A de novo synthesis of 2,6-dideoxy-C-aryl glycosides based on ring closing metathesis and diastereoselective epoxide cleavage/anomerization reactions

Concise Stereoselective Total Synthesis of (-)-Hedycoropyran A

A concise and stereoselective total synthesis of (-)-hedycoropyran A was accomplished in a substrate-controlled manner from a readily available alkene. Highlights of the synthesis include a highly diastereoselective dehydrogenative cycloetherification to construct the trans-2-aryl-6-alkyl-3,6-dihydro-2H-pyran framework and late-stage substrate-controlled trans-dihydroxylation at C(3,4).

Diastereoselective Construction of trans-2-Alkyl-6-aryl-3,6-dihydro-2H-pyrans via Dehydrogenative Cycloetherification Promoted by DDQ

A diastereoselective synthesis of trans-2-alkyl-6-aryl-3,6-dihydro-2H-pyrans has been described. Dehydrogenative cycloetherification of (E)-(±)-1-aryl-5-hydroxy-1-alkenes promoted by DDQ proceeded cleanly via 6-endo cyclization to afford trans-2-alkyl-6-aryl-3,6-dihydro-2H-pyrans (32 examples) in good yield (up to 89%) and with moderate to excellent diastereoselectivity (up to 99:1). The synthetic utility of the method was illustrated by the second total synthesis of (±)-(2R,6S)-3,4-dehydro-1,7-bis(4-hydroxy phenyl)-4'-de-O-methyl centrolobine and a total synthesis of (±)-centrolobine.

Preparation of C-arylglycals via Suzuki-Miyaura cross-coupling of dihydropyranylphosphates

The preparation of C-arylglycals has been accomplished employing the Suzuki-Miyaura cross-coupling reaction of dihydropyranylphosphates with arylboronate esters. The reaction is tolerant of both electron-donating (EDG) and electron-withdrawing (EWG) groups on the aromatic ring and affords the corresponding C-arylglycals in good to excellent yields (68-97%). Additionally, the ketene acetal phosphate derived from 6-deoxy-3,4-di-O-benzyl-L-rhamnal also couples efficiently to yield C-arylglycals in excellent yields.

A sequential indium-mediated aldehyde allylation/palladium-catalyzed cross-coupling reaction in the synthesis of 2-deoxy-beta-C-aryl glycosides

Indium-mediated allylation of aldehydes with 2-chloro-3-iodopropene, followed by a palladium-catalyzed cross-coupling reaction with triarylindium reagents or arylboronic acids, leads to aryl-substituted homoallylic alcohols in good to excellent yields and diastereoselectivities. The products obtained from reactions conducted with d-glyceraldehyde acetonide can be transformed into 2-deoxy-beta-C-aryl ribofuranosides in high overall yields. Similarly, 2-deoxy-beta-C-aryl allopyranosides may be prepared efficiently from 2,4-O-benzylidene erythrose.

Stereocontrolled [4+2]-Annulation Accessing Dihydropyrans: Synthesis of the C1a-C10 Fragment of Kendomycin

[reaction: see text] Development of new organosilane reagents bearing C-centered chirality where the stereocenter is fully substituted, and their use in the stereocontrolled synthesis of cis- and trans-dihydropyrans containing a trisubstituted olefin is described. The reagents participate in Lewis acid promoted [4+2]-annulations providing useful levels of selectivity with both aliphatic and aromatic aldehydes. A stereoselective synthesis of the C1a-C10 fragment of kendomycin (1) is also described.

A Straightforward and Flexible [4 + 2] Route to β-C-Naphthyl-2-deoxy-glycosides through Tandem Hydroboration-Ketal Reduction: De Novo Access to C-Naphthyl-6-fluoro and 6,6-Difluoro 2-Deoxyglycosides

[reaction: see text] Under standard hydroboration-oxidation conditions, the dihydropyrans 4 underwent a highly stereocontrolled tandem reaction, involving the expected hydration of the double bond together with the reduction of the ketal moiety. This unprecedented transformation gives rise to a short, [4 + 2]-based synthetic route to (+/-)-beta-C-naphthyl-2-deoxyglycosides 5, which allows a significant structural and functional diversity at C-6. We thus described the first synthesis of (+/-)-C-aryl-6-fluoro and -6,6-difluoro olivosides, via the allylic mono- and difluorides produced by regioselective fluorination of, respectively, hydroxyalkyl and oxoalkyl dihydropyran derivatives.

