Intramolecular hydrogen abstraction. Iodosobenzene diacetate, an efficient and convenient reagent for alkoxy radical generation

A hydrogen peroxide based access to qinghaosu (artemisinin)

Attachment of H(2)O(2) onto the highly hindered quaternary C-12a in an advanced qinghaosu (artemisinin) precursor has been achieved through a facile perhydrolysis of a spiro epoxy ring with the aid of a previously unknown molybdenum species without involving any special equipment or complicated operations. The resultant β-hydroxyhydroperoxide can be further elaborated into qinghaosu, illustrating an entry fundamentally different from the existing ones to this outstanding natural product of great importance in malaria chemotherapy.

If C-H bonds could talk: selective C-H bond oxidation

C-H oxidation has a long history and an ongoing presence in research at the forefront of chemistry and interrelated fields. As such, numerous highly useful articles and reviews have been written on this subject. Logically, these are generally written from the perspective of the scope and limitations of the reagents employed. This Minireview instead attempts to emphasize chemoselectivity imposed by the nature of the substrate. Consequently, many landmark discoveries in the field of C-H oxidation are not discussed, but hopefully the perspective taken herein will allow C-H oxidation reactions to be more readily incorporated into synthetic planning.

A synthesis of the carbocyclic core of maoecrystal V

An approach toward the synthesis of the complex polycyclic diterpene maoecrystal V (1) is described. Construction of the advanced tetracyclic core structure (i.e., 19) was achieved in 13 steps from 3,3-dimethylcyclohexanone (6) by employing a stereoselective Nazarov cyclization followed by a Diels-Alder reaction to forge the two contiguous quaternary stereocenters.

Pushing Radical Cyclization from Regioselective to Regiospecific: Cyclization of Amidyl Radicals Controlled by Vinylic Halogen Substitution

Efficient and regiospecific 6-exo, 7-endo, 7-exo, and even 8-endo amidyl radical cyclizations can be accomplished by the direct reactions of unsaturated N-H amides if they bear a vinylic halogen (Cl, Br, I) substituent. This remarkable halogen-substitution effect could be rationalized in terms of lone pair-lone pair electron repulsion between the N radical and the vinylic halogen atom, in addition to the well-known steric and radical-stabilizing effect. [reaction: see text]

Synthesis of aminocyclopentanols: α-d-galacto configured sugar mimics

Four aminocyclopentanols, as mimics of putative intermediates in the hydrolysis of alpha-d-galactosides, have been synthesized through a number of stereoselective transformations using the cis-fused cyclopentane-1,4-lactone (1R, 5S, 7R, 8R)-7,8-dihydroxy-2-oxabicyclo[3.3.0]oct-3-one as a chiral building block. The compounds were tested towards various glycosidases but showed no anomer selectivity in the inhibition of alpha- and beta-galactosidases.

Studies on the Total Synthesis of (−)-CP-263,114

Alkoxyl radicals have a wide range of applications in organic synthesis due to their remarkable chemical properties in molecular transformation. The present study shows two types of alkoxyl radicals (primary vs tertiary) to selectively undergo dehydrogenation and beta-scission to give rise to key structural elements of (-)-CP-263,114 (1). By alkoxyl radical transformation followed by installation of the C19-C25 (CP numbering) side chain and the bridged bisacetal unit, the functionalized CP precursor 2 was obtained.

Asymmetric Iodolactamization Induced by Chiral Oxazolidine Auxiliary

Asymmetric iodolactamization reactions of unsaturated amides with oxazolidines as the chiral auxiliaries were investigated. With (4S)-4-((2R)-2-butyl)-2,2-dimethyloxazolidine as the auxiliary and LiH as the base, a number of unsaturated amides underwent iodolactamization smoothly to afford the corresponding gamma- and delta-lactams in 30-98% yield with de values up to 97%.

Biomimetic Enantioselective Total Synthesis of (−)-Siccanin via the Pd-Catalyzed Asymmetric Allylic Alkylation (AAA) and Sequential Radical Cyclizations

(-)-Siccanin (1), a natural product possessing significant antifungal properties, was synthesized enantioselectively via a biomimetic route. This synthetic route features two sequential radical cyclizations: a Ti(III)-mediated radical cyclization of epoxyolefin 48 to construct the B-ring, and a Suarez reaction to establish the tetrahyrofuran ring. Chiral chroman moiety of siccanin was prepared based on our recent development of the Pd-catalyzed asymmetric allylic alkylation (AAA) of phenol trisubstituted allyl carbonates. Several other members of the siccanin family were also synthesized including siccanochromenes A (2), B (3), E (6), F (7), and the methyl ether of siccanochromene C (55). These studies may shed light on the biosynthesis of this novel family of compounds.

Facile 5-Endo Amidyl Radical Cyclization Promoted by Vinylic Iodine Substitution

[reaction: see text] BF(3).OEt(2)-catalyzed atom-transfer radical addition of iodoacetamides to alkynes yielded the corresponding vinyl iodides, which upon treatment with (t)BuOCl and I(2) afforded gamma-lactam derivatives in moderate to good yield. The mechanism was proposed to be 5-endo amidyl radical cyclization, and vinylic iodine substitution provided the driving force for the cyclization.

Synthesis of new 18-substituted analogues of calcitriol using a photochemical remote functionalization

A novel convergent synthetic approach to new analogues of calcitriol modified at the C-18 position is reported. The key step in the synthesis is the 20-hydroxyl-directed photochemical iodination of the 18-methyl group in the presence of (diacetoxyiodo)benzene. Using this methodology, two new analogues of calcitriol were prepared: the first contains a hydroxylated alkyl side chain attached at C-18 with the natural side chain replaced by an isopropylidene group; the second is a conformationally locked analogue due to an extra oxacycle between the C-18 and C-20 positions.

Intramolecular 1,5- versus 1,6-hydrogen abstraction reaction promoted by alkoxy radicals in carbohydrate models

[reaction: see text] The alkoxy radical generated by reaction of 3,7-anhydro-2-deoxyoctitols with (diacetoxyiodo)benzene (DIB) and iodine abstracts regioselectively either the proton at C7 or that at C4 depending on the electronegativity of the substituent at C4. The correct election of this substituent can switch the reaction to give 2,9-dioxabicyclo[3.3.1]nonane or hexahydro-2H-furo[3,2-b]pyran ring systems.

The first total synthesis of (corrected) ritterazine M

Hecogenin acetate was converted to ritterazine M in 16 operations with an average yield per opearation of 87%. The overall linear yield was 12%. This confirmed 1 as the corrected structure for ritterazine M by total synthesis.

Redox refunctionalization of steroid spiroketals. Structure correction of ritterazine M

The structure of the North spiroketal moiety of ritterazine M has been corrected from 1a to 1b. This was accomplished by comparison of published spectra of the natural product with five synthetic spiroketal-alcohols. Synthesis of these models was efficiently accomplished by reductive cleavage of the spiroketal and Sharpless asymmetric dihydroxylation of an isopentyl, methyl 1,1-disubstituted olefin, followed by Suarez iodine[III] oxidative spirocyclization of monoprotected 1 degree,3 degree 1,2 diols.

Remote functionalization reactions in steroids

Work published over the last thirty years on the functionalization of unactivated positions in the steroid skeleton has been reviewed.