Versatile Syntheses of Alkynyl- and Substituted Alkynylcyclopropanes: 2-Alkoxyethynylcyclopropanes for the Anellation of Bicyclo[3.3.0]octane Fragments
作者:Hans-Christian Militzer、Sten Schömenauer、Carola Otte、Carsten Puls、Jürgen Hain、Stefan Bräse、Armin de Meijere
DOI:10.1055/s-1993-25988
日期:——
Enol ethers 1a-f are brominated at - 78° C and the resulting alkyl 1,2-dibromoethyl ethers are regioselectively coupled with propargyl-magnesium bromide to give 4-alkoxy-5-bromo-1-pentynes 2a-f, which are protected at the acetylenic terminus with a trimethylsilyl group. These 4-alkoxy-5-bromo-1-trimethylsilyl-1-pentynes 3a-f readily cyclize by γ-elimination to give trimethylsilyl protected 2-alkoxyethynylcyclopropanes 4a-f in excellent overall yields. Under appropriate conditions (E)-diastereomers (E)-4a-f can be prepared selectively. The diastereoselectivity of the bromine addition onto enantiomerically pure chiral enol ethers 1e and 1f, however, is 64% (for 2f) at best. 2-Alkoxy-1-trimethylsilylethynyl-cyclopropanes (E/Z)-4 can be deprotonated with LDA or butyllithium to give configurationally stable 1-lithio derivatives (E)-6, which are trapped by reactive electrophiles (protons, carbonyl compounds, tosyl halides) with retention of configuration on the three-membered ring. 2-Alkoxy-1-ethynylcyclopropanes 5 can be used to anellate a bicyclo[3.3.0]octane fragment onto a cycloalkene in a domino-type reaction sequence, as demonstrated by the Pauson-Khand cycloaddition of (E)-5c to norbornene (14) as well as deltacyclene (20), and subsequent interconversion to the tetracyclic and hexacyclic compounds anti/syn-19 and anti/syn-25/27 with a reasonable degree of diastereoselectivity.
Enol醚1a-f在-78°C下进行溴化,生成的烷基1,2-二溴乙基醚经区域选择性偶联反应与炔丙基-镁溴反应,得到4-烷氧基-5-溴-1-戊炔2a-f,其在炔端用三甲基硅基保护。这些4-烷氧基-5-溴-1-三甲基硅基-1-戊炔3a-f通过γ-消除易于环化,以优异的总收率得到三甲基硅基保护的2-烷氧基乙炔基环丙烷4a-f。在适当条件下,可以选择性地制备(E)-4a-f。然而,溴对纯手性enol醚1e和1f的加成反应的立体选择性最多为64%(对于2f)。2-烷氧基-1-三甲基硅基乙炔基环丙烷(E/Z)-4可通过LDA或丁基锂去质子化,得到构型稳定的1-锂衍生物(E)-6,这些衍生物通过保留三元环的构型被活性亲电试剂(质子、羰基化合物、甲苯磺酰卤化物)捕获。2-烷氧基-1-乙炔基环丙烷5可用于将双环[3.3.0]辛烷片段通过多米诺式反应序列结合到环烯上,如通过(E)-5c与降冰片烯(14)以及δ环烯(20)的Pauson-Khand环加成反应,随后通过合理的立体选择性相互转化为四环和六环化合物anti/syn-19以及anti/syn-25/27所证明。