Abstract:
A new synthetic route to the half-ester (14), usually prepared from a Stobbé Condensation has been investigated. Both of the cis and trans paraconic esters, (25) and (26) respectively, can be ring-opened with sodium methoxide in methanol to give a good yield of the half-ester (14). This also provides further evidence for the intermediacy of paraconic esters in the Stobbé Condensation. A number of partially methylated compounds, (81), (82) and (85) have been isolated from the synthetic route leading to the dimethoxy aldehyde (55). The aldehyde (55) has been successfully employed in a Wittig reaction with the dimethoxy ylide (70), giving a 96% yield of the α,β-unsaturated ester (86). If left in solution, this alkene photoisomerises to a 1:1 mixture containing the E and Z esters, (86) and (87). Reduction of the E isomer (86) with 10% Pd/C, gives a product (93) of over-reduction. The desired product (96) was obtained when the catalyst was pre-hydrogenated and washed with a dilute base solution. The yield of the half-ester (59) has been improved to 85%. A two step reductive methylation of the quinone carbonyls has been used with moderate success to prepare (106) and (110). Asymmetric hydrogenation of the half-ester (14) was successful giving a 53.5% ee using a rhodium-DIOP catalyst. The dimethoxy ester (86) was less successful as a substrate giving a quantitative yield of the reduction product, but the absence of any optical activity. A Diels-Alder route has been linked to a synthetic route developed in this department involving sequential reductive Claisen rearrangements. A "one pot" route to the key intermediate (164) has been developed. A new preparation of 1,2-anthracenedione (192) in a 73% yield from alizarin is reported. A synthetic route to the aldehyde (194) from 1,2,5-trihydroxyanthraquinone (177) involving protection of 1,2-diphenols as benzophenone ketals is discussed. A number of unexpected products on this sequence are reported.