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Additional resources for Advanced Organic Chemistry, Part B: Reactions and Synthesis;
104 one hundred and five 106 J. G. Solsona, P. Romea, F. Urpi, and J. Villarrasa, Org. Lett. , five, 519 (2003). B. M. Trost and H. Urabe, J. Org. Chem. , fifty five, 3982 (1990). C. H. Heathcock and S. Arseniyadis, Tetrahedron Lett. , 26, 6009 (1985) and Erratum Tetrahedron Lett. , 27, 770 (1986); N. A. Van Draanen, S. Arseniyadis, M. T. Crimmins, and C. H. Heathcock, J. Org. Chem. , fifty six, 2499 (1991). 104 (CH3)3C OLi bankruptcy 2 (CH3)3C Reactions of Carbon Nucleophiles with Carbonyl Compounds TMS H CH3 + RCH O H OTMS H O CH3 (CH3)3C Li R CH3 TBSO O R OH O O significant 2,2 ′-anti:syn R i-Pr > 95:5 t-Bu > 95:5 Ph > 95:5 PhCH2OCH2 > 95:5 The corresponding di-n-butylboron enolate offers the 2,2′ -syn adduct. The nonchelating boron is assumed to react via a TS within which the conformation of the substituent is managed via a dipolar impact. The E-titanium enolate used to be ready by way of deprotonation with TMP-MgBr, by way of response with i-PrO three TiCl within the presence of HMPA. The TS for addition is usually ruled through a polar impact and provides and 2,2′ -anti product. O (CH3)3C CH3 OTBDMS H 1) TMPMgBr TBDMSO 2) HMPA H (i PrO)3TiCl4 R three) RCH O CH3 C(CH3)3 Ti(Oi-Pr)3 O OH O (CH3)3C TBDMSO O R CH3 H a sign of the relative effectiveness of oxygen substituent in selling chelation of lithium enolates is located within the enolates 3a–d. The order of choice for the chelation-controlled product is CH3 OCH2 O > TMSO > PhCH2 O > TBDMSO, with the nonchelation product favourite for TBDMSO. 107 OR (CH3)2CH 1) LDA TMEDA CH3 O CH3 OR CH3 OR 2) (CH3)2CHCH CH3 O O H R + OH O 2′,3-anti chelation-control OH O 2′,3-syn R CH(CH3)2 O Li O (CH3)2CH 3a CH3OCH2 93:7 b PhCH2 75:25 c TMS 88:12 d TBDMS 24:76 chelated TS 107 C. Siegel and E. R. Thornton, Tetrahedron Lett. , 27, 457 (1986); A Choudhury and E. R. Thornton, Tetrahedron Lett. , 34, 2221 (1993). Tin(II) enolates having three′ -benzyloxy substituents are topic to chelation regulate. The enolate from 2-(benzyloxymethyl)-3-pentanone gave generally 2,2′ -syn-2,3-syn product, a outcome that's in keeping with a chelated TS. 108 OCH2Ph CH3 O Sn(OTf)2 CH3 O Et3N Sn O CH3Ph O CH2Ph OCH2Ph H O Sn OTf O CH3 H H O O Sn R R CH3 + H H CH2Ph 2,2′-anti -2,3-syn O H CH2Ph O OH R OTf OCH2Ph OH O 2,2′-syn-2,3-syn CH3 O R OH O CH3 H CH3 CH3 CH3 CH3 RCH=O R OTf CH3 CH3 Polar results seem to be very important for three′ -alkoxy substituents in enolates. 3-Benzyloxy teams improve the facial selectivity of E-boron enolates, and this can be attributed to a TS I during which the benzyloxy workforce faces towards the impending aldehyde. This constitution is assumed to be optimal to another conformation J, that could be destabilized through electron pair repulsions among the benzyloxy oxygen and the enolate oxygen. 109 CH2Ph H R CH3 CH3 CH3 PhCH2O BR2 R2B O O O O R CH3 J I This impression is noticeable in relation to ketone four, the place the stereoselectivity of the benzyloxy spinoff is far larger than the compound missing the benzyloxy staff. a hundred and ten O PhCH2O CH3 CH3 (c -C6H11)2BCl PhCH O 2 Et3N four O(c -C6H11)2 O CH3 CH O OH PhCH2O CH3 CH3 CH3 CH3 CH3 an identical -alkoxy impression seems to be operative in a 2’-methoxy substituted process.