Tandem inter [4+2]/intra [3+2] nitroalkene cycloadditions. 4. Cycloadditions with (E)- and (Z)-1-propenyl ethers
摘要:
The tandem [4 + 2]/[3 + 2] cycloaddition of nitroalkene 1 was initiated efficiently by 1-propenyl ethers as the dienophile. Ethyl(E)-1-propenyl ether reacted highly selectively to give a single nitro so acetal cycloadduct 6. Hydrogenolysis of 6 afforded a single isomer of alpha-hydroxy lactam 8 wherein the newly installed methyl group occupied the beta-configuration. Similarly, ethyl(Z)-1-propenyl ether produced an anomeric mixture of cycloadducts 7a and 7b which afforded a single alpha-hydroxy lactam 9 upon hydrogenolysis. The methyl group in 9 occupied the alpha-orientation. The high and complementary selectivity for production of 8 and 9 was explained on the basis of an extreme endo preference for the ethoxy group of the 1-propenyl ether in the [4 + 2] cycloaddition.
Stereoselective Radical Carbon−Carbon Bond Forming Reactions of β-Alkoxy Esters: Atom and Group Transfer Allylations under Bidentate Chelation Controlled Conditions
作者:Y. Guindon、B. Guérin、C. Chabot、W. Ogilvie
DOI:10.1021/ja9541241
日期:1996.1.1
The radical allylation of a series of beta-alkoxy esters using allyltrimethylsilane in the presence of MgBr2 . OEt(2) is described. Under bidentate chelation-controlled conditions, allyltrimethylsilane rivals allyltributyltin in efficiency and is a superior reagent from ecological and practical perspectives. The reactions work with iodides and bromides as well as phenylselenides. The isolation of gamma-phenylseleno intermediates indicates that the reaction proceeds by an atom transfer process. These reactions require initiation with Et(3)B and can be inhibited by galvinoxyl, m- and p-dinitrobenzene indicating that this atom transfer sequence involves the intermediacy of radicals.
Rate enhancement in the Wacker oxidation of hydroxy-α,β-unsaturated esters: A fast neutral method for the preparation of masked β-ketoesters
作者:Simon X Auclair、Michelle Morris、Michael A. Sturgess