Intramolecular [4+3] Cycloadditions. Studies of Relative Asymmetric Induction
摘要:
Treatment df both the E and Z isomers of alkoxyallylic sulfones 20a and 20b with TiCl4 results in the formation of 4 + 3 cycloadducts by intramolecular cycloaddition. The distribution of diastereomers is different from each isomer of educt. This suggests that cycloaddition occurs faster than any isomerization process of the intermediate allylic cations. Both the E and Z isomers of 20c lead to the same 4 + 3 cycloadduct with essentially complete diastereoselectivity. Inherently high simple diastereoselection and a strong conformational bias in the allylic cation intermediates for both isomers account for this selectivity. The importance of allylic cation stereochemistry in these reactions is underscored by the cyclization of 20d. The Z isomer give only 4 + 3 cycloadducts with excellent relative but poor simple diastereoselection, suggestive of a concerted reaction. The E isomer give a [3 + 2] cycloadduct, enol ether 60, as the major cycloadduct as well as 4 + 3 cycloadducts and chloride-trapping product 61. This result is indicative of a stepwise reaction. The data reflect the importance of allylic cation stereochemistry in the regiochemical and stereochemical outcomes of intramolecular 4 + 3 cycloaddition reactions.
Intramolecular [4+3] Cycloadditions. Studies of Relative Asymmetric Induction
摘要:
Treatment df both the E and Z isomers of alkoxyallylic sulfones 20a and 20b with TiCl4 results in the formation of 4 + 3 cycloadducts by intramolecular cycloaddition. The distribution of diastereomers is different from each isomer of educt. This suggests that cycloaddition occurs faster than any isomerization process of the intermediate allylic cations. Both the E and Z isomers of 20c lead to the same 4 + 3 cycloadduct with essentially complete diastereoselectivity. Inherently high simple diastereoselection and a strong conformational bias in the allylic cation intermediates for both isomers account for this selectivity. The importance of allylic cation stereochemistry in these reactions is underscored by the cyclization of 20d. The Z isomer give only 4 + 3 cycloadducts with excellent relative but poor simple diastereoselection, suggestive of a concerted reaction. The E isomer give a [3 + 2] cycloadduct, enol ether 60, as the major cycloadduct as well as 4 + 3 cycloadducts and chloride-trapping product 61. This result is indicative of a stepwise reaction. The data reflect the importance of allylic cation stereochemistry in the regiochemical and stereochemical outcomes of intramolecular 4 + 3 cycloaddition reactions.
Rapid and Stereoselective Synthesis of Spirocyclic Ethers via the Intramolecular Piancatelli Rearrangement
作者:Leoni I. Palmer、Javier Read de Alaniz
DOI:10.1021/ol303263q
日期:2013.2.1
The first example of a Piancatelli rearrangement of alcohols is demonstrated utilizing dysprosium(III) triflate as a catalyst to access oxaspirocycles in a highly diastereoselective manner. The cascade reaction constructs the spirocyclicether ring system and the tertiary stereocenter in a single operation and is experimentally easy to perform.
1,6,7-trioxa-spiro[4.5]decanes were synthesized using the hydrogen peroxide in UHP (urea–H2O2 complex) as the source of the peroxy bond. Incorporation of H2O2 into the organic molecular framework was facilitated by the potential to form a five- or six memberd cyclic hemiketal. Saturation of the double bond in a substituted γ-keto-butenal, a step required in all the syntheses, was achieved in one case
使用过氧化氢在UHP(尿素-H 2 O 2络合物)中合成了四个1,2,7-三氧杂螺[5.5]十一烷和两个1,6,7-三氧杂螺[4.5]癸烷。过氧键。通过形成五元或六元环状半缩酮的潜力,可以促进将H 2 O 2引入有机分子骨架中。在一种情况下,用NaI /浓盐酸在低温下实现了取代的γ-酮-丁醛中双键的饱和,这是所有合成所需要的步骤,而在室温下未裂解TBS保护基或引起过多的副反应。温度。所有这些过氧化物均显示出与天然过氧苯甲酸和已知类似物相当的体外抗疟活性。
Multiphotocatalyst Cascades: From Furans to Fused Butyrolactones and Substituted Cyclopentanones
relies on a new and mild method for butenolide synthesis mediated by thiols. The initial photooxygenation and butenolide synthesis have been merged with subsequent photoredox reactions to achieve rare dual-photocatalyst cascades affording various fused butyrolactones. Ground state Lewis acid activity for methyleneblue has been unveiled and then exploited in the synthesis of substituted cyclopentanones
Substituent Effects in the Intramolecular Diels−Alder Reaction of 6-Furylhexenoates
作者:Michael E. Jung、Mehrak Kiankarimi
DOI:10.1021/jo9721554
日期:1998.5.1
The series of furyl enone esters, 4a-c, were synthesized from furan or furfural by straightforward routes. Their thermal intramolecular Diels-Alder reactions to give the tricyclic ketones 5a-c were studied in acetonitrile and toluene at two temperatures and the kinetics of the reactions determined. A comparison of these data with that obtained for the corresponding esters 2 to give the lactones 3 indicates that the rate enhancements seen for the esters (rate of dimethyl 310 times that of monomethyl) are much larger than those seen for the ketones (rate of dimethyl 6.8 times that of monomethyl). Thus, this is additional evidence for the earlier hypothesis that the presence of the oxygen atom in the tether is a factor responsible for the larger than normal rate enhancements.