Monobenzylation Of 1,n-Diols via Dibutylstannylene Intermediates

A synthetic cyclitol-nucleoside conjugate polyphosphate is a highly potent second messenger mimic

Reactions that form sec-sec ethers are well known, but few lead to compounds with dense functionality around the O-linkage. Replacement of the α-glucopyranosyl unit of adenophostin A, a potent d-myo-inositol 1,4,5-trisphosphate (IP3R) agonist, with a d-chiro-inositol surrogate acting substantially as a pseudosugar, leads to "d-chiro-inositol adenophostin". At its core, this cyclitol-nucleoside trisphosphate comprises a nucleoside sugar linked via an axial d-chiro-inositol 1-hydroxyl-adenosine 3'-ribose ether linkage. A divergent synthesis of d-chiro-inositol adenophostin has been achieved. Key features of the synthetic strategy to produce a triol for phosphorylation include a new selective mono-tosylation of racemic 1,2:4,5-di-O-isopropylidene-myo-inositol using tosyl imidazole; subsequent conversion of the product into separable camphanate ester derivatives, one leading to a chiral myo-inositol triflate used as a synthetic building block and the other to l-5-O-methyl-myo-inositol [l-(+)-bornesitol] to assign the absolute configuration; the nucleophilic coupling of an alkoxide of a ribose pent-4-ene orthoester unit with a structurally rigid chiral myo-inositol triflate derivative, representing the first sec-sec ether formation between a cyclitol and ribose. Reaction of the coupled product with a silylated nucleobase completes the assembly of the core structure. Further protecting group manipulation, mixed O- and N-phosphorylation, and subsequent removal of all protecting groups in a single step achieves the final product, avoiding a separate N6 protection/deprotection strategy. d-chiro-Inositol adenophostin evoked Ca2+ release through IP3Rs at lower concentrations than adenophostin A, hitherto the most potent known agonist of IP3Rs.

Synthesis of sulfated brassinosteroids

A number of water soluble sulfates of 24-epibrassinolide including the 2α,3α-disulfate and all possible monosulfates were synthesized. The target compounds were isolated in crystalline form as the corresponding sodium salts. Pyridine-sulfur trioxide complex was used as sulfating agent followed by transformation of the resulting pyridinium salts into the sodium sulfates by treatment with NaOH. The control of the regioselectivity was achieved by an appropriate use of acetyl and benzyl protecting groups.

Synthesis of brassinosteroids with a keto group in the side chain

The aim of this work was to prepare 24-epicryptolide and 22-dehydro-24-epibrassinolide as possible metabolites of 24-epibrassinolide. The main synthetic problem to be solved was the differentiation of functional groups in brassinosteroids. Distinguishing 2α,3α-diol function from another diol group in 24-epibrassinolide was achieved via selective hydrolysis of 2α,3α-cyclic carbonate or via regioselective reaction of boric acid with the functional groups in the side chain. The hydroxyl at C-23 was more reactive than the 22-OH in the oxidation with bromine in the presence of bis(tributyltin) oxide and in the benzylation reaction that resulted in the predominant formation of the corresponding α-hydroxy ketone derivatives with the ratio ranging from 4:1 to 1.5:1.

Synthesis of fatty acyl derivatives of 24-epibrassinolide

A number of fatty acid (palmitic, myristic and lauric) esters (both 3α- and 3β-isomers) of epibrassinolide has been prepared as reference compounds for metabolic studies. Selective protection of the three of four hydroxyl groups of epibrassinolide was successively performed first as cyclic 22,23-methylboronates and then as 2α-benzyl ethers. α,β-Inversion of C-3 hydroxyl group was achieved through a consecutive oxidation-reduction reactions or by a nucleophilic substitution of the 3α-mesylates. Treatment of the 3α- and 3β-alcohols with palmitic, myristinic or lauric acid chlorides gave the corresponding esters. The hydrolysis of 22,23-methylboronates was performed after their transformation into 2-hydroxy-1,3,2-dioxaborolanes using a cation exchange column with DOWEX 50WX8 in NH4(+) form. Hydrogenolysis of the benzyl ethers catalyzed by palladium yielded the target compounds. This article is part of a Special Issue entitled "Synthesis and biological testing of steroid derivatives as inhibitors".

Syntheses of monohydroxy benzyl ethers of polyols: tri-O-benzylpentaerythritol and other highly benzylated derivatives of symmetrical polyols

Symmetrical polyols can be converted into benzyl ethers with one free hydroxyl group in good yield by reaction of the monodibutylstannylene acetal with excess benzyl bromide in the presence of tetrabutylammonium bromide and diisopropylethylamine in xylene. The reaction pathway involves initial benzylation of the dibutylstannylene acetal to give benzyl and bromodibutylstannyl ethers; if a hydroxyl group remains unsubstituted, the latter ether ring closes and reacts further.