Taxane Synthesis through Intramolecular Pinacol Coupling at C-1−C-2. Highly Oxygenated C-Aromatic Taxanes
摘要:
Chiral, nonracemic intramolecular pinacol coupling substrates 3/20 and 30 have been prepared from ethyl isopropyl ketone and acryloyl chloride, which provide the A-ring and either o-iodobenzyl alcohol or 2,4-dimethoxybenzyl alcohol, which provide the respective aromatic C-rings, in 14-16 linear steps in overall yields of approximately 20%. Potential pinacol coupling substrate 23 could not be made available for investigation due to intervening pinacol rearrangement in the acetonide formation step. 3/20 undergo stereoselective cyclizations mediated by TiCl4-Zn in which the C-9 oxygen substituent plays the dominant role in determining the stereochemical outcome at C-l and C-2 in the respective tricyclic products 4 and 21. The formation of 21 is the more stereoselective process. The reagent of choice for the transformation of 30 into 31 is SmI2, which, although less stereoselective than TiCl4-Zn, leads to higher yielding carbon-carbon bond formation relative to carbonyl reduction. These pinacol cyclizations are interpreted to occur through endo boat-chair transition structures that prefer to orient the developing C-2 substituent and the preexisting C-9 substituent equatorially. Pinacol product 31 was converted through three additional steps into 40 having a B-ring closely related to that of taxol. We believe that these studies indicate pinacol cyclizations at C-1-C-2 to have considerable potential for producing advanced intermediates for syntheses of taxol and related complex taxanes.
Pinacol closure of oxygenated taxane skeleta at C-1-C-2 with stereoinduction by oxygen substituents at C-9 and C-10
摘要:
The treatment of keto aldehydes 4 and 5 with low-valent Ti results in stereoselective intramolecular pinacol couplings that lead to optically active oxygenated taxane skeleta 2 and 3.
Taxane Synthesis through Intramolecular Pinacol Coupling at C-1−C-2. Highly Oxygenated C-Aromatic Taxanes
作者:Charles S. Swindell、Weiming Fan
DOI:10.1021/jo9519367
日期:1996.1.1
Chiral, nonracemic intramolecular pinacol coupling substrates 3/20 and 30 have been prepared from ethyl isopropyl ketone and acryloyl chloride, which provide the A-ring and either o-iodobenzyl alcohol or 2,4-dimethoxybenzyl alcohol, which provide the respective aromatic C-rings, in 14-16 linear steps in overall yields of approximately 20%. Potential pinacol coupling substrate 23 could not be made available for investigation due to intervening pinacol rearrangement in the acetonide formation step. 3/20 undergo stereoselective cyclizations mediated by TiCl4-Zn in which the C-9 oxygen substituent plays the dominant role in determining the stereochemical outcome at C-l and C-2 in the respective tricyclic products 4 and 21. The formation of 21 is the more stereoselective process. The reagent of choice for the transformation of 30 into 31 is SmI2, which, although less stereoselective than TiCl4-Zn, leads to higher yielding carbon-carbon bond formation relative to carbonyl reduction. These pinacol cyclizations are interpreted to occur through endo boat-chair transition structures that prefer to orient the developing C-2 substituent and the preexisting C-9 substituent equatorially. Pinacol product 31 was converted through three additional steps into 40 having a B-ring closely related to that of taxol. We believe that these studies indicate pinacol cyclizations at C-1-C-2 to have considerable potential for producing advanced intermediates for syntheses of taxol and related complex taxanes.
Pinacol closure of oxygenated taxane skeleta at C-1-C-2 with stereoinduction by oxygen substituents at C-9 and C-10
作者:Charles S. Swindell、Weiming Fan、Peter G. Klimko
DOI:10.1016/s0040-4039(00)73292-8
日期:1994.7
The treatment of keto aldehydes 4 and 5 with low-valent Ti results in stereoselective intramolecular pinacol couplings that lead to optically active oxygenated taxane skeleta 2 and 3.