A. Forbes Cameron

Structural investigations of ylides. Part VI. Crystal and molecular structures of two resonance-stabilized Wittig reagents 2-carboxy-1-methoxycarbonylethyltriphenylphosphorane and its t-butyl ester

The structures of the title compounds (3) and (4) have been determined by three-dimensional X-ray analyses by use of 3356 (3) and 2883 (4) independent reflexions measured by diffractometer. Crystals of both compounds are monoclinic, space group P21/n; for (3)Z= 4, unit cell dimensions a= 8.579(1),b= 13.893(4),c= 16.768(3)A, β= 97.31(3); for (4),Z= 4, cell dimensions a= 15.731(5),b= 11.941(2), c= 13.194(2)A, β= 97.22(1)°. The structure of (3) was solved by the heavy-atom method, and of (4) by the symbolic addition method. Both structures have been refined by least-squares calculations to R0.054(3) and 0.076(4). The results show that the structures are stabilised in part by delocalisation of the formal negative charge on the P+–C– group into the carbomethoxy-function.

Isolation, and crystal and molecular structure of cytochalasin G: an [11]cytochalasan containing an indole group

The structure of cytochalasin G, an [11]cytochalasan containing a 10-(indol-3-yl) group, and produced by an unidentified Nigrosabulum sp., has been determined by X-ray methods, by use of 1656 independent data collected on a diffractometer. Crystals are orthorhombic, a= 25·437(2), b= 7·480(1), c= 13·458(1)A, space group P212121, with Z= 4. The structure was solved by the symbolic-addition method and refined by least-squares techniques to R 0·102.

Laurencia natural products, part II. Crystal structure and absolute stereochemistry of laurinterol acetate, A bicyclo[3,1,0]hexane derivative

The crystal structure and the absolute stereochemistry of laurinterol acetate, a crystalline derivative of laurinterol, have been determined by a single-crystal X-ray analysis using three-dimensional diffractometer data. The crystals are monoclinic, space group P21, with Z= 2 in a unit cell of dimensions a= 10·26, b= 7·28, c= 12·22 A, β= 114·1°. The structure was solved by the heavy-atom method and refined by least-squares calculations to R 0·086 for 1097 data. The absolute stereochemistry of the related compound aplysin has been established by consideration of anomalous dispersion effects. Laurinterol contains a trisubstituted bicyclo[3,1,10]hexane system which adopts an overall boat-like conformation.

Crystal and molecular structures of 8,9-dibromo-1,4,4,8-tetramethyltricyclo[5,4,0,0]undecane and 8,9-dibromo-2,2,4,8-tetramethyltricyclo[5,3,1,0]undecane, derivatives of rearrangement products of caryophyllene dihydrochloride and isocaryophyllene respectively

The structures of the title compounds (I) and (IV) have been determined by single-crystal X-ray analyses. Crystals of both are orthorhombic, space group P21212 for (I), P212121 for (IV), with Z= 4 in unit cells of dimensions: (I), a= 15·47(2), b= 14·02(1), c= 7·01(1)A; (IV), a= 6·63(1), b= 8·96(1), c= 26·35(2)A. Both structures were solved from photographic data by the heavy-atom method, and refined by least-squares calculations to R= 0·106 for 1336 independent reflexions (I), and R 0·096 for 860 independent reflexions (IV). In each case the absolute stereochemistry was determined by consideration of anomalous dispersion effects. The analyses of the two dibromo-derivatives have allowed us to deduce the structures and stereochemistries of their parent hydrocarbons [(II) and (V)], and hence to postulate possible mechanisms by which these latter may be formed from caryophyllene dihydrochloride (III) and (–)-isocaryophyllene (VI) respectively.

Structural investigations of ylides. Part III. Crystal and molecular structure of N-(dimethylsulphonio)toluene-p-sulphonamidate

The crystal and molecular structure of the title compound has been determined from X-ray photographic data. Crystals are monoclinic, space group P21/c, with Z= 4 in a unit cell of dimensions a= 12·93(2), b= 7·17(1), c= 12·50(2)A, β= 104·2(1)°. The structure was solved by the heavy atom technique and refined by least-squares calculations to R 0·107 for 1085 independent data. The molecular geometry is compared with those of other similar ylides in relation to the possibility of d-orbital involvement within the S–N–S bonding system.

