Dynamics of alkene radical cation/phosphate anion pair formation from nucleotide C4' radicals. The DNA/RNA paradox revisited

The fragmentation of nucleotide C4' radicals generated by thiyl radical addition to C4'C5' exocyclic glycals has been re-examined and found to be a function of the thiol and, probably, the initiating system employed. It has been demonstrated that C4' radicals of DNA and RNA models fragment even in the very nonpolar benzene solution if the correct (aliphatic) thiol is employed. (17)O-Labeling experiments are used to demonstrate that the fragmentation of nucleotide C4' radicals (2-deoxyribo- and ribo-) to contact ion pairs is either irreversible or so rapidly reversible as to preclude prior reorganization of the contact ion pair. Formation of the solvent-separated ion pair is an irreversible step, with all such ion pairs proceeding to product formation.

1-Benzenesulfinyl piperidine/trifluoromethanesulfonic anhydride: a potent combination of shelf-stable reagents for the low-temperature conversion of thioglycosides to glycosyl triflates and for the formation of diverse glycosidic linkages

The combination of 1-benzenesulfinyl piperidine (BSP) and trifluoromethanesulfonic anhydride (Tf(2)O) forms a new, powerful, metal-free thiophile that can readily activate both armed and disarmed thioglycosides, via glycosyl triflates, in a matter of minutes at -60 degrees C in dichloromethane, in the presence of 2,4,6-tri-tert-butylpyrimidine (TTBP). The glycosyl triflates are rapidly and cleanly converted to glycosides, upon treatment with alcohols, in good yield and selectivity.

Why are the hydroxy groups of partially protected N-acetylglucosamine derivatives such poor glycosyl acceptors, and what can be done about it? A comparative study of the reactivity of N-acetyl-, N-phthalimido-, and 2-azido-2-deoxy-glucosamine derivatives in glycosylation. 2-Picolinyl ethers as reactivity-enhancing replacements for benzyl ethers

Competition experiments were used to determine that the 4-OH of a 2-deoxy-2-azidoglucose derivative is more reactive than that of the corresponding N-phthalimido glucose derivative which, in turn, is more easily glycosylated than the N-acetyl derivative. Glycosylation of the 4-OH groups of the N,N-diacetyl and N-acetyl-N-benzyl glucosamine was also found to be superior to that of the simple N-acetyl substance. The 3-O-picolinyl ether of a 4,6-O-benzylidene-protected N-acetylglucosamine was shown to have a strong intramolecular hydrogen bond to the adjacent acetamide group. This interaction does not persist in the 3-O-picolinyl-6-O-benzyl N-acetylglucosamine derivative, owing to a probable competing hydrogen bond between the 4-OH and the picolinyl ether. However, in the 3-O-picolinyl-4-O-benzyl N-acetylglucosamine regioisomer the picolinyl-acetamide hydrogen bond persists and leads to an enhancement of reactivity of the 6-OH, over and above that in the corresponding 3-O-benzyl ether, due to disruption of the typical intermolecular amide hydrogen bonding scheme. It is demonstrated that the picolinyl ether is readily removed by hydrogenolysis at atmospheric pressure and room temperature.

Generation and reaction of alkene radical cations under nonoxidizing conditions: synthesis of the pyrrolizidine nucleus

[see reaction]. Stable beta-phosphatoxy nitroalkanes, readily assembled by the Henry reaction and subsequent phosphorylation, serve as good precursors to alkene radical cations on treatment with triphenyltin or tributyl hydride and AIBN in benzene at reflux. When the beta-phosphatoxy nitroalkane is suitably functionalized with nucleophilic groups, substitutions can be achieved with the formation of heterocyclic rings. When the nucleophile is an allylamine, tandem processes occur giving pyrrolizidines.

Product studies and laser flash photolysis on alkyl radicals containing two different beta-leaving groups are consonant with the formation of an olefin cation radical

