Studies on vitamin D (calciferol) and its analogs. 38. [1,7]-Sigmatropic hydrogen shifts of A-norvitamin D analogs: ring size and substituent effects on the previtamin D-vitamin D equilibrium

The biosynthesis of steroids and triterpenoids

“…900 Incorporation of an 11-hydroxy group into the previtamin D 3 nucleus and expansion of the A-ring to a heptacycle resulted in a rate acceleration and a shifting of the equilibrium towards the vitamin side which was explained as a result of increased A-ring strain, 901 whilst contraction of the A-ring to a pentacycle also led to a rate acceleration, for reasons which are not immediately obvious. 902 (Similarly, an enhanced rate of conversion for 1 -hydroxyprevitamin D 3 diacetate in benzene as compared with other solvents is not easily explained. 903 ) Substitution of the 3-hydroxy group in previtamin D 2 by electron-withdrawing groups shifts the equilibrium to the previtamin side, which has been explained in terms of *interactions.…”

An Asymmetric Synthetic Approach to the A-ring of the Taxol Family of Anti-Cancer Compounds

“…Therefore, in order to improve the yield of the IMDA reaction and at the same time introduce functionality that has to be present in the A-ring we decided to prepare the molecule 38, starting from the already known 2-methyltetronic acid 32 [18] (Scheme 12). The corresponding triflate 33 [19] was coupled with tributyl(vinyl)tin to give 34 [20] which was reduced [21] to the corresponding diol 35. We now intended to protect the hydroxyl group at C-9' and then apply the chemistry we had already developed in the previous scheme to the unprotected hydroxyl moiety.…”

Pd‐Mediated Reactions of Epoxides

“…In contrast, reactions of 2-(2-ethynylaryl)-2-methyloxiranes reported by Mirco and co-workers [Equation (93)] occurred through the ring-opening/heterocyclization/alkoxycarbonylation sequence depicted in Scheme 32. [138] ynyl esters [Equation (95)], [141][142][143] would involve the cationic allenylpalladium complex 33A (Scheme 33), which would accept two electrons from SmI 2 to afford Pd 0 and the allenyl anion 33B. Ring-opening of this andsubsequent workup would lead to the 2-en-4-yn-1-ol, so the opening of the oxirane would not be mediated by palladium.…”