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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.

Osmylation of the enantiopure conjugated enones 4 and 15 proceeded predominantly or exclusively from the α-face to give the 2',3'-diols 6 and 16. Ensuing conversion into the acetonide allowed for sequential stereocontrolled reduction with L-Selectride, and esterification with triflic anhydride was utilized to form triflates 11 and 19. The heightened electrophilicity of these intermediates made possible S N 2 displacements with several nucleobases as promoted with potassium hydride in N,N-dimethylformamide at room temperature. Suitable deprotection measures led to the targeted compounds.

A new strategy for implementing α-ketol rearrangements under mild conditions is presented. The reactants are mono- and disilylated stereoisomers of α,α′- and α,β-dihydroxycycloheptanones and -cyclooctanones. Compounds of this class experience ready deprotection upon treatment with tetrabutylammonium fluoride. In certain examples, this process is accompanied by structural isomerization. Since the product diols are stable to the reaction conditions, these chemical changes have been attributed to kinetically controlled events following transient alkoxide generation resulting from cleavage of the OSi bond proximal to the carbonyl group. The experimental findings are evaluated against a backdrop of calculated (MM3) steric energies of the silicon-free products and their response to equilibration under strongly basic conditions.

The stereochemical course of the epoxidation of the silyl enol ethers of 2-tert-butyldimethylsilyloxycycloheptanone and -cyclooctanone has been investigated and shown to proceed exclusively anti to the existing α-substituent. 2-(Benzyloxy)cyclooctanone behaves similarly, and the presence of a transannular double bond does not alter the outcome. α-Ketol rearrangements are seen to operate during ensuing fluoride ion-induced removal of the silyl protecting groups in select examples. The preferred means for generating the cis isomers of the α,α‘-dihydroxy cycloalkanones involves methylenation of the monoprotected trans-dihydroxy ketones with the Nysted reagent, followed by oxidation and subsequent reduction with sodium borohydride. Ozonolysis and fluoride ion-induced desilylation complete the route.

For enzymatic hydrolyses of advanced synthetic intermediates, the use of an organic cosolvent is often warranted. As a model study, the cosolvent dependence (at 10 volume%) of the key enzymatic step in our asymmetric synthesis of (−)-podophyllotoxin was examined. At high protein concentrations and rt, DMSO and the polyethereal organic cosolvents dioxane, diglyme and triglyme emerged as the best organic cosolvents, in terms of yield, % ee and catalytic efficiency (V max /K m ). However, only in 10% triglyme was optimal efficiency maintained at both lower protein concentration and higher temperature.

[reaction: see text] Apha,alpha'-dihydroxy ketones having two OH groups that can come into close proximity react with phosgene in the presence of DMAP to produce 1,3-dioxane-2,5-diones. When covalent bonding in this manner is prohibited, alternate reaction pathways can in principle be adopted. Of these, the generation of spiro epoxy carbonates is shown to be feasible in three examples. In other cases, the alpha-ketol rearrangement precedes diacylation.

Admixture of vicinal di-, tri-, and tetraketones with ethereal diazomethane results in one-time methylene transfer to the less hindered face of the sterically most accessible and electron-deficient carbonyl group to deliver epoxy ketones in a highly selective manner unless ring strain effects contravene.

The stereochemical course of the epoxidation of the silyl enol ethers of 2-tert-butyldimethylsilyloxycycloheptanone and -cyclooctanone has been investigated and shown to proceed exclusively anti to the existing alpha-substituent. 2-(Benzyloxy)cyclooctanone behaves similarly, and the presence of a transannular double bond does not alter the outcome. Alpha-ketol rearrangements are seen to operate during ensuing fluoride ion-induced removal of the silyl protecting groups in select examples. The preferred means for generating the cis isomers of the alpha,alpha'-dihydroxy cycloalkanones involves methylenation of the monoprotected trans-dihydroxy ketones with the Nysted reagent, followed by oxidation and subsequent reduction with sodium borohydride. Ozonolysis and fluoride ion-induced desilylation complete the route.

[reaction: see text] Apha,alpha'-dihydroxy ketones having two OH groups that can come into close proximity react with phosgene in the presence of DMAP to produce 1,3-dioxane-2,5-diones. When covalent bonding in this manner is prohibited, alternate reaction pathways can in principle be adopted. Of these, the generation of spiro epoxy carbonates is shown to be feasible in three examples. In other cases, the alpha-ketol rearrangement precedes diacylation.

The two title reactions have been evaluated in order to maximize the efficiency with which pyrimidine and purine nucleobases can be introduced into 2',3'-dideoxyspirocarbanucleosides.