David C. Baker

Synthesis of branched-chain sugar derivatives related to aldgarose

Abstract Ethynylation of 1,2:5,6-di- O -isopropylidene-α- D -ribo-hexofuranos-3-ulose ( 1 ) gave the 3- C -ethynyl allo derivative 2 , together with an adduct ( 3 ) resulting from interaction of two molecules of 1 with one of acetylene. Lithium aluminum hydride reduced the acetylenes 2 and 3 to the corresponding alkenes 4 and 8 ; on sequential ozonolysis-borohydride reduction, these both gave 3- C -(hydroxymethyl)-1,2:5,6-di- O -isopropylidene-α- D -allofuranose ( 6 ), further characterized as its 3,3 1 -cyclic carbonate 9 . Ozonolysis of the acetylene 2 gave the 3 1 ,5-lactone ( 5 ) of the 3- C -carboxy analog, thus establishing the stereochemistry of 2, which was independently established by n.m.r. spectroscopy employing a lanthanide shift-reagent. Treatment of 2 with mercuric acetate in ethyl acetate, followed by hydrogen sulfide, gave a mixture of the 3- C -acetyl-3- O -acetyl derivative 10 and a product ( 11 ) derived from internal cyclization of 5,6-deacetonated, O -deacetylated 10 . Reduction of 10 with lithium aluminum hydride gave a separable mixture of diastereoisomeric 3- C -(l-hydroxy-ethyl) derivatives ( 12a , 12b ) that were individually converted into their corresponding 3,3 1 -cyclic carbonates 13a and 13b , products that contain the branch functionality of the unusual, branched-chain sugar aldgarose.

Photolysis of azido sugar derivatives. Generation of 5′-aldehydes from 5′-azido derivatives of adenosine and uridine

Abstract 5′-Azido-5′-deoxy-2′,3′- O -isopropylideneadenosine ( 4 ) [prepared by way of N 6 -formyl-2′,3′- O -isopropylidene-5′- O - p -tolylsulfonyladenosine ( 2 )], upon photolysis and mild treatment with acid, gives the nucleoside 5′-aldehyde derivative 5 , characterized by reduction with borohydride to 2′,3′- O -isopropylideneadenosine ( 1 ) and with borodeuteride to give the 5′-deuterated analog ( 6 ) of 1 . Similarly, photolysis of 5′-azido-2′,3′- O -benzylidene-5′-deoxyuridine ( 9 ), followed by treatment with acid, gave the nucleoside 5′-aldehyde derivative 10 , characterized by borohydride reduction to 2′,3′- O -benzylideneuridine ( 7 ) and borodeuteride reduction to the 5′-deuterated analog ( 11 ) of 7 . The position of deuterium labeling in 6 and 11 was verified by high-resolution n.m.r. and mass spectrometry.

SYNTHESIS AND EVALUATION OF ACYCLIC SUGAR NUCLEOSIDES

Acylated aldose dialkyl dithioacetals with bromine undergo replacement of one alkylthio group by bromine. These unstable bromides react, as by fusion with 2,4-bis(trimethylsilyloxy)pyrimidine, to give acylated 1-(pyrimidin-1-yl) derivatives that upon saponification afford acyclic sugar nucleoside analogues, some as separable mixtures of 1-epimers. Systemic stereochemical variants have been conducted. Pmr conformational studies show that the sugar chain is extended in certain examples, whereas others favor folded (sickle) conformations, in line with a general rationale developed for acyclic-sugar derivatives. Condensation of the bromides with purines gives 9-substituted acyclic-sugar nucleoside analogues; synthesized systematically for various series, these include the D-pentoses in combination with 6-mercaptopurine. In vitro and in vivo biological activities vary according to stereochemistry of the sugar. The position of substitution of the sugar chain, the chirality at C-1, and the tautomeric form of the heterocycle, were established by x-ray crystallography of the product from D-arabinose and 6-mercaptopurine. The x-ray data permit correlation of C-1 chirality throughout the series and pmr data indicate the favored conformations.

Synthesis and stereochemical characterization of a series of five-carbon, acyclic-sugar derivatives of 1,6-dihydro-6-thioxopurine (6-mercaptopurine)☆☆☆

