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.