By Rizzo C.J.
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Derived from the well known, Encyclopedia of Reagents for natural Synthesis (EROS), the similar editors have created a brand new guide which makes a speciality of chiral reagents utilized in uneven synthesis and is designed for the chemist on the bench. This new guide follows an identical structure because the Encyclopedia, together with an advent and an alphabetical association of the reagents.
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1978, 17, 569. ) Acid Chlorides O O O O N N + N R R OH R R N Cl + N N acyl pyridinium ion (more reactive) 1. SOCl2 2. PCl5 3. (COCl)2 Anhydrides O O P2O 5 2 R OH R O O R Activating Agents: Carbonyl Diimidazole O O O R OH N + N R N N N N Acyl Imidazole NH + CO + N PROTECTING GROUPS 63 Dicyclohexylcarbodiimide C 6H11 O NH O R + OH R N C N O O Nu: O C N R +C 6H11 Nu N H N H C 6H11 C 6H11 Ketene formation is a common side reaction- scambling of chiral centers C 6H11 O NH R R O C N H C O "ketene" C 6H11 Hydroxybenzotriazole (HOBT) - reduces ketene formation C 6H11 O R O N NH + O C N N N O N R N N OH C 6H11 N-Hydroxysuccinimide (NHS) O C 6H11 O R O O NH HO + O C N R N O N O C 6H11 O 2,2'-Dipyridyl Disulfide (Aldrithiol, Corey Reagent) Aldrichimica Acta 1971, 4 , 33 O R OH O Ph3P: + N S S R N + S + N N Ph3P=O SH Mukaiyama's Reagent (2-Chloro-1-methyl pyridinium Iodide or 2-Fluoro-1methyl pyridinium p-toulenesulfonate) Aldrichimica Acta 1987, 20 , 54 Chem.
5 mol % Catalyst 50 : 1 150 : 1 33 : 1 52 : 1 OH REDUCTIONS 34 Olefin Isomerization: CH3 (3 : 1) OH OH olefin isomerization O OH major product - Conducting the hydrogenation at high H 2 pressures supresses olefin isomerization and often gives higher diastereoselectivity. Other Lewis basic groups can direct the hydrogenation. (Ir seems to be superior to Rh for these cases) OMe CO2Me OMe Ir CO2Me + Ir+ 99 : 1 O O CO2H Ir+ 7 : 1 Rh+ 1 : 1 O O N O O N O 130 : 1 1:1 Acyclic Examples Rh+ (2 mol %) OH Me CH 3 JCSCC 1982, 348 H2 (15 psi) Me L L M OH (97:3) H O OH H H Ph H 3C H H L CH3 Ph M L Ph anti H H O H 32 : 1 CO2H > 99 : 1 O > 99 : 1 Rh+ OH 1,2-strain Ph syn REDUCTIONS L L R3 H OH R3 35 H O M R2 R1 OH favored R3 H syn R1 R2 R1 R2 R3 H L H R1 R2 M OH disfavored R3 1,2-strain R1 R2 O H L anti - Supression of olefin isomerization is critical for acyclic stereocontrol !
REDUCTIONS 37 General Mechanism: J. Halpern Science 1982, 217, 401 Asymmetric Synthesis 1985, vol 5, 41. CO2Me Ph S P Rh P Ph NHAc S P Rh NH O P (fast equilibrium) CO2Me CH3 rate limiting CO2Me Ph H2 (slow) NHAc Ph P H Rh P O S Ph H CO2Me NH S P (fast) P CO2Me H NH Rh O CH3 CH3 Detailed Mechanism: P * P MeO2C + S Ph CO2Me NHAc NH P * S Rh Ph Rh P P Ph CH3 O CO2Me HN H 3C P O major complex minor complex H2 (slow) MeO2C H H P * P H2 (slow) NH NH Ph Rh CH3 O H 3C Rh * S H CH3 NH Ph H N H 3C O P P * S CO2Me P H Rh O CH3 O CO2Me H Ph S P * Rh CO2Me Ph (R) Minor product HN MeO2C O S P Rh * H P Ph MeO2C Ph H N CH3 O (S) Major product REDUCTIONS + MeO2C H NH H Ph Rh P P Free Energy * NH 38 CH3 O + CO2Me H H Ph + CO2Me HN Rh O P P * P Ph H 3C H 3C Rh * P O minor complex MeO2C * Ph Rh P + NH P CH3 O major complex Reaction Coordinate Ph Ph O O P Ru O P O Ph Ph ACR 1990, 23, 345.
Advanced Organic Reactions by Rizzo C.J.