Laurencia natural products. Part I. Crystal structure and absolute stereochemistry of laurencin

The crystal structure and the absolute stereochemistry of laurencin have been determined by a single-crystal X-ray analysis. The crystals are orthorhombic, space group P212121 with four molecules of C17H23O3Br in a unit cell of dimensions a= 7·70, b= 9·70, c= 22·93 A. The structure was solved by the heavy-atom method and refined by least-squares calculations. R is 0·103 for 1152 reflexions. The absolute stereochemistry was determined by consideration of anomalous dispersion effects. The molecule is unusual in that it has an eight-membered ether-ring containing a cis-double bond. Laurencin provides an example of strong intermolecular–C⋮CH ⋯ O hydrogen-bonding in the solid state as revealed by i.r. absorption data. In the crystal, the terminal ethynyl group of a trans-hex-3-ene-5-yne side-chain is involved in a bifurcated hydrogen bond with ether and acetate oxygen atoms of a neighbouring molecule.

Absolute configuration of (+)-trans-chrysanthemic acid. Crystal structure analysis of a p-bromoanilide derivative

The absolute configuration of (+)-trans-chrysanthemic acid, deduced previously from chemical considerations as 1R,2R has been confirmed by a three-dimensional X-ray analysis of a p-bromoanilide derivative using Bijvoets anomalous dispersion method. Crystals are orthorhombic, space group P212121, with Z= 4 in a unit cell of dimensions a= 9.58(2), b= 6.06(2), c= 27.53(5)A. The structure was solved from photographic data by the heavy-atom method, and refined by least-squares calculations to R 0.109 for 400 reflexions.

Molecular structure and absolute stereochemistry of a 1-phenylethylisoquinoline alkaloid from Colchicum cornigerum: X-ray analysis of a methiodide derivative

The alkaloid CC-2 (Colchicum cornigerum alkaloid number 2) has been shown to possess the molecular structure and absolute stereochemistry (I) by single-crystal X-ray analysis of a methiodide derivative. Crystals are orthorhombic, space group P212121, with Z= 4 in a unit cell of dimensions a= 7·75(2), b= 14·57(2), c= 21·55(4)A. The structure was solved by the heavy-atom method by use of 1739 visually estimated independent reflexions, and refined by block-diagonal least-squares calculations to R 0·13. The absolute configuration was determined by considering anomalous scattering effects.

Crystal and molecular structure of the p-chloroanilide derivative of bicyclo[5,3,1]undec-7-en-11-one-1-carboxylic acid

The structure of the p-chloroanilide derivative of bicyclo[5,3,1]undec-7-en-11-one-1-carboxylic acid (I, R =p-ClC6H4·NH·CO) has been determined by a single-crystal X-ray analysis. The crystals are orthorhombic, space group Pbcn, with Z= 8 in a unit cell of dimensions a= 19·97, b= 7·67, and c= 21·35 A. The structure was solved by the heavy-atom method and refined by least-squares calculations to R= 0·092 for 983 independent reflexions. The bridgehead position of the double bond produces molecular strain which not only results in distortion of the double bond itself, but also results in a molecular conformation in which there is incomplete delocalisation of the π-electrons in the enone system. This is reflected by abnormal spectroscopic and chemical properties for the compound (I, R = H and R = CO2Et) which may be correlated with the molecular geometry determined by the structure analysis.

Conformation of a bicyclo[3,2,2]non-6-ene system: crystal and molecular structure of syn-3-exo-p-bromobenzoyloxybicyclo[3,2,2]non-6-ene-8,9-endo-cis-dicarboxylic acid anhydride

The structure of syn-3-exo-p-bromobenzoyloxybicyclo[3,2,2]non-6-ene-8,9-endo-cis-dicarboxylic acid anhydride has been determined by a single-crystal X-ray analysis. The crystals are monoclinic, space group P21/c with Z= 4 in a unit cell of dimensions a= 13·47, b= 12·31, c= 10·63 A, β= 117·5°. The structure was solved by the heavy-atom method and refined by least-squares calculations to R= 0·089 for 1977 reflections. This structure determination allows the identification of the epimeric products of the reaction of 1-benzoyloxyclohepta-3,5-diene and maleic anhydride. Twisting of the bicyclic skeleton about the axis through the bridgehead carbon atoms is not observed in the present instance, the relief of molecular strain being suggested by systematic valency-angle increases and by flattening of the three-carbon bridge.