Taxane diterpenes 3: Formation of the eight-membered B-ring by semi-pinacol rearrangement

One-step synthesis of furan-2,5-dicarboxylic acid from furan-2-carboxylic acid using carbon dioxide

“…[18][19][20][21] Furan ring deprotonisation of furan-2-carboxylic acid derivatives with strong bases such as n-butyllithium or LDA (lithiumdiisopropylamide) has been also published. 22,23 The first work concerning the generation of dianions directly from furan-2-carboxylic acid (2) was published by Knight and Nott.…”

Recent advances in the semi-pinacol rearrangement of α-hydroxy epoxides and related compounds

“…Further studies by the same research group went on to apply second, third and fourth semi-pinacol rearrangement-based approaches to related substrates in the hope of securing the rearranged product at the correct oxidation level for subsequent manipulations. 18 In summary, it was found that the eight-membered B-ring of the taxanes could be isolated in very high yield and regiospecificity, albeit by having to activate the alcohol, by the formation of the extremely reactive triflate, to force the rearrangement.…”

“…17 Subsequent studies by the same group showed that the semipinacol rearrangement can also be effective towards the synthesis of other ring-expanded products of the taxane diterpene core and an excellent report on this subject has been published. 18 In the above example, it should be noted that the less substituted of the two possible migrating partners migrates; in general, in acyclic systems, the migratory aptitude of a tert-butyl group is 240 times greater than that of an ethyl group, 19 and thus the more substituted C-11 in 18 might be expected to migrate preferentially. However, in cyclic systems, such as 18, these rearrangements are often controlled by powerful stereoelectronic effects whereby the observed migration is governed by the relative alignment of the molecular orbitals involved.…”

“…Other applications of the semi-pinacol rearrangement to ringexpansions in natural product synthesis can be found in their approaches towards the Taxol TM framework by Magnus et al 17,18 In their efforts to secure a suitable method for the construction of the eight-membered B-ring of the taxanes, a semi-pinacol rearrangement was proposed. In the event, activation of the C-10 alcohol of 18, by formation of a triflate, followed by treatment with aqueous acidic trifluoroethanol resulted in a rapid and clean conversion to the spiro-hemiketal 19, in an impressive 100% yield (Scheme 9).…”

“…However, in cyclic systems, such as 18, these rearrangements are often controlled by powerful stereoelectronic effects whereby the observed migration is governed by the relative alignment of the molecular orbitals involved. 18 In such cases, when one hopes to predict the outcome of a planned 1,2-alkyl shift, careful thought must be given to the orientation of the bonds involved. The orientation of the donating electron pair should be anti-periplanar to the bond that is migrating and that this migrating bond is also anti-periplanar to the leaving group.…”