Synthesis of C-glycopyranosylphloroacetophenone derivatives and their anomerization facilitated by 1,3-diaxial interactions

The reaction of 2,3,4-tri-O-benzyl-6-deoxy-alpha-D-glucopyranosyl fluoride, 2,3,4,6-tetra-O-benzyl-alpha-D-allopyranosyl fluoride, and 2,3,4-tri-O-benzyl-alpha-L-fucopyranosyl fluoride with 2,4-di-O-benzylphloroacetophenone, in the presence of boron trifluoride diethyl etherate, afforded, respectively, the corresponding 3-C-beta-D-glycopyranosylphloroacetophenone derivatives exclusively in anomerically pure form. Alternatively, the reaction of 2,3,4,6-tetra-O-benzyl-alpha-D-gulopyranosyl fluoride with 2,4-di-O-benzylphloroacetophenone afforded both the 3-C-beta-D-gulopyranosylphloroacetophenone derivative (4C(1) conformation) as the major product and the 3-C-alpha-D-gulopyranosylphloroacetophenone derivative (1C(4) conformation) as the minor product under identical conditions. Including the previously prepared C-glycosylphloroacetophenone derivatives that contain 3-C-beta-D-glucosyl, 3-C-beta-D-xylosyl, 3-C-beta-2-deoxy-D-arabino-hexosyl, 3-C-beta-D-galactosyl, 3-C-beta-L-arabinosyl, and 3-C-alpha-L-arabinosyl moieties, the conformation is dictated primarily by the preference of the bulky aromatic aglycon to orient equatorially, due to the strong repulsion of the aglycon. The anomerization is directed secondarily by the presence of 1,3-diaxial interactions in the sugar moiety.

Synthesis of C-mannopyranosylphloroacetophenone derivatives and their anomerization

The reaction of 2,3,4-tri-O-benzyl-alpha-L-rhamnopyranosyl fluoride (6-deoxy-2,3,4-tri-O-benzyl-alpha-L-mannopyranosyl fluoride) with 2,4-dibenzylphloroacetophenone, in the presence of boron trifluoride.diethyl etherate, afforded both the 3-C-alpha-L- and the 3-C-beta-L-rhamnopyranosylphloroacetophenone derivatives. The 3-C-alpha-L-rhamnoside was produced as a major product, while the 3-C-beta-L-rhamnoside was produced as a minor product via anomerization of the 3-C-alpha-L-rhamnoside. Alternatively, the reaction of 2,3,4,6-tetra-O-benzyl-alpha-D-mannopyranosyl fluoride with 2,4-dibenzylphloroacetophenone afforded both the 3-C-alpha-D- and the 3-C-beta-D-mannnopyranosylphloroacetophenone derivatives under identical conditions. The 3-C-beta-D-mannoside was produced as a major product via anomerization of the 3-C-alpha-D-mannoside during the reaction. These differences in composition result apparently from the magnitude of the 1,3-diaxial interactions between the C-3 and C-5 positions in these sugar moieties.

Diastereoselective ring-closing metathesis in the synthesis of dihydropyrans

An investigation into the factors influencing the diastereochemical outcome of the ring-closing metathesis based synthesis of dihydropyrans is presented in this paper. Divinyl carbinols derived from alpha-hydroxy carboxylic acid esters are elaborated to trienes with two diastereotopic vinyl moieties. Depending on the steric demand of the oxo substituent of the divinyl carbinol moiety (either unprotected OH, TBDMS, or benzyl ether) different diastereomers are preferrably formed upon ring-closing metathesis. An extension to diastereoselective double ring-closing metathesis in the formation of spirocycles has also been investigated.