The synthesis of C-13 functionalized pleuromutilins via C-H amidation and subsequent novel rearrangement product

Multi-gram quantities of C-13 functionalized pleuromutilin intermediates were synthesized under mild conditions via C-H amidation chemistry. The initial work was performed using rhodium catalysis. However, the highest yields were achieved using silver catalysis, resulting in stereoselective oxazolidinone formation. The ring closure was performed in the presence of a C-12 vinyl group, which differentiates it from historical methods involving the saturated pleuromutilin skeleton. A highlight of this vinyl template is a unique tetracyclic rearrangement product that can be generated from its treatment with hydrochloric acid and heat.     

3-Hydroxydihydrobenzofuran glucosides from Gnaphalium polycaulon

A new 3-hydroxydihydrobenzofuran glucoside, gnaphaliol 9-O-β-D-glucopyranoside (2), was isolated from the aerial parts of Gnaphalium polycaulon together with 1-{(2R*,3S*-3-(β-D-glucopyranosyloxy)-2,3-dihydro-2-[1-(hydroxyl methyl)vinyl]-1-benzofuran-5-yl}-ethanone or gnaphaliol 3-O-β-D-glucopyranoside (1), (Z)-3-hexenyl O-β-D-glucopyranoside (3) and adenosine (4). The absolute configurations at C-2 and C-3 positions of compound 1 were determined to be 2R and 3R. The structures of these compounds were elucidated on the basis of their physical and spectroscopic data.     

The Role of the C-3 Substituent in the Asymmetric Dihydroxylation of Hexo-5-Enofuranosides

ABSTRACT

Asymmetric dihydroxylation of vinyl furanosides 1-6 by use of OsO₄, AD-mix-α® and β® is described yielding the corresponding hexofuranose sugars. Vinyl furanosides 2 and 3, with an ester group at C-3, and vinyl manno furanoside 5 on asymmetric dihydroxylation with AD-mix α® exhibited high R diastereoselectivity at C-5. Reversal in diastereoselectivity at C-5 was observed for the 3-deoxy vinyl furanoside 6 giving furanosaccharide 6S with the S configuration at C-5.     

Diastereoselective C-C bond formation at C-5 of vinyl sulfone-modified hex-5-enofuranosyl carbohydrates: diversity-oriented synthesis of branched-chain sugars and beyond

This is the first report on the diastereoselective addition of carbon nucleophiles to vinyl sulfone-modified hex-5-enofuranosides. The stereoelectronic properties of the substituents at the C-3 position and their interactions with the incoming carbon nucleophiles control the diastereoselectivity of addition at the C-5 position, favoring the formation of l-ido derivatives as major products in most of the cases studied. This new concept of stereocontrolled carbon-carbon bond formation in vinyl sulfone-modified carbohydrates is general in nature. The novel chirons generated by this diversity-oriented synthetic method have been implemented in the preparation of a wide range of hexofuranosyl C-5 branched-chain sugars, bicyclic derivatives, chirally pure enals, and densely functionalized carbocycles.     

A diastereoselective and general route to 5-amino-5-deoxysugars: influence of C-3 substitution on the addition of amines to C-5 of vinyl sulfone-modified hex-5-enofuranosyl carbohydrates

In the synthesis of vinyl sulfone-modified hex-5-enofuranosides, the E/Z ratios of the products are influenced by the stereoelectronic property of a group present at the C-3 position. This observation has been utilized to influence the diastereoselectivity of addition of amines to C-5 of vinyl sulfone- modified hex-5-enofuranosides, which are efficient Michael acceptors. The stereoelectronic effect of OMe attached to the beta-face of C-3 (gluco derivative) is sufficient to impose diastereoselectivity overwhelmingly in favor of l-ido-aminosugars when the Michael acceptor is reacted with both primary and secondary amines. 3-O-Benzylated gluco derivative is also effective in producing l-ido-aminosugars but only in reactions with primary amines. The selectivity is lost when an allo derivative with OBn at the alpha-face of C-3 is used. Selected products were desulfonated to establish this new approach as a general and versatile strategy for accessing 5-amino-5-deoxysugars.     

