Stereochemical Features of Lewis Acid-Promoted Glycosidations Involving 4‘-Spiroannulated DNA Building Blocks

Design, synthesis, and evaluation of DNA topoisomerase II-targeted nucleosides

We developed novel nucleoside-based topoisomerase II selective inhibitors and showed that small structural units, such as catechols, are essential for DNA topoisomerase II inhibitory activity. Moreover, nucleoside analogues containing TBS and 1,3-dithian moieties had potent and selective DNA topoisomerase II inhibitory activities. In further experiments, compound 25b having a beta configuration of the thymine moiety showed relatively strong growth inhibitory activity against cancer cell lines, and was more potent against all cancer cell lines than compound 26b, which carries a thymine moiety in the alpha configuration.

Syntheses of new spirocarbocyclic nucleoside analogs using iminonitroso Diels-Alder reactions

N-Cbz- and Boc-protected spirocyclic dienes were prepared by dialkylation of cyclopentadiene. These dienes coupled efficiently in a series of iminonitroso Diels-Alder reactions to produce a series of new spirocyclic adducts. Hydrogenolysis of these adducts afforded new spirocycles that contain multiple handles for further functionalization. Furthermore, stereocontrolled dihydroxylation and reductive cleavage of the spirocyclic adducts generated versatile scaffolds for the syntheses and derivatization of novel spirocyclic carbocyclic nucleoside analogs.

Synthesis and Lewis acid induced isomerization of mono-, di-, and tri-spiro α-keto tetrahydro-furans and -pyrans

The stereoselectivity of the acid-catalyzed ring expansion of dihydrofuranyl and dihydropyranyl 1,2-adducts to spirocyclic ketones 8 and 9 has been examined. These substrates have become readily available by the utilization of 1,3-dichloroacetone as a synthetic equivalent of cyclopropanone. The kinetically controlled isomerizations result in ring expansion to dispirocyclopentanones. Third-stage conversion to trispirocyclohexanones was shown to be possible in select examples. On a different front, the syn and anti pairs proved capable of interconversion by heating in the presence of boron trifluoride etherate. A push–pull fragmentation of the ketone ring is proposed to account for these epimerizations. Calculations have been performed to simulate transition state energies in the oxonium ion intermediates and to gain some idea of the preferred ground-state geometries of the final products.Key words: oxonium ion rearrangements, ring expansions, acid-catalyzed epimerizations, cyclopropanone equivalent, spirocyclic ethers.

Highly Stereoselective β-Anomeric Glycosidation of a 2‘-Deoxy Syn-5‘-Configured 4‘-Spironucleoside

A direct enantioselective pathway that delivers exclusively the beta-anomer of a 4'-spironucleoside has been successfully developed. The key starting material is the enantiomerically pure dihydroxy lactone 19, which has proven amenable to conversion to glycal 22 via the chloro acetonide. This intermediate is then capped as the 3,5-O-(tetraisopropyldisiloxane-1,3-diyl) glycal. The latter can enter into N-iodosuccinimide-promoted glycosidation with persilylated thymine. Only the beta anomer is formed. Ensuing deiodination and desilylation proceed quantitatively to furnish the targeted, previously elusive anomer.

Stereoselective Synthesis of Conformationally Constrained 2‘-Deoxy-4‘-thia β-Anomeric Spirocyclic Nucleosides Featuring Either Hydroxyl Configuration at C5‘

An enantioselective approach to 2'-deoxy-4'-thia spirocyclic nucleosides featuring an alpha- or beta-hydroxyl substituent at C-5' of the carbocyclic ring is detailed. The starting point is the mandelate acetal 8. The overall strategy involves the stereocontrolled dihydroxylation of this dihydrothiophene, subsequent generation of the keto acetonide 12 followed by its Meerwein-Ponndorf-Verley reduction and beta-elimination, protection of the resulting dihydroxy thiaglycal, electrophilic glycosidation according to the Haraguchi protocol, reductive removal of the phenylseleno group, and end-game global deprotection. Acquisition of the alpha- and beta-5'-isomers is equally facile. Various 1D and 2D NMR techniques are used for assigning configuration.

Practical Synthesis of Enantiopure Spiro[4.4]nonane C-(2‘-Deoxy)ribonucleosides

The levorotatory diol 7 has been sequentially monosilylated, dehydrated, and oxidized at the allylic methylene group to provide (+)-12. The enantiomeric dextrorotatory diol 7 has been directed down a different sequence of steps involving monosilylation, dehydration, hydroboration, Swern oxidation, and regioselective introduction of a conjugated double bond to generate (-)-33. The novel feature of these transformations is that two key deoxycarbospironucleoside intermediates of the proper absolute configuration have been made available from enantiomerically related precursors. Also reported is a highly practical and reliable means for the formation of novel 2'-deoxyribonucleosides of novel structural type from these spirocyclic cyclopentenones.