•Tamao–Fleming oxidation conditions to prepare N,O-acetals.•Preparation of N,O-acetals linked to the peptide bond nitrogen.•Traceless linker for potential application to bioconjugates and prodrugs.Tamao–Fleming oxidation of the N-dimethylphenylsilylmethyl group linked to the nitrogen of a peptide bond enables access to dipeptide N,O-acetal functionality. The N-silylmethyl functionality serves as a latent form of the N,O-acetal which is revealed after peptide bond construction.Download high-res image (59KB)Download full-size image

A configuration-encoded strategy provides unambiguous stereocontrol in an efficient synthesis of 1,5-polyols. To access the anti,syn-1,5,7-triol moiety of the C15–C25 fragment of tetrafibricin, a fibrinogen receptor antagonist, a strategy is introduced, which sequences the 1,5-polyol synthesis with selective desilylation and diastereoselective intramolecular conjugate addition. For tetrafibricin, assembly of the C15–C25 anti-1,5-diol in five steps is followed by the conjugate addition to introduce a syn-1,3-diol to complete the anti,syn-1,5,7-triol unit and to provide the functionality and stereochemistry required for tetrafibricin synthesis.

Synthesis of tubuphenylalanine and tubuvaline (Tuv), α-substituted γ-amino acid building blocks for tubulysin family of antimitotic compounds, has been improved using a radical addition reaction in the presence of unprotected hydroxyl functionality. The key carbon–carbon bond construction entails stereoselective Mn-mediated photolytic additions of alkyl iodides to the C=N bond of chiral N-acylhydrazones, and generates the chiral amines in high yield with complete stereocontrol. Reductive N–N bond cleavage and alcohol oxidation converted these amino alcohols into the corresponding γ-amino acids. The route to Tuv proceeded via peptide coupling with serine methyl ester, followed by a high-yielding sequence to convert the serine amide to a thiazole. Finally, peptide bond construction established the tubulysin framework in the form of a C-terminal alcohol analog. Attempted oxidation to the C-terminal carboxylate was unsuccessful; control experiments with dipeptide 18 showed a cyclization interfered with the desired oxidation process.

Sequencing a free radical addition and nucleophilic substitution enables [3 + 2] and [4 + 2] annulations of N-acylhydrazones to afford substituted pyrrolidines and piperidines. Photolysis of alkyl iodides in the presence of Mn2(CO)10 leads to chemoselective iodine atom abstraction and radical addition to N-acylhydrazones without affecting alkyl chloride functionality. Using radical precursors or acceptors bearing a suitably positioned alkyl chloride, the radical addition is followed by further bond construction enabled by radical–polar crossover. After the alkyl radical adds to the imine bond, the resulting N-nucleophile displaces the alkyl chloride leaving group via 5-exo-tet or 6-exo-tet cyclizations, furnishing either pyrrolidine or piperidine, respectively. When both 5-exo-tet and 6-exo-tet pathways are available, the 5-exo-tet cyclization is preferred. Isolation of the intermediate radical adduct, still bearing the alkyl chloride functionality, confirms the order of events in this radical–polar crossover annulation. A chiral oxazolidinone stereocontrol element in the N-acylhydrazones provides excellent stereocontrol in these reactions.

The Z-selective ruthenium-catalyzed addition of aromatic carboxylic acids to alkynes was followed by dioxirane epoxidation to furnish enol ester epoxides with cis configuration. Upon treatment of enol ester epoxides with tert-butyldimethylsilyl triflate in the presence of 2,6-lutidine, synthetically useful α-silyloxyaldehydes were obtained. This novel transformation was facilitated by microwave irradiation.

Stereocontrolled Mn-mediated addition of alkyl iodides to chiral N-acylhydrazones enables strategic C–C bond constructions at the stereogenic centers of chiral amines. Applying this strategy to quinine suggested complementary synthetic approaches to construct C–C bonds attached at the nitrogen-bearing stereogenic center using multifunctional alkyl iodides 6a–d as radical precursors, or using multifunctional chiral N-acylhydrazones 26a–d as radical acceptors. These were included among Mn-mediated radical additions of various alkyl iodides to a range of chiral N-acylhydrazone radical acceptors, leading to the discovery that pyridine and alkene functionalities are incompatible. In a revised strategy, these functionalities are avoided during the Mn-mediated radical addition of 6d to chiral N-acylhydrazone 22b, which generated a key C–C bond with complete stereochemical control at the chiral amine carbon of quinine. Subsequent elaboration included two sequential cyclizations to complete the azabicyclo[2.2.2]octane ring system. Group selectivity between two 2-iodoethyl groups during the second cyclization favored an undesired azabicyclo[3.2.1]octane ring system, an outcome that was found to be consistent with transition state calculations at the B3LYP/6-31G(d) level. Group differentiation at an earlier stage enabled an alternative regioconvergent pathway; this furnished the desired azabicyclo[2.2.2]octane ring system and afforded quincorine (21b), completing a formal synthesis of quinine.

Stereocontrolled Mn-mediated radical addition of alkyl iodides to chiral N-acylhydrazones enables strategic C–C bond disconnection of chiral amines. This strategy was examined in the context of a total synthesis of quinine, generating new findings of functional group compatibility leading to a revised strategy. Completion of a formal synthesis of quinine is presented, validating the application of Mn-mediated radical addition as a useful new C–C bond construction method for alkaloid synthesis. The Mn-mediated addition generates the chiral amine substructure of quinine with complete stereocontrol. Subsequent elaboration includes two successive ring closures to forge the azabicyclo[2.2.2]octane ring system of quincorine, linked to quinine through two known reactions.

The isolated stereogenic centers of 1,5-polyol-containing natural products present challenges to synthesis and structure determination. To address this problem, a configuration-encoded strategy defines each configuration within a simple 4-(arylsulfonyl)butyronitrile building block, a repeat unit that is reliably and efficiently coupled in iterative fashion to afford 1,5-polyols of defined stereochemistry. For example, the C27−C40 subunit of tetrafibricin is prepared in five steps and 42% yield. This strategy is amenable to rapid and unambiguous preparation of all configurational permutations of 1,5-polyols with equal facility.

Addition of allyltributylstannane to 2,7-dioxabicyclo[2.2.1]heptane in the presence of TiCl4 produces 5-allyl-2-(hydroxymethyl)tetrahydrofuran with a trans/cis ratio of 93:7. The trans-selectivity is also observed in additions of various other carbon nucleophiles.

Stereocontrolled Mn-mediated radical addition of alkyl iodides to chiral N-acylhydrazones enables strategic C–C bond disconnection of chiral amines. This strategy was examined in the context of a total synthesis of quinine, generating new findings of functional group compatibility leading to a revised strategy. Completion of a formal synthesis of quinine is presented, validating the application of Mn-mediated radical addition as a useful new C–C bond construction method for alkaloid synthesis. The Mn-mediated addition generates the chiral amine substructure of quinine with complete stereocontrol. Subsequent elaboration includes two successive ring closures to forge the azabicyclo[2.2.2]octane ring system of quincorine, linked to quinine through two known reactions.