MECHANISMS. Croomine. Key reaction is the vinylogous Mannich reaction. (CH 2 ) 4 Br H N P. CO 2 Me. Iminium ion formation via decarboxylation
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1 MECAM Croomine Key reaction is the vinylogous Mannich reaction T C 2 Me T C 2 Me (C 2 ) 4 C 2 Me minium ion formation via decarboxylation C 2 Cl 3 Cl ndanomycin The Julia lefination Classical Julia Ar = h ' 2 Ar " 73TL4833; (M.Julia) C a / g ' 2 Ar Li + " sterics normally control " ' trans predominates mproved Julia Ar = ' 2 Li " 2 91TL1175 (. Julia) Allylic bromination n 2' driving force = formation Gareth owlands (g.rowlands@sussex.ac.uk) Ar402, etrosynthesis
2 MECAM F-ing Altohyrtin A oxymercuration with oxidative removal g(ac) 2 gl n 2 gl n gln mercury activates alkene to nucleophilic attack hydride source initiates radical decomposition ab 4 g g regioselective epoxide opening Bn Ti(ir) 4 δ+ Ti (ir) 3 h n 2 attack with inversion activation of the 3 position via chelation (ir) 3 Ti Ch Ch crotyl group prefers to be far from methyl substituents own Crotylation B vs B favoured interaction between B-substituents Gareth owlands (g.rowlands@sussex.ac.uk) Ar402, etrosynthesis
3 MECAM Epothilone A uzkui coupling " reductive elimination d(0) - oxidative addition " d L 2 d L 2 '-BY 2 a' " BY 2 d L 2 ' a enolate geometry controls syn or anti Z-enolate formed on steric grounds apamycin Evan's asymmetric aldol ' ' " B Bu B Bu " " Bu Bu ' h h h aldehyde approach controlled by geometry auxiliary alignment due to dipoles? ozaki-takai-iyama-kishi reaction Me + " TB CrCl 2, icl 2 (cat.) Me TB ' " catalytic reduces nickel i() + 2 Cr() i(0) X i()x Cr() transmetalltion Cr() 2 Cr() oxidative addition ' ' Gareth owlands (g.rowlands@sussex.ac.uk) Ar402, etrosynthesis
4 MECAM V Epoxydictymene Favourski rearrangement only one isomer; base equilibration Me C 2 Me ame icholas reaction 1. Co 2 (C) 8, Et 2 (C) 3 Co Co(C)3 TM Et cobalt complex can be used as alkyne protecting group here serves 3 purposes facilitates icholas cation formation, allowing neutral nucleophile to attack bends 'alkyne' so that reactive centres are in proximity facilitates the auson-khand reaction Et TM 1. TMTf, Et 2 reaction sets up taxing 8-ring sets up auson-khand precursor (C) 3 Co Co(C)3 (C) 3 Co TM Co(C) 3 Et lost in preference to allyl ether possibly because it is less sterically hindered so readily coordinates the Lewis acid possibly because the tertiary alkoxy group is more electron rich and encourages oxonoim ion formation no conclusion as mechanism very variable Gareth owlands (g.rowlands@sussex.ac.uk) Ar402, etrosynthesis
5 MECAM V Epoxydictymene auson-khand eaction L Co 2 (C) 8 (C) 3 Co (C) 3 Co L - C (C) 3 Co (C) 2 Co L insertion controls regiochemistry with substituents staying apart + C (C) 4 Co L Co(C) 3 reductive elimination (C) 3 Co + C (C) 3 Co L + C alkyl migration (or C insertion) (C) 3 Co (C) 3 Co L - Co 2 (C) 7 decomplexation L tandard Conditions: C under atmosphere of C luggish and very messy (low yield) Modern Conditions: Acclererated by -oxides (faster, milder, better yields) Also catalytic variants ecently asymmetric version eason for -oxide Acceleration M C Me 3 M Me 3 M + C 2 + Me 3 removes C from metal thus frees up coordination centre auson-khand eaction: Comp. rg. yn. Vol 5, Ch 9.1, p Accelerating Methods: ynthesis 1989, 472; JMC 1988, 354, 233; Tet. Lett. 1990, Catalytic: JAC 1994, Asymmetric: Angew. Chem. nt. Ed. 1998, 911. Gareth owlands (g.rowlands@sussex.ac.uk) Ar402, etrosynthesis
6 transmetallation or ET MECAM V Erythronolide B β-elimination Zn Zn sufficiently good leaving group Claisen rearrangement boat-like transition state TM TM LDA, TMCl C 2 Altohyrtin A Julia Methylenation 2 h ' Mg BuLi 2 h Mg ' ' "one-pot" bisalkylation TB tbume 2 i ' TB ' TB Gareth owlands (g.rowlands@sussex.ac.uk) Ar402, etrosynthesis
7 MECAM V kadiac acid spiroketalisation most thermodynamically stable complimentary anomeric effects Me Ac ' ' spiroketalisation ' Me ()h 2 C most thermodynamically stable complimentary anomeric effects Me Bn Ar LDA, tbuk Me Me Bn Ar deprotects BDA & forms oxonium ion TFA Bn Ar Bn Ar spiroketalisation h 2 BuLi h 2 cleaves protecting group and forms oxonium species spontaneous antiperiplanar elimination CA, Me most thermodynamically stable complimentary anomeric effects Gareth owlands (g.rowlands@sussex.ac.uk) Ar402, etrosynthesis
8 MECAM V ( )-tenine ossible explanation for the selectivity of the MDA conformation fixed by chelation and sterics Al Al MB(C 2 ) 3 endo selective h MB(C 2 ) 3 auxiliary blocks one face h MB(C 2 ) 4 * Curtius rearrangement (h) 2 3 Me 3 (h) 2 migration with retension Gareth owlands (g.rowlands@sussex.ac.uk) Ar402, etrosynthesis
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