Six-membered ring derivatives

As we have seen previously (see Section II,A,3), the four-membered ring heterocycle 20 is a precursor of germene 15 that has been characterized by its dimerization and by insertion into the Ge-S bond of 2029 (Scheme 7). However, the formation of 63 could also arise by insertion of 21 into a Ge-C bond of the 1,3-digermacyclobutane 62, and 64 could be formed from germene 15 and a four-membered ring (Me2Ge-S)2 intermediate.29 The six-membered ring derivative 61 has also been observed. 

Dihydrothiophenes have also been used to provide access to six-membered ring derivatives. A carbenoid species might be involved in the ring expansion of the dihydrothienyl ketone (203) to give a 1 8 mixture of (555) and (556) (79JA5059). 

The same general methodology, the reaction of the ditellurolate dianion 68 with tetrachloroethylene, was employed for the synthesis of the thiophene-annelated TTeF. Compound 69 was obtained in 75% yield (83MI1 84JA8303 85MI2). No formation of the possible six-membered ring derivative was reported. 

While many enantioselective processes utilizing optically pure acyclic sulfoxide derivatives have been studied extensively among the synthetic community, their cyclic counterparts have, by comparison, received little attention. Cyclic sulfoxide systems which have received the most attention are five- and six-membered ring derivatives, and it is the chemistry of such compounds which forms the subject of this discussion. 

Under these conditions the stereoselectivity is higher, as the attack of the reagent occurs equatorially, leading to axial amines (72-92%). A comparison with the results of reduction of 4-f-butylcyclohexanone shows a complete inversion of stereoselectivity (16-27% of equatorial attack). Reductions of 2-alkylcyclohexanone enamines also proceed with high stereoselectivity, leading predominantly to the cis amines in the six-membered ring derivatives (66-96%), and almost exclusively in the case of the five-membered enamines (95-98%) (Scheme 84). The A1,3 strain112 in fact would shift the conformational equilibrium towards the axial (or pseudo xial) orientation of the substituent. 

Ca,Mg7 25Si,4 - view of the crystal structure along the [001] direction (Si red spheres, Ca green spheres, Mg small blue spheres), disordered model there are isolated Si atoms and planar six membered ring derivatives Si 2... 

AIBN, the desired six-membered ring derivative was produced via intramolecular... 

Kawaguchi and coworkers at Teijin have prepared a series of polymers based on poly(diallyl amine), its copolymer with sulfur dioxide, and various terpolymers.39 The chemistry of this polymer synthesis is shown below. The patent description shows the diallylamine polymers to be polypiperidine (six-membered ring) derivatives, but there are a number of publications that show this monomer to produce preferably polypyrrolidine (five-membered ring) structures ... 

Formation of the five-membered ring from an existing six-membered ring derivative 330... 

Sulfonated polyimides (sPI) are another type of ionomers that have been prepared [5] by copolymerization of 4,4 -diamino-biphenyl 2,2 -disulphonic acid (BDSA), 4,4 -oxydianiline (ODA), and naphtalenic or phthalic anhydrides. Polyimide structures based on six-membered rings derived from naphtalenics anhydrides, such as 1,4,5,8-naphtalene tetracarboxylic dianhydride (NTDA), shown in Fig. 6.8, are more hydrolitically stable than the five-membered rings basedoxy-diphthalic anhydrides (ODPA). 

Distinction should be made between the self-assembled dimeric or trimeric species dted above and authentic heterocycles in which the metal(III) is three-coordinate, e.g. the four-membered ring derivatives R2M2E2R 2, 20, and the six-membered ring derivatives R3M3E3R 3, 21. 

In 1999, an efficient DKR in the context of baker s yeast mediated reductions of 2-benzenesulfonylcycloalkanones was reported. Again, the best results were observed with five- or six-membered ring derivatives, whereas the seven-and eight-membered ring compounds showed much less efficiency (Scheme 3.3). 

Furan is the five-membered ring heterocycle containing one oxygen. Thiophene is the five-membered ring heterocycle containing one sulfur. Pyran and thiopyran are the six-membered ring derivatives. 

Control of the diastereochemistry of the DA reaction by means of a chiral center in the substrate is a versatile and reliable mean for stereoselective synthesis of six-membered ring derivatives with multistereogenic centers. A reaction of fu-marate having two chiral auxiliaries with 2-methyl-l,3-butadiene afforded virtually complete stereocontrol in titanium-promoted DA reaction to give an optically active cydohexene derivative [147] (Scheme 14.58). 

Our stereochemical studies have involved the use of five-and six-membered ring derivatives of trivalent phosphorus, 10... 

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