The Preparation of Optically Pure 7-Oxabicyclo [2.2.1]hept-2-ene Derivatives. The CD Spectrum of (+)-(1R)-7-Oxabicyclo [2.2.1]hept-5-en-2-one

Masked Ketenes as Dienophiles in the Diels–Alder Reaction

The Diels–Alder reaction is one of the most powerful, well-established, and versatile reactions in organic chemistry; however, its application in certain settings remains a challenge as a result of functional group incompatibility. In this review, we examine the methods in which masked ketenes can be employed as dienophiles, taking particular note of applications in complex settings.

Stereochemistry of ring-opening/cross metathesis reactions of exo- and endo-7-oxabicyclo[2.2.1]hept-5-ene-2-carbonitriles with allyl alcohol and allyl acetate

The ROCM reactions of exo- and endo-2-cyano-7-oxanorbornenes with allyl alcohol or allyl acetate promoted by different ruthenium alkylidene catalysts were studied. The stereochemical outcome of the reactions was established. The issues concerning chemo- (ROCM vs ROMP), regio- (1-2- vs 1-3-product formation), and stereo- (E/Z isomerism) selectivity of reactions under various conditions are discussed. Surprisingly good yields of the ROCM products were obtained under neat conditions.

Cyano(ethoxycarbonothioylthio)methyl Benzoate: A Novel One-Carbon Radical Equivalent

[reaction: see text] Cyano(ethoxycarbonothioylthio)methyl benzoate 3 has been prepared and shown to be an excellent one-carbon radical equivalent that can be applied for the introduction of an acyl unit via xanthate transfer radical addition to olefins. The corresponding adducts can be further elaborated. A rare 1,5-nitrile translocation was also observed during the study.

Synthesis ofd- andl-2,3-trans-3,4-cis-4,5-trans-3,4-Dihydroxy-5-hydroxymethylproline and Tripeptides Containing Them

Enantiomerically pure (-)-(1R,4R,5R,6S)- and (+)-(1S,4S,5S,6R)-7-(tert-butoxycarbonyl)-5,6-exo-isopropylidenedioxy-7-azabicyclo[2.2.1]hept-2-one ((-)-3 and (+)-3) have been obtained from the Diels-Alder adduct of N-(tert-butoxycarbonyl)pyrrole and 2-bromo-1-(p-toluenesulfonyl)acetylene, including the Alexakis optical resolution of ketone (+/-)-3 via formation of cyclic aminals with (1R,2R)-diphenylethylenediamine. Compounds (-)-3 and (+)-3 were converted into d- and l-2,3-trans-3,4-cis-4,5-trans-N-(tert-butoxycarbonyl)-5-hydroxymethyl-3,4-isopropylidenedioxyprolines (-)-4 and (+)-4, respectively. Applying the Boc and Fmoc strategies of peptide synthesis, these compounds were used to construct two tripeptides containing the d- or l-2,3-trans-3,4-cis-4,5-trans-3,4-dihydroxy-5-hydroxymethylproline.

Transition metal-catalyzed enantioselective ring-opening reactions of oxabicyclic alkenes

Over the past five years, several metal-catalyzed asymmetric ring-cleaving reactions have been developed that generate ring-opened products in high yield and enantiomeric excess. These reactions can be carried out with a range of nucleophiles including hydride, stabilized and nonstabilized carbanions, alcohols, amines, and carboxylates. To achieve these results, three different transition metals have been employed, namely nickel, palladium, and rhodium, leading not only to synthetically useful transformations but also to a greater understanding of the reaction manifolds possible with these metals. This Account covers the work by our group and by others toward the development of new metal-catalyzed asymmetric ring-opening reactions of oxabicyclic alkenes.

