The nuclear magnetic resonance method of isotopic perturbation can distinguish between single- and double-well potentials in intramolecularly hydrogen-bonded monoanions of dicarboxylic acids. These are classic cases of a "strong," symmetric hydrogen bond in the crystal. The observed carbon-13 isotope shifts induced by the substitution of oxygen-18 demonstrate that these monoanions exist as a single symmetric structure in a nonpolar solvent but as two equilibrating tautomers in aqueous solution. The change is attributed to the disorder of the aqueous environment. These are simple counterexamples to the hope that the crystal structure reveals the actual molecular structure in aqueous solution.

http://science.sciencemag.org/content/sci/266/5191/1665.full.pdf

Symmetries of hydrogen bonds in solution.

The reaction of specifically protected anhydroalditols with (diacetoxyiodo)benzene or iodosylbenzene and iodine is a mild and selective procedure for the synthesis of chiral 6,8-dioxabicyclo[3.2.1]octane and 2,7-dioxabicyclo[2.2.1]heptane ring systems under neutral conditions. This reaction can be considered to be an intramolecular glycosidation that goes through an intramolecular hydrogen abstraction promoted by an alkoxy radical followed by oxidation of the transient C-radical intermediate to an oxycarbenium ion. This methodology is useful not only for the preparation of chiral synthons but also for the selective oxidation of specific carbons of the carbohydrate skeleton, constituting a good procedure for the synthesis of protected uloses.

Intramolecular hydrogen abstraction reaction promoted by alkoxy radicals in carbohydrates. Synthesis of chiral 2,7-dioxabicyclo[2.2.1]heptane and 6,8-dioxabicyclo[3.2.1]octane ring systems.

Described is a study of the preparation, reactivity, and diastereoselectivity of chiral 1-amino-3-siloxy-1,3-butadienes in the Diels−Alder reaction. These dienes were easily prepared in good yield from the corresponding enantiomerically pure substituted pyrrolidines. All the dienes described underwent cycloadditions readily with several activated dienophiles under mild reaction conditions. An amino siloxy diene containing a C2-symmetric 2,5-diphenylpyrrolidine auxiliary was found to provide high diastereofacial control, even at or above room temperature. Upon hydrolysis of the cycloadducts, 4-substituted and 4,5-disubstituted cyclohexenones were obtained with ee's ranging from 85% to >98%. A simple model based primarily on steric arguments was developed to rationalize and predict the absolute configurations of final products obtained by this sequence. The synthetic utility of the methodology was illustrated through a concise enantioselective synthesis of (−)-α-elemene. The synthesis also served to establi...

Chiral Amino Siloxy Dienes in the Diels−Alder Reaction: Applications to the Asymmetric Synthesis of 4-Substituted and 4,5-Disubstituted Cyclohexenones and the Total Synthesis of (−)-α-Elemene

The kibdelones A-C (1-3) and their isomeric metabolites (cf. isokibdelone C 4) are hexacyclic tetrahydroxanthone natural products recently isolated by Capon and coworkers from the rare Australian actinomycete Kibdelosporangium sp. (Figure 1).[1] An interesting property is the facile equilibration of kibdelones B 2, and C 3 to a mixture of 1-3via keto/enol tautomerizations followed by quinone/hydroquinone redox reactions.[1] Related natural products include simaomicin α 5 which has been shown to sensitize cancer cells to cytotoxic agents including bleomycin at nanomolar concentrations and to exhibit potent antimalarial and anticoccidial activities. [2] Evaluation of the kibdelones in the NCI 60-cell panel of human cancer cell lines revealed that they are active at low nanomolar concentrations against a number of human tumor cell lines. For example, kibdelone A has a GI50 of 1.2 nm against a SR (leukemia) tumor cell line and <1 nm (GI50) against SN12C (renal) cell carcinoma.[1] In addition, the kibdelones were shown to display novel COMPARE analysis profiles for cancer cell growth inhibition.

/pdf/synthesis-and-biological-evaluation-of-abcd-ring-fragments-1atfkyiwr8.pdf

Synthesis and Biological Evaluation of ABCD Ring Fragments of the Kibdelones

Intramolecular diels-alder reaction by self-assembly of the components on a lewis acid template

Diene-dienophile hydrogen-bonding control of Diels-Alder reactions with dienes bearing a remote stereogenic center. Amplification of the receptor nature of dienes by conformational tuning leading to high diastereofacial discrimination

Asymmetric Diels-Alder reactions with chiral dienes. Control of facial selectivity through hydrogen bonding

Diene-Dienophile Hydrogen-Bonding Control of Diels-Alder Reactions with Dienes Bearing a Remote Stereogenic Center. Amplification of the Receptor Nature of Dienes by Conformational Tuning Leading to High Diastereofacial Discrimination.

Rabindranath Tripathy

Papers

Diene-dienophile hydrogen-bonding control of Diels-Alder reactions with dienes bearing a remote stereogenic center. Amplification of the receptor nature of dienes by conformational tuning leading to high diastereofacial discrimination

Asymmetric Diels-Alder reactions with chiral dienes. Control of facial selectivity through hydrogen bonding

Diene-Dienophile Hydrogen-Bonding Control of Diels-Alder Reactions with Dienes Bearing a Remote Stereogenic Center. Amplification of the Receptor Nature of Dienes by Conformational Tuning Leading to High Diastereofacial Discrimination.

Asymmetric Diels-Alder Reactions with Chiral Dienes. Control of Facial Selectivity Through Hydrogen Bonding.