Molybdenum Oxides as Highly Effective Dehydrative Cyclization Catalysts for the Synthesis of Oxazolines and Thiazolines
作者:Akira Sakakura、Rei Kondo、Kazuaki Ishihara
DOI:10.1021/ol050543j
日期:2005.5.1
In the presence of molybdenum oxide the dehydrative cyclization of N-acylserines, N-acylthreonines, and N-acylcysteines can be carried out under Dean-Stark conditions in toluene to give oxazolines and thiazolines. The ammonium salts (NH(4))(6)Mo(7)O(24).4H(2)O and (NH(4))(2)MoO(4) have excellent catalytic activities for the dehydrative cyclization of serine and threonine derivatives, and the acetylacetonate
available molybdenum(VI) oxides such as (NH4)2MoO4, (NH4)6Mo7O24·4H2O, MoO2(acac)2, and MoO2(TMHD)2 are highly effective dehydrative cyclization catalysts for the synthesis of a variety of oxazolines. The reaction proceeds with a complete retention of configuration at the β-position. For the dehydrative cyclization of cysteine derivatives, bis(2-ethyl-8-quinolinolato)dioxomolybdenum(VI) shows remarkable
(NH 4)2 MoO 4,(NH 4)6 Mo 7 O 24 ·4H 2 O,MoO 2(acac)2和MoO 2(TMHD)2等市售氧化钼(VI)是高效的脱水环化反应用于合成各种恶唑啉的催化剂。反应进行时将构型完全保留在β位。对于半胱氨酸衍生物的脱水环化反应,双(2-乙基-8-喹啉酮基)二氧钼(VI)显示出显着的催化活性,并在没有明显损失立体化学完整性的情况下提供了噻唑啉。C 2-外甲硫氨酸的位置。
Titanium(IV)-Mediated Tandem Deprotection−Cyclodehydration of Protected Cysteine <i>N</i>-Amides: Biomimetic Syntheses of Thiazoline- and Thiazole-Containing Heterocycles
作者:Prakash Raman、Hossein Razavi、Jeffery W. Kelly
DOI:10.1021/ol000178q
日期:2000.10.1
[GRAPHICS]The scope and limitations of TiCl4-mediated Delta(2)-thiazoline synthesis via tandem deprotection-dehydrocyclization of trityl-protected cysteine N-amides is presented. While chemical yields are acceptable (53-96%), the stereochemical outcomes vary on the basis of structural considerations and reaction conditions (22-99% ee). Racemization at the C(2)-exomethine position limits the utility of this method for the formation of a thiazoline within a peptide. Treatment of a tritylated Cys-Cys dipeptide with TiCl4 afforded the corresponding thiazole-thiazoline heterocycle 12 (38% yield, 97% ee).