1-Bromo-2-methoxy-1-phenylpropan-2-yl (3) and 2-methoxy-1-phenyl-1-diphenylphosphatopropan-2-yl (4) were generated under continual photolysis from the respective PTOC precursors in a mixture of acetonitrile and methanol. The radicals undergo heterolytic fragmentation of the substituent in the beta-position to generate the olefin cation radical (5). Z-2-Methoxy-1-phenylpropene (15) is the major product formed in the presence of 1,4-cyclohexadiene, and is believed to result from hydrogen atom transfer to the oxygen of the olefin cation radical, followed by deprotonation. Laser flash photolysis experiments indicate that reaction between 5 and 1,4-cyclohexadiene occurs with a rate constant of approximately 6 x 10(5) M(-1) s(-1). 2,2-Dimethoxy-1-phenylpropane (18) is observed as a minor product. Laser flash photolysis experiments place an upper limit on methanol trapping of 5 at k <1 x 10(3) M(-1) s(-1) and do not provide any evidence for the formation of reactive intermediates other than 5. The use of two PTOC precursors containing different leaving groups to generate a common olefin cation radical enables one to utilize product analysis to probe for the intermediacy of other reactive intermediates. The ratio of 15:18 is dependent upon hydrogen atom donor concentration, but is independent of the PTOC precursor. These observations are consistent with the proposal that both products result from trapping of 5 that is formed via heterolysis of 3 and 4.

S-(4-methoxyphenyl) benzenethiosulfinate (MPBT)/trifluoromethanesulfonic anhydride: a convenient system for the generation of glycosyl triflates from thioglycosides

[structure] The combination of S-(4-methoxyphenyl) benzenethiosulfinate (MPBT, 1) and trifluoromethanesulfonic anhydride forms a powerful, metal-free, thiophile which readily activates thioglycosides, via glycosyl triflates, at -60 degrees C in dichloromethane, in the presence of 2,6-di-tert-butyl-4-methylpyridine. The glycosyl triflates are rapidly and cleanly converted to glycosides, upon treatment with alcohols, in good yield and selectivity.

Efficient, diastereoselective chemical synthesis of a beta-mannopyranosyl phosphoisoprenoid

[reaction: see text] Tetrabutylammonium benzyl dihydrophytylphosphate was coupled to S-phenyl 2,3-di-O-benyl-4, 6-O-benzylidene-1-thio-alpha-D-mannopyranoside S-oxide on activation with triflic anhydride in toluene at -78 degrees C to give the corresponding beta-mannosyl phosphate in 56% yield with no detectable formation of the alpha-anomer. Treatment with sodium in liquid ammonia then afforded the unprotected beta-mannosyl phosphoisoprenoid.

Highly diastereoselective alpha-mannopyranosylation in the absence of participating protecting groups

S-Phenyl 2,6-di-O-benzyl-3,4-O-(2',3'-dimethoxybutane-2', 3'-diyl)-1-thia-alpha-D-mannopyranoside and its sulfoxide, following activation at -78 degrees C with benzenesulfenyl triflate or triflic anhydride, respectively, provide the corresponding alpha-mannosyl triflate as demonstrated by NMR spectroscopy. On addition of an acceptor alcohol alpha-mannosides are then formed. Similarly, S-phenyl 2,3-O-carbonyl-4, 6-O-benzylidene-1-thia-alpha-D-mannopyranoside and ethyl 3-O-benzoyl-4, 6-O-benzylidene-2-O-(tert-butyldimethylsilyl)-1-thia-alpha-D-mannopyr anoside both provide alpha-mannosides on activation with benzenesulfenyl triflate followed by addition of an alcohol. These results stand in direct contrast to the highly beta-selective couplings of comparable glycosylations with 2,3-di-O-benzyl-4, 6-O-benzylidene protected mannosyl donors and draw attention to the subtle interplay of reactivity and structure in carbohydrate chemistry.

Direct stereoselective synthesis of beta-thiomannosides

A highly diastereoselective synthesis of beta-thiomannopyranosides is described in which S-phenyl 2,3-di-O-benzyl-4, 6-O-benzylidene-1-deoxy-1-thia-alpha-D-mannopyranoside S-oxide is treated with triflic anhydride and 2, 6-di-tert-butyl-4-methylpyridine in CH(2)Cl(2) at -78 degrees C leading to the formation of an intermediate alpha-mannosyl triflate. Addition of primary, secondary, or tertiary thiols then leads to the beta-thiomannosides, by an S(N)2-like displacement, in good yield and with excellent stereoselectivity. Deprotection is achieved either by Birch reduction or by Zemplen deacetylation, of the acetyl protected aglycons, followed by hydrogenolysis over Pearlman's catalyst. The assignment of configuration of the beta-thiomannopyranosides is discussed in terms of the chemical shift of the mannose H5 resonance and the (1)J(CH) of the mannose anomeric carbon.

