Other Heteroatoms in the Ring

A recent example for the preparation of tellurosugars was provided by Schiesser and coworkers 

Phosphosugars attracted interest for potential biochemical applications but have not met with expectations. Because of the lack of stability of the phosphane oxidation state (+ 3), the phosphane oxide is usually preferred in phosphosugar syntheses. The literature on phosphosugars until 1984 was reviewed by Yamamoto and Inokawa [228]. 

SCHEME 9.21 a MeNOj, then AcjO, NaOAc, b MePh(=0)0Me, benzene, 80°C, c Hj, PtOj, then NaNOj, HOAc, d TrCl, pyr., e sodium dihydrobis-(2-methoxyethoxy)aluminate, then HCl, EtOH and f AC2O, pyr. 

SCHEME 9.22 a O = PH(MeO)2, DBU, b MeOCOCOCl, DMAP, MeCN, then BusSnH, AIBN, toL, c sodium dihydrobis-(2-methoxyethoxy)aluminate, HCl, H2O2 and d AC2O, pyr., then CH2N2. 

Considering the size and number of diverse areas covered in this chapter, it was not possible to aim for comprehensiveness. Consequently, to fill the gaps between somewhat arbitrarily chosen examples, additional reading will guide the interested reader towards more specialized information for deeper insight. 

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Nitrogen-donor macrocycles have also been prepared with other heteroatoms in the ring but not coordinated to the metal. Schrauzer (124) observed that chelate complexes (LVI) could be obtained when certain metal salts were present in the reaction of borinic acid ester... 

Related to the crown ethers are compounds, such as hexamethyl-[14]-4,ll-diene (6), which differ by the replacement of one or more of the oxygen atoms by other kinds of donor atoms, particularly N or S. MacrocycHc amine and thioether compounds have been synthesized. Compounds having more than one kind of heteroatom in the ring are called mixed-donor macrocycles. The naturally occurring metaboUtes nonactin [6833-84-7] and monactin [7182-54-9] have both ether and ester groups incorporated in the macrocyclic stmcture. 

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

Only one heterocyclic cycloproparene having the heteroatom in the ring adjacent to the cyclopropene has been isolated, namely the l,l-dimethylcyclopropa[c]pyri-dine derivative 170 which is available in moderate yield upon photochemical extrusion of Nj from the 3Ff-indazole 169. In contrast, irradiation of 171 produces no cycloproparene 173, but other products derived from the intermediate biradical 172. Apparently, the radical centers in 172 are too far apart to allow ring-closure. ... 

The azoles (oxazole, imidazole, and thiazole) are five-membered aromatic heterocycles that have two heteroatoms in the ring. One of the heteroatoms in each of these heterocycles is an sp2-hybridized nitrogen that contributes one electron to the 6n aromatic system and has a basic nonbonded lone pair. The other heteroatom (oxygen, nitrogen, or sulfur) contributes two electrons to the 6n system. The imidazole skeleton is present in the amino acid histidine. The thiazole ring occurs in thiamin (vitamin B. ... 

A compound with two heteroatoms in the ring 66, needed for a synthesis of mannicone 65, illustrates the other two oxidation states. Disconnecting both C-N bonds noting that C-l is at the acid oxidation level while C-3 is an enamine and so at the ketone oxidation level gives the keto-ester 67 and hydrazine 68. Where two heteroatoms are joined in a ring, it is usually better not to disconnect the bond between them but to look for a starting material containing both. 

One of the most fascinating reaction modes of disilenes is the ready formation of disiliranes, three-membered ring systems containing two silicon atoms and one heteroatom in the ring, which are hardly accessible by other routes. They are mostly formed by [2 + 1] cycloadditions of atoms or molecular fragments to the Si=Si double bond. One of the few examples of other accesses to this ring system is the reaction of AMithio-2,4,6-trimethylanilide 53 with the disilane 52, which affords the azadisilacyclopropane 54 in modest yield (equation 8)75. 

Other examples of 7-membered ring heterocycles with three heteroatoms in the ring include the chiral titanium complex 311 [01SL1889], the novel phosphoramidite ligand 312 [01SL1375], and the cyclipostin natural products from a Streptomyces species (e.g. Cyclipostin N, 313), which is a hormone-sensitive lipase inhibitor [02JAN480]. 

P increments (a -P), phosphenium cations, phosphaalkenee and heteroatom analogues, X3-phosphinines, phospholes with further heteroatoms in the ring (cr2-P) and, finally, the large field of phosphoranes with two, doubly bonded other elements as well as the X.5-phosphaalkynes (cx3-P).8... 

Anhydro-pyranoses with other than an oxygen heteroatom in the ring are known (although not common). Thiolevoglucosan has been known for many years [15]. An interesting example of the synthesis of 1,6-anhydrothiomannose (9) was presented recently [16]. 

Besides the thermodynamic feasibility, there should also be a kinetic pathway for the ring to open, facilitating polymerization. Cycloalkanes, for example, have no bond in the ring structiire that is prone to attack and thus lack a kinetic pathway. This is in marked contrast to the cyclic monomers such as lactones, lactams, cyclic ethers, acetals, and many other cyclic monomers that have a heteroatom in the ring where a nucleophilic or electrophilic attack by an initiator species can take place to open the ring and initiate polymerization. Both thermodynamic and kinetic factors are thus favorable for these monomers to polymerize (Odian, 1991). 

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