Dissecting the Stereocontrol Elements of a Catalytic Asymmetric Chlorolactonization: <i>Syn</i> Addition Obviates Bridging Chloronium
作者:Roozbeh Yousefi、Kumar Dilip Ashtekar、Daniel C. Whitehead、James E. Jackson、Babak Borhan
DOI:10.1021/ja4072145
日期:2013.10.2
absolute and relative stereochemistry of addition in enantioselective chlorolactonizations of 4-phenyl-4-pentenoic acid and its related t-butyl ester, catalyzed by (DHQD)2PHAL. Predominant syn addition of the chlorenium and the nucleophile across the olefin is observed. As shown by isotopic labeling, NMR spectroscopy, and derivative studies, the two new stereocenters formed by addition across the double
AbstractThe main fragmentation pathway of ionized hydroxyallenes (1) consists of a methyl loss. Extensive deuterium‐labelling experiments indicate that the terminal allenic carbon is implied in this fragmentation. Collisional activation spectra indicate a propenyl‐acylium structure (a) for these [M – CH3]+ ions which can originate from a 1,4‐hydroxyl migration followed by hydrogen rearrangements. Isomeric hydroxyacetylenes (2) behave similarly, also giving rise, by methyl loss, to acylium ions a. It is proposed that 2+ ˙ is irreversibly isomerized into 1+ ˙ by a 1,3‐hydrogen transfer ‘catalysed’ by the hydroxy group. The proposed internal proton‐bound complex justifies also the easier loss of water from 2+˙. Ethyl loss is also a prominent fragmentation for the hydroxyallene and hydroxy‐acetylene homologues.