Synthesis of termini-differentiated 6-carbon stereotetrads: an alkylative oxidation strategy for preparation of the c21-c26 segment of apoptolidin(1)

[reaction: see text] Two methods have been developed for the synthesis of epoxide 36. The first uses (+)-pulegone 25 as an enantiopure starting material and introduces the requisite intricacy of target 22 in 12 operations. The second method employs an enantiospecific catalytic Jacobsen epoxidation of 1a and is five operations shorter. The second sequence features an oxygen-directed alkylative oxidation reaction that re-establishes the dienyl sulfone functionality with concomitant 1,3-transposition of the sulfone moiety.

Building a small polypropionate library. Synthesis of all possible stereotetrads (building blocks for polyketide synthesis) from furan

The all diastereomeric stereotetrads (polypropionate fragments) were synthesized in a stereodivergent fashion, starting from the Diels-Alder adducts of furan with acrylic acid and (-)-2-camphanoxyacrylonitrile, respectively. The key intermediates of these sequences were the oxanorbornenic sulfones 7, (-)-53 and (+)-54. Regio- and stereocontrolled alkylative ring opening of these intermediates afforded the cyclohexenyl sulfones 14, (+)-55 and (-)-58 which were transformed into the desired stereotetrads 30, 31, 40, 41, (+)-62, (+)-63, (-)-66, and (+)-69.

The C-disaccharide alpha-C(1-->3)-mannopyranoside of N-acetylgalactosamine is an inhibitor of glycohydrolases and of human alpha-1,3-fucosyltransferase VI. Its epimer alpha-(1-->3)-mannopyranoside of N-acetyltalosamine is not

The radical C-glycosidation of (-)-(1S,4R,5R, 6R)-6-endo-chloro-3-methylidene-5-exo-(phenylseleno)-7-ox abi cyclo[2. 2.1]heptan-2-one ((-)-4) with 2,3,4, 6-tetra-O-acetyl-alpha-D-mannopyranosyl bromide gave (+)-(1S,3R,4R, 5R,6R)-6-endo-chloro-5-exo-(phenylseleno)-3-endo-(1',3',4', 5'-tetra-O-acetyl-2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)-7-oxabi cyc lo[ 2.2.1]hept-2-one ((+)-5) that was converted into (+)-(1R,2S,5R, 6R)-5-acetamido-3-chloro-2-hydroxy-6-(1',3',4',5'-tetra-O-acetyl)-2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)cyclohex -3-en- 1-yl acetate ((+)-10) and into (+)-(1R,2S,5R, 6S)-5-bromo-3-chloro-2-hydroxy-6-(1',3',4',5'-tetra-O-acetyl-2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)cyclohex -3-en- 1-yl acetate ((+)-19). Ozonolysis of (+)-10 and further transformations provided 2-acetamido-2,3-dideoxy-3-C-(2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)-D-galac tos e (alpha-C(1-->3)-D-mannopyranoside of N-acetylgalactosamine (alpha-D-Manp-(1-->3)CH(2)-D-GalNAc): 1). Displacement of the bromide (+)-19 with NaN(3) in DMF provided the corresponding azide ((-)-20) following a S(N)2 mechanism. Ozonolysis of (-)-20 and further transformations led to 2-acetamido-2,3-dideoxy-3-C-(2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)-D-talose (alpha-C(1-->3)-D-mannopyranoside of N-acetyl D-talosamine (alpha-D-Manp-(1-->3)CH(2)-D-TalNAc): 2). The neutral C-disaccharide 1 inhibits several glycosidases (e.g., beta-galactosidase from jack bean with K(i) = 7.5 microM, alpha-L-fucosidase from human placenta with K(i) = 28 microM, beta-glucosidase from Caldocellum saccharolyticum with K(i) = 18 microM) and human alpha-1, 3-fucosyltransferase VI (Fuc-TVI) with K(i) = 120 microM whereas it 2-epimer 2 does not. Double reciprocal analysis showed that the inhibition of Fuc-TVI by 1 displays a mixed pattern with respect to both the donor sugar GDP-fucose and the acceptor LacNAc with K(i) of 123 and 128 microM, respectively.