Stereostructure

Reaction of lithium diphenylcuprate with optically active 2-bromobutane yields 2-phenylbu-tane, with high net inversion of conhguration. When the 2-bromobutane used has the stereostructure shown, will the 2-phenylbutane formed have the R or the S conhguration ... 

T. M. Kamenecka and S. J. Danishefsky, The Total Synthesis of Himastatin Confirmation of the Revised Stereostructure, Angew. Chem., Int. Ed. Engl., 37, 2995 (1998). We thank Professor Danishefsky for providing us with preprints of the himastatin communications here and in ref. 31(d). 

Stereostructure and synthesis of aplyronine A (macrolide with 24-member lactone ring), an antitumor substance of marine origin 96YGK1077. 

The relative stereostructure of 9-acetyl-7-hydroxy-l,2-dimethyl-7-meth-oxycarbonyl-4-phenyl-6-oxo-l, 4,7,8-tetrahydro-6/7-pyrido[l, 2-u]pyri-midine-3-carboxylate 122 was justified by an X-ray diffraction analysis (97JOC3109). The stereochemistry and solid state structure of racemic trans-6,9-//-l, 6-dimethyl-9 z-ethoxy-9-hydroxy-4-oxo-l,6,7,8,9,9 z-hexahydro-4//-pyrido[l,2- z]pyrimidine-3-carboxylate (123), adopting a cw-fused conformation, were determined by X-ray investigations (97H(45)2175). 

Stereostructure of ethyl c/>4u,7-H-7-phenyl-8-cyano-2-oxo-2,3,4,4u,7,8-hexahydropyrido[l,2-6][l,2]oxazine-8-carboxylate was confirmed by X-ray analyses. It justified a trans ring junction of the bicycle (00OL4007). 

Structure of 4//-pyrido[l,2-a]pyrazines 348-350 was confirmed by X-ray investigations (99JPR332). The stereostructure of 1,3,4,6,1 l,lla-hexahydro-2/f-pyrazino[l,2-A]isoquinoline-l,4-dione 351 was determined by X-ray investigation (01TL543). 

The coordination number and stereostructure of the metal complex are specific for a given metal ion species. 

Figure 8 Stereostructures of polypropylene and its relationship with metallocene structures. (From Ref. 3, with permission from Elsevier Trend Journals, Cambridge, U.K.)...

Goto, T., Takase, S., and Kondo, T., PMR spectra of natural acylated anthocyanins determination of stereostructure of awobanin, shisonin and violanin (Commelina communis, Viola tricolor, Perilla orimoides). Tetrahedron Lett, 27, 2413, 1978. Agrawal, P.K., NMR spectroscopy in the structural elucidation of ohgosaccharides and glycosides, Phytochemistry, 31, 3307, 1992. 

Elucidation of the stereostructure - configuration and conformation - is the next step in structural analysis. Three main parameters are used to elucidate the stereochemistry. Scalar coupling constants (mainly vicinal couplings) provide informa-hon about dihedral bond angles within a structure. Another way to obtain this information is the use of cross-correlated relaxation (CCR), but this is rarely used for drug or drug-like molecules. 

Fig. I. Stereostructural representation of koumine (47) according to Khuong-Huu et at. (28). Reprinted with permission from Ref. 28. 

Fig. 2. Stereostructure of koumine (48) according to Liang and co-workers (26,29). Also shown is the three-dimensional projection of koumine (49). Reprinted with permission from C. T. Liu, Q. W. Wang and C. H. Wang, J. Am. Chem. Soc., 1981,103,4635, Copyright (1981) American Chemical Society. 

Fig. 3. Stereostructure of neodihydrokoumine (55) and intermolecular bonding in the unit cell. (With permission from Ref. 47.)...

Fig. 3 Stereostructure and electron-delocalization in hypothetical aromatic phospholes...

Stereostructures of a co-crystal of (li )-l- 4-[(9aA)-perhydropyrido[l,2- ]pyrazin-2-yl]phenyl -2-phenyl-7-hydroxy-l, 2,3,4-tetrahydroisoquinoline with ERa-LBD301-553/C — S triple mutant <2005JME364> and iV-[2-(4-hydroxyphenyl)ethyl]-a-propyl-3-[(4-hydroxyphenyl)methyl]-l,4-dioxo-l,2,3,4,ll,l la-hexahydro-67/-pyrazino[l,2- ]isoquinoline-3-acetamide with fructose-1,6-biphosphatase <2003JBC51176> were determined by X-ray crystallography. The structure of a complex formed from 3-[( -methylphenyl)amino]-4-[(4-methylphenyl)imino]-4//-pyrido[l,2-tf]pyrazine with sodium bis(trimethylsilyl)amide and (norbornadiene)Mo(CO)4 in THF was characterized by single crystal X-ray diffraction <1995JPR38>. 