Proton transfer from C-6 of uridine 5'-monophosphate catalyzed by orotidine 5'-monophosphate decarboxylase: formation and stability of a vinyl carbanion intermediate and the effect of a 5-fluoro substituent

The exchange for deuterium of the C-6 protons of uridine 5'-monophosphate (UMP) and 5-fluorouridine 5'-monophosphate (F-UMP) catalyzed by yeast orotidine 5'-monophosphate decarboxylase (ScOMPDC) at pD 6.5-9.3 and 25 °C was monitored by (1)H NMR spectroscopy. Deuterium exchange proceeds by proton transfer from C-6 of the bound nucleotide to the deprotonated side chain of Lys-93 to give the enzyme-bound vinyl carbanion. The pD-rate profiles for k(cat) give turnover numbers for deuterium exchange into enzyme-bound UMP and F-UMP of 1.2 × 10(-5) and 0.041 s(-1), respectively, so that the 5-fluoro substituent results in a 3400-fold increase in the first-order rate constant for deuterium exchange. The binding of UMP and F-UMP to ScOMPDC results in 0.5 and 1.4 unit decreases, respectively, in the pK(a) of the side chain of the catalytic base Lys-93, showing that these nucleotides bind preferentially to the deprotonated enzyme. We also report the first carbon acid pK(a) values for proton transfer from C-6 of uridine (pK(CH) = 28.8) and 5-fluorouridine (pK(CH) = 25.1) in aqueous solution. The stabilizing effects of the 5-fluoro substituent on C-6 carbanion formation in solution (5 kcal/mol) and at ScOMPDC (6 kcal/mol) are similar. The binding of UMP and F-UMP to ScOMPDC results in a greater than 5 × 10(9)-fold increase in the equilibrium constant for proton transfer from C-6, so that ScOMPDC stabilizes the bound vinyl carbanions, relative to the bound nucleotides, by at least 13 kcal/mol. The pD-rate profile for k(cat)/K(m) for deuterium exchange into F-UMP gives the intrinsic second-order rate constant for exchange catalyzed by the deprotonated enzyme as 2300 M(-1) s(-1). This was used to calculate a total rate acceleration for ScOMPDC-catalyzed deuterium exchange of 3 × 10(10) M(-1), which corresponds to a transition-state stabilization for deuterium exchange of 14 kcal/mol. We conclude that a large portion of the total transition-state stabilization for the decarboxylation of orotidine 5'-monophosphate can be accounted for by stabilization of the enzyme-bound vinyl carbanion intermediate of the stepwise reaction.     

Conformational analysis: IX–a 300 MHz study of 4-vinylbutyrolactone

The 300 MHz ¹H NMR spectrum of 4-vinylbutyrolactone has been recorded and analysed. The results showed that the compound is a mixture of two rapidly interconverting envelope forms in which (C-3) is the flap atom. The vinyl group favours the pseudo-equatorial orientation by 1·9 ± 0.3 kJ mol^?1, as shown by the dependence of the vicinal coupling constants on temperature.     

Stereospecific Pd(0)-catalyzed arylation of an allylic hydroxy phosphonate derivative: formal synthesis of (S)-(+)-ar-turmerone

Reaction of the (1S)-allylic hydroxy phosphonate 1(S) with methyl chloroformate in pyridine yields the corresponding carbonate 3(S). The carbonate 3(S) undergoes a palladium-catalyzed arylation with p-tolyl tributylstannane to give both the 1E and 1Z vinyl phosphonates 6 (85:15). The E and Z vinyl phosphonates 6 were shown to have the opposite configuration at C-3. The major vinyl phosphonate isomer (3S,1E), was converted to (3S)-3-(p-tolyl)-butanal 8(S), completing a formal total synthesis of (S)-ar-turmerone 5a.     

A convergent synthesis of (±)daunomycinoni

A convergent AB+CD synthesis of(±)daunomycinone 1 in 14% overall yield from hydroxy phthalan 10 is described. Methyl vinyl ketone reacts with 4,7-dimethoxy isobenzofuran (generated from 10) to provide the adduct 16 which is developed into the AB synthon 30. The cyanophthalide 32 the CD half of the molecule, is attached with good regiocontrol and a subsequent tetracyclic C6-C7 acetonide 40, oxygenated a C-9 to eventually produce a 5:6 mixture 1 and its C-7 epimer 8.     