Isolation and identification of poly-alpha-(1-->4)-linked 3-O-methyl-D-mannopyranose from a hot-water extract of Mycobacterium vaccae

A polysaccharide around 3.6 kDa has been identified as the major carbohydrate moiety of a antineoplastic protein-polysaccharide complex (PS4A) obtained by boiling intact cells of Mycobacterium vaccae in water. 1H and 13C NMR spectra of this polysaccharide suggested it was a highly homogeneous polymer composed substantially of one monomer, probably an alpha-linked O-methylated mannose. Comparison of the COSY spectra of the original and acetylated polymer indicated that the glycosidic linkage and the methyl ether were interchangeable, at O-3 and O-4. Further study demonstrated that the benzyolated hydrolysate of the polymer was 1,2,4,6-tetra-O-benzoyl-3-O-methyl-beta-mannopyranose. The hydrolysate was 3-O-methyl-alpha, beta-mannopyranose and the polymer was therefore poly-alpha-(1-->4)-linked 3-O-methyl-D-mannopyranose. This conclusion was further confirmed with an authentic sample of the monomer, which had spectral data identical to those of the hydrolyzate and co-eluted from an ion-exchange HPLC with the major sugar in the hydrolysate.

Heterolytic cleavage of a beta-phosphatoxyalkyl radical resulting in phosphate migration or radical cation formation as a function of solvent polarity

[formula: see text] The 2-(diethylphosphatoxy)-2-(p-methoxyphenyl)-1,1-dimethylethyl radical (1) reacted to give the benzylic radical product from phosphate migration or a radical cation (or a mixture of the two) as a function of solvent. Smooth acceleration in rates of reactions of 1 in solvents of increasing polarity and consistent entropies of activation indicate that radical 1 reacts by common mechanism irrespective of the final products formed, specifically by initial heterolysis to a radical cation-phosphate anion pair.

Designed Chiral Acyl Radical Equivalents. Preparation and Cyclizations of Disymmetrically Substituted 1,3-Dioxabicyclo[4.4.0]decan-2-yl Radicals

The diastereoselectivity of 5-exo-trigonal cyclizations of 2-(4-penten-1-yl)-1,3-dioxolan-2-yl and 2-(4-penten-1-yl)-1,3-dioxan-2-yl radicals is investigated. When dioxolanes or dioxanes derived from C(2) symmetrically substituted diols are employed the diastereoselectivity is poor. In the dioxanyl series this is a consequence of the cyclization occurring through a twist-boat conformer. Disymmetrically substituted dioxanyl radicals, derived from the alcohols 21 and 41, are, however, constrained to chairlike conformations and accordingly give rise to highly diastereoselective cyclizations. Conditions are described for the hydrolysis of the resulting spiroacetals and for determination of the ee of the resulting 2-methylcyclopentanones.

Antiovulatory antagonists of LHRH related to antide

We report 104 analogues of the potent antiovulatory antagonist of LHRH, N-Ac-D-Nal-D-Cpa-D-Pal-Ser-Lys(Nic)-D-Lys(Nic)-Leu-Ilys-Pro-D-Ala- NH2, Antide. We replaced the Nic group in Antide with other acyl substituents to modulate size, hydrophilicity or basicity of the molecule, we also replaced the Lys residues with shorter basic amino acids, and made cyclic 5/6 analogues as well as position 5 or 6 dimers. We substituted Ilys8 with other alkyl groups and acyl derivatives. When injected in 0.1% DMSO in water in a typical antiovulatory (AO) assay. Antide gives six rats ovulating out of eight (6/8) at 2 micrograms, 4/8 at 4 micrograms, and in the histamine release assay (HRA). ED50 is > 300 micrograms/ml; [Lys(N-Isobutyl)8]Antide gave 2/8 at 2 micrograms/rat; [Lys (8-Qis)5]Antide gave 1/8 at 1 microgram, and 0/8 at 2 micrograms, and in the HRA ED50. 22 micrograms/ml; [D-Lys(8-Qis)5]Antide gave 4/8 at 1 microgram and 0/8 at 2 micrograms, and in the HRA, ED50 was 27 micrograms/ml; [Lys(8-Qic)8] gave 5/8 at 1 microgram 1/8 at 2 micrograms/ [Lys(2-Pyc)6]Antide gave 3/8 at 1 microgram, and 0/8 at 2 micrograms, and in the HRA ED50 was 116 micrograms/ml; [D-Lys (2-Pyc)5]Antide gave 5/8 at 1 microgram and in the HRA, ED50 was 100- > 300 micrograms/ml; [Lys(2-Pyc)5.D-Lys(2-Pyc)6]Antide gave 2/8 at 1 microgram. The substitutions of the Nic groups of Antide at Lys5 or D-Lys6 with 8-Qis or with 2-Pyc groups seem to give highly potent antiovulatory antagonists of LHRH and constitute significant new leads to generate potent antiovulatory compounds endowed with moderate or low histamine release.