Structures of 3-[(4-methylphenyl)amino]-47/-pyrido[l,2-tf]pyrazine-4-thione, iV-tosyl-3-[(4-methylphenyl)a-mino]-4//-pyrido[l,2-tf]pyrazine 4-imine, and 246 were confirmed by X-ray investigations <1999JPR332>. The stereostructures of (4A,llaA)-lla-ethoxycarbonylT,3,4,6,lla-hexahydro-l,3,4,6,ll,lla-hexahydro-4-methox-ycarbonyl-l-oxo-[l,4]oxazino[4,3-A isoquinoline <1995ZK787>, 2//-pyrazino[l,2- ]isoquinolinc-l,4-dione 247 <2001TL543>, 4-benzyl-2-methyl-1,3,4,6,7,11 b-hcxahydro-2//-pyrazino[2,l -zz] isoqu incline-3,6-dione... 

The relative configurations and predominant conformations of [l,2,3]oxathiazino[4,3- ]isoquinoline derivatives 177-182 were studied by means of H and 13C NMR spectroscopy with the applications of DNOE, 2D HSC, and 2D-COSY measurements. Their stereostructure includes an equilibrium between the conformers cis-1, trans and cis-2. In the /ra -structure, the B/C rings are trans-ane-WatcA with H-l lb and the N-5 lone electron pair trans-diaxial. In the cis structures, the hetero-rings are air-anellated in the cis-1 conformation C-l is in inside position, while in cis-2... 

Partial structures I and II thus obtained were connected through the carbonyl group by the strong HMBC correlations of H-7 (8 4.05ppm) and H-9 (8 3.18ppm) to C-8 carbonyl carbon (8 208.5 ppm). Thus, the total planar structure of 46, including the partial relative stereostructure of the decalin system, was elucidated as shown in Figure 4. 

Several examples of the monocyclic isothiocyano sesquiterpenoids having the bisabolane (83) skeleton are known. Along with the hydrocarbon theonellin (84), isothiocyanate 86 and formamide 87 were obtained from the Okinawan sponge Theonella cf. swinhoei. It seems remarkable, but not unusual, that not only was the amide the major constituent, but the isonitrile 85 was the missing member of the triad [57], Relative stereostructures were indicated by NMR analysis of theonellin formamide (87) and its transformation products. 

From the mixture, a minor component whose double bond was at first assigned incorrectly [61], was isolated as the major metabolite from a Halichondria sp. and identified as 8-isocyano-15,20-cycloamphilect-l(12)-ene (103) [22], X-ray crystallography of 103 and its minor co-metabolites, 104-106, provided relative stereostructures. 

Portoghese, P. S. Relationships between stereostructure and pharmacological activity Annu. Rev. Pharmacol. 10 51-76, 1970. 

Chain Migratory Insertion Mechanism. For a given catalytic model, the stereoselectivity of each insertion step does not assure its stereospecificity (i.e., to lead to a stereoregular polymer). In fact, the possible presence as well as the kind of stereospecificity depends on possible differences between stereostructures of transition states of two successive insertion steps. 

These stereostructures are related to the best geometry of coordination of the growing chain at the site in the absence of the monomer molecule (as at the end of each polymerization step) as well as to the stability of such geometry before the coordination of a new monomer molecule. 

The possible occurrence of a back-skip of the chain for catalytic systems based on C2-symmetric metallocenes would not change the chirality of the transition state for the monomer insertion and hence would not influence the corresponding polymer stereostructure. On the contrary, for catalytic systems based on Cs-symmetric metallocenes, this phenomenon would invert the chirality of the transition state for the monomer insertion, and in fact it has been invoked to rationalize typical stereochemical defects (isolated m diads) in syndiotactic polypropylenes.9 376 60 This mechanism of formation of stereoerrors has been confirmed by their increase in polymerization runs conducted with reduced monomer concentrations.65 In fact, it is reasonable to expect an increase in the frequency of chain back-skip by reducing the monomer concentration and hence the frequency of monomer insertion. 

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