1-Substituted beta-carboline-3-carboxylates with high affinities to the benzodiazepine recognition site

Naturally occurring derivatives of beta-carboline-3-carboxylic acid bearing acetyl or vinyl groups at C-1 were prepared by Pd-catalyzed cross-coupling reactions of methyl 1-chloro-beta-carboline-3-carboxylate with appropriate organostannanes. Esters with chloro or acetyl groups at C-1 showed high affinity for the brain benzodiazepine recognition site. Thus, in contrast to 1-alkyl and 1-aryl analogs, these beta-carboline-3-carboxylates with electron-withdrawing substituents at C-1 show high affinities.     

Synthesis of oxepane ring containing monocyclic, conformationally restricted bicyclic and spirocyclic nucleosides from D-glucose: a cycloaddition approach

Carbohydrate-derived substrates having (i) C-5 nitrone and C-3-O-allyl, (ii) C-4 vinyl and a C-3-O-tethered nitrone, and (iii) C-5 nitrone and C-4-allyloxymethyl generated tetracyclic isoxazolidinooxepane/-pyran ring systems upon intramolecular nitrone cycloaddition reactions. Deprotection of the 1,2-acetonides of these derivatives followed by introduction of uracil base via Vorbrüggen reaction condition and cleavage of the isooxazolidine rings as well as of benzyl groups by transfer hydrogenolysis yielded an oxepane ring containing bicyclic and spirocyclic nucleosides. The corresponding oxepane based nucleoside analogues were prepared by cleavage of isoxazolidine and furanose rings, coupling of the generated amino functionalities with 5-amino-4,6-dichloropyrimidine, cyclization to purine rings, and finally aminolysis.     

A general approach to the synthesis of enantiopure 19-nor-Vitamin D(3) and its C-2 phosphate analogs prepared from cyclohexadienyl sulfone

A synthesis of enantiopure 19-nor-Vitamin D(3) and its C-2 substituted cyclic phosphate analogs is achieved via in situ trapping of an α-sulfonyl anion with a CD-ring allyl chloride and 1,2-eliminative desulfonylation exploiting the basic properties of TBAF. The A-ring is prepared via anti-selective dithiane addition to vinyl sulfone and LiBH(4) mediated sequential bis reduction of an epoxy vinyl sulfone.     

Thermolysin catalyses the synthesis of cyclodextrin esters in DMSO

Fatty acid esters of cyclodextrins (CDs) were synthesised in a one-step reaction with native CDs as acyl acceptors and vinyl-activated fatty acid esters as acyl donors. Immobilised preparations of thermolysin, subtilisin, the alkaline protease AL-89 and Candida antarctica lipase B were investigated for their catalytic properties regarding transesterification in solvents of increasing hydrophilicity. The synthesis of cyclodextrin fatty acid esters was proved to be catalysed enzymatically by thermolysin in DMSO. The obtained products were analysed by TLC and their structures characterised by NMR, MS and FTIR spectroscopy. With vinyl decanoate as acyl donor β-CD was esterified at all seven glucose C-2 positions resulting in heptakis(2-O-decanoyl)-β-cyclodextrin as the major product. With vinyl butyrate, substitution occurred at all the C-2 and partially at the C-3 or C-6 positions resulting in an average degree of substitution of nine. Between 20% and 25% (w/w) of the acyl donor was converted to esters in 20 h corresponding to an estimated total conversion of the acyl acceptor in the case of maltosyl-β-CD. In the subtilisin and AL-89 catalysed reactions, product formation was simultaneously catalysed non-enzymatically by inorganic buffer salts in aprotic, hydrophilic solvents and with the lipase no products were formed in any of the solvents investigated.     

Radical dearomatising spirocyclisations onto the C-2 position of benzofuran and indole

Spirolactams were obtained via an intramolecular radical ipso-type spirocyclisation in benzofuran and indole systems. Alkyl, vinyl and aryl radicals, tethered at the C-2 position of the heterocycle underwent radical cyclisation to produce novel tricyclic partially dearomatised heterocycles in moderate yields. Fragmentation of the furan ring was observed subsequent to spirocyclisation of a vinyl radical onto a benzofuran.     

