Biomedical subjects

V H Rawal

Publications and source records attributed to V H Rawal.

7 recordsLinked to original sources

PubMed2001

Efficient assembly of the phomactin core via two different macrocyclization protocols.

[reaction--see text] The core structure of phomactins C and D was assembled by an efficient strategy starting from 3,4-dimethylcyclohexen-2-one. Key reactions include (1) a high yielding and highly diastereoselective Michael addition of a mixed cuprate, (2) a carbonylative alkyne-enoltriflate coupling or an intramolecular addition of an acetylide onto an aldehyde to form the macrocycle, (3) chemoselective Michael addition of a cuprate to an enynone, to give the carbon framework of desformyl phomactin C or D, and finally (4) regioselective addition of a thia-nucleophile to the more reactive beta-position of the resulting dienone.

Crystallography, X-Ray↗

PubMed2000

Racemic and asymmetric Diels-Alder reactions of 1-(2-oxazolidinon-3-yl)-3-siloxy-1,3-butadienes.

Achiral and chiral 1-(2-oxazolidinon-3-yl)-3-siloxy-1,3-butadienes were prepared from readily available starting materials. Although more stable than the parent 1-amino-3-siloxy dienes, the 1-(2-oxazolidinon-3-yl)-3-siloxy-1,3-butadienes are still very reactive in Diels-Alder reactions, somewhat more than 1,3-dialkoxy-1, 3-butadienes (e.g., Danishefsky's diene). The cycloadditions of the achiral and chiral dienes with several different dienophiles were examined. The reactions proceeded in good yield, with modest to high endo selectivity. The chiral dienes exhibited excellent facial selectivity in cycloadditions with alpha-substituted acroleins, maleic anhydride and N-phenylmaleimide. Upon reduction and hydrolysis of the cycloadducts, substituted cyclohexenones were obtained with ee's ranging from 22% to >98%.

Acrolein↗

PubMed2000

Hetero Diels-Alder reactions of 1-amino-3-siloxy-1,3-butadienes under strictly thermal conditions.

The hetero Diels-Alder reaction of 1-amino-3-siloxy-1,3-butadiene (1a) with a range of unactivated aldehydes proceeds readily under remarkably mild conditions: at room temperature and in the absence of Lewis acid catalysts. The cycloadducts are formed in good yields and can be converted directly to the corresponding dihydro-4-pyrones using acetyl chloride. Ketones and imines are also reactive in hetero Diels-Alder reactions with this diene.

Butadienes↗

PubMed2000

Expeditious syntheses of (+/-)-5-oxosilphiperfol-6-ene and (+/-)-silphiperfol-6-ene.

[reaction: see text]Stereocontrolled syntheses of 5-oxosilphiperfol-6-ene (1) and silphiperfol-6-ene (2) have been accomplished in four and five steps, with overall yields of 32% and 26%, respectively, from 1-acetyl-3-methylcylopentene. The strategy features two pivotal reactions: (a) a Diels-Alder reaction between 1,3-dimethylcyclopentadiene and 1-acetyl-3-methylcyclopentene, which proceeds with remarkable regio-, endo-, and diastereofacial selectivities, and (b) an intramolecular Paterno-Büchi reaction to snap together the triquinane framework.

Indicators and Reagents↗

PubMed1999

Regiocontrolled synthesis of carbocycle-fused indoles via arylation of silyl enol ethers with o-nitrophenylphenyliodonium fluoride.

[formula: see text] A new, regiocontrolled synthesis of carbocycle-fused indoles has been developed. The two-step procedure involves first the regiospecific arylation of silyl enol ethers with o-nitrophenylphenyliodonium fluoride (1). Reduction of the nitro group on the aromatic ring with TiCl3 followed by spontaneous condensation of the aniline with the ketone then affords the indole products.

Indoles↗

PubMed1993

Structure of diphenanthro[1,2-b;2',1'-d]-furan at 191 K.

Diphenanthro[1,2-b;2',1'-d]furan crystallizes in space group Pnma (No. 62) with the mirror plane bisecting the molecule and passing through the furan O atom. Although exhibiting packing similar to that of dibenzofuran, this diphenanthrofuran showed no disorder of the sort found in that close molecular analog. The C atoms of the individual phenanthrene rings of the title compound have a mean deviation of 0.027 (21) A from the best least-squares plane describing the rings. The configuration and conformation of the individual phenanthrene rings in this diphenanthrofuran closely match the theoretical and experimental results for phenanthrene itself. Overall, this diphenanthrofuran molecule adopts a distinct V shape, the dihedral angle between the least-squares best-fit planes of the phenanthrene rings being 11.0 (1.2) degrees.

Chemical Phenomena↗

PubMed1991

Structure of 4-dibenzofurancarboxaldehyde, C13H8O2.

Mr = 196.21, monoclinic, P2(1)/n, a = 6.589 (1), b = 9.784 (2), c = 14.394 (2) A, beta = 95.69 (1) degrees, V = 923.4 (3) A3, Z = 4, Dx = 1.41 g cm-3, lambda(Mo K alpha) = 0.71073 A, mu = 0.89 cm-1, F(000) = 408, T = 296 K, R = 0.048 for 964 unique reflections having I greater than 3 sigma I. The average C-C bond length within the benzenoid rings in 4-dibenzofurancarboxaldehyde is 1.388 (9) A; the average interior angle is 120.0 (2.6) degrees. While two of the three rings of 4-dibenzofurancarboxaldehyde are planar, the benzenoid ring to which the carbonyl group is attached is slightly non-planar. The dihedral angle between the two benzenoid rings measures 1.4(1) degrees. The structure is composed of pairs of 4-dibenzofurancarboxaldehyde molecules related by inversion centers. This structure is further characterized by approaches of both the carbonyl and the furan O atoms to ring H atoms with separations which are slightly less than the sum of the relevant van der Waals radii.

Benzofurans↗