Abstract The four acetylated d -aldose diethyl dithioacetals ( 1a-d ) were treated with bromine to give the corresponding, unstable 1-bromides ( 2a-d ), which were immediately condensed with 6-chloro-9-(chloromercuri)purine ( 3 ) to furnish the respective, protected nucleosides ( 4a-d ). Subsequent treatment with thiourea gave the crystalline 6-mercaptopurine analogs ( 5a-d ) which, upon deacetylation in butylamine-tetrahydrofuran, gave the free, acyclic-sugar nucleosides ( 6a-d )- With the exception of the arabino derivative 4b , the 6-chloropurine derivatives were mixtures of 1′-epimers. Crystallization of the acylated 6-mercaptopurine derivatives afforded single 1′-epimers for 5a-c ; 5d remained an epimeric mixture. A positive Cotton effect for both the ribo and arabino analogs 5a and 5b , and a negative Cotton effect for the xylo derivative 5c , suggested the (1′ R ) configuration for 5a and 5b , and the (1′ S ) configuration for 5c . Proof of the stereochemistry at C-1 (as well as proof of derivatization at N-9 of the purine ring) was afforded by X-ray crystallographic analysis of the arabino derivative 5b , which was demonstrated to have the (1′ R ) configuration. Use of the Generalized Heterocycle Rule also supported the stereochemical attributions at C-1′ for the remaining compounds. The arabino derivative 5b adopts an extended, planar, zigzag conformation of the side chain in solution, as well as in the crystalline state, whereas the ribo ( 5a ) and xylo ( 5c ) derivatives exist in solution as non-extended, sickle conformations; the lyxo analog ( 5a ) was found to be a mixture of 1′-epimers, both of which adopt in solution an extended, planar, zigzag arrangement of the sugar chain.

Biochemistry and pharmacology of 7α-substituted androstenediones as aromatase inhibitors ☆

The inhibition of aromatase, the enzyme responsible for converting androgens to estrogens, is therapeutically useful for the endocrine treatment of hormone-dependent breast cancer. Research by our laboratory has focused on developing competitive and irreversible steroidal aromatase inhibitors, with an emphasis on synthesis and biochemistry of 7alpha-substituted androstenediones. Numerous 7alpha-thiosubstituted androst-4-ene-3,17-diones are potent competitive inhibitors, and several 1,4-diene analogs, such as 7alpha-(4-aminophenylthio)-androsta-1,4-diene-3,17-di one (7alpha-APTADD), have demonstrated effective enzyme-activated irreversible inhibition of aromatase in microsomal enzyme assays. One focus of current research is to examine the effectiveness and biochemical pharmacology of 7alpha-APTADD in vivo. In the hormone-dependent 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary carcinoma model system, 7alpha-APTADD at a 50 mg/kg/day dose caused an initial decrease in mean tumor volume during the first week, and tumor volume remained unchanged throughout the remaining 5-week treatment period. This agent lowers serum estradiol levels and inhibits ovarian aromatase activity. A second research area has focused on the synthesis of more metabolically stable inhibitors by replacing the thioether linkage at the 7alpha position with a carbon-carbon linkage. Several 7alpha-arylaliphatic androst-4-ene-3,17-diones were synthesized by 1,6-conjugate additions of appropriate organocuprates to a protected androst-4,6-diene or by 1,4-conjugate additions to a seco-A-ring steroid intermediate. These compounds were all potent inhibitors of aromatase with apparent Kis ranging between 13 and 19 nM. Extension of the research on these 7alpha-arylaliphatic androgens includes the introduction of a C1-C2 double bond in the A-ring to provide enzyme-activated irreversible inhibitors. The desired 7alpha-arylaliphatic androsta-1,4-diene-3,17-diones were obtained from their corresponding 7alpha-arylaliphatic androst-4-ene-3,17-diones by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). These inhibitors demonstrated enzyme-mediated inactivation of aromatase with apparent k(inact)s ranging from 4.4 x 10(-4) to 1.90 x 10(-3) s(-1). The best inactivator of the series was 7alpha-phenpropylandrosta-1,4-diene-3,17-dione, which exhibited a T(1/2) of 6.08 min. Aromatase inhibition was also observed in MCF-7 human mammary carcinoma cell cultures and in JAr human choriocarcinoma cell cultures, exhibiting IC50 values of 64-328 nM. The 7alpha-arylaliphatic androgens thus demonstrate potent inhibition of aromatase in both microsomal incubations and in choriocarcinoma cell lines expressing aromatase enzymatic activity. Additionally, the results from these studies provide further evidence for the presence of a hydrophobic binding pocket existing near the 7alpha-position of the steroid in the active site of aromatase. The size of the 7alpha-substituent influences optimal binding of steroidal inhibitors to the active site and affects the extent of enzyme-mediated inactivation observed with androsta-1,4-diene-3,17-dione analogs.

Synthesis and stereochemical correlation of acyclic sugar nucleoside analogues: X-ray crystal structure of (1R)-2,3,4,5-tetra-O-acetyl-1-S-ethyl-1-(1,6-dihydro-6-thioxopurin-9-yl)-1-thio-D-arabinitol

A crystal structure determination has been used to establish the absolute stereochemistry of a mixed acetal derivative, a nucleoside analogue (1) having purine-6-thione (‘6-mercaptopurine’) attached to an acyclic D-arabinose derivative.