Synthetic Studies on Taxane Diterpenoids.

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We have designed a total synthesis of taxol as the optically pure form. Our strategy was highly dependent on the 8-membered B ring cyclization between dienol ethers and acetal, and it was realized at earlier stage by using model compounds containing aromatric C ring to produce the desired endo tricarbocycle with correct stereochemistry. Our next problem was how to make an optically pure alpha-hydroxyaldehyde as A ring fragment which may play important roles to control the enantioselectivity at each site. For such purpose, we developed a useful synthetic method of optically pure A ring fragment by employing Sharpless AD methodology to cyclohexanedione-aldehyde derivative. Further, a useful method for introduction of 7-OH group was also explored.By combing these results, A fragment was connected with two types of C fragments such as cyclohexenone or anisol derivative, and then B ring cyclization was examined. The reaction took place nicely to give the desired tricarbocycles in good yields with correct stereochemistry. Stereoselective introduction of 7-OH group was readily performed by using our above methodology and we have succeeded to prepare two types of useful synthetic intermediates for taxol in optically pure forms. Both intermediates will be easily converted by applying the following transformation ; introduction of 1) C-8 methyl and 2) C-10 OAc group, 3) construction of D ring, and 4) acylation of C-13 hydroxy group.Since most of them are already dissolved by us and others, it will be very promising to complete an efficient total synthesis of taxol in a near future.It has also been proved our B ring cyclization methodology is very useful for the synthesis of taxusin and taxinine, and their synthetic studies will also complete in a short time.