Efficient mercury-free preparation of vinyl and isopropenyl ethers of chiral secondary alcohols and α-hydroxyesters

Modular ketal-linked prodrugs and biomaterials enabled by organocatalytic transisopropenylation of alcohols

Isopropenyl ethers are critical intermediates for accessing medicinally valuable ketal-based prodrugs and biomaterials, but traditional approaches for the synthesis of isopropenyl ethers suffer from poor functional group compatibility and harsh reaction conditions. Here, we develop an organocatalytic transisopropenylation approach to solve these challenges, enabling the synthesis of isopropenyl ethers from various hydroxyl-group-containing small-molecule drugs, polymers, and functional building blocks. The method provides a straightforward and versatile synthesis of isopropenyl ethers, features excellent tolerance of diverse functional groups, applies to a wide range of substrates, and allows scalable synthesis. The development of this organocatalytic transisopropenylation approach enables access to modular preparation of various acid-sensitive ketal-linked prodrugs and functionalized ketalated biomaterials. We expect our syntheses and transformations of isopropenyl ethers will find utility in several diverse fields, including medicinal chemistry, drug delivery, and biomaterials.

Isopropenyl ethers (IPPEs) are essential intermediates for obtaining ketal-based prodrugs and biomaterials, but traditional approaches for their synthesis are limited by poor functional group compatibility and harsh reaction conditions. Here, the authors report an organocatalytic transisopropenylation approach for IPPE synthesis in mild reaction conditions and with wide range of substrates, and use it to prepare acid-sensitive ketal-linked prodrugs and biomaterials.

Design and synthesis of a G-protein-coupled receptor antagonist library of aryloxyalkanolamines using a polymer-supported acyclic acetal linker

A G-Protein-coupled receptor-targeted library of aryloxypropanolamines and aryloxybutanolamines was efficiently executed using a novel, polymer-supported acyclic acetal linker, producing compounds in good yields and purities.

Stereospecific synthesis of 1,2-cis glycosides by vinyl-mediated IAD

[reaction: see text] Stereospecific 1,2-cis glycosylation of 2-O-vinyl thioglycosides, synthesized from the corresponding alcohols by Ir-catalyzed transvinylation with vinyl acetate, is achieved by iodine-mediated tethering of a range of primary and secondary carbohydrate acceptors, followed by intramolecular aglycon delivery (IAD). The use of such an intramolecular glycosylation strategy furnishes the desired alpha-gluco and beta-manno disaccharides in an entirely stereoselective manner.

Diastereoselective Diels-Alder reactions of N-sulfonyl-1-aza-1,3-butadienes with optically active enol ethers: an asymmetric variant of the 1-azadiene Diels-Alder reaction

The first detailed study of a room-temperature asymmetric Diels-Alder reaction of N-sulfonyl-1-aza-1,3-butadienes is reported enlisting a series of 19 enol ethers bearing chiral auxiliaries, with many providing highly diastereoselective (endo and facial diastereoselection) reactions, largely the result of an exquisitely organized [4+2] cycloaddition transition state. Three new, readily accessible, and previously unexplored auxiliaries rationally emerged from the studies and provide remarkable selectivities (two of these give 49:1 endo:exo and 48:1 facial selectivity) that promise to be useful in systems beyond those detailed.

Total Synthesis of Gambierol: The Generation of the A–C and F–H Subunits by Using a C-Glycoside Centered Strategy

Gambierol, a representative of the marine ladder toxin family, consists of eight ether rings, 18 stereocenters, and two challenging pyranyl rings having methyl groups that are in a 1,3-diaxial orientation to one another. Herein we describe the generation of gambierol's A-C and F-H ring systems and demonstrate the versatility of the glycosyl anhydride, enol ether-olefin RCM strategy to fused polycyclic ethers. This work has both enabled us to generate sufficient quantities of the gambierol precursors and has enabled us to better understand the chemical transformations that were key to these efforts. Fundamental work included efforts to C-glycosides and C-ketosides, Claisen rearrangements, and enol ether-olefin RCM reactions.

Enantioselective [4 + 2] cycloadditions of o-quinone methides: total synthesis of (+)-mimosifoliol and formal synthesis of (+)-tolterodine

The first example of an enantioselective cycloaddition of an o-quinone methide (o-QM) with a chiral enol ether is described along with the total synthesis of (+)-mimosifoliol and the formal synthesis of (+)-tolterodine. These syntheses exemplify a three-component, one-pot benzopyran approach for the construction of chiral benzylic junctions. Cycloadditions of various enol ethers and o-QMs are examined, and diastereoselectivities >95% are obtained with trans-2-phenyl-1-cyclohexanol and 2,2-diphenylcyclopentanol vinyl ethers.

Stereoselectivity in formation of oxacephams from 1,3-alkylidene-threitols

The [2+2]cycloaddition of CSI to the (Z)-propenyl ethers derived from respective 1,3-methylidene- and 1,3-ethylidene-threitols, contrary to the corresponding erythritol derivatives, is characterized by a low stereoselectivity and a lack of stereospecificity. On the other hand, the alternative method of the oxacepham formation, based on the 4-vinyloxy-azetidinone, proceeds with an excellent stereoselectivity. The CD-spectroscopy offers an attractive tool for determination of the absolute configuration of the bridgehead carbon atom at the 5-oxacepham skeleton.

Six-membered cyclic sulfites derived from glucofuranose and 1,2,4-butanetriol

Six-membered cyclic sulfites derived from glucofuranose derivatives 5, 6 and from 1-O-tert-butyldimethylsilil-1,2,4-butanetriol 12 were synthesized and separated into pure diastereomers which were in turn subjected to the sequence of reactions leading to the introduction of the terminal vinyl ether fragment. Reactivity and applicability of cyclic sulfites as intermediates in [2+2]cycloaddition of chlorosulfonyl isocyanate (CSI) to vinyl ethers were studied. The cycloaddition to vinyl ethers 19 and 20 proceeded in low yield and low asymmetric induction, in the case of the former, and moderate yield and pronounced asymmetric induction, in the case of the latter. The (1)H-NMR spectra of sulfites 13-16 reveal a preference of the sulfite oxygen atom for the axial position. Thus, well-defined conformation in solution for compounds 13 and 15 and a mixture of the two possible chair forms for sulfites 14 and 16 could be assigned. The stretching frequency of the S-->O bond in stable conformation with an axial sulfite oxygen occurs in the range 1,160-1,210 cm(-1), whereas conformationally mobile sulfites exhibit corresponding absorption above 1,220 cm(-1). The absolute configuration assignments of sulfites 7, 8, 15, 16 and 25-28 were done empirically based on the combined analysis of the NMR, IR, X-ray, and dichroic data. It was demonstrated that the sign of the Cotton effect around 194 nm correlated with the absolute stereochemistry at the sulfur atom in a sulfite chromophore.

An Approach to Clavams and 1-Oxacephams from Hydroxy Acids

[2 + 2] Cycloaddition of chlorosulfonyl isocyanate to chiral alkyl vinyl ethers bearing a sterogenic center in the alkyl part of the ether afford the corresponding azetidin-2-ones with relatively good asymmetric induction in certain cases. Reactions were shown to depend on steric requirements of the ligands at the stereogenic center. The model that rationalizes the stereochemical outcome is based on the s-cis conformation of the vinyl ether in which the bulkiest of the ligands is situated in the plane of the double bond, and the next most demanding substituent is placed gauche to the double bond.