合成路线1

Alitretinoin esters (IIa) and (IIb) can be prepared by a variety of methods. Horner-Emmons reaction of either the ionylideneacetaldehyde (V) (3, 4) or its tricarbonyl iron complex (XII) (5) with diethyl 3-(ethoxycarbonyl)-2-methylprop-2-enylphosphonate (XIII), optionally followed by decomplexation using CuCl2 in EtOH, provides alitretinoin ethyl ester (IIa). Alternatively, Wittig reaction of aldehyde (V) with iodomethylenetriphenylphosphorane, followed by in situ elimination of HI using an excess of sodium hexamethyldisilazide, and then addition of lithium butyl(tributylstannyl)cyanocuprate to the obtained terminal acetylene, leads to the unstable vinyl stannane (XIV). Subsequent Stille coupling of crude stannane (XIV) with the vinyl triflate (XV) (derived from ethyl acetoacetate) furnishes the target 9-cis-retinoate (IIa) (6). In a different route, the addition of lithium butyl(tributylstannyl)-cyanocuprate (generated from Bu3SnH, BuLi and CuCN) to (Z)-3-methyl-2-penten-4-yn-1-ol (XVI) and MnO2 oxidation of the allyl alcohol function results in the stannyl dienal (XVII). Subsequent Horner-Emmons reaction of aldehyde (XVII) with phosphonate (XIII) followed by iododestannylation of the obtained adduct furnishes the tetraenyl iodide (XVIII). 2,2,6-Trimethylcyclohexanone (XIX) is reacted with hydrazine hydrate and Et3N in boiling EtOH to produce the corresponding hydrazone, which is converted to vinyl iodide upon treatment with iodine and DBN. Metalation of the vinyl iodide with t-BuLi followed by trapping with trimethyl borate generates an unstable intermediate, assumed to be the boronic ester (XX). Then, Suzuki coupling between the in situ-generated boronate (XX) and freshly prepared tetraenyl iodide (XVIII) provides the target cis-retinoate (IIa) in satisfactory yields (7, 8). Two related synthetic strategies, useful for introducing tritium labeling onto the cyclohexenyl ring of (II), are based on the catalytic hydrogenation of the conjugated cyclohexadiene compound (XXIa) or its nonconjugated analogue (XXIb) in the presence of Rh(PPh3)3Cl (9). Compound (IIb), along with some labeled derivatives, can be prepared by the following route. Condensation of 2,2,6-trimethylcyclohexanone (XIX) with the bromomagnesium acetylide of (Z)-3-methyl-2-penten-4-yn-1-ol (XVI) affords the propargylic alcohol adduct (XXII). After reduction of the triple bond of (XXII) with LiAlH4, oxidation of the primary alcohol group using MnO2 in dry CH2Cl2 yields the hydroxy dienal (XXIII). Wittig olefination of aldehyde (XXIII) with (methoxycarbonylmethylene)triphenylphosphorane followed by smooth dehydration of the tertiary alcohol with 80% formic acid in hexane then leads to the tetraenoate ester (XXIV). After reduction of ester (XXIV) with DIBALH and reoxidation of the obtained alcohol to aldehyde with MnO2, the addition of methylmagnesium bromide results in the secondary alcohol (XXV). The deuterium and tritium analogues of (XXV) can be similarly obtained by using the corresponding labeled Grignard reagents. Subsequent oxidation of (XXV) with MnO2 followed by condensation of the resulting methyl ketone with methyl diethylphosphonoacetate leads to the 9-cis-methylretinoate (IIb) (10, 11). Scheme 2.

