Study On The Chemical Synthesis And Hypnotic Activity Of PFP Analogues

(E)-dimethyl 5,5'-(but-1-ene-1,3-diyl)bis(4-methylnicotinate)(PFP) is a newpyridine alkaloid isolated from the root of Jasminum sambac (L.) Aiton by our group.Preliminary hypnotic evaluation showed that PFP significantly prolonged thepentobarbital sodium induced sleeping time in mice. However, preparation of thiscompound is difficult because of its low content in the original plant. Thus, it isnecessary to develop a synthetic method for preparation of PFP and its structuralanalogues in order to find potential lead compound.There are two major issues in the total synthesis of PFP. The first is how to formthe 3,4,5-trisubstituted pyridine ring. Pyridine ring occur aromatic electrophilicsubstitution on very harsh reaction conditions and with low yields. Furthermore,pyridine ring can't occur Friedel-Crafts reaction. Pyridine ring is prone to occurnucleophilic substitution but mainly at theα- orγ-C of pyridine ring. So it is verydifficult to add substituting groups at proper site of the pyridine moiety by substitutionreactions. For this reason, it is better to prepare precursors which carry appropriatesubstituting groups and then create the pyridine ring by cyclization reaction. Accordingto the known synthetic methods for pyridine derivatives, a feasible approach is tocondense an appropriate substituted acetylacetic ester with cyanoacetamide to afford amultiple substituted pyridine derivative, which can be converted into the3,4,5-trisubstituted pyridine moiety in PFP by modifying the functional groups. Theother issue in the synthesis of PFP is that there is a trans carbon-carbon double bond inthe molecule, and adjacent to the double bond is a tertiary carbon. The double bond cannot be established by elimination reactions since it will shift to the tertiary carbon toform the thermodynamically more stable product. Therefore, Wittig-Horner reaction ischosen for creation of the double bond, since it is known to have relatively higher position and stereo selectivities, the products have the tendency to form E-configuration.Retrosynthetic analysis show that the structure of PFP can be divided into twoprecursors of Wittig-Horner reaction, one is an aldehyde, that is, 4-methy-5-(1-oxopropan-2-yl)-nicotinonitrile (08), and the other is a phosphonate ester, that is,diethyl- (5-cyano-4-methylpyridin-3-yl)-methylphosphonate (16). For the synthesis of08, following steps were carried out:(1) Reaction of 1-bromo-2-methoxyethane with ethyl acetoacetate to give ethylα-(β-methoxyethyl)-acetoacetate, then condense with cyanoacetamide to afford2,6-dihydroxy-3-cyano-5-(2-methoxyethyl)-4-methylpyriding (01). Chlorination of thephenolic hydroxyls of 01 followed by selective dechlorination, cleavage of the etherbond and oxidation of the aliphatic hydroxyl can afford 4-methy-5-(1-oxoethyl)nicotinonitrile (07). Methylation of an imine derivative of 07 would givethe key intermediate 08. However, chlorination of the phenol groups of 01 was failedeven if most of the common chlorination conditions have been tried.(2)α-Acetyl-γ-butyrolactone was reacted with cyanoacetamide to give2,6-dihydroxy-5-(2-hydroxyethyl)-4-methylnicotinonitrile (03). Chlorination of 03 give2,6-dichloro-5-(2-chloroethyl)-4-methylnicotinonitrile (04). Selective removal thechlorines on the pyridine ring afforded 5-(2-chloroethyl)-4-methylnicotinonitrile (05)which is subjected to KI in acetone in order to get5-(2-iodoethyl)-4-methylnicotinonitrile (06). It was planed to oxidize 06 with DMSO toform 07, and then transform it into 08. But the iodination of 05 have not given theaimed compound 06 but 4-methyl-5-vinylnicotinonitrile (09) in nearly quantitativeyield.(3) Compound 05 was let to react with sodium formate to form a formate ester,which would be transformed into compound 07 by hydrolyzation and oxydation. But thereaction of 05 with sodium formate also led to compound 09 quantitatively. (4) Compound 09 was converted into 5-(1-hydroxyethyl)-4-methylnicotinonitrile(10) by hydroxy mercury-reduction reaction and then oxidized to 5-acetyl-4-methylnicotinonitrile (11). Compound 11 was converted to the enol ether(E)-5-(1-methoxyprop-1-en-2-yl)-4-methylnicotinonitrile (12) by wittig reaction.Compound 12 was hydrolyzed to the key intermediate 08. In this course, the yield of thehydroxyl mercury-reduction reaction was very low and hard to reduplicate even if thereaction condition was carefully controlled.(5) Compound 09 was converted to 5-formyl-4-methylnicotinonitrile (13) throughozonation. The reaction of 13 with methylmagnesium bromide gave 10 which was thenoxidized to 11. Compound 11 was converted to the enol ether 12 by Wittig reaction andthen hydrolyzed to the key intermediate 08 in excellent yield.Started from compound 13, the other key intermediate 16 was obtained viareduction, chlorination and Arbuzov rearrangement reaction.To connect the key intermediates 08 and 16 by Wittig-Horner reaction, most of thecommon conditions of Wittig-Horner reaction have been tested, but the yield of olefinproducts was less than 5%. The yield of olefin products increased to 60% only whenDMPU was added to the reaction system as a co-solvent. In the process of transformingthe cyano group into an ester group, the olefin bond translocated to the tertiary carbon,forming predominant products with a thermodynamicly more stable trisubstituteddouble bond, whether under acidic or alkaline conditions. Only alkaline conditionsafforded a very small amount of target product PFP. This result revealed that PFP isquite unstable. Because the proton of the methine group connecting with the pyridinering is relatively active, the double bond is very easy to shift to the tertiary carbon.The hypnotic activity of six PFP analogues have been evaluated by thepentobarbital sodium induced sleeping time test in mice. Compound 18 and 19 showedsignificant hypnotic activity, comparable to that of PFP.