E and Z isomers [6], In turn, small- and medium-sized cyclic ketones afford the E isomer. 

Major advances were also made in the field of ARCM. Notable examples of this process are shown in Scheme 2 and include AROM <02JA10779>, and the desymmetrization reactions of acetals <00TL9553> and ethers <02JA2868>. Especially noteworthy are the ARCM to yield medium-sized, cyclic amines that occur in high yield and enantiomeric excess in the absence of solvent <01JA6991> [—m indicates the site of RCM]. 

Carbodiimides have chiral structures similar to allenes, i.e., they can exist in optically active forms. Schloegl and Mechtler were the first to report a partial optical separation of N,N"-diferrocenylcarbodiimide into enantiomers by chromatography on acetylated cellulose, but other authors doubt the validity of these results. According to theoretical calculations a separation of carbodiimide enantiomers is not possible. N,N -diferrocenylcarbodiimide was also obtained in optically active form by kinetic resolution in the reaction with (-)-S-6,6"-dinitrodiphenic acid. Cervinka and coworkers isolated both enantiomers of (R,S)-N,N"-bis(o -phenylethyl)carbodiimide, and they found that they undergo racemization at room temperature. A recent study on the racemization mechanism of macrocyclic carbodiimides indicates that the open chain as well as the large ring carbodiimides racemize by nitrogen inversion or tra 5-rotation, while medium size cyclic carbodiimides racemize by cw-rotation. ... 

Medium sized cyclic /3-lactames are obtained via an intramolecular Staudinger ligation using DCC or EDC in the presence of Dabco. ... 

Medium-sized cyclic ketones. A method for synthesis of 8-12 membered cyclic ketones is formulated in equation 1. ... 

Medium-sized cyclic ketones have been enantioselec-tively alkylated via their chiral lithioenamines to yield 2-alkylcycloalkanones in 80-100% ee. This procedure has also furnished a,a -dialkyl cyclohexanones in good enantiomeric excess (eq 2). Based on this protocol, regiospecific deutera-tion of 3-methylcyclohexanones has been achieved with good enantioselectivity. ... 

In contrast with medium-sized cyclic ketones, alkylation of macrocyclic ketones can afford either optical antipode depending on whether the lithioenamine is formed via kinetic ( -) or thermodynamic conditions (Z-enamine) (eq 3). Optically active a-alkyl macrocyclic ketones have been formed in 30-82% enantiomeric... 

However, the products of the reaction of medium sized cyclic olefins, such as cyclooctene, with iodine azide did decompose to give the vinyl azides... 

The use of oxocarbenium ions provides an efficient method for the preparation of heterocycles, especially of medium-sized cyclic ethers. 

Attempts to apply Rh(II)-catalysis conditions similar to those in the synthesis of piperidones 164 (n = 1) to substrates 163 ( = 2, 3 R = Ph) with one or two additional methylene units in the linker met with little success [164 (n = 2 R = Ph) 35% yield, (/i = 3 R = Ph) 0% yield]. Intramolecular C —H insertion to give cyclopentanones appeared to be the major pathway in these cases. Use of Cu(acac)2-catalyzed decomposition of 163 (n = 2, 3 R = Ph) furnishes 164 ( = 2, 3 R = Ph) in 61 and 58% yield, suggesting a remarkably efficient and selective capture of the copper carbenoid by the amine to give a medium-size cyclic ylide in preference to other carbenoid pathways (94JOC6892). 

Wharton fragmentation Base-induced formation of medium-sized cyclic alkenes from 1,3-diol monosulfonates. 480... 

Cope, A. C., Ciganek, E., Howell, C. F., Schweizer, E. E. Amine oxides. VIII. Medium-sized cyclic olefins from amine oxides and quaternary ammonium hydroxides. J. Am. Chem. Soc. 1960, 82,4663-4669. 

A very interesting synthesis of medium-sized cyclic amines has been performed by selective ring cleavage of sulfonylated bicyclic amines.246 A Julia-type desulfonylation of an activated p-amino sulfone is the key step in this method, which takes place even in the presence of a hydroxyl leaving group in the P position (Eq. 140). 

Organic boron molecules with multidentate Lewis acidic sites such as (2) and (3) bind strongly with fluoride ion so as to form a pyramidalized chelate (4) (Scheme 3.32). The incorporation of the anion into a medium-sized cyclic structure releases ring strain via the pyramidalization of planar boron centers. It is interesting to see the binding energies of... 

New synthetic methods for construction of medium-sized cyclic ethers and convergent coupling of polyether fragments 01YGK193. 

More recently, [1,3] shifts at room temperature have been reported under these conditions with medium-sized cyclic vinylic carbinols. For example, (3) rearranges predominantly to the ring enlarged ketones (4) and (5) (equation I). 

With medium-sized cyclic compounds, transannular reactions have been observed. The reaction of cyclooctene with carbon tetrachloride and bromotri-chloromethane is an interesting example. As shown in the equation below, bromo-trichloromethane adds in a completely normal manner, but carbon tetrachloride gives some 4-chloro-l-trichloromethylcyclooctane as well as the expected product ... 

Medium-Sized Cyclic Ethers (7- to 10-Membered Rings)... 

Scales, L. E. Semiyen, J. A., Equilibrium Ring Concentrations and the Statistical Conformations of Polymer Chains Part 14. Calculation of the Concentrations of Medium Sized Cyclics in Poly(dimethylsiloxane) Equilibrates. Polymer 1976,17, 601-604. 

Table 4.2. Number of partial chirotopes generated for medium sized cyclic hydrocarbons.

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