Vinyl sulfone- and vinyl sulfoxide-modified tetrahydrofurans: a preliminary account of the enantiomeric synthesis of and diastereoselectivity of addition to new classes of Michael acceptors

Enantiomerically pure 2-hydroxymethylene substituted-2,5-dihydro-3-(arylsulfonyl)- and 2-hydroxymethylene substituted-2,5-dihydro-3-(arylsulfinyl)-furans have been prepared from easily accessible carbohydrate derivatives for the first time. The strategy for accessing both these sulfones and sulfoxides is more efficient than the methods reported so far for the synthesis of this type of compounds. Hydroxymethylene group is sufficient to impose diastereoselectivity on the addition of a wide range of nucleophiles to vinyl sulfone-modified tetrahydrofurans. The benzyl protected hydroxymethylene group also suppresses the influence of chirally pure sulfoxides as two diastereomeric vinyl sulfoxide-modified tetrahydrofurans afforded the Michael adducts with same configurations at C-3 and C-4; this has been established by oxidizing the adducts, which were found to be identical to the products obtained by adding the same nucleophiles to the corresponding vinyl sulfones. These highly reactive Michael acceptors may be considered as a new addition to the arsenals of synthetic chemists interested in the functionalization of tetrahydrofurans.     

Synthesis of reactive cytidine derivatives as building blocks for cross-linking oligonucleotides

6-Vinylcytidine derivative (1) possessing good Michael acceptor properties has been synthesized through C-6 formylation and subsequent Wittig reaction. In view of introducing the reactive nucleoside into the oligonucleotide sequence, protection of the vinyl group as ethylthio derivative was proved to be effective for the masking and subsequent regeneration of the reactive vinyl moiety.     

Ionic hydrogenation-directed stereoselective construction of C-20(H) stereogenic center in steroid side chains: Scope and limitations

Stereoselective synthesis of steroidal C-20 tertiary alcohols with n-butyl, vinyl, furyl, thienyl, thiazolyl, aryl and pyridyl side chains via Grignard reaction or organolithium reagents have been realized starting from readily available 16-dehydropregnenolone acetate. The ionic hydrogenation of steroidal C-20 tertiary alcohols having furyl, methylfuryl, thienyl, phenyl and 4-methoxyphenyl side chains, resulted into the deoxygenated product with C-20 natural configuration in excellent yields. However, the alkyl, thiazolyl and pyridyl incorporated steroidal C-20 tertiary alcohols were failed under the same reaction condition. The scope of ionic hydrogenation is further highlighted through the stereoselective reduction of steroidal C-20,21-ene compounds with furyl, thienyl and 4-methoxyphenyl side chains gave the saturated compounds with C-20 natural configuration.     

Synthesis of pyrrolo[2,3-d]pyrimidine-2,4-diones by sunlight photolysis of N-(5-vinyluracil-6-yl)sulfilimines

Uracil derivatives having a vinyl group at the C-5 position and a sulfilimine moiety at the C-6 position were prepared and cyclized to 1,3,6-trisubstituted pyrrolo[2,3-d]pyrimidine-2,4-diones by sunlight photolysis in good yields.     

Structurally simplified zaragozic acid (squalestatin): Stereoselective preparation of a 3,4-unsubstituted derivative

Stereoselective preparation of a structurally simplified 3,4-unsubstituted zaragozic acid derivative was achieved starting from D-glucose. The present approach involved the stereoselective addition of a vinyl Grignard reagent, selective cleavage of 1,2-diol, and regioselective acylation of the hydroxyl group at C-6.     

Late-stage intermolecular CH activation for lead diversification: a highly chemoselective oxyfunctionalization of the C-9 position of potent bryostatin analogues

Treatment of highly potent and densely functionalized bryostatin analogue 1 with dimethyldioxirane afforded the C-9 hydroxylated hemiketal 2 via oxyfunctionalization of the C9-CH bond, one of 12 CH bonds geminal to an oxygen substituent in 1. When bryostatin analogue 3 was subjected to identical conditions, oxidation of a C-26 secondary hydroxyl group was found to compete with C-9 hydroxylation. Complete selectivity for C-9 hydroxylation was restored upon acylation of the C-26 secondary alcohol.