合成路线2

The dehydro precursors (XXIa) and (XXIb) are prepared by the following sequences. Allylic bromination of β-cyclocitral (XXXVII) with N-bromosuccinimide in cold CH2Cl2 in the presence of CaO and NaHCO3 followed by elimination of HBr in boiling collidine gives α-safranal (XXXVIII). Subsequent cyclization of aldehyde (XXXVIII) with the lithium carbanion derived from ethyl 3,3-dimethylacrylate (XXXIX) in cold THF provides lactone (XLa). The analogous lactone containing an unconjugated cyclohexadiene ring (XLb) is obtained by MichaelWittig tandem reaction of ethyl 2-isopropylideneacetoacetate (XLI) with the phosphorous ylide derived from allyltriphenylphosphonium chloride (XLII) to produce the cyclic ester (XLIII). Subsequent LiAlH4 reduction of ester (XLIII) followed by Swern oxidation gives aldehyde (XLIV). After isomerization of aldehyde (XLIV) upon treatment with catalytic DBU in CH2Cl2 at room temperature, the isomeric cyclohexadienal (XLV) is condensed with ethyl 3,3-dimethylacrylate (XXXIX) as above to furnish lactone (XLb). Reduction of either lactone (XLa) or (XLb) with DIBALH followed by acid-catalyzed ring opening of the resulting lactols leads to the respective tetraenic aldehydes (XLVIa) and (XLVIb), which undergo Horner-Emmons olefination with diethyl 3-(ethoxycarbonyl)-2-methylprop-2-enylphosphonate (XIII) to furnish the corresponding esters (XXIa) and (XXIb) (9). Scheme 4.

合成路线3

Addition of methyllithium to 3,5-di-tert-butylbenzoic acid (I) at -78 C gave acetophenone (II). The Horner-Emmons condensation of acetophenone (II) with diethyl (cyanomethyl)phosphonate (III) resulted in a mixture of isomeric alpha,beta-unsaturated nitriles (IV). After isolation of the major E-isomer by preparative TLC, cyano group reduction employing DIBAL in CH2Cl2 at -78 C provided aldehyde (V). This was subjected to a new Horner-Emmons condensation with phosphonate (VI) to provide the octatrienoate ester (VII). The target carboxylic acid was then prepared by saponification of ester (VII) with methanolic KOH.

合成路线4

The tetrahydronaphthalene derivative (III) was prepared by condensation of 1-bromo-2-ethoxybenzene (II) with 2,5-dichloro-2,5-dimethylhexane (I) in the presence of AlCl3. Lithium-bromine exchange in (III), followed by treatment with trimethyl borate and acid quenching, provided boronic acid (IV). This was subjected to palladium-catalyzed coupling with 3-bromo-3-buten-1-ol (V) to afford the homoallylic alcohol (VI). A Simmons-Smith reaction of (VI) with chloroiodomethane and diethylzinc furnished the cyclopropyl alcohol (VII), which was further oxidized to aldehyde (VIII) employing PCC. Horner-Emmons condensation of aldehyde (VIII) with phosphonate (IX) gave rise to the dienoate ester (X). Finally, basic hydrolysis of the ester function of (X) yielded the corresponding carboxylic acid.

合成路线5

Horner-Emmons condensation of aldehyde (XI) with phosphonate (XII) produced the diene ester (XIII). Finally, ethyl ester hydrolysis of (XIII) gave rise to the title carboxylic acid.

合成路线6

Addition of methyllithium to 3,5-di-tert-butylsalicylic acid (I) affords ketone (II). Wittig condensation of the ortho-hydroxyacetophenone (II) with (carbethoxymethylene)triphenylphosphorane leads to the coumarin derivative (III). Subsequent reductive opening of the lactone ring of (III) by means of LiAlH4 provides diol (IV). The phenolic hydroxyl of (IV) is then alkylated by 1,1-difluoro-2-bromoethane (V) in the presence of CsF to furnish the difluoroethyl ether (VI). Oxidation of the allylic alcohol function of (VI) to aldehyde (VII) is accomplished by treatment with tetrapropylammonium perruthenate and N-methylmorpholine-N-oxide. Wadsworth-Emmons condensation of the unsaturated aldehyde (VII) with phosphonate (VIII) then provides the triene adduct (IX). The ethyl ester group of (IX) is finally hydrolyzed under alkaline conditions to the desired carboxylic acid.

合成路线7

Alkylation of 2,4-di-tert-butyl-6-iodophenol (I) with bromopropane by means of NaH (1) or CsF (2) provides the corresponding propyl ether (II). Subsequent Suzuki coupling of aryl iodide (II) with 2-formylbenzeneboronic acid (III) leads to the biphenyl aldehyde (IV). This is then condensed with phosphonate (V) producing the dienoate ester (VI). Finally, the ethyl ester group of (VI) is hydrolyzed with LiOH to the target carboxylic acid (1,2).

合成路线8