Gopinath Kartha

Crystal structure of sulfamerazine

The structure of sulfamerazine, C11H12N4O2S, a very important sulfonamide drug, has been solved by direct methods with X-ray diffraction data collected using CuKα radiation. The crystals are orthorhombic, witha=9.145(1),b=11.704(1),c=22.884(2) Å and space groupPbca. The structure was refined with 2082 observed independent reflections, measured on a CAD-4 diffractometer, to a finalR value of 0.078. The molecule has similar conformational features to those observed in other sulfonamide drugs.

Crystal structure and absolute configuration of yohimbine hydrochloride, C21H27ClN2O3

Yohimbine (C21H26N2O3) is an important indole alkaloid found in the Indian snake rootrauwolfia serpentina, and is very closely related to reserpine. Yohimbine hydrochloride (C21H27ClN2O3) is orthorhombic:P212121,a = 11·54,b = 24·88,c = 7·00 Å,Z = 4. X-ray diffraction data to a Bragg angle of 80 ° were collected on a G.E. diffractometer using CuKα radiation. The structure was solved by direct methods, and refined to a reliability index of 9·9 % by the block-diagonal least-squares method. Yohimbine has a ring system very similar to that of reserpine, though the conformations of the ringsC andD about the C—N bond are different in the two molecules. The absolute configuration of the molecule, determined by X-ray anomalous scattering, confirms the 3α, 15α, 20β configuration assigned earlier by conformational analysis.

Synthetic studies on terpenoids. Part 19. Synthesis of 3β,10α,14β-trimethyl-1βH,11βH-tricyclo[9.3.0.0]tetradec-6-en-5-one, a tricyclic ketone related to the ophiobolins

The tricarbocyclic ketone (36) with well defined stereochemistry at each of the four contiguous asymmetric centres has been synthesised, and the stereochemistry of these centres has been deduced mechanistically and confirmed by X-ray crystallographic analysis of the bicyclic acidic precursor (22). The orientation of the tertiary methyl group in structure (36) has been deduced from force-field calculations and confirmed by model studies. The configuration of the two ring junction hydrogen atoms and of the two methyl groups in structure (36) are the same as found in the ophiobolins. The conformations of substituted cis-perhydroindanones, bicyclo[3.3.1 ]nonanes, cyclo-octenes and cyclo-octanones, are described.

c-Thioacylation by the Willgerodt–Kindler reaction. Structure of the anomalous products from salicylaldehydes. X-Ray crystal structure of the betaine from 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine, sulphur, and 3,5-dichlorosalicylaldehyde

1,2-Dimethyl-1,4,5,6-tetrahydropyrimidine (1), on reaction with salicylaldehydes (3) in the presence of sulphur, does not lead to the expected C-thioacylated products (4); instead, the products obtained are the betaines (5). The structure of (5b) from 3,5-dichlorosalicylaldehyde has been established by X-ray diffraction analysis.

Structure and absolute configuration of enhydrin bromohydrin

X-ray study of enhydrin bromohydrin, C23H29BrO10, reveals that the crystal has space groupP212121 with cell parametersa =10.08,b = 26.97, andc = 8.94 Å. There are four molecules in the unit cell. The structure was solved by direct methods, with three-dimensional CuKα diffractometer data to θ = 75 °, and refined by block-diagonal least squares to anR index of 11.4% for 2254 reflections. The molecule is a germacronolide sesquiterpene having acis sterochemistry at the C(1), C(10) double bond and atrans configuration at the epoxide ring, C(4)-O-C(5). The absolute configuration established on the basis of the anomalous scattering of CuKα. radiation by the bromine atom shows that the asymmetric atoms have the 4R, 5R, 6S, 7S, 8S, 9S, 20S, and 22S configurations.

A novel ring-opening of a 2-alkyl-5-imino-4-nitro-2,5-dihydroisothiazole by cyanide ion

Reaction of 2-methyl-3-methylamino-4-nitro-5-phenylimino-2,5-dihydroisothiazole (1b) with cyanide ion proceeds with two successive sequences of ring-opening and ring-closure with the ultimate generation of 2,5-bis(methylamino)-4-nitro-3-phenylimino-3H-pyrrole (2b) or its tautomer (2a).

Crystal structure of (+)-isomenthyl p-bromophenylcarbamate and absolute configuration and conformation of isomenthol by X-ray diffraction and nuclear magnetic resonance studies

Crystals of (+)-isomenthyl p-bromophenylcarbamate, C10H19O·CO·NH·C6H4Br are orthorhombic, space group P212121, and unit cell dimensions a= 25.58 (5)A, b= 13.18(2)A. c= 16.70(3)A. There are 12 molecules in the unit cell, 3 in each asymmetric unit. The structure was solved by the heavy-atom method, and refined by block-diagonal least-squares to R 0.087 for 2927 reflections. In the crystalline state, the isomenthol of the three molecules in the asymmetric unit exist in two different types of chair conformations. Two have the methyl substituent at C(1) axial, while the p-bromophenylcarbamate and the isopropyl substituents at C(3) and C(4) are both equatorial. The third molecule has the alternate chair conformation, i.e., methyl equatorial at C(1), p-bromophenylcarbamate axial at C(3), and isopropyl axial at C(4). 1H and 13C n.m.r. spectra of isomenthol in carbon disulphide and dimethyl sulphoxide solutions show it exhibits conformational mobility involving the two chair forms. The isomenthol conformer with the hydroxy- and the isopropyl-groups equatorial is the major component, that with methyl equatorial and the other two substituents axial the minor. The absolute configurations at the asymmetric carbons as derived from anomalous dispersion method are 1R,3S,4R. There is no flattening observed in the cyclohexane ring as a whole, only some distortions at the substituted positions. The p-bromophenylcarbamate chain is almost planar forming angles of 53° 43′, 53° 13′, and 81° 10′ with the planes of the isomenthol for molecules (